Voltage Drop Calculator

It is a recommendation, not a mandatory requirement. NEC Article 210.19(A) Informational Note No. 4 states that conductors should be sized to limit voltage drop to 3% on branch circuits and 5% combined on feeders plus branch circuits. Informational Notes are advisory, not enforceable code language. However, some local amendments and certain product standards (such as UL-listed EV charging equipment) may impose their own voltage drop requirements. Even where not legally required, exceeding 3% drop consistently shortens equipment life, increases operating costs, and can void manufacturer warranties on voltage-sensitive loads.

Yes. Aluminum's resistivity at 75°C is approximately 21.2 ohm-cmil/ft versus 12.9 for copper - about 64% higher. To achieve the same voltage drop as a given copper AWG, you need approximately two AWG sizes larger in aluminum. For example, AWG 2 aluminum (66,360 cmil) drops about the same voltage as AWG 4 copper (41,740 cmil) at the same current, because 21.2 ÷ 12.9 ≈ 1.64 and the increased area compensates. Aluminum conductors are widely used for service entrance feeders (200A residential service, 400A commercial) where the weight and cost savings are significant, but they require aluminum-rated terminations and anti-oxidant compound to prevent connection resistance from building up over time.

The absolute voltage drop (in volts) is the same for a given current, wire size, and length regardless of system voltage - it is purely a function of resistance and current. But the percentage drop scales inversely with voltage. A 10 V drop on a 120 V circuit is 8.3%, while the same 10 V drop on a 240 V circuit is only 4.2%, and on a 480 V circuit it is 2.1%. This is why higher-voltage distribution (277/480 V three-phase versus 120/208 V) dramatically reduces voltage drop problems on long industrial runs - the same conductor carries the same power at half the current (P = VI), reducing resistive drop by half and I²R losses by 75%.

Copper and aluminum resistance increases with temperature. The NEC formula uses resistivity values at 75°C (the standard conductor temperature rating for common insulation types like THHN). At lower temperatures, resistance and drop are slightly less; at higher temperatures they are greater. A copper conductor in a hot attic conduit at an ambient of 50°C may have its temperature rating derated to 60°C per NEC Table 310.15, and its actual resistance at operating temperature can be 5–10% higher than the 75°C value used in the formula. For precise calculations in extreme environments, apply the temperature correction factor from NEC Chapter 9, Table 9 notes.

The most effective options in decreasing order of impact are: (1) reduce the run length by moving the panel closer to the load or adding a sub-panel at an intermediate point; (2) increase system voltage - upgrading from 120 V single-phase to 240 V halves the current for the same power, cutting drop by half; (3) for three-phase systems, balance loads so all three phases are equally loaded and the neutral carries minimal current. If none of these is practical, upsizing conductor AWG is the only remaining option - going up two AWG sizes (for example from #12 to #8) roughly doubles the circular mil area and halves the drop.