What size cable do I need for an electric shower?

For electric showers: 7.5kW needs 6mm² cable with 32A MCB, 8.5kW needs 6mm² with 40A MCB, 9.5kW needs 10mm² with 40A MCB, and 10.5kW+ needs 10mm² with 45A MCB. Calculate current by dividing power by 230V (e.g., 9500W ÷ 230V = 41.3A). Cable run should be less than 20m for 6mm² or 30m for 10mm² to stay within voltage drop limits.

What cable size for a cooker circuit?

Cooker cable sizes depend on the appliance rating: Single oven (2-3kW) needs 2.5mm² with 20A MCB, Double oven (5-6kW) needs 4mm² with 32A MCB, Range cooker (8-10kW) needs 6mm² with 40A MCB, and Large range (12kW+) needs 10mm² with 45A MCB. Most domestic cooker circuits use 6mm² cable as standard.

What cable size for an EV charger?

For EV chargers: 3.6kW (16A) uses 2.5mm² cable, 7.4kW (32A) uses 6mm² for runs up to 20m or 10mm² for 20-40m runs, and 22kW 3-phase uses 6mm² 4-core SWA. Most home chargers are 7.4kW single phase. SWA (Steel Wire Armoured) cable is recommended for outdoor/underground runs to the charger location.

What is 6mm cable used for?

6mm² cable (twin & earth or SWA) is commonly used for: Electric showers up to 9kW, Small cookers up to 8kW, EV chargers at 7.4kW (32A) for runs under 20m, Hot tubs up to 32A, Outbuilding supplies, and High-power radial circuits. It has a current rating of 47A clipped direct or 41A in conduit.

What is 10mm cable used for?

10mm² cable is used for high-power installations: Large electric showers (9.5kW+), Range cookers (10-12kW), EV chargers with long cable runs, Large outbuildings/workshops, Sub-main distribution, and Hot tubs/spas over 32A. Current rating is 64A clipped direct or 57A in conduit.

What cable size do I need for a 32A circuit?

For a 32A circuit, you typically need 4mm² copper cable for runs up to 23 meters (clipped direct installation). For longer runs or installation in conduit, you may need 6mm² to stay within voltage drop limits. Ring main circuits use 2.5mm² cable with 32A MCB because current is shared between both legs of the ring.

How do I calculate voltage drop in a cable?

Voltage drop = (mV/A/m × Current × Length) ÷ 1000. The mV/A/m value is found in BS 7671 tables and varies by cable size and type. For example, 2.5mm² copper has 18 mV/A/m. Our calculator automatically performs this calculation and checks BS 7671 compliance (≤3% for lighting, ≤5% for other circuits).

What is the difference between Twin & Earth and SWA cable?

Twin & Earth (6242Y) is flat grey cable for internal domestic wiring - used for lighting, sockets, and fixed appliances inside buildings. SWA (Steel Wire Armoured) has steel wire armour protection and is used for underground burial, external runs, and industrial installations. SWA can be buried directly (450mm deep) while twin & earth must be protected in conduit if external.

What installation method should I choose?

Common installation methods:Clipped Direct (Method C): Cable clipped directly to wall/ceiling - best heat dissipationIn Conduit (Method B): Cable in plastic/metal conduit - reduced current capacityIn Insulation (Method 103): Cable touching thermal insulation - significant derating requiredUnderground (Method D): Direct burial or in ducts - good heat dissipation

Why is my recommended cable size larger than expected?

Several factors increase cable size requirements: Long cable runs (voltage drop), Installation in conduit/trunking (reduced cooling), Cables touching insulation (derating), High ambient temperatures, or Multiple circuits grouped together. BS 7671 requires conservative sizing to ensure safety and compliance.

What does the recommended MCB rating mean?

The MCB (Miniature Circuit Breaker) rating is the maximum protective device size suitable for the cable. The MCB must protect the cable from overload, so its rating cannot exceed the cable's current-carrying capacity. For example, 2.5mm² copper (clipped direct) carries 27A, so the maximum MCB is 25A or 32A depending on installation factors.

What cable size for a garden office or outbuilding?

For outbuildings, use SWA cable buried 450mm deep: Light use (lighting + sockets) needs 4mm² SWA with 32A MCB, With electric heating needs 6mm² with 40A MCB, and Workshop with power tools needs 10mm² with 63A MCB. Install a small consumer unit in the outbuilding with RCD protection for all circuits.

Cable Size Calculator - BS 7671 Compliant

Calculate the correct cable size for electrical installations according to British Standards BS 7671:2018+A2:2022

Last updated: 25 April 2026

Calculate Cable Size

Enter your installation parameters to calculate the required cable size

Cable Type

Flat twin and earth cable with PVC insulation • Domestic fixed wiring - lighting, sockets, cookers

Load Input Method

Load Current*

Amps

The maximum current the circuit will carry

Cable Length*

meters

One-way cable run distance from distribution board to load

Voltage*

Installation Method*

How the cable will be installed (affects current carrying capacity)

Conductor Material

Copper (Most common)Aluminium

What Size Cable Do I Need? Quick Answer

Cable size in the UK depends on the circuit's design current, installation method, and cable run length per BS 7671:2018+A2:2022. For domestic installations: use 1.0mm² or 1.5mm² for lighting circuits (6A-10A MCB), 2.5mm² for socket ring mains (32A MCB), 6.0mm² for electric showers up to 9.5kW, and 10.0mm² for showers above 9.5kW or long cable runs where voltage drop exceeds the 5% limit. All cable sizes must be verified against BS 7671 current-carrying capacity tables with correction factors applied.

kW to Cable Size Conversion (Single Phase, 230V)

To convert kW to cable size, divide the power in watts by 230V to find the design current in amps, then select the cable from BS 7671 tables. This table shows typical cable sizes for common load ratings at 230V single phase, assuming clipped direct installation at 30°C ambient with no grouping.

Load (kW)	Current (A)	Cable Size	MCB Rating	Typical Application
1-2 kW	4.3-8.7A	1.0-1.5mm²	6-10A	Lighting circuits
3 kW	13.0A	2.5mm²	16A	Immersion heater, small appliances
5-6 kW	21.7-26.1A	4.0mm²	32A	Small cooker, hob
7-8.5 kW	30.4-37.0A	6.0mm²	40A	Electric shower, EV charger (7kW)
9.5-10 kW	41.3-43.5A	10.0mm²	45-50A	Large shower, cooker, EV charger (10kW)
12-15 kW	52.2-65.2A	16.0mm²	63A	Sub-main to outbuilding, 3-phase loads

Cable sizes assume short cable runs (<15m) with no grouping or thermal insulation derating. For longer runs, voltage drop may require upsizing — use the calculator above to check. Reference: BS 7671 Table 4D5 (twin & earth copper conductors, 70°C PVC insulation).

How to Calculate Cable Size UK - Complete Guide

Calculating the correct cable size for UK electrical installations requires consideration of several critical factors according to BS 7671 wiring regulations. This guide explains the complete process step-by-step.

Step 1: Determine the Design Current (Ib)

The design current is the maximum current your circuit will carry under normal operating conditions.

For known loads: Divide the power (watts) by voltage. Example: 9.5kW shower ÷ 230V = 41.3A
For socket circuits: Use the circuit breaker rating (e.g., 32A for ring main)
For motors: Check the motor nameplate for full load current

Step 2: Apply Correction Factors

BS 7671 requires cable sizing to account for installation conditions that affect heat dissipation:

Installation Method (Ca)

Clipped direct (Method C): Factor = 1.0 (best cooling)
In conduit on walls (Method B): Factor = 0.95
In thermal insulation (Method A): Factor = 0.5-0.89 (worst cooling)

Ambient Temperature (Cg)

Standard is 30°C. For loft spaces (45°C) or other hot environments, apply temperature correction from BS 7671 Table 4B1.

Grouped Circuits (Ci)

Multiple cables grouped together generate more heat. Correction factors from BS 7671 Table 4C1.

Step 3: Calculate Required Current Carrying Capacity (It)

Divide your design current by the overall correction factor:

It = Ib ÷ (Ca × Cg × Ci)

Example: 32A circuit, in conduit (0.95), at 30°C (1.0), with 2 cables (0.8):
It = 32A ÷ (0.95 × 1.0 × 0.8) = 42.1A required capacity

Step 4: Select Cable Size from BS 7671 Tables

Choose the cable size from the appropriate BS 7671 table based on your cable type and installation method:

Table 4D5: Twin and earth cables (most common domestic)
Table 4D4A: Steel wire armoured (SWA) cables
Table 4E4A: Single-core cables in conduit

Select the smallest cable size that has a current rating ≥ It (your calculated capacity)

Step 5: Check Voltage Drop

BS 7671 Regulation 525 limits voltage drop to prevent poor performance:

Lighting circuits: Maximum 3% (6.9V on 230V system)
Other circuits: Maximum 5% (11.5V on 230V system)

Voltage Drop Formula (Single Phase):

Vd = (mV/A/m × Length × Current) ÷ 1000

mV/A/m values are found in BS 7671 voltage drop tables (e.g., Table 4D5)

If voltage drop exceeds limits, increase cable size and recalculate.

Real-World Example: 9.5kW Electric Shower

Given: 9.5kW shower, 230V, 20m cable run, clipped direct, 30°C ambient

Step 1: Design current (Ib) = 9500W ÷ 230V = 41.3A

Step 2: Correction factors: Ca=1.0, Cg=1.0, Ci=1.0

Step 3: Required capacity: It = 41.3A ÷ 1.0 = 41.3A

Step 4: From Table 4D5, 6mm² twin & earth = 47A (clipped direct) ✓

Step 5: Voltage drop: (18 × 20 × 41.3) ÷ 1000 = 14.87V = 6.5%

⚠️ Voltage drop exceeds 5% limit!

Solution: Upgrade to 10mm² cable (voltage drop = 3.9%) ✓

Final answer: 10mm² twin & earth cable with 45A MCB

Important BS 7671 Cable Sizing Requirements

Cable must be able to carry the design current indefinitely (Regulation 512.1.5)
Protective device must disconnect before cable temperature becomes dangerous
Account for thermal insulation contact (Regulation 523.7)
Consider fault current and short circuit protection (Regulation 434.5.2)
All calculations must reference current BS 7671:2018+A2:2022 tables

💡 Pro Tip: Always use the calculator above for accurate results. Manual calculations are prone to errors, especially when multiple correction factors apply. The calculator automatically applies all BS 7671 requirements and checks voltage drop compliance. For full BS 7671 cable current rating tables and correction factor reference data, see the SparkyHub cable ratings guide.

The Cable Sizing Hierarchy: Ib ≤ In ≤ I_z

BS 7671 Regulation 433.1.1 expresses cable sizing as a coordination inequality. Every correctly sized UK circuit satisfies three currents in ascending order — memorising this framework is how electricians verify a cable choice in one pass.

I_b ≤ I_n ≤ I_z

I_b — Design Current

The current the circuit is designed to carry in normal service. Calculated from load power (W ÷ V) or taken from the protective device rating after diversity.

I_n — Nominal Protective Device Rating

The rated current of the MCB, RCBO or fuse protecting the circuit (BS EN 60898, BS EN 61009, BS 88, BS 3036). Must equal or exceed I_b.

I_z — Cable Current-Carrying Capacity

The tabulated cable rating I_t (Appendix 4, Tables 4D1–4J4) after all correction factors: I_z = I_t × C_a × C_g × C_i × C_c.

The Four Correction Factors (C_a, C_g, C_i, C_c)

Factor	Accounts For	BS 7671 Reference	Typical Range
C_a	Ambient temperature (if not 30°C)	Table 4B1	0.71–1.22
C_g	Grouping of circuits on the same surface or enclosure	Table 4C1–4C5	0.40–1.00
C_i	Thermal insulation (cable buried in insulation)	Reg 523.9, Table 4A2	0.50–0.89
C_c	Type of protective device (0.725 for BS 3036 rewirable fuses)	Reg 433.1.1	0.725–1.00

For modern MCBs (BS EN 60898) and RCBOs (BS EN 61009), C_c = 1.0 and can be omitted. It only applies to semi-enclosed rewirable fuses still found in some legacy domestic consumer units.

Worked Example: Verifying the Hierarchy

Circuit: 32A ring final, 2.5mm² T&E, in plasterboard ceiling with >100mm insulation on one side

I_b: 32A (design current)

I_n: 32A Type B MCB (BS EN 60898) → I_b ≤ I_n ✓

I_t: 27A (Table 4D5, Method C clipped direct)

Corrections: C_a=1.0 (30°C), C_g=1.0 (not grouped), C_i=0.75 (Method 101), C_c=1.0 (MCB)

I_z: 27 × 0.75 = 20.25A

✗ I_n (32A) > I_z (20.25A) — FAILS the hierarchy. Cable must be upsized to 4.0mm² (I_t=37A, I_z=27.75A ≥ 32A ✓).

Pro tip: the calculator above runs the full hierarchy check automatically. Use this framework when reviewing manual designs or inspecting existing installations to spot under-sized cables fast.

Cable Size Chart UK - Quick Reference Guide

This cable size chart shows current ratings for common UK cable types. Values are for copper conductors at 30°C ambient with PVC insulation (70°C). Always verify with BS 7671 tables for your specific installation.

Twin & Earth Cable (6242Y) - Current Ratings

Cable Size	Clipped Direct	In Conduit	Max MCB	Typical Use
1.0mm²	15.5A	13.5A	10A	Lighting circuits
1.5mm²	20A	17.5A	16A	Lighting, immersion heaters
2.5mm²	27A	24A	20A	Ring mains, radial sockets
4.0mm²	37A	32A	32A	Cooker circuits, high-power radials
6.0mm²	47A	41A	40A	Electric showers, small cookers
10.0mm²	64A	57A	50A	Large cookers, EV chargers
16.0mm²	85A	76A	63A	Sub-mains, large loads

Reference: BS 7671 Table 4D5 (Flat twin and earth cables, copper conductors, 70°C PVC)

SWA Cable (Steel Wire Armoured) - Current Ratings

Cable Size	Clipped/Tray	Buried Direct	Max MCB	Typical Use
2.5mm²	28A	35A	25A	Garden lighting, small outbuildings
4.0mm²	37A	45A	32A	Garages, workshops
6.0mm²	47A	57A	40A	EV chargers, hot tubs
10.0mm²	64A	78A	50A	Larger outbuildings, sub-mains
16.0mm²	84A	102A	63A	Commercial sub-mains
25.0mm²	110A	133A	100A	Main supplies, large installations

Reference: BS 7671 Table 4D4A (Armoured 70°C PVC cables, 2-core for single phase)

Fire-Rated, Mineral-Insulated and Flexible Cables — UK Sizing Reference

BS 7671 cable sizing tables cover more than twin & earth and SWA. Commercial installations, fire alarm systems, escape-route lighting and machinery wiring all use cables that electricians must size correctly under BS 5839-1, BS 5266-1 and BS EN 60204-1.

Fire-Rated Cables (FP200, FP400, Firetuf, Prysmian FP PLUS)

Fire-resistant cables maintain circuit integrity during a fire — essential for fire alarms (BS 5839-1), emergency lighting (BS 5266-1), smoke control systems and sprinkler pumps. Ratings are specified by BS 8491 / BS EN 50200 survival times (PH30, PH60, PH120) and the "Enhanced" category (BS 8434-2).

Cable Size	Clipped Direct (A)	In Conduit (A)	Typical Use
1.0mm²	15A	11.5A	Fire alarm detector loops, interface units
1.5mm²	19.5A	15A	Emergency lighting circuits (230V)
2.5mm²	27A	20A	Sounder circuits, powered escape signs
4.0mm²	36A	27A	Smoke extract fans, sprinkler pumps (small)
6.0mm²	46A	34A	Fire pump sub-mains, stair pressurisation

Typical values — always verify against manufacturer data sheet. Standard FP-range cables meet PH30/PH60 per BS EN 50200; use "Enhanced" grade (BS 8434-2) where specified by the fire strategy (e.g. high-rise residential common areas).

Mineral-Insulated Copper Cable (MICC / Pyro) — BS 7671 Table 4H1A–4H4A

MICC (branded Pyrotenax, Wrexham MIMS) uses compacted magnesium-oxide insulation in a copper sheath — it is non-combustible, withstands temperatures up to 250°C in service and passes BS 6387 CWZ. Used on fire alarms in heritage buildings, process plant and where the highest fire-survival rating is specified.

Conductor Size	500V MICC (A)	750V MICC (A)	Reference
1.0mm²	20A	21A	Table 4H1A
1.5mm²	25A	26A	Table 4H1A
2.5mm²	34A	35A	Table 4H1A
4.0mm²	45A	46A	Table 4H1A
10.0mm²	84A	86A	Table 4H1A

Values for bare MICC, single-core, exposed to touch. Sheathed (LSF overall) cable uses lower ratings — see Table 4H2A. MICC has extremely low voltage drop (higher conductor operating temperature raises resistance only modestly).

Flexible Cables — SY, H07RN-F, 3183Y, 3184Y

Flexibles are used for machinery connections (BS EN 60204-1), generators, temporary power, festoon lighting and appliance leads. Sizing uses Table 4F3A (light PVC flex) and Table 4H3A (heavy-duty rubber flex).

Cable Type	Jacket	Typical Application	1.5mm² / 2.5mm² Rating
3183Y / 3184Y	PVC	Appliance leads, pendant luminaires	16A / 25A
H05VV-F	PVC (harmonised)	Class II appliance flex, indoor use	16A / 25A
H07RN-F	Rubber (EPR)	Generator leads, site power, outdoor use, mobile machinery	20A / 27A
SY (CY/YY)	PVC + braided screen	Machinery control, inverter/VSD motor cables	16A / 21A
Arctic grade (3183A)	PVC (-25°C cold flex)	110V site extension leads, cold-store cabling	16A / 25A

Note: SY cables are not armoured in the BS 7671 sense — the braid is a mechanical screen, not a BS EN 60228 Class 4 earth. Do not rely on the screen as a CPC. BS 7629 "braided screen" cables are equivalent.

LSZH / LSF Insulation — What's the Difference?

LSZH (Low Smoke Zero Halogen) meets IEC 60754-1/-2 — emits no corrosive halogen gases if burned. Used on London Underground, hospitals, data centres, high-rise escape routes (Approved Document B). LSF (Low Smoke & Fumes) is a lower-specification modified PVC — still contains halogens but reduces smoke density. For current-carrying capacity both are treated as 70°C thermoplastic insulation (Tables 4D/4E), so the BS 7671 ratings are identical to standard PVC.

Common Cable Sizes for UK Installations

Here are typical cable sizes for common domestic and commercial installations. Always calculate for your specific cable length and installation method.

Electric Shower

7.5kW shower: 6mm² cable, 32A MCB

8.5kW shower: 6mm² cable, 40A MCB

9.5kW shower: 10mm² cable, 40A MCB

10.5kW+ shower: 10mm² cable, 45A MCB

Current = Power ÷ 230V (e.g., 9500W ÷ 230V = 41.3A)

Electric Cooker/Oven

Single oven (2-3kW): 2.5mm², 20A MCB

Double oven (5-6kW): 4mm², 32A MCB

Range cooker (8-10kW): 6mm², 40A MCB

Large range (12kW+): 10mm², 45A MCB

Note: Apply diversity for cookers with multiple rings

EV Charger

3.6kW (16A): 2.5mm² cable

7.4kW (32A): 6mm² cable (up to 20m)

7.4kW (32A): 10mm² cable (20-40m)

22kW (32A 3-phase): 6mm² 4-core SWA

Most home EV chargers are 7.4kW single phase

Immersion Heater

Standard (3kW): 2.5mm², 16A MCB

Long run (3kW, 20m+): 4mm², 16A MCB

Heat-resistant flex required at the heater connection

Ring Main (Sockets)

Standard ring: 2.5mm², 32A MCB

Floor area: Max 100m² per ring

Spurs: Max 1 per socket, same cable size

Radial circuits: 2.5mm²/20A (20m²) or 4mm²/32A (50m²)

Lighting Circuit

Domestic: 1.0mm² or 1.5mm², 6A MCB

Max per circuit: 12 lighting points

Voltage drop: Max 3% (6.9V)

Use 1.5mm² for longer runs or LED driver loads

Hot Tub / Spa

13A plug-in: Existing socket circuit

32A dedicated: 6mm² SWA

40A large spa: 10mm² SWA

RCD protection required - outdoor special location

Outbuilding / Garden Office

Light use: 4mm² SWA, 32A MCB

With heating: 6mm² SWA, 40A MCB

Workshop: 10mm² SWA, 63A MCB

SWA buried 450mm deep or in ducting

Air Conditioning Unit

Small split (1-2kW): 2.5mm², 16A MCB

Medium (3-5kW): 4mm², 20A MCB

Large (7kW+): 6mm², 32A MCB

Check manufacturer specs - motor starting current

Important: These are typical values only. Always calculate cable size for your specific installation considering cable length, installation method, ambient temperature, and grouping with other cables. Use the calculator above for accurate sizing.

Worked Examples - Step-by-Step Cable Size Calculations

Follow these detailed calculations for the most common UK electrical installations. All examples use BS 7671:2018+A2:2022 (18th Edition) standards.

1Cable Size for 7.4kW EV Charger (32A)

Scenario:

Load: 7.4kW single-phase EV charger
Cable run: 25 meters from consumer unit to garage
Installation: 6mm² SWA cable, buried direct 450mm deep
Voltage: 230V single-phase
Ambient: 20°C (buried underground)

Step 1 - Calculate Design Current:

Ib = 7400W ÷ 230V = 32.2A

Step 2 - Apply Correction Factors:

Installation method (buried direct): Ca = 1.0 (from Table 4D4A)
Ambient temp (20°C underground): Cg = 1.0
Grouping (single circuit): Ci = 1.0
Overall factor: 1.0 × 1.0 × 1.0 = 1.0

Step 3 - Required Capacity:

It = 32.2A ÷ 1.0 = 32.2A

Step 4 - Select Cable from BS 7671 Table 4D4A (SWA):

6mm² SWA buried direct = 57A rating ✓ (exceeds 32.2A)

Step 5 - Check Voltage Drop:

From Table 4D4A: 6mm² SWA = 7.3 mV/A/m

Vd = (7.3 × 25 × 32.2) ÷ 1000 = 5.88V

Percentage = (5.88 ÷ 230) × 100 = 2.56% ✓ (under 5% limit)

✓ Final Answer:

Use 6mm² 3-core SWA cable with 32A Type B MCB

Note: Ensure RCD protection (30mA) as per BS 7671 Regulation 722.531.2

2What Size Cable for 8.5kW Electric Shower?

Scenario:

Load: 8.5kW electric shower
Cable run: 12 meters from consumer unit
Installation: Twin & earth, clipped direct to wall
Voltage: 230V single-phase
Ambient: 30°C (standard)

Step 1 - Calculate Design Current:

Ib = 8500W ÷ 230V = 37A

Step 2 - Apply Correction Factors:

Installation method (clipped direct, Method C): Ca = 1.0
Ambient temp (30°C): Cg = 1.0
Grouping (single circuit): Ci = 1.0
Overall factor: 1.0

Step 3 - Required Capacity:

It = 37A ÷ 1.0 = 37A

Step 4 - Select Cable from BS 7671 Table 4D5:

6mm² twin & earth clipped direct = 47A ✓ (exceeds 37A)

Note: 4mm² = 37A would be borderline, use 6mm² for safety margin

Step 5 - Check Voltage Drop:

From Table 4D5: 6mm² = 7.3 mV/A/m

Vd = (7.3 × 12 × 37) ÷ 1000 = 3.24V

Percentage = (3.24 ÷ 230) × 100 = 1.41% ✓ (well under 5%)

✓ Final Answer:

Use 6mm² twin & earth cable with 40A Type B MCB

3Cable Size for 10kW Electric Cooker

Scenario:

Load: 10kW double oven + hob
Cable run: 8 meters from consumer unit
Installation: Twin & earth, clipped direct
Apply diversity (BS 7671 Appendix 15)

Step 1 - Calculate Design Current with Diversity:

Total load: 10kW = 43.5A at 230V

Apply cooker diversity:

First 10A at 100% = 10A
Remaining 33.5A at 30% = 10A
Plus 5A for socket outlet = 5A

Design current (Ib) = 10 + 10 + 5 = 25A

Step 2 - Required Capacity:

It = 25A ÷ 1.0 = 25A (no derating needed)

Step 3 - Select Cable:

4mm² twin & earth = 37A ✓ OR 6mm² = 47A for longer runs

Step 4 - Voltage Drop Check (using 6mm²):

Vd = (7.3 × 8 × 25) ÷ 1000 = 1.46V = 0.63% ✓

✓ Final Answer:

Use 6mm² twin & earth cable with 32A Type B MCB

Note: Diversity applied per BS 7671 Appendix 15 for cooking appliances

46mm Cable Amps - Current Rating Guide

The current rating of 6mm² cable depends on the installation method. Here are the ratings from BS 7671:

Installation Method	Current Rating	Max MCB	Reference
Twin & Earth - Clipped Direct	47A	40A	Table 4D5 Method C
Twin & Earth - In Conduit/Trunking	41A	32A	Table 4D5 Method B
SWA - Clipped to Surface	47A	40A	Table 4D4A Method C
SWA - Buried Direct	57A	50A	Table 4D4A Method D

Important: These ratings assume 30°C ambient temperature and single circuit. Apply correction factors for grouped cables or high temperatures.

510mm Cable Rating - How Many Amps Can It Handle?

10mm² cable is commonly used for larger loads like big cookers, sub-mains, and high-power equipment:

Cable Type & Method	Rating (Amps)	Max MCB	Typical Use
Twin & Earth - Clipped Direct	64A	50A	Large cookers, shower pumps
Twin & Earth - In Conduit	57A	50A	Protected high-power circuits
SWA - Buried Direct	78A	63A	Outbuilding sub-mains

Real-world example: A 40A shower (9.2kW) with 15m cable run would need 10mm² cable to keep voltage drop under 5%. Using 6mm² would result in 6.8% voltage drop (too high).

💡 Quick Cable Size Selection Tips

Always check voltage drop - this often determines cable size more than current rating
For long runs, increase cable size by one or two steps to reduce voltage drop
High-power loads (showers, cookers, EV chargers) typically need 6mm² or 10mm²
Use the calculator above for accurate results considering all BS 7671 requirements
When in doubt, go up a cable size - the cost difference is minimal compared to rewiring

3-Phase Cable Size Calculator — BS 7671 Worked Examples

Three-phase cable sizing follows the same BS 7671 hierarchy as single-phase but uses 400V line-to-line (230V line-to-neutral), the √3 factor when converting between kW and line current, and Table 4D4A columns for 3- or 4-core cables. For the full industrial 3-phase treatment — motor full-load currents, star/delta starting, harmonic neutral sizing, and 4-core SWA derating — see the dedicated 3-Phase Cable Size Calculator.

Three-Phase Current and Voltage Drop Formulas

Line Current (balanced load):

I_L = P ÷ (√3 × V_LL × PF)

Where V_LL = 400V, PF = power factor (1.0 for resistive loads, 0.8 typical for motors)

3-Phase Voltage Drop:

V_d = (mV/A/m × L × I_L × √3) ÷ 1000

Use the 3-phase mV/A/m column in Table 4D4B / 4E4B (typically ~87% of single-phase value for the same cable)

122kW Three-Phase EV Charger (32A per phase)

Scenario:

Load: 22kW three-phase EV charger, balanced across L1/L2/L3
Cable: 4-core SWA, buried direct 600mm
Run length: 45m from sub-main to garage
Voltage: 400V line-to-line, PF = 1.0 (modern PFC-corrected charger)

Step 1 — Line current:

I_L = 22000 ÷ (√3 × 400 × 1.0) = 22000 ÷ 692.8 = 31.8A per phase

Step 2 — Select protective device:

32A Type B 3-pole MCB (BS EN 60898) — I_n = 32A ≥ I_b

Step 3 — Current-carrying capacity check (Table 4D4A, 4-core SWA, Method D buried direct):

6mm² = 38A (column 7); I_z = 38 × 1.0 × 1.0 × 1.0 = 38A ≥ 31.8A ✓

Step 4 — Voltage drop (3-phase, Table 4D4B):

6mm² 4-core SWA 3-phase mV/A/m = 6.4

V_d = (6.4 × 45 × 31.8 × √3) ÷ 1000 = 15.88V

As % of 400V line-to-line = 3.97% ✓ (under 5%)

✓ Final answer:

Use 6mm² 4-core SWA with 32A Type B 3-pole MCBand 30mA Type B RCD (BS 7671 Section 722 — EV-specific RCD requirement).

Note: a single-phase 32A 7.4kW charger on the same 45m run would need 10mm² SWA (Vd = 4.3% at 6mm², 2.6% at 10mm²). Three-phase halves the cable size because each phase only carries a third of the total power.

2100A Three-Phase Sub-Main to Workshop

Scenario:

Load: 100A TP&N sub-main feeding workshop distribution board
Cable: 4-core SWA, clipped to tray
Run length: 35m, budget ≤ 2% voltage drop (leaving 3% for final circuits)

Step 1 — Capacity: I_b = 100A, I_n = 100A (BS 88-3 fuse or moulded-case CB)

Step 2 — First-pass cable (Table 4D4A, Method E clipped to tray, column 5):

25mm² 4-core SWA = 110A ✓; 16mm² = 83A ✗ (insufficient)

Step 3 — Voltage drop at 25mm² (Table 4D4B, 3-phase z column):

25mm² 3-phase mV/A/m = 1.50

V_d = (1.50 × 35 × 100 × √3) ÷ 1000 = 9.09V

As % of 400V = 2.27% — over 2% budget

Step 4 — Upsize to 35mm²:

mV/A/m = 1.10; V_d = (1.10 × 35 × 100 × √3) ÷ 1000 = 6.67V = 1.67% ✓

✓ Final answer:

Use 35mm² 4-core SWA with 100A BS 88-3 fuse (or MCCB). Armour continuity checked against adiabatic equation for CPC duty; add 16mm² external earth if Z_s calc demands it.

kW → 3-Phase Current Quick Reference (400V, balanced, PF=1.0)

Load (kW)	Line current (A)	Typical MCB	Typical cable (≤30m)
11 kW	15.9A	16A / 20A TP	2.5mm² 4-core SWA
22 kW	31.8A	32A TP	6mm² 4-core SWA
30 kW	43.3A	50A TP	10mm² 4-core SWA
45 kW	65A	80A TP fuse	16mm² 4-core SWA
69 kW	100A	100A BS 88-3	25–35mm² 4-core SWA

For PF < 1.0 (typical 0.85 for motor-heavy loads), scale up the current by 1/PF. Example: 22kW motor at 0.85 PF → 37.4A per phase, step up to 10mm² SWA.

Critical Cable Sizing Mistakes UK Electricians Must Avoid

These common errors can result in failed inspections, dangerous installations, or expensive rewiring. Understanding these traps is essential for BS 7671 compliance.

1The Thermal Insulation Trap (Reference Methods 100-103)

Modern homes with 270mm+ loft insulation create a critical derating scenario that many electricians miss. When cables are buried in thermal insulation, heat cannot dissipate.

Reference Method	Scenario	Derating Factor (Ci)
Method 100	Clipped to joist, <100mm insulation	0.89
Method 101	Clipped to joist, >100mm insulation	0.5
Method 102	In stud wall, not touching inner wall	0.89
Method 103	In stud wall, touching inner wall	0.5

Real Impact: A 2.5mm² cable rated at 27A clipped direct becomes just 13.5Aunder Method 101. This is insufficient for a 32A ring circuit. The circuit requires upsizing to 4mm² or 6mm².

Reference: BS 7671 Regulation 523.9 and Table 4A2 (Amendment 3)

2The 1.45 Rule and Rewirable Fuses (BS 3036)

BS 7671 Regulation 433.1.1 requires that the current causing effective operation of the protective device (I₂) must not exceed 1.45 times the cable's current-carrying capacity (Iz).

I₂ ≤ 1.45 × Iz

For modern MCBs (BS EN 60898), this is automatically satisfied because I₂ = 1.45 × In. However, for BS 3036 rewirable fuses, the fusing factor is approximately 2.0.

Critical: When using rewirable fuses, you must apply an additional derating factor of 0.725 to the cable capacity. A 27A cable becomes effectively 19.6A.

Many older domestic consumer units still use rewirable fuses. Always check before sizing cables.

390°C Cable vs 70°C Terminals

SWA cables with XLPE insulation are rated for 90°C operation and have higher current ratings than 70°C PVC cables. However, this creates a hidden problem.

The Problem: Most domestic consumer units, MCBs, and terminal blocks are rated for70°C maximum. Running an XLPE cable at 90°C will conduct excessive heat into the terminals, causing nuisance tripping or terminal damage.

Solution: When using XLPE/SWA cables with standard 70°C terminals, use the 70°C current ratings from BS 7671 tables, not the 90°C ratings. The benefit of XLPE is only realised when both the cable AND terminals are rated for 90°C.

4Ignoring Cumulative Voltage Drop

Voltage drop is cumulative through an installation. Many electricians calculate the final circuit drop but forget the sub-main contribution.

Total Voltage Drop = Sub-main + Final Circuit

Example: 1.5% (sub-main) + 4% (final circuit) = 5.5% (FAIL)

Best Practice: Budget your 5% allowance carefully:

Sub-mains: Allow maximum 1.5-2%
Final circuits: Allow maximum 3-3.5%
Total: Keep under 5% with margin

5The Physical Reality Problem

Calculations may suggest a cable size that is electrically correct but physically impossible.

Example: High derating factors on a 32A ring circuit might suggest 10mm² cable. However, 10mm² cable will not fit into the terminals of a standard 13A double socket. Maximum terminal capacity is typically 2.5mm² or 4mm².

Solutions:

Use junction boxes with larger terminals for cable transitions
Consider radial circuits instead of rings for heavily derated installations
Route cables to avoid severe derating conditions

Key Takeaway

Cable sizing is not just about current capacity. It requires simultaneous verification of thermal conditions, voltage drop, protective device coordination, physical terminations, and earth fault loop impedance. Use this calculator and always verify calculations against your specific installation conditions.

UK Trade Practice vs BS 7671 Minimum — When Sparkies Always Upsize

BS 7671 gives the minimum legally-compliant cable size. On real jobs, UK electricians habitually upsize beyond the minimum for four reasons: voltage-drop headroom on future loads, margin for installation errors, ease of future diversity changes, and avoiding call-backs. These are the canonical "trade rules" that every experienced sparky applies.

1. 8kW+ Electric Showers → 10mm² (even when 6mm² calculates)

BS 7671 minimum for an 8.5kW shower on a short run is 6mm² T&E. Trade practice installs 10mm² on anything 8kW or above. Reasons: homeowners often upgrade to a 9.5kW or 10.5kW unit later, power-shower boost pumps add load, and the extra cost (~£2/m) is less than the cost of returning to re-pull cable.

This is the most widely-cited unwritten rule in UK domestic wiring — referenced in JIB, NICEIC and Elecsa guidance alike.

2. EV Chargers → default 10mm² for home wallbox (OZEV / future 11kW upgrade)

A 7.4kW 32A wallbox on a 15m clipped run calculates as 6mm² SWA. Trade practice pulls 10mm² because (a) householders commonly upgrade to a second phase or 11kW charger within 5 years, (b) 10mm² keeps voltage drop under 3% for PEN-protection algorithms (BS 7671 722.411.4.1 iii), and (c) grant-funded OZEV installations are often inspected against conservative interpretations.

3. Garage / Outbuilding Sub-Main → 10mm² SWA minimum

A 32A outbuilding with 2kW of lighting and a few sockets calculates as 6mm² SWA. Trade practice goes to 10mm² because workshops gain heating, compressors, welders and EV chargers over time. 10mm² also gives headroom for the armour to serve as the sole CPC under the adiabatic equation when Z_s is high.

4. Lighting Circuits > 8 metres → 1.5mm² (not 1.0mm²)

BS 7671 permits 1.0mm² T&E on a 6A lighting circuit up to ~45m for voltage drop. Trade practice defaults to 1.5mm² on any new-build rewire because (a) future LED → incandescent retrofits are rare but inrush on LED driver banks can be substantial, (b) 1.5mm² handles 10A if a circuit is later split, and (c) the unit-price difference is under £5 on a typical property.

5. Ring Final in a Kitchen → 4mm² radial preferred over 2.5mm² ring

Modern kitchens with induction hob, oven, microwave, kettle and dishwasher can approach 32A on a single appliance bank. Many installers now prefer a 4mm² radial on a 32A MCB (or two 2.5mm² radials on 20A MCBs) over the traditional ring final — better voltage stability under simultaneous load, easier fault finding and less copper than a large ring.

6. Submain into a Consumer Unit → 16mm² tails even when 10mm² suffices

DNO 80A/100A cutouts feed consumer unit tails. BS 7671 allows 16mm² 6181Y meter tails up to 80A and 25mm² for 100A. Trade practice (and many DNOs) specifies 25mm² tails on every new installation regardless of maximum demand, because the tails are rarely replaced when the DNO upgrades the cutout from 80A to 100A years later.

Why this matters: a calculator can only tell you the minimum BS 7671-compliant cable. The best UK electricians treat the calculator as a floor, not a target — and build in headroom for the next 10 years of load growth. If a homeowner adds an EV, hot tub, or heat pump and the rewire was sized exactly to today's load, the upgrade becomes a full re-pull rather than a consumer-unit swap.

Advanced Cable Sizing Considerations - BS 7671

For commercial, industrial, and complex residential installations, these advanced factors must be considered beyond basic current and voltage drop calculations.

Harmonic Currents and Neutral Sizing

Modern LED lighting, IT equipment, and Variable Speed Drives (VSDs) are non-linear loads that create harmonic currents. The third harmonic (150Hz) is particularly problematic in three-phase systems.

The Physics: Third harmonic currents from all three phases are in-phase (0° apart instead of 120°). They don't cancel at the neutral point—they add together.

3rd Harmonic Content	Phase Reduction Factor	Neutral Sizing
0-15%	1.0	Standard (same as phase)
15-33%	0.86	Standard (same as phase)
33-45%	0.86	Neutral = Phase size
>45%	By calculation	Neutral may exceed phase

Reference: BS 7671 Regulation 523.6.3 and Appendix 4, Section 11

The 30% Grouping Rule - Reduce Cable Sizes Legitimately

BS 7671 contains an often-overlooked provision that can significantly reduce cable costs on commercial installations.

The Rule: Cables carrying no more than 30% of their grouped ratingmay be excluded from grouping factor calculations.

Practical Application:

If 6 cables are grouped but 2 are lighting circuits running at only 2A (10% of capacity), these can be excluded from grouping
This effectively reduces the group from 6 circuits (Cg=0.57) to 4 circuits (Cg=0.65)
The remaining 4 cables can be smaller, saving significant material costs

Reference: BS 7671 Table 4C1, Note 3

Motor Circuits - Inrush Current Considerations

Induction motors draw 6-8 times their Full Load Current (FLC) during Direct-On-Line (DOL) starting. While cable current capacity is sized for running current, voltage drop during starting can cause problems.

Starting Voltage Drop: A cable sized for 5% voltage drop at running current will experience 30-40% drop during starting (6-8× current). If this exceeds 15%, the motor may stall or the contactor may chatter.

Sizing Approach for Motors:

Size current capacity for running FLC
Check voltage drop at starting current (not running)
Ensure starting drop does not exceed 15% for sensitive motors
Consider soft starters or VFDs for long cable runs

Adiabatic Equation - CPC Sizing for Fault Protection

The Circuit Protective Conductor (CPC/Earth) must withstand the thermal stress of a fault without damage. BS 7671 Regulation 543.1.3 provides the adiabatic equation:

S = √(I²t) / k

Variables:

S = Minimum CPC size (mm²)
I = Fault current (A)
t = Disconnection time (s)
k = Material factor

k values:

Copper/PVC: 115
Copper/XLPE: 143
Steel armour: 51

SWA Cable Note: The steel armour of SWA cables often serves as the CPC. Due to the lower k factor (51 vs 115), the armour may be insufficient for high fault currents. Always verify with the adiabatic equation and add external earth cable if needed.

Earth Fault Loop Impedance (Zs) and Cable Selection

Cable selection must ensure the earth fault loop impedance (Zs) is low enough to guarantee protective device operation within required disconnection times.

Zs = Ze + (R1 + R2) × L × Cr / 1000

Ze = External earth fault loop impedance

TN-C-S (PME): 0.35Ω typical

TN-S: 0.8Ω typical

R1+R2 = Resistance per metre (mΩ/m)

L = Cable length (m)

Cr = Temperature correction (1.2 for 70°C)

Practical Impact: Type C MCBs require much lower Zs than Type B for the same rating. This often dictates using larger cables or shorter runs with Type C devices.

MCB Rating	Type B Max Zs	Type C Max Zs
16A	2.73Ω	1.37Ω
32A	1.37Ω	0.68Ω
63A	0.70Ω	0.35Ω

Professional Resources

For complex installations, always reference:

BS 7671:2018+A2:2022 Appendix 4 - Current-carrying capacities and voltage drop
IET Guidance Note 1 - Selection and Erection of Equipment
IET On-Site Guide - Quick reference for inspection and testing
Manufacturer cable data sheets for specific XLPE/SWA properties

How to Use the Cable Size Calculator

Enter the load current - The maximum current (in Amps) that will flow through the cable. This can be calculated from the power rating of your equipment.
Specify the cable length - The one-way distance (in meters) from your distribution board or consumer unit to the load.
Select the voltage - Choose 230V for single-phase or 400V for three-phase installations.
Choose the installation method - How the cable will be installed affects its current-carrying capacity due to different cooling conditions.
Select conductor material - Copper is most common in UK installations. Aluminium has higher resistance but is lighter and cheaper for large cables.

Understanding the Results

Recommended Cable Size: The minimum cross-sectional area (in mm²) required for your installation
Voltage Drop: The voltage lost over the cable run as a percentage. Must be ≤5% for most circuits, ≤3% for lighting
Maximum Current: The current-carrying capacity of the selected cable with derating factors applied
Recommended MCB: The maximum circuit breaker rating suitable for the cable

BS 7671 Compliance Information

This calculator complies with BS 7671:2018+A2:2022 (18th Edition) - Requirements for Electrical Installations (IET Wiring Regulations). BS 7671 is the UK implementation of the harmonised international standard IEC 60364-5-52 (Selection and erection of electrical equipment — Wiring systems), so current-carrying capacities and voltage-drop methodology align with European and international practice.

Regulation 525 - Voltage Drop

Under normal service conditions, the voltage drop between the origin of the installation and the socket outlet or appliance shall not exceed:

3% of the nominal voltage for lighting circuits
5% of the nominal voltage for other circuits

Current-Carrying Capacity Considerations

The calculator applies derating factors for different installation methods as per Appendix 4 of BS 7671. Factors considered include:

Installation method (clipped direct, in conduit, etc.)
Ambient temperature (assumed 30°C)
Thermoplastic insulation (70°C rating)

Important: This calculator is a tool for initial sizing estimates. Final cable selection should be verified by a qualified electrician considering all installation-specific factors including grouping, thermal insulation, and harmonic currents.

Frequently Asked Questions

Recommended Products

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Free: BS 7671 Quick Reference Card

Max Zs values, diversity factors, cable ratings, voltage drop — one printable page. Plus occasional emails with calculator updates and useful tips.

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Complete Calculation Workflow

Follow this workflow for a complete electrical installation calculation:

Load Calculator

Calculate total electrical load and demand

Diversity Calculator

Apply BS 7671 diversity factors

Cable Size Calculator

Select correct cable size

You are here

Voltage Drop CalculatorNext Step

Verify voltage drop compliance

Max Zs Calculator

Check earth fault loop impedance

Calculate Cable Size

What Size Cable Do I Need? Quick Answer

kW to Cable Size Conversion (Single Phase, 230V)

How to Calculate Cable Size UK - Complete Guide

Step 1: Determine the Design Current (Ib)

Step 2: Apply Correction Factors

Installation Method (Ca)

Ambient Temperature (Cg)

Grouped Circuits (Ci)

Step 3: Calculate Required Current Carrying Capacity (It)

Step 4: Select Cable Size from BS 7671 Tables

Step 5: Check Voltage Drop

Real-World Example: 9.5kW Electric Shower

Important BS 7671 Cable Sizing Requirements

The Cable Sizing Hierarchy: Ib ≤ In ≤ Iz

The Four Correction Factors (Ca, Cg, Ci, Cc)

Worked Example: Verifying the Hierarchy

Cable Size Chart UK - Quick Reference Guide

Twin & Earth Cable (6242Y) - Current Ratings

SWA Cable (Steel Wire Armoured) - Current Ratings

Fire-Rated, Mineral-Insulated and Flexible Cables — UK Sizing Reference

Fire-Rated Cables (FP200, FP400, Firetuf, Prysmian FP PLUS)

Mineral-Insulated Copper Cable (MICC / Pyro) — BS 7671 Table 4H1A–4H4A

Flexible Cables — SY, H07RN-F, 3183Y, 3184Y

LSZH / LSF Insulation — What's the Difference?

Common Cable Sizes for UK Installations

Electric Shower

Electric Cooker/Oven

EV Charger

Immersion Heater

Ring Main (Sockets)

Lighting Circuit

Hot Tub / Spa

Outbuilding / Garden Office

Air Conditioning Unit

Worked Examples - Step-by-Step Cable Size Calculations

1Cable Size for 7.4kW EV Charger (32A)

2What Size Cable for 8.5kW Electric Shower?

3Cable Size for 10kW Electric Cooker

46mm Cable Amps - Current Rating Guide

510mm Cable Rating - How Many Amps Can It Handle?

💡 Quick Cable Size Selection Tips

3-Phase Cable Size Calculator — BS 7671 Worked Examples

Three-Phase Current and Voltage Drop Formulas

122kW Three-Phase EV Charger (32A per phase)

2100A Three-Phase Sub-Main to Workshop

kW → 3-Phase Current Quick Reference (400V, balanced, PF=1.0)

Critical Cable Sizing Mistakes UK Electricians Must Avoid

1The Thermal Insulation Trap (Reference Methods 100-103)

2The 1.45 Rule and Rewirable Fuses (BS 3036)

390°C Cable vs 70°C Terminals

4Ignoring Cumulative Voltage Drop

5The Physical Reality Problem

Key Takeaway

UK Trade Practice vs BS 7671 Minimum — When Sparkies Always Upsize

1. 8kW+ Electric Showers → 10mm² (even when 6mm² calculates)

2. EV Chargers → default 10mm² for home wallbox (OZEV / future 11kW upgrade)

3. Garage / Outbuilding Sub-Main → 10mm² SWA minimum

4. Lighting Circuits > 8 metres → 1.5mm² (not 1.0mm²)

5. Ring Final in a Kitchen → 4mm² radial preferred over 2.5mm² ring

6. Submain into a Consumer Unit → 16mm² tails even when 10mm² suffices

Advanced Cable Sizing Considerations - BS 7671

Harmonic Currents and Neutral Sizing

The 30% Grouping Rule - Reduce Cable Sizes Legitimately

Motor Circuits - Inrush Current Considerations

Adiabatic Equation - CPC Sizing for Fault Protection

Earth Fault Loop Impedance (Zs) and Cable Selection

Professional Resources

How to Use the Cable Size Calculator

Understanding the Results

BS 7671 Compliance Information

Regulation 525 - Voltage Drop

Current-Carrying Capacity Considerations

Frequently Asked Questions

What size cable do I need for an electric shower?

What cable size for a cooker circuit?

What cable size for an EV charger?

What is 6mm cable used for?

What is 10mm cable used for?

What cable size do I need for a 32A circuit?

The Cable Sizing Hierarchy: Ib ≤ In ≤ I_z

The Four Correction Factors (C_a, C_g, C_i, C_c)