BS 7671 Voltage Drop Calculator
Estimate cable voltage drop and verify compliance with BS 7671 Appendix 4 limits for UK electrical installations. Handles single and three-phase designs.
BS 7671 Voltage Drop Calculator
How to Use BS 7671 Voltage Drop Calculator
Ensuring cable sizes comply with voltage drop limits is a fundamental requirement of BS 7671 (the IET Wiring Regulations). Follow these step-by-step instructions to verify compliance:
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1Select supply phase. Choose single-phase (230 V) or three-phase (400 V) based on your installation type.
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2Select circuit type. Select whether the installation is for lighting (3% maximum allowable limit) or power/heating (5% limit).
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3Enter cable length. Input the physical routing length of the cable path, measured in meters.
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4Enter current. Input the design current (Ib) of your connected load in Amps.
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5Choose mV/A/m value. Select the specific voltage drop factor from the dropdown, which represents your cable's size and resistivity.
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6Interpret compliance results. Click calculate to see the pass or fail compliance status, along with absolute and percentage values.
BS 7671 Standards for Voltage Drop
Under BS 7671 (IET Wiring Regulations), regulations specify the maximum voltage drop allowed between the origin of the installation (usually the consumer unit or distribution board) and any individual outlet or load. The standard limits depend on the type of supply and the circuit's purpose to ensure safety and system efficiency.
Standard Recommended Limits
For installations supplied directly from a low-voltage public distribution network, the limits are:
- Lighting Circuits: Maximum 3% voltage drop. On a 230 V single-phase supply, this equates to 6.9 V. On a 400 V three-phase supply, it is 12.0 V. This prevents visible lamp flicker.
- Other Circuits (Power/Heating/Sockets): Maximum 5% voltage drop. On a 230 V single-phase supply, this equates to 11.5 V. On a 400 V three-phase supply, it is 20.0 V. This ensures heaters, motors, and appliances function optimally.
Private Supplies and General Exceptions
If the electrical installation has its own private generation or transformer supply (not directly connected to the public grid), BS 7671 permits higher limits: up to 6% for lighting and 8% for other power circuits. Furthermore, temporary inrush starting currents of motors or equipment are generally excluded from these limits, provided the voltage drop during start-up does not exceed safe operating parameters for other connected devices.
How to Calculate BS 7671 Voltage Drop
Under the IET Wiring Regulations, design requirements require sizing conductors to guarantee the voltage drop does not affect the performance of connected devices. The equations used in this UK voltage drop calculator are derived directly from Appendix 4.
Formula 1 — Calculate Voltage Drop (V)
The total voltage drop in volts along a radial circuit is calculated by multiplying the cable's specific resistivity factor (mV/A/m) by the design current and the routing path distance, and then dividing by 1,000 to convert millivolts to volts:
Formula 2 — Calculate Voltage Drop (%)
The percentage drop compares the calculated drop against the supply nominal voltage (230 V for a single-phase supply or 400 V for a three-phase supply):
Step-by-Step Engineering Example
Suppose you are analyzing a radial power circuit on a single-phase supply (230 V) with the following parameters:
- Nominal Supply Voltage: 230 V (single phase)
- Circuit Category: Power Circuit (5% maximum drop limit)
- Design Current: 32 A
- Route Length: 45 m
- mV/A/m factor: 11 mV/A/m
Step 1 — Calculate Voltage Drop
Apply the values to the primary calculation equation:
Step 2 — Calculate Percentage Drop
Compare this drop with the 230 V nominal supply:
Step 3 — Compliance Verification
The calculated drop of 6.89% exceeds the 5% BS 7671 limit for power circuits (which is 11.5 V). Therefore, this cable sizing results in a FAIL compliance status. A larger cable size with a smaller mV/A/m factor must be selected.
BS 7671 Voltage Drop Chart
This reference chart displays calculated voltage drops and percentage values at a standard single-phase 230 V supply across different mV/A/m factors, load currents, and installation lengths. All sample data points are selected to fully comply with BS 7671 voltage drop limits.
| Cable Size (mm²) | mV/A/m | Current (A) | Length (m) | Voltage Drop (V) | Voltage Drop (% at 230V) | BS 7671 Status |
|---|---|---|---|---|---|---|
| 1.0 mm² | 44 | 6 A | 15 m | 3.96 V | 1.72% | PASS |
| 1.5 mm² | 29 | 10 A | 20 m | 5.80 V | 2.52% | PASS |
| 2.5 mm² | 18 | 16 A | 25 m | 7.20 V | 3.13% | PASS |
| 4.0 mm² | 11 | 20 A | 30 m | 6.60 V | 2.87% | PASS |
| 6.0 mm² | 7.3 | 32 A | 35 m | 8.18 V | 3.56% | PASS |
| 10 mm² | 4.4 | 40 A | 40 m | 7.04 V | 3.06% | PASS |
| 16 mm² | 2.8 | 50 A | 45 m | 6.30 V | 2.74% | PASS |
| 25 mm² | 1.8 | 63 A | 50 m | 5.67 V | 2.47% | PASS |
| 35 mm² | 1.3 | 80 A | 60 m | 6.24 V | 2.71% | PASS |
| 50 mm² | 0.93 | 100 A | 70 m | 6.51 V | 2.83% | PASS |
Note: Cable voltage drop values above are calculated using standard single-phase 230 V installations. Sizing adjustments using our BS7671 cable voltage drop tool help verify parameters quickly prior to issuing an Electrical Installation Condition Report (EICR).
BS 7671 Voltage Drop Frequently Asked Questions
Under BS 7671 guidance, the maximum permitted voltage drop for installations supplied directly from a low-voltage public distribution system is 3% for lighting circuits and 5% for other circuits (such as socket outlets, heaters, and power loads).
Yes, the total voltage drop is evaluated from the origin of the installation (main distribution board or intake point) to the furthest terminals of the load. If sub-mains are present, drops in both the sub-mains and final radial or ring final circuits must be summed together.
Yes. BS 7671 specifies a recommended 3% limit for lighting circuits to prevent visible lumen variations, flickering, and startup drops on luminaire ballasts, ensuring consistent operation and meeting compliance standards for UK electrical installations.
The unit mV/A/m stands for millivolts drop per Ampere of current per Meter of run. This standardized metric is tabulated in BS 7671 Appendix 4 for different cable sizes, core arrangements, insulation temperatures, and installation methods.
To reduce voltage drop in your circuits, you can select a larger conductor size (cross-sectional area) to decrease resistance, shorten the overall cable run routing, reduce the load current, or balance loads more evenly across three phases.
Exceeding recommended voltage drop limits can lead to equipment malfunctions, failure of inductive loads to start up, dim or flickering luminaires, excess heat generation in cables, and failures to pass an Electrical Installation Condition Report (EICR).
