What is the maximum voltage drop allowed by BS 7671?

BS 7671 limits voltage drop to 3% for lighting circuits and 5% for all other circuits from the origin of the installation. For a 230V supply, this means maximum 6.9V for lighting and 11.5V for power circuits. The total voltage drop from the supply point must not exceed these values. Note that these limits apply only to installations supplied from a public low-voltage network — for private LV generation (e.g. site generators, large solar), limits increase to 6% lighting / 8% other (Appendix 12).

How do I calculate voltage drop in a cable?

Voltage drop is calculated using the formula: Vd = (mV/A/m × Current × Length) / 1000. The mV/A/m value comes from BS 7671 Appendix 4 tables and depends on cable size, type (copper/aluminium), and installation method. For three-phase circuits, use the 3-phase mV/A/m column and multiply by √3. Importantly, BS 7671 does not publish the formula itself — it is derived from Ohm's law and the tabulated impedance per metre.

What is mV/A/m in voltage drop calculations?

mV/A/m (millivolts per amp per metre) is the voltage drop coefficient for cables. It represents how many millivolts are lost per ampere of current per metre of cable length. Values are found in BS 7671 Tables 4D1A-4J4A. The value combines both conductor resistance and inductive reactance at the cable's operating temperature — it is a complex impedance figure, not just resistance. Lower values mean less voltage drop — larger cables have lower mV/A/m values.

Why is voltage drop important in electrical installations?

Excessive voltage drop causes problems including dim lights, motors running slow or overheating, equipment malfunction, increased energy waste, and non-compliance with BS 7671. Power loss is quadratic: the energy wasted in a cable is I²R, so a 5% voltage drop on a sub-main carrying 100A continuously wastes around 5% of delivered power as cable heat — often tens of pounds per year on a commercial circuit.

How do I reduce voltage drop in a circuit?

Five tactics, in order of cost-effectiveness: 1) Upsize the cable (doubling cross-section roughly halves voltage drop). 2) Shorten the route — reposition accessories or run cleats rather than long detours. 3) Reposition the distribution board closer to the load centre. 4) Add a sub-distribution board — splits the run into sub-main + shorter final circuits, often cheaper than a larger single cable for big installations. 5) Split the load across multiple circuits. For three-phase loads, converting from single-phase also dramatically cuts voltage drop thanks to the √3 factor.

What is the voltage drop for 2.5mm cable?

2.5mm² twin & earth cable (copper) has a voltage drop of approximately 18 mV/A/m (single-phase, clipped direct, 70°C thermoplastic). For a 20A load over 20m, voltage drop = (18 × 20 × 20) / 1000 = 7.2V or 3.1% of 230V. This passes the 5% limit for power circuits but would fail for lighting (3% limit).

Does voltage drop affect ring final circuits?

Ring circuits have lower voltage drop than radials because current flows both ways around the ring. Calculate voltage drop to the furthest point using half the current and half the cable length. For a properly balanced ring, voltage drop is approximately one quarter of an equivalent radial circuit. This is why rings are still widely used in the UK on 32A 2.5mm² despite the cable's moderate current capacity.

How do you calculate voltage drop for a 3-phase circuit?

For a balanced 3-phase load: Vd = (mV/A/m × L × I × √3) ÷ 1000, using the 3-phase mV/A/m column from BS 7671 (typically around 87% of the single-phase value). The result is the line-to-line drop (measured against 400V). Example: 32A 3-phase EV charger on 6mm² 4-core SWA over 45m with 3-phase mV/A/m = 6.4: Vd = (6.4 × 45 × 32 × √3) ÷ 1000 = 15.97V = 3.99% of 400V ✓. Note that a single-phase 32A circuit on the same cable and length would produce 5.32% drop — three-phase effectively halves the voltage drop for the same conductor size.

What is the voltage drop for SWA cable?

SWA (steel wire armoured) cables use the same Table 4D4B mV/A/m values as comparable twin & earth for voltage drop — the steel armour contributes negligible resistance to the phase conductors. Typical 2-core SWA values (single-phase): 2.5mm² = 18 mV/A/m, 4mm² = 11, 6mm² = 7.3, 10mm² = 4.4, 16mm² = 2.8, 25mm² = 1.75. For 4-core SWA on 3-phase loads, use the 3-phase column (~13% lower). SWA's main voltage-drop advantage is its suitability for long outdoor runs — a 50m 10mm² SWA buried-direct run to an outbuilding has the same drop as the equivalent T&E indoors.

How do I calculate voltage drop for an EV charger?

Use single-phase: Vd = (mV/A/m × L × 32) ÷ 1000 for a 7.4kW wallbox. Aim for under 3% drop (not the full 5%) because the supply voltage can already be at 216V (ESQCR minimum) before cable losses. For a 45m 7.4kW (32A) run: 6mm² SWA → 4.6% (fails headroom test), 10mm² SWA → 2.8% (pass), 16mm² SWA → 1.75% (future-proof for 11kW upgrade). For 22kW 3-phase chargers, divide by √3 and use the 3-phase mV/A/m column.

Does voltage drop cause energy loss?

Yes — voltage drop represents real power dissipated as heat in the cable, governed by P = I²R. At a 5% voltage drop, roughly 5% of the power delivered to the load is wasted in the cable. On a 100A continuous industrial sub-main at 400V running 4000 hours/year, a 5% drop wastes ~£140/year in energy at 15p/kWh. Upsizing the cable can pay back in under 5 years on heavily-loaded circuits, which is why Regulation 132.5 and Appendix 17 encourage voltage-drop budgets well below the 5% ceiling for energy-efficient design.

What is cumulative voltage drop?

Cumulative voltage drop is the total drop from the installation origin (supply terminals) to the point of use. BS 7671's 3%/5% limits apply to this total — not to individual circuits. A typical multi-stage budget: 1% tails + 1.5% sub-main + 2.5% final circuit = 5% total. If a sub-main has already used 2% of the budget, final circuits downstream only have 3% to work with. Always trace the full path when designing.

Voltage Drop Calculator - BS 7671 Compliant

Calculate voltage drop and check BS 7671 compliance for electrical installations

Last updated: 25 April 2026

Calculate Voltage Drop

Check if your cable installation meets BS 7671 voltage drop requirements

How to Calculate Voltage Drop UK - Complete Guide

Voltage drop calculation is essential for ensuring electrical installations comply with BS 7671 regulations. This guide explains the complete process for UK electrical systems.

Step 1: Understand BS 7671 Voltage Drop Limits

BS 7671 Regulation 525 specifies maximum voltage drop limits:

Lighting circuits: Maximum 3% of nominal voltage (230V × 3% = 6.9V maximum drop)
Other circuits: Maximum 5% of nominal voltage (230V × 5% = 11.5V maximum drop)
Three-phase systems: Measured line-to-line (400V system)

Step 2: Identify the Required Values

To calculate voltage drop, you need:

Cable length (L): One-way distance in meters from source to load
Design current (Ib): Maximum current the circuit will carry in Amps
Cable size: Cross-sectional area in mm²
mV/A/m value: From BS 7671 voltage drop tables (Table 4D5, 4E4A, etc.)
Phase type: Single-phase (230V) or three-phase (400V)

Step 3: Use the Voltage Drop Formula

The formula differs between single-phase and three-phase systems:

Single-Phase Formula:

Voltage Drop (V) = (mV/A/m × Length × Current) ÷ 1000

For 230V single-phase circuits (most domestic installations)

Three-Phase Formula:

Voltage Drop (V) = (mV/A/m × Length × Current × √3) ÷ 1000

For 400V three-phase circuits (commercial/industrial)

Then convert to percentage: (Voltage Drop ÷ Nominal Voltage) × 100

Step 4: Find mV/A/m Values from BS 7671 Tables

The millivolt drop per amp per meter (mV/A/m) is found in BS 7671 tables:

Table 4D5: Twin & earth cables (PVC insulated)
Table 4E4A: Single-core cables in conduit
Table 4D4A: SWA armoured cables
Table 4J4A: Mineral insulated cables

These values account for both conductor resistance and reactance at operating temperature. For the complete tables, see the BS 7671 voltage drop reference tables.

Important: BS 7671 itself does not publish the voltage drop formula — it only publishes the mV/A/m values. The formula V_d = (mV/A/m × L × I) ÷ 1000 is derived from Ohm's law (V = IR), with mV/A/m representing the complex impedance (resistance + reactance) per metre. UK trade practice for domestic work commonly skips temperature correction and uses tabulated values directly as a conservative worst-case.

Real-World Example: 32A Shower Circuit

Given: 32A shower, 15m cable run, 6mm² twin & earth, 230V

Step 1: From Table 4D5, 6mm² cable = 7.3 mV/A/m

Step 2: Calculate voltage drop:

Vd = (7.3 × 15 × 32) ÷ 1000 = 3.504V

Step 3: Convert to percentage:

(3.504 ÷ 230) × 100 = 1.52%

✓ Result: 1.52% is under the 5% limit - compliant with BS 7671

Common Mistakes to Avoid

Using cable length as return distance - always use one-way length
Forgetting to divide by 1000 (mV to V conversion)
Using incorrect mV/A/m values for the cable type
Not accounting for the √3 factor in three-phase calculations
Checking only current capacity and forgetting voltage drop

Voltage Drop Tables UK - mV/A/m Values BS 7671

Quick reference voltage drop values (mV/A/m) from BS 7671 for common UK cable types. These values are used in voltage drop calculations.

Twin & Earth Cable (6242Y) - Voltage Drop Values

Cable Size	mV/A/m	Example: 10m @ 20A	Max Length @ 5%
1.0mm²	44 mV/A/m	8.8V (3.8%)	26m @ 20A
1.5mm²	29 mV/A/m	5.8V (2.5%)	40m @ 20A
2.5mm²	18 mV/A/m	3.6V (1.6%)	64m @ 20A
4.0mm²	11 mV/A/m	2.2V (1.0%)	52m @ 32A
6.0mm²	7.3 mV/A/m	1.46V (0.6%)	79m @ 32A
10.0mm²	4.4 mV/A/m	0.88V (0.4%)	87m @ 40A

Reference: BS 7671 Table 4D5 (2-core cables with protective conductor, 70°C thermoplastic)

SWA Cable - Voltage Drop Values

Cable Size	mV/A/m (2-core)	mV/A/m (3-core)	Typical Use
2.5mm²	18 mV/A/m	18 mV/A/m	Garden lighting
4.0mm²	11 mV/A/m	11 mV/A/m	Garage sub-mains
6.0mm²	7.3 mV/A/m	7.3 mV/A/m	EV chargers, outbuildings
10.0mm²	4.4 mV/A/m	4.4 mV/A/m	Large outbuildings
16.0mm²	2.8 mV/A/m	2.8 mV/A/m	Commercial sub-mains

Reference: BS 7671 Table 4D4A (Armoured 70°C thermoplastic cables)

💡 Quick Tip: Lower mV/A/m values mean less voltage drop. For long cable runs, use larger cable sizes to reduce voltage drop and stay within BS 7671 limits.

Voltage Drop Calculation Examples - Step by Step

Follow these worked examples to understand voltage drop calculations for common UK installations.

132A Ring Main Circuit - 2.5mm² Cable

Scenario:

Circuit: 32A socket ring main
Cable: 2.5mm² twin & earth
Length: 28m (one leg of ring)
Load: 20A (typical maximum on one leg)

Calculation:

mV/A/m for 2.5mm² = 18 mV/A/m (Table 4D5)

Vd = (18 × 28 × 20) ÷ 1000 = 10.08V

Percentage = (10.08 ÷ 230) × 100 = 4.38%

✓ Result: 4.38% - Within 5% limit for power circuits

2Lighting Circuit - Checking 3% Limit

Scenario:

Circuit: 6A lighting circuit
Cable: 1.5mm² twin & earth
Length: 35m to furthest light
Load: 5A (LED lighting)

Calculation:

mV/A/m for 1.5mm² = 29 mV/A/m (Table 4D5)

Vd = (29 × 35 × 5) ÷ 1000 = 5.075V

Percentage = (5.075 ÷ 230) × 100 = 2.21%

✓ Result: 2.21% - Within 3% limit for lighting

3EV Charger Long Run - Voltage Drop Exceeds Limit

Scenario:

Circuit: 32A EV charger (7.4kW)
Cable: 6mm² SWA
Length: 40m to garage
Load: 32A continuous

Initial Calculation (6mm²):

mV/A/m for 6mm² SWA = 7.3 mV/A/m (Table 4D4A)

Vd = (7.3 × 40 × 32) ÷ 1000 = 9.344V

Percentage = (9.344 ÷ 230) × 100 = 4.06%

Upgrade to 10mm²:

mV/A/m for 10mm² SWA = 4.4 mV/A/m

Vd = (4.4 × 40 × 32) ÷ 1000 = 5.632V

Percentage = (5.632 ÷ 230) × 100 = 2.45%

✓ Solution: Upgrade to 10mm² SWA for 2.45% voltage drop

Note: 6mm² would work for runs under 28m at 32A

💡 Voltage Drop Calculation Tips

Always check before installation - voltage drop often requires larger cables than current capacity
Lighting circuits are stricter - 3% limit instead of 5%
For long runs, increase cable size to reduce voltage drop
Use the calculator above for quick verification of your calculations
Remember: One-way length, not return distance

3-Phase Voltage Drop Calculator UK — Formula and Worked Examples

Three-phase voltage drop calculations follow BS 7671 Appendix 4 Section 6, but use a √3 factor and a distinct 3-phase mV/A/m column. This section covers the formula, how 3-phase compares to single-phase for the same cable, and a full worked example for a typical UK commercial sub-main. For full 3-phase cable sizing (including motor FLCs, starting-current considerations, and 4-core SWA derating tables), use the 3-Phase Cable Size Calculator.

3-Phase Voltage Drop Formula (Line-to-Line)

V_d = (mV/A/m × L × I_L × √3) ÷ 1000

Variables:

mV/A/m — from 3-phase column of Table 4D4B / 4E4B / 4D5
L — cable length in metres (one-way)
I_L — line current (balanced load)
√3 ≈ 1.732 (phase-to-line relationship)

Convert to % of 400V line-to-line:

% drop = (V_d ÷ 400) × 100

BS 7671 limits: 3% lighting (12V), 5% other (20V) on 400V systems.

Why 3-Phase Halves Voltage Drop (for the same cable and load)

A 22kW load on single-phase 230V draws 95.7A; the same 22kW balanced across three phases draws only 31.8A per phase. Because voltage drop is proportional to current, spreading the load cuts drop dramatically — even after applying the √3 factor. This is why industrial and commercial installations are almost always 3-phase over any distance.

Scenario	Current	Cable	Vd over 40m	As % of V_nom
22kW single-phase, 230V	95.7A	16mm² T&E	10.7V	4.66%
22kW 3-phase, 400V (balanced)	31.8A/phase	6mm² 4-core SWA	14.1V	3.52%

The 3-phase cable is also a third of the single-phase cross-section. Copper cost is lower, cable weight is lower, and the 30mA RCD sizing is easier.

Worked Example: 63A 3-Phase Sub-Main to Factory Unit

Scenario:

Load: 63A per phase TP&N sub-main, 400V, PF=0.9
Cable: 16mm² 4-core SWA, clipped to cable tray
Length: 55m from main LV panel to sub-DB
Voltage-drop budget: 2% on sub-main (leaving 3% for final circuits)

Step 1 — Identify mV/A/m (3-phase):

16mm² 4-core SWA, 3-phase column (Table 4D4B) = 2.4 mV/A/m

Step 2 — Apply formula:

V_d = (2.4 × 55 × 63 × √3) ÷ 1000 = 14.41V

Step 3 — Convert to percentage of 400V:

(14.41 ÷ 400) × 100 = 3.60%

Step 4 — Check against budget:

✗ Fails the 2% sub-main budget — upsize to 25mm² 4-core SWA.

Step 5 — Recalculate with 25mm²:

25mm² 4-core SWA 3-phase mV/A/m = 1.5

V_d = (1.5 × 55 × 63 × √3) ÷ 1000 = 9.00V

As % of 400V = 2.25% — still slightly over. Try 35mm²:

35mm² mV/A/m = 1.10; V_d = (1.10 × 55 × 63 × √3) ÷ 1000 = 6.60V = 1.65% ✓

✓ Final answer:

Use 35mm² 4-core SWA for 1.65% drop on the sub-main, leaving 3.35% headroom for final circuits in the sub-DB. Protective device: 63A 3-pole MCCB or BS 88-3 fuse.

SWA Cable Voltage Drop — UK Reference & Long-Run Guidance

Steel wire armoured (SWA) cable is the default for UK outdoor and underground runs — outbuildings, garages, EV chargers, sub-mains and industrial circuits. Its voltage drop values are essentially identical to same-size copper PVC cables because the armour carries no phase current, but several SWA-specific factors affect real-world voltage drop calculations.

SWA mV/A/m Values (Single-Phase & 3-Phase)

Size	2-core (1φ)	3-core (1φ)	4-core (3φ)	Typical application
2.5mm²	18	18	15	Garden lighting, small outbuildings
4.0mm²	11	11	9.5	Garage sub-mains, small workshops
6.0mm²	7.3	7.3	6.4	7kW EV chargers, hot tubs, 22kW 3φ EV
10mm²	4.4	4.4	3.8	Long EV runs, larger outbuildings
16mm²	2.8	2.8	2.4	Commercial sub-mains, workshop feeds
25mm²	1.75	1.75	1.50	100A TP&N sub-mains
35mm²	1.25	1.25	1.10	Long-distance 3φ sub-mains

Values for 70°C PVC-insulated SWA cables reproduced from BS 7671:2018 Table 4D4B. BS 7671 publishes two voltage-drop columns: two-core single-phase AC and three-/ four-core three-phase AC. For a 3-core SWA used on a single-phase circuit (L/N/E) the accepted industry convention is to use the two-core single-phase value shown here — the extra core carries no current. 90°C XLPE SWA (BS 6724 / Table 4E4B) values are within 3% of these figures at design temperature. For 25mm² and above, the tabulated z is the total complex impedance (resistance + reactance).

Worked Example: Long-Run SWA to Outbuilding (7.4kW EV + Lighting)

Scenario:

Load: 40A total (32A EV charger + 8A for lighting/sockets diversity)
Cable: 2-core SWA buried direct 600mm
Length: 65m from CU to detached garage consumer unit
Voltage-drop target: ≤ 3% (tight, to protect EV charger PEN-detection algorithms)

Try 10mm² 2-core SWA:

V_d = (4.4 × 65 × 40) ÷ 1000 = 11.44V = 4.97% ✗ (over 3% target)

Try 16mm² 2-core SWA:

V_d = (2.8 × 65 × 40) ÷ 1000 = 7.28V = 3.17% ✗ (still just over)

Try 25mm² 2-core SWA:

V_d = (1.75 × 65 × 40) ÷ 1000 = 4.55V = 1.98% ✓

✓ Final answer:

Use 25mm² 2-core SWA for 1.98% drop. Over 65m, the copper cost difference between 10mm² and 25mm² is significant (~£200 extra) but saves a site visit if the EV charger rejects the installation due to voltage fluctuation. For 3-phase (22kW charger): 10mm² 4-core SWA delivers ~2.1% drop over the same run — a much cheaper option.

SWA-Specific Voltage-Drop Gotchas

Armour as CPC is fine for voltage drop but a concern for Z_s. The steel armour carries fault current, not phase current, so it doesn't affect voltage drop. However, SWA's armour has a higher resistance per metre than the phase conductor, so long runs may need a separate CPC for earth fault loop impedance compliance.
Buried-direct runs stay cooler — voltage drop is usually lower than tabulated. BS 7671 mV/A/m values assume 70°C conductor. Buried cables often operate at 40–50°C in UK soil, reducing conductor resistance by ~10%. Worst-case calculation is still safer for design.
3-core SWA single-phase vs 4-core SWA 3-phase — different columns. A 3-core SWA used single-phase (L/N/earth) uses the single-phase mV/A/m column; a 4-core SWA on balanced 3-phase uses the lower 3-phase column. Don't mix them.
Neutral conductor loading on 3-phase 4-wire SWA. Harmonic-rich loads (LED banks, VSD drives) can push neutral current above phase current — affecting both voltage drop and cable heating. See BS 7671 Appendix 4 §11.

How to Fix Excessive Voltage Drop — 5 Remediation Strategies

When a voltage drop calculation exceeds BS 7671 limits (or your design target), upsizing the cable is the default fix — but it isn't always the cheapest or cleverest option. These are the five remediation strategies UK electricians use in practice, ranked roughly from simplest to most involved.

1Upsize the Cable (one or two sizes)

The default fix. Moving from 6mm² to 10mm² roughly halves voltage drop (4.4 → 7.3 mV/A/m — approximately 60% reduction). Typical extra cost: £2–6/m for the extra copper. Works best when the run isn't already at the maximum cable size the terminals will accept (2.5mm² at socket terminals, 10–16mm² at MCB terminals).

When to use: short-to-medium runs (≤30m) with straightforward upsize path. The calculator above automatically suggests the smallest compliant size.

2Shorten the Cable Route

Voltage drop is directly proportional to length. A 20% shorter route = 20% less drop. On long runs with unnecessary detours (over, around and back through loft spaces), re-routing can solve marginal cases for free. Also consider running cables through floors instead of around them, or using external wall routes where compliant.

When to use: new builds and rewires where cable routes are not yet fixed. Rarely viable on retrofit unless major building works are planned.

3Reposition the Distribution Board

If most circuits are long because the consumer unit is tucked in a corner cupboard, relocating the CU closer to the load centre (e.g. central utility room instead of far-corner garage) can reduce average run length by 30–50%. Typical cost: £200–500 plus meter-tail extension. Applies disproportionately to older bungalows and 1970s-plan semis where the CU ended up next to the incoming service head for convenience.

When to use: major refurbishments, CU upgrades, or fuse-board replacements where tails are being replaced anyway.

4Add a Sub-Distribution Board

For large installations (multi-unit residential, commercial, outbuildings, barns), a heavy sub-main to a second DB at the load centre is often cheaper than oversizing every final circuit. Run a 25mm² SWA sub-main to the new DB, then short 2.5mm² and 6mm² final circuits locally. Key Regulation 433.2.2: each DB must have its own overcurrent protection at origin.

When to use: outbuildings, upstairs extensions, garages with heavy loads. The classic UK application is a 40m+ run to a workshop or granny annexe.

5Split the Load Across Multiple Circuits

If one 32A circuit is failing voltage drop at 40m, two 20A radial circuits at the same length and cable size will easily comply — each carries less current, and voltage drop is proportional to current. Especially useful for kitchen circuits (split high-current appliances across two 20A radials), long outbuilding circuits, and EV chargers where a second dedicated circuit can future-proof the installation.

When to use: heavily-loaded kitchens, outbuildings with mixed loads, and when the CU has spare ways. Less practical when CU is already full.

Decision rule of thumb

If the run is under 30m and one cable size bigger fixes the calculation — upsize. If the run is over 50m and you need to go up three cable sizes — split to two circuits or add a sub-main. If the whole installation has long runs — relocate the distribution board. Voltage drop is a design problem, not just a cable-size problem.

Voltage Drop Budget Table — Cumulative Drop From Origin to Load

BS 7671 Appendix 12 specifies voltage drop from the origin of the installation (the DNO cutout or private supply origin) to the final point of use — not per circuit. Real installations use multiple voltage-drop stages. A thoughtful budget allocates the 5% allowance across those stages so no one stage consumes the entire headroom.

Typical UK Installation Budgets

Installation Type	Tails	Sub-main	Final circuit	Total (max)
Small domestic (single CU)	~0.5%	—	≤4.5%	5% (power)
Domestic + outbuilding DB	~0.5%	≤2%	≤2.5%	5% (power)
Small commercial (meter → DB → finals)	~0.5%	≤1.5%	≤3%	5% (power)
Commercial lighting (to furthest fitting)	~0.5%	≤1%	≤1.5%	3% (lighting)
Industrial 3-phase (main panel → sub-DB → motor)	—	≤1.5%	≤3.5%	5% (power)
Private LV supply (site generator, large solar)	up to 1%	up to 3%	up to 4%	8% (private)

Energy Cost of Voltage Drop — Power Loss is Quadratic

Voltage drop isn't a cosmetic number — it's real power dissipated as cable heat, governed by P_loss = I²R. At 5% drop, roughly 5% of delivered power is wasted in the cable. For heavily-loaded commercial circuits, the energy cost of undersized cables can justify upsizing within a few years.

Circuit	Load hours/yr	Vd	Est. cable energy loss (£/yr at 30p/kWh)
Domestic ring final, 20A typical	~500h	3%	~£0.70
Commercial office lighting, 30A	~3,000h	3%	~£21
Industrial 100A 3φ sub-main	~4,000h	5%	~£400
Data centre 3φ sub-main, 300A continuous	~8,760h	5%	~£7,800

Estimates assume balanced resistive load and average 30p/kWh commercial electricity. Regulation 132.5 and BS 7671 Appendix 17 encourage design below the 5% ceiling specifically to reduce this energy waste. Upsizing one cable step often pays back in under 5 years on high-duty-cycle circuits.

Design rule: always verify voltage drop at the furthest point of use, not just at the sub-DB input. For lighting circuits the stricter 3% limit applies — so commercial lighting designs often run finals in 1.5mm² instead of 1.0mm² specifically to keep cumulative drop under 3%.

Advanced Voltage Drop Considerations - UK Regulations

Professional electricians need to understand these advanced concepts for complex installations and edge cases that affect voltage drop compliance.

ESQCR Supply Voltage Limits - The Real Starting Point

The Electricity Safety, Quality and Continuity Regulations (ESQCR) 2002 define UK supply voltage limits that affect how you budget voltage drop.

Nominal	Tolerance	Range
230V	+10% / -6%	216.2V to 253V
400V (3-phase)	+10% / -6%	376V to 440V

The Critical Insight: If supply arrives at 216.2V (minimum allowed), a 5% installation drop leaves only 205.4V at the load. Many appliances specify minimum 207V operation. For critical loads, aim for 3-4% total drop, not 5%.

Cumulative Voltage Drop - Sub-Main + Final Circuit

BS 7671 limits apply to total voltage drop from origin to load, not just the final circuit. For installations with sub-distribution boards, you must budget carefully.

Voltage Drop Budget Example:

Total allowed: 5% (11.5V)

Sub-main (meter to DB): 1.5% (3.45V)

Final circuit (DB to socket): 3.5% (8.05V)

Total: 5.0% ✓ Compliant

Common Mistake: Calculating final circuit drop without checking what drop already exists on the sub-main. A 40m sub-main at 100A on 25mm² cable already uses 2.1% of your budget.

Understanding mV/A/m - Resistance vs Impedance

The mV/A/m values in BS 7671 tables account for more than just conductor resistance. They include the complex impedance at operating temperature.

What mV/A/m Includes:

• Conductor DC resistance at 70°C (PVC) or 90°C (XLPE)
• Skin effect (significant above 120mm²)
• Proximity effect in grouped cables
• Inductive reactance (XL)

Temperature Correction:

• Tables assume conductor at max operating temperature
• Actual drop may be lower if cable runs cool
• For lightly loaded cables, actual drop could be 80-90% of calculated

For precise design calculations, BS 7671 Appendix 4 provides separate r (resistance) and x (reactance) components for complex impedance calculations.

Motor Starting Voltage Drop - The 6-8× Factor

Induction motors draw 6-8 times Full Load Current (FLC) during Direct-On-Line (DOL) starting. This causes temporary voltage drop that can affect starting performance.

Running Vd	Starting Vd (×7)	Impact
2%	14%	Acceptable
3%	21%	Marginal
5%	35%	May stall

Rule of Thumb: For motors, limit running voltage drop to 2-3% so starting drop stays under 20%. Use soft starters or VFDs for long cable runs.

Power Factor and Reactive Loads

For resistive loads (heaters, kettles), the simple mV/A/m calculation is accurate. For inductive loads (motors, transformers), power factor affects both current and voltage drop characteristics.

Impact on Design:

• Low PF loads draw more current for same kW
• A 10kW motor at 0.8 PF draws 54.3A (not 43.5A as pure resistive)
• Higher current = proportionally higher voltage drop
• BS 7671 mV/A/m values assume unity PF for resistive loads

For accurate motor circuit design: Use the actual current (Watts ÷ Volts ÷ PF) not the theoretical resistive current, when calculating voltage drop.

EV Charger Voltage Drop - Real World Considerations

EV chargers present unique voltage drop challenges because they operate at high current for extended periods, often at the end of long cable runs.

7.4kW Charger (32A)

Up to 25m: 6mm² SWA (3.2% drop)
25-50m: 10mm² SWA (2.8% drop)
50m+: Consider 16mm² or voltage boost

22kW Charger (32A 3-phase)

Up to 40m: 6mm² 4-core SWA
40-80m: 10mm² 4-core SWA
Lower voltage drop per phase (√3 factor)

OZEV grant regulations require voltage drop compliance certification. Oversizing cable avoids costly corrections and future-proofs for higher-power chargers.

Electric Shower Voltage Drop - Why It Matters More

Electric showers are particularly sensitive to voltage drop because the heating element output is proportional to V². A 5% voltage drop causes approximately 10% reduction in heating power.

Power Reduction Example (9.5kW shower):

At 230V:

9.5kW output

At 218.5V (5% drop):

8.55kW output (-10%)

At 210V (8.7% drop):

7.9kW output (-17%)

Recommendation: For electric showers, aim for maximum 3% voltage drop to maintain adequate hot water temperature, especially in winter.

Professional Best Practice

When designing circuits, consider voltage drop early in the process:

Calculate voltage drop before selecting cable size for current capacity
Budget sub-main drop before designing final circuits
Use 3% as a practical target, saving margin for supply variations
Document voltage drop calculations on design records for compliance

How to Use the Voltage Drop Calculator

Use this calculator to verify that your cable installation meets BS 7671 voltage drop requirements before energizing the circuit.

Enter the load current - The actual current (in Amps) flowing through the cable under normal operation.
Specify the cable length - The one-way distance (in meters) from the distribution board to the load point.
Select the cable size - The cross-sectional area of the conductor in mm² (e.g., 2.5mm², 4.0mm², etc.).
Choose the voltage - 230V for single-phase or 400V for three-phase installations.
Select conductor material - Copper is standard in the UK. Aluminium has higher resistance.

Understanding Voltage Drop

Voltage drop is the reduction in voltage that occurs as electrical current flows through a conductor due to the conductor's resistance. Excessive voltage drop can cause:

Poor performance of electrical equipment
Dimming of lights
Overheating of motors
Reduced equipment lifespan

BS 7671 Limits

Lighting circuits: Maximum 3% voltage drop
Other circuits: Maximum 5% voltage drop

BS 7671 Regulation 525 - Voltage Drop

This calculator checks compliance with BS 7671:2018+A2:2022 Regulation 525 - Voltage drop in consumers' installations.

Regulation 525.1

Under normal service conditions, the voltage drop between the origin of the installation (usually the supply terminals of the consumer unit) and any socket outlet or other point of utilization shall not exceed the values given in Appendix 12:

3% for lighting circuits - To prevent visible dimming
5% for other uses - Power circuits, heating, etc.

How Voltage Drop is Calculated

The calculator uses the following formula:

Single-phase: V_drop = 2 × I × L × R

Three-phase: V_drop = √3 × I × L × R

Where: I = Current (A), L = Length (m), R = Resistance (Ω/m)

Note: Voltage drop is only one consideration. Cable selection must also account for current-carrying capacity, overcurrent protection, and fault protection requirements.