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

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.