Voltage Drop CalculatorSWA cable size calculator for sheds and garden rooms
Voltage drop on a steel wire armoured cable to a shed, garden room, garage or EV charger, in mm² and metres, against the BS 7671 figures: 3% for lighting and 5% for other uses.
Result
Copper, 75 °C basis| Wire size | 6 mm² |
| Voltage drop | 5.72 V |
| Percent drop | 2.49 % |
| At the load | 224.28 V |
Within 3% target
6 mm². Drop 5.72 volts, 2.49 percent. Within 3% target.
How this is worked out
V = 2 × ρ × I × L ÷ A
2 × 0.02145 × 32 A × 25 m ÷ 6 mm² = 5.72 V
5.72 V ÷ 230 V × 100 = 2.49 %. At the load: 230 V − 5.72 V = 224.28 V.
- L is the one-way length; the 2 counts the return conductor.
- ρ is 0.02145 for copper and 0.03524 for aluminum, in ohm·mm² per metre: the same 75 °C basis as K = 12.9 and 21.2 in US units.
- Not checked here: temperature derating, conduit fill, power factor and code compliance.
What BS 7671 asks for
- 3% for lighting, 5% for everything else, on a normal public supply. A design within these figures is deemed to comply with Section 525. On 230 V they are 6.9 V and 11.5 V.
- Measured over the whole path, from the origin of the installation, where the supply comes in, to the equipment. The SWA and the circuits in the outbuilding share one budget.
- Deemed to satisfy, not a hard limit. The requirement is that equipment gets a voltage it can work with. A larger drop is accepted while a motor starts or for high-inrush equipment, as long as the equipment stays within its own tolerance.
- The current edition is BS 7671:2018+A4:2026 (18th Edition, Amendment 4), published in April 2026. The earlier version was withdrawn in October 2026.
Worked example: a garden office 25 m away
A 32 A supply to a garden office, 25 m of copper SWA from the house, at 230 V:
| SWA size | Drop | Left for lighting | Left for other uses |
|---|---|---|---|
| 4 mm² | 3.73% | none | 1.27% |
| 6 mm² | 2.49% | 0.51% | 2.51% |
| 10 mm² | 1.49% | 1.51% | 3.51% |
6 mm² is the smallest size within 3% (5.72 V), which is where the calculator above starts. But that leaves the lights in the office almost nothing, so if the office has lighting circuits, a larger SWA leaves room for them.
Splitting the budget
Decide how much of the 3% and 5% the SWA may use, then enter that share as a custom target in the calculator. Whatever the SWA uses, the circuits in the outbuilding get the rest:
| SWA drop | Left for lighting | Left for other uses |
|---|---|---|
| 1% | 2% | 4% |
| 1.5% | 1.5% | 3.5% |
| 2% | 1% | 3% |
| 2.5% | 0.5% | 2.5% |
| 3% | 0% | 2% |
The cable from the meter to the consumer unit in the house counts too, but on a short run its share is small.
The mV/A/m method and this calculator
UK electricians work out drop with the mV/A/m figures in BS 7671 Appendix 4: drop in volts = mV/A/m × design current × route length in metres ÷ 1000. For multicore armoured copper cable the figures are in Tables 4D4B (cable rated at 70 °C) and 4E4B (rated at 90 °C).
This calculator works from conductor resistance instead (the formula and constants are on the method page), so you don’t need the tables. Two differences:
- Temperature. The tables assume the conductor at its full operating temperature, so they are cautious. This calculator uses a 75 °C copper basis, so its result can come out a few percent below a 90 °C table figure. BS 7671 also gives a more exact method for a lightly loaded cable that runs cooler. If you are close to the limit, check the run with the tables or an electrician.
- Reactance. Up to 16 mm² the tables use resistance alone, as this calculator does. From 25 mm² they add reactance, which this calculator leaves out.
SWA cable types
- BS 5467 is the usual SWA: XLPE insulation, PVC sheath, 600/1000 V, 90 °C maximum conductor temperature, with standard metric conductor sizes. Most equipment terminals are rated 70 °C, so designers often rate it as if it were a 70 °C cable.
- BS 6724 is the low-smoke zero-halogen version, used inside public buildings.
- BS 6346, PVC-insulated SWA, was withdrawn in 2011.
Voltage drop is one of several checks
- Current rating. The cable must carry the current for the way it is installed (buried, clipped, in a duct). Enter that rating in the optional ampacity field; the ampacity guide explains which check governs.
- Fault protection. A fault must be cleared fast enough, which depends on the earth fault loop impedance of the whole run. On a long SWA this can decide the size too. This calculator does not check it.
- Earthing. The steel armour can serve as the earth (circuit protective conductor) if an electrician confirms it is large enough. On a PME supply, the earthing at an outbuilding needs its own design, often a 3-core SWA or a separate earth electrode.
Common sizes
6 mm² and 10 mm² are the sizes most often used for sheds and garden offices, with 10 mm² for longer runs or heavier loads. That is common practice, not a recommendation: put your own current and length into the calculator.
Frequently asked questions
What voltage drop does BS 7671 allow?
A design is deemed to comply when the drop from the origin of the installation to the equipment is within 3% for lighting and 5% for other uses, on a normal public supply. On 230 V that is 6.9 V and 11.5 V. These are deemed-to-satisfy figures from Appendix 4, not hard legal limits: what the regulations require is that each piece of equipment gets a voltage it can work with.
What size SWA do I need for a shed or garden office?
It depends on the current and the length. 6 mm² and 10 mm² are the sizes most often used for sheds and garden offices, with 10 mm² for longer runs or heavier loads, but that is common practice, not a rule. Enter your own run above, then check the cable’s current rating and the earth fault loop impedance with an electrician.
Do I enter 230 V or 240 V?
230 V, the UK nominal voltage. The supply may vary from about 216 V to 253 V (230 V +10% / −6%), and the drop in volts is the same whatever the supply voltage is.
What current do I enter?
The design current: the most the outbuilding will draw at once. If you do not know it, the rating of the breaker that feeds the SWA is a safe upper figure.
Does this calculator check the cable’s current rating?
No. It holds no BS 7671 current-rating tables. Enter the rating for your cable and installation method, from the tables or the cable maker’s datasheet, in the optional ampacity field, and the calculator checks your current against it.
Do I need to tell building control?
In England a new circuit to an outbuilding is notifiable under Part P: use a registered electrician or tell building control before you start. In Wales new circuits and new outdoor wiring are notifiable. Scotland has no Part P; a new circuit in a typical house needs no building warrant but must still meet the building regulations. As of November 2026; check with your council.
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