Voltage Drop Explained (BS 7671)
Voltage drop is the small loss of voltage along a cable under load — and if it grows too large, equipment at the far end won’t work properly or safely.
Every conductor has resistance, so every cable loses a little voltage as current flows through it. Push more current, or run the cable further, and the loss grows. BS 7671 places limits on that loss so appliances receive close to their rated voltage, and Appendix 4 of the Regulations gives the tabulated mV/A/m figures you use to check it.
This guide explains what voltage drop is, the 3% and 5% limits recommended for a 230 V supply, how the millivolt-per-amp-per-metre method works with a full worked example, why temperature and load correction matter, and the two practical ways to fix a circuit that fails.
Key takeaways
- Voltage drop is the voltage lost along a cable under load — it rises with current, length and resistance.
- BS 7671 recommends a maximum drop of 3% for lighting and 5% for other circuits from a 230 V supply.
- 3% of 230 V is 6.9 V; 5% is 11.5 V — measured from the origin to the furthest point of use.
- You calculate it with the mV/A/m figure from Appendix 4: drop = (mV/A/m × current × length) ÷ 1000.
- Fix a failing circuit by increasing the cable size or shortening the run — nothing else reliably cures excessive drop.
What voltage drop is and why it matters
When current flows along a conductor, the conductor’s own resistance causes some of the supply voltage to be dropped before it reaches the load. The higher the current and the longer the cable, the bigger the loss. A motor fed with too little voltage runs hot and struggles to start; dimmed lighting, slow-charging equipment and heaters that never reach temperature are all symptoms of excessive drop.
Voltage drop is not a safety fault in the way a poor earth or missing RCD is, but it is a compliance and performance requirement. Regulation 525 requires that the voltage at the terminals of any current-using equipment is not so low as to impair safe function, and Appendix 4 gives the recommended limits that satisfy it in most installations.
The 3% and 5% limits
For a low-voltage installation supplied at 230 V, BS 7671 recommends the voltage drop between the origin and any point of use does not exceed 3% for lighting circuits and 5% for all other circuits. Lighting gets the tighter figure because dimming and flicker are noticeable at small losses, whereas socket and power circuits tolerate a little more.
The percentages are easier to work with as volts. The table below converts them and shows the mV/A/m values for common copper conductor sizes — the figure you multiply by current and length to get the actual drop.
| Circuit type / cable size | Limit or mV/A/m | Notes |
|---|---|---|
| Lighting limit (3% of 230 V) | 6.9 V | Maximum drop origin to furthest fitting |
| Power/other limit (5% of 230 V) | 11.5 V | Sockets, cookers, showers, etc. |
| 1.0 mm² (single-phase) | 44 mV/A/m | Small lighting circuits |
| 1.5 mm² (single-phase) | 29 mV/A/m | Lighting and small power |
| 2.5 mm² (single-phase) | 18 mV/A/m | Ring and radial socket circuits |
| 4.0 mm² (single-phase) | 11 mV/A/m | Larger radials, cookers |
| 6.0 mm² (single-phase) | 7.3 mV/A/m | Showers, sub-mains |
Which distance do you use?
The limit applies from the origin of the installation to the furthest point of use — so for a ring final circuit measure to the furthest socket, and remember a ring shares the load across two legs, which halves the effective drop compared with the same length of radial.
The mV/A/m method with a worked example
The tabulated figure is the drop in millivolts for each amp of design current flowing through each metre of cable. To find the actual drop, multiply the three together and divide by 1000 to convert millivolts to volts: drop (V) = (mV/A/m × Ib × L) ÷ 1000, where Ib is the design current and L is the route length.
Worked example: a 2.5 mm² radial socket circuit carries a design current of 20 A over a 25 m run. From the table, 2.5 mm² is 18 mV/A/m. Drop = (18 × 20 × 25) ÷ 1000 = 9.0 V. As a percentage of 230 V that is 3.9%, comfortably inside the 5% (11.5 V) limit for a power circuit, so the cable passes on voltage drop.
If that same 20 A load ran 40 m instead, the drop becomes (18 × 20 × 40) ÷ 1000 = 14.4 V, or 6.3% — now over the 11.5 V limit and a fail. That single change of length shows how sensitive drop is to distance.
Let the numbers do the work
TradePlanr’s voltage drop calculator applies the correct Appendix 4 mV/A/m figure for your cable and gives the volts and percentage instantly, and the cable sizing calculator flags when a size fails so you can step up before you order.
Temperature and load correction
The tabulated mV/A/m values assume the conductor is at its full operating temperature. Copper resistance rises with temperature, so a lightly loaded cable running cool actually drops a little less than the table suggests — for accuracy on circuits loaded well below their rating you can apply a correction factor to reduce the tabulated figure, though using the table value uncorrected is the safe, conservative approach.
For circuits longer than about 100 m the reactance of the cable also starts to matter, and the mV/A/m figure then splits into resistive and reactive components (mVr and mVx) which are combined. For everyday domestic and small commercial runs the simple resistive figure is all you need.
How to fix a circuit that fails
There are only two reliable cures for excessive voltage drop, and both attack the equation directly. The first is to increase the cable size: a larger cross-sectional area has a smaller mV/A/m figure, so the drop falls in proportion. Stepping a failing 2.5 mm² radial up to 4.0 mm² takes the mV/A/m from 18 to 11, cutting the drop by nearly 40%.
The second is to shorten the run — feed the circuit from a nearer distribution point, or split a long radial so no single leg travels as far. Reducing the load current helps too, but you rarely control that. What does not work is anything to do with the protective device or terminations; voltage drop is a function of conductor size, length and current alone.
Frequently asked questions
Is the 3%/5% voltage drop limit a legal requirement?
The percentages are recommended maxima in Appendix 4 of BS 7671, not absolute limits. The actual requirement in Regulation 525 is that equipment receives enough voltage to function safely. In practice, designing to 3% for lighting and 5% for other circuits is the accepted way to satisfy that, and exceeding it needs justification.
Do I measure voltage drop from the meter or the consumer unit?
From the origin of the installation to the furthest point of use on the circuit. For most jobs that means from the consumer unit, since the drop on the supplier’s side is their responsibility. Include the full route length of the final circuit conductors, not the straight-line distance.
Why does a ring circuit have less voltage drop than a radial?
A ring final circuit feeds each point from two directions, so the load current divides between the two legs. This roughly halves the effective drop compared with a radial of the same total cable length carrying the same load, which is one reason rings suit longer socket circuits.
From guidance to action
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Cable Sizing to BS 7671
The full cable sizing method — design current, correction factors, tabulated ratings and volt drop.
Max Zs Values Table (BS 7671 18th Edition)
Full maximum Zs tables for Type B/C/D MCBs and RCBOs, the Cmin formula and the 80% rule.
Initial Verification: The BS 7671 Testing Sequence
The correct dead-then-live testing sequence for initial verification, step by step.
