Free Voltage Drop Calculator – Wire Size, % Drop & Voltage at Load

How Voltage Drop Is Calculated and Why It Matters

A voltage drop calculator tells you how much voltage a cable loses along its length before the electricity reaches the load. Every conductor has resistance, and pushing current through that resistance uses up part of the supply voltage as heat. The longer the run and the higher the current, the bigger the loss, so a light or motor at the far end of a long cable sees less voltage than the panel it started from. Electricians, solar installers and anyone wiring a long garden or workshop run use this figure to decide whether a chosen wire size is adequate or whether they need thicker cable. The method here is the standard approximate one. For a single-phase or DC circuit the drop is Vd = 2 × L × I × R ÷ 1000, and for a balanced three-phase circuit it is Vd = √3 × L × I × R ÷ 1000. L is the one-way length in metres, I is the load current in amps, and R is the conductor resistance in ohms per kilometre. The factor of 2 in the single-phase case accounts for current travelling out and back along two conductors, while three-phase uses √3 (about 1.732) because of the phase geometry. Resistance values come from the NEC Chapter 9 Table 8 basis at 75 °C, which is why the calculator states the temperature — resistance rises with heat, and a table read at a different temperature is the usual reason two calculators disagree. Here is a worked case you can follow. Take 10 AWG copper, which is about 4.0773 Ω/km, carrying 15 amps over a 25 metre one-way run on a 120 volt single-phase supply. The drop is 2 × 25 × 15 × 4.0773 ÷ 1000 = 3.058 volts, which is 2.55 percent of 120 volts, leaving 116.94 volts at the load. Because 2.55 percent is below the NEC 3 percent branch-circuit recommendation, this run passes. Swap in a three-phase example: 50 mm² copper at 0.4193 Ω/km carrying 100 amps over 100 metres on 400 volts gives √3 × 100 × 100 × 0.4193 ÷ 1000 = 7.26 volts, only 1.82 percent, so it also passes comfortably. The results drive real decisions. A solar installer running DC from rooftop panels to a distant inverter checks that the drop stays low enough to avoid wasting harvested energy. A site electrician feeding a submersible pump 80 metres down a borehole confirms the motor still gets enough voltage to start under load. A caravan or boat builder sizing 12 volt wiring, where even a couple of volts lost is a large percentage, uses it to pick cable that keeps lights bright. When a run fails, the calculator suggests the next larger standard size that would pass, so you can size up without guesswork. A few limits are worth knowing. This model uses conductor resistance only and ignores reactance, so it is accurate for smaller cables and near-unity power factor loads but slightly optimistic for very large feeders and low power-factor motor loads. The NEC 3 percent branch and 5 percent total figures are recommendations in informational notes, not hard code limits, so a custom percentage lets you match IEC or local rules. The most common mistake is entering the round-trip length instead of the one-way length — this tool already doubles it for single phase, so give the one-way distance only. A second frequent slip is picking aluminium in the size list but reading a copper resistance from memory, since aluminium of the same size drops noticeably more voltage. Treat every answer as an engineering estimate: mains wiring must follow your local electrical code and be carried out by a qualified electrician. Everything runs in your browser, so nothing you enter is uploaded or stored.

Voltage Drop Calculator Formula & Method

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