Voltage Drop Explained: BS 7671 Limits and a UK Worked Example

Cable has resistance. Push current through it and some voltage is lost as heat along the way, so the voltage at the end of a run is always a little lower than at the start. On a short lighting drop that loss is trivial. On a 40 m submain to an outbuilding it can be the difference between kit that runs and kit that browns out.

The loss depends on three things: how much current flows, how long the cable is, and the cable's resistance per metre, which falls as the cross-sectional area rises. That's why the standard fix for excessive voltage drop is a bigger cable, not a different route.

Takeaway: voltage drop is a length-and-current problem. The longer the run and the higher the current, the more you lose.

BS 7671, the UK wiring regulations, recommends limits rather than hard-coding a single number. For lighting circuits the drop from the origin of the installation to the far point should stay within 3% of nominal voltage. For everything else the figure is 5%.

On a standard 230 V single-phase supply that works out at about 6.9 V for lighting and 11.5 V for power. Lighting gets the tighter limit because dimming and flicker show up quickly, and because lamps are sensitive to running below their rated voltage. These are the numbers most UK installers check against, and they're measured across the whole circuit, not just the final leg.

Takeaway: 3% for lighting, 5% for the rest, measured all the way to the far end.

BS 7671 makes the calculation easy by publishing a figure for each cable size called mV/A/m, the millivolts dropped per amp of current per metre of run. You'll find it in the Appendix 4 tables alongside the current-carrying capacities.

The sum is: volts dropped = (mV/A/m × current × length) ÷ 1000. Divide by 1000 because the table value is in millivolts. Compare the result against your limit in volts, or turn it into a percentage of the supply voltage. For three-phase you use a slightly different table value, because the geometry of a three-phase circuit changes the drop.

A worked feel for the numbers helps. A 2.5 mm² cable has an mV/A/m of about 18. A 10 mm² cable is about 4.4. So swapping 2.5 mm² for 10 mm² cuts the drop per metre to roughly a quarter, which is exactly why upsizing fixes a failing run.

Takeaway: one table value, one short sum. Get the mV/A/m for your cable size and the rest is arithmetic.

A homeowner in Bristol wants power to a summerhouse at the bottom of the garden. The supply is a 20 A radial, single-phase 230 V, run in 2.5 mm² twin and earth for 35 m. The load is fine for 2.5 mm² on current-carrying capacity, so on paper the cable passes. The question is voltage drop.

Take the mV/A/m for 2.5 mm², which is about 18. The design current is 20 A and the run is 35 m. So the drop is (18 × 20 × 35) ÷ 1000, which comes to 12.6 V. As a percentage of 230 V that's about 5.5%. The 5% limit for a power circuit is 11.5 V, so 2.5 mm² fails, but only just.

Step up to 4 mm², which has an mV/A/m of about 11. The same sum gives (11 × 20 × 35) ÷ 1000, or 7.7 V, about 3.3%. That's comfortably inside 5%, so 4 mm² is the cable that works. Notice the load never changed. The length alone pushed the size up.

Takeaway: a circuit can pass on current and still fail on voltage drop. Always run both checks on a long radial.

Long garden runs, EV charge points on the far side of a property, workshop and garage submains, and agricultural feeds are the usual culprits. Anything that leaves the house and keeps going. In practice most UK electricians will size the cable for current, then immediately check voltage drop on any run past about 20 m, because that's roughly where the two checks start to disagree.

Cable price makes this a real decision rather than an academic one. A run of 4 mm² twin and earth from a wholesaler like Screwfix or CEF costs noticeably more than 2.5 mm² over 35 m, so you don't want to over-specify every job, but you also can't afford a callback because the summerhouse lights flicker. The calculation is how you land on the size that's right rather than the size that's cheap or the size that's lazy.

Takeaway: check voltage drop on every long run before you buy cable, not after the flicker complaint.

Here's the point competing guides often gloss over. The 3% and 5% figures in BS 7671 are recommendations, not absolute limits like a current-carrying capacity you must never exceed. The regulation's real requirement is that the voltage at equipment stays within the range it needs to work safely and correctly. In some cases, with a documented assessment, a designer can justify a drop slightly outside the guideline if the connected equipment still operates within its own tolerance.

That isn't a licence to ignore the limits. It's the opposite of the "5% is a brick wall" framing you'll read elsewhere. The wall is whether the equipment works and stays safe; the percentages are the sensible shortcut that gets you there without a full assessment on every circuit. For everyday work, treat 3% and 5% as your targets. Just know why they exist.

Takeaway: hit the 3% and 5% targets by default, but understand they serve the real requirement, which is equipment that works within its rated voltage.

What is the maximum voltage drop allowed in the UK? BS 7671 recommends 3% of nominal voltage for lighting and 5% for other circuits, measured from the origin to the far point. On 230 V that's about 6.9 V and 11.5 V.

How do I calculate voltage drop? Use volts dropped = (mV/A/m × current × length) ÷ 1000, taking the mV/A/m value for your cable size from the BS 7671 tables, then compare against your limit.

Why does a longer cable drop more voltage? Because more metres of cable means more total resistance in the circuit, and voltage drop rises directly with length for a given current and cable size.

Does a bigger cable reduce voltage drop? Yes. A larger cross-sectional area has a lower mV/A/m, so it drops less voltage per metre. Upsizing is the standard fix for a run that fails on voltage drop.

Is voltage drop the same for three-phase? No. Three-phase circuits use a different mV/A/m value from the tables because the drop relationship differs, so use the three-phase figure rather than the single-phase one.

The example above is exactly what the Voltage Drop Calculator does: enter cable size, current and length and it returns the drop in volts and as a percentage against the 3% or 5% limit. If you're sizing the circuit from scratch rather than checking a size you've already chosen, the Cable Sizing Calculator (BS 7671) works out the cable and runs the voltage drop check together. For the rest of a job, browse all BuildByJai calculators.

Professional advice notice: this article is general guidance, not a design. Electrical work in the UK is subject to Part P of the Building Regulations. Have circuits designed, installed and certified by a competent person where required, for example an NICEIC or NAPIT registered electrician.