Maximum Voltage Drop Allowed in the UK: BS 7671 3% and 5% Limits Explained

BS 7671:2018+A2:2022, the 18th Edition Amendment 2, sets recommended voltage drop limits in Appendix 12. For a low-voltage installation supplied directly from the public network, which covers almost every UK home, the figures are 3% for lighting circuits and 5% for other uses such as socket outlets, cookers, showers and EV chargers.

On a standard 230 V single-phase supply, those percentages turn into real volts. 3% of 230 V is 6.9 V. 5% of 230 V is 11.5 V. So a lighting circuit shouldn't drop more than 6.9 V from the intake to the last fitting, and a power circuit shouldn't drop more than 11.5 V to the furthest point of use.

There's a separate set of figures if your installation is fed from a private supply, such as your own transformer or generator rather than the grid. In that case Appendix 12 allows 6% for lighting and 8% for other uses, because the starting voltage is under your own control. For the vast majority of homes, though, it's the 3% and 5% figures that apply.

This is where people get caught out. The limit applies to the whole route from the origin of the installation, your main intake, all the way to the equipment. It's not measured per circuit in isolation.

That matters when there's more than one leg. If you feed a detached garage from a sub-main and then run a final circuit inside the garage, the voltage drop of the sub-main and the final circuit add up, and the total has to stay within the 5% limit. Size the garage final circuit on its own and you can pass the final circuit while failing the installation as a whole.

The other detail is that voltage drop is driven by three things: the current the circuit carries, the length of the run, and the cross-sectional area of the cable. You can't do much about the first two once the design is set, so cable size is the lever you actually pull.

Here's the part most pages skip. The 3% and 5% figures live in an informative appendix, and they're recommendations, not an absolute pass or fail. The actual requirement sits in Regulation 525, which says the voltage at the terminals of any piece of equipment must stay within the range that equipment needs to work properly.

In practice that means two things. A design can technically exceed 5% and still comply, if the equipment's manufacturer confirms it tolerates the lower voltage and the reasoning is documented. And, more importantly the other way round, staying under 5% is no guarantee everything works. Voltage-sensitive equipment, motors on start-up, and long runs to demanding loads can misbehave at a drop that's comfortably inside the recommendation.

So treat 5% as the sensible design target it is, not as a magic threshold. If a circuit lands at 4.9% you haven't necessarily "passed", and if it lands at 5.2% you haven't automatically broken the law. Design to what the equipment at the end actually needs, and use the recommendation as your default unless you've a documented reason to depart from it.

Take a common 2026 job. You're installing a 7.4 kW EV charger, which draws about 32 A, and the run from the consumer unit to the charger is 40 m, a realistic distance to a driveway or detached garage.

Start with 6 mm² twin and earth. From the BS 7671 Appendix 4 tables, 6 mm² has a voltage drop of about 7.3 mV per amp per metre. The formula is:

voltage drop = (mV/A/m × design current × length) ÷ 1000

So: (7.3 × 32 × 40) ÷ 1000 = 9.34 V.

As a percentage of 230 V, that's 9.34 ÷ 230 = 4.06%. The limit for a power circuit is 5%, or 11.5 V, so 6 mm² passes with a bit of headroom.

Now see what happens if you drop to 4 mm², which has a higher figure of about 11 mV/A/m: (11 × 32 × 40) ÷ 1000 = 14.08 V, which is 6.12%. That fails the 5% recommendation, and at over 14 V lost the charger could throttle or fault on a cold morning. So 6 mm² is the right call here, and a 50 m drum of it from a wholesaler like CEF or Screwfix, in a brand such as Doncaster Cables, is a modest cost for the certainty. The Cable Sizing Calculator checks this alongside current-carrying capacity, which is the other half of the decision.

For decades voltage drop was mostly an on-paper check that passed without drama, because runs were short and loads were modest. That's changed. The circuits that fail the 5% limit in modern homes are almost always the long, hungry ones: EV chargers on driveways, hot tubs and garden rooms at the end of the garden, workshops in detached garages, and electric showers a long way from the board.

In practice, most electricians size these runs a notch bigger than the current alone would demand, precisely because voltage drop, not current-carrying capacity, is the thing that catches them. A 32 A circuit might be fine on 4 mm² for its load, and still need 6 mm² once the length is in. The length is the input people underestimate, because they measure the straight line across the garden and forget the cable goes up, along, down and around.

If your calculation comes back above 3% or 5%, you have three levers. Increase the cable cross-sectional area, which is the usual fix and the one the worked example shows. Shorten the run if the route allows a more direct path. Or reduce the load, which is rarely practical for a fixed appliance. Going up a cable size is almost always the cleanest answer, and it's cheap relative to redoing the job.

One thing not to do is ignore it because "it's only informative". Excess voltage drop shows up as dim lights, slow-heating showers, EV chargers that de-rate, and motors that run hot. It's a real performance and, over time, a real safety issue, not a box-ticking formality.