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.

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 tables and depends on cable size, type (copper/aluminium), and installation method. For three-phase circuits, multiply single-phase result by 0.866 (√3/2).

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. 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. Maintaining voltage within limits ensures equipment operates correctly and efficiently.

How do I reduce voltage drop in a circuit?

Reduce voltage drop by: 1) Using larger cable (lower mV/A/m), 2) Reducing circuit length, 3) Splitting load across multiple circuits, 4) Using a higher voltage supply for long runs, 5) Ensuring connections are tight (high resistance increases drop). Sometimes a submain board closer to loads is the best solution.

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). 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.

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.

Voltage Drop Calculator - BS 7671 Compliant

Calculate voltage drop and check BS 7671 compliance for electrical installations

Last updated: 15 January 2025

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.

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

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.