How the sum works
Every real wire has some resistance, so pushing current through it always loses a small amount of voltage along the way. That loss depends on how much current is flowing, how resistive the metal is, how long the cable run is, and how thick the conductor is. Put those four together and you get the standard voltage drop formula used across electrical trade references:
The 2× at the front is not a fudge factor. Current has to get to the load and back again, so it travels the length of the run twice over two conductors (the line conductor out, and the neutral or return conductor back), and both of those conductors are adding their own resistance. A run measured as 20 metres one way is really 40 metres of current-carrying wire, and the sum accounts for that by doubling the length rather than asking you to measure the return conductor separately.
Resistivity is a fixed property of the metal at a given temperature. This calculator uses the standard annealed-copper figure of 1.724 × 10-8 Ω·m and the standard aluminium figure of 2.82 × 10-8 Ω·m, both at 20°C, which is the reference temperature most published conductor tables use. Real cable warms up under load, and resistance rises a little with temperature, so a cable working hard in a hot loft will drop slightly more than this calculator predicts. Treat the result as the standard reference-temperature figure, which is what the guidance thresholds below are written against.
Why long 12V runs are brutal
The formula does not care what the supply voltage is, only what current is flowing, and that is exactly why low-voltage runs punish a thin cable so much harder than mains does. A device pulls power, not current, and current is power divided by voltage. Halve the voltage and the same wattage needs double the current, and voltage drop scales directly with current. So a 120W load takes about 0.5A at 230V but around 10A at 12V, and that tenfold jump in current means a tenfold jump in the volts lost to the same length of the same cable.
That is the trap behind a lot of LED strip, solar panel and campervan wiring complaints: a run that would be a non-issue on a mains circuit turns into dim lights or a battery monitor reading lower than the panel is actually producing, purely because the same absolute voltage drop is a much bigger slice of a 12V supply than of a 230V one. A 2V drop is under 1% of 230V but over 16% of 12V. Low-voltage systems need noticeably thicker cable, or a shorter run, to keep the percentage drop sane.
Choosing the right wire size
Once you know the drop is too high, there are only three ways to bring it down: shorten the run, cut the current, or use a bigger conductor. Wire size is usually the practical lever, because the run length and the load are normally fixed by the job. Stepping up one standard size (say, from 2.5 mm² to 4 mm², or from 12 AWG to 10 AWG) increases the cross-sectional area by roughly 50-60%, which cuts the voltage drop by a similar fraction for the same current and length, since drop is inversely proportional to area.
This calculator lets you type the wire size either way round. AWG gauges are a US convention where, confusingly, a smaller number means a thicker wire; the common gauges from 14 down to 1/0 are listed with their cross-section already worked out. The metric mm² sizes are the ones printed directly on UK and European cable, and are the more intuitive of the two because the number is simply the area.
Copper versus aluminium
Copper is the better conductor: aluminium's resistivity is about 1.6 times higher, so an aluminium conductor of the same cross-section drops roughly 60% more voltage than copper over the same run at the same current. Aluminium is still used, mainly in larger feeders and overhead lines, because it is lighter and cheaper for a given current-carrying capacity, but it needs a noticeably bigger cross-section than copper to match the same voltage drop, and it brings its own connection and expansion considerations that a qualified electrician accounts for. This calculator only compares the resistivity difference; it does not size a conductor for current-carrying capacity (ampacity), which is a separate check.
DC versus AC, and what this calculator does not do
For DC and for single-phase AC, the resistive voltage drop formula above is the same sum. The one thing this calculator does not model is reactance: AC current flowing through a conductor also meets inductive reactance, which adds to the resistive drop, and that reactance becomes more significant as the conductor gets larger. As a rough rule of thumb repeated across electrical references, conductors up to around 50 mm² are resistance-dominated enough that a resistance-only estimate stays reasonably close, while larger conductors increasingly need a full impedance (resistance plus reactance) calculation from published cable tables. This tool also does not check three-phase circuits, which use a different (smaller) multiplying factor than the 2× used here, and it does not check whether the cable is rated to carry the current safely in the first place. It answers one question only: how much voltage does this specific run lose.
Reading the pass or exceeds verdict
Two commonly cited planning figures are built into the threshold toggle above. In the US, the National Electrical Code's informational notes recommend keeping voltage drop to around 3% for a branch circuit and around 5% for the combined feeder and branch circuit together; these are recommendations for satisfactory operation, not code requirements enforced everywhere. In the UK, BS 7671 (the IET Wiring Regulations) gives an equivalent commonly cited guideline of around 3% for lighting circuits and around 5% for other uses, when supplied from the public low-voltage network. Both sets of figures are planning guidance rather than hard physical limits, and the verdict shown here should be read the same way: a useful sanity check, not a certificate of compliance.
Questions people ask
Why is the length doubled in the formula?
Because current needs a complete circuit: it travels the one-way run length out to the load on one conductor, and the same length back on the other. Both conductors add resistance, so the total resistive path is twice the one-way distance you measure and enter.
Does this calculator check if my cable is safe to carry the current?
No. This is a voltage drop check only. Whether a cable can safely carry a given current (its current-carrying capacity, or ampacity) depends on installation method, ambient temperature, grouping with other cables and insulation type, and is a separate calculation covered by cable rating tables in BS 7671 or the NEC, best confirmed by a qualified electrician for any fixed wiring.
Why does aluminium show a bigger drop than copper for the same size?
Aluminium is a less conductive metal than copper: its resistivity is roughly 1.6 times higher. For the identical cross-sectional area, length and current, an aluminium conductor loses about 60% more voltage than a copper one.
Is 3% or 5% the correct limit for my circuit?
Neither figure is a universal legal maximum; they are widely cited planning recommendations. 3% is generally quoted for a single branch or lighting circuit, and 5% for the combined run from the source through a feeder and branch circuit together. Local wiring rules and equipment manufacturers can specify their own tighter figures, so treat both as a sensible starting point rather than the final word.
Does this work for three-phase circuits?
No. Three-phase circuits use a different multiplying factor (the square root of 3, roughly 1.73, instead of the 2× used here for a two-conductor DC or single-phase run), because the current-return arrangement is different. This calculator covers DC and single-phase AC only.
Why does my 12V system need such thick cable compared to mains wiring?
The same power at a lower voltage always means a higher current, and voltage drop scales directly with current, not power. A load that draws well under an amp at 230V can draw ten amps or more at 12V, and that extra current needs a noticeably thicker conductor to keep the percentage drop within a sensible range.
Does temperature affect the result?
Yes, in reality. This calculator uses standard resistivity figures quoted at 20°C, which is the reference temperature published conductor tables use. A cable running hot under sustained load has slightly higher resistance than at 20°C, so its real drop will be a little higher than this estimate.
This calculator estimates voltage drop from the figures you enter. It does not check current-carrying capacity, does not cover three-phase circuits, and does not replace a qualified electrician's design for fixed wiring. Any permanent or fixed electrical installation should be designed, installed and inspected by a qualified electrician in line with the wiring regulations that apply where you live.