Voltage drop calculator

What voltage drop is, and why 3 percent

Every conductor has resistance, and current through resistance costs volts. A 120 volt circuit that delivers 112 volts at the far end of a long run has dropped 8 volts, about 7 percent, and the load at the end runs hot, slow or not at all: motors draw more current to deliver the same power, incandescent light dims, electronic power supplies may cut out. The National Electrical Code leaves the limit to the designer for most circuits, but its informational notes to 210.19(A) and 215.2(A) recommend no more than 3 percent on a branch circuit and 5 percent for the feeder and branch circuit together. Those are the targets the calculator uses by default.

The formula the calculator applies

The estimate the trade uses is the circular-mil method: VD = 2 x K x I x L / CM for single-phase and DC, 1.732 x K x I x L / CM for three-phase. K is the resistance of a conductor one foot long and one circular mil in area, 12.9 ohm-circular-mils per foot for copper and 21.2 for aluminium at 75 C; I is the current in amps; L the one-way length in feet; CM the cross-section of the wire in circular mils, from Chapter 9, Table 8 of the Code. The result is an estimate, close enough for sizing a residential or light commercial run. The K method is meant for conductors up to 1/0 AWG; above that, alternating current flows mostly near the surface of the conductor (skin effect) and the formula takes an adjustment factor Q, the ratio of the AC resistance in Table 9 to the DC resistance in Table 8. The calculator does not apply Q, so for 2/0 and larger it understates the drop slightly. It also ignores power factor, the exact conductor temperature and the type of conduit, which is why engineers use the Table 9 impedance method for large or critical feeders.

How far can each wire size go at 3 percent

Computed with the formula above for copper, single-phase, at a common circuit for each size, loaded to the breaker rating. The real load is usually lower than the breaker, so these are conservative.

L = 0.03 x V x CM / (2 x 12.9 x I), rounded down. Ampacity is a separate limit (NEC Table 310.16): the circuits shown are the usual pairing of breaker and wire size, not a rule.

Voltage drop and ampacity are two different checks

Ampacity is the current a conductor can carry without overheating its insulation, fixed by Table 310.16 and the termination temperature. Voltage drop is about what arrives at the load. A short run only needs the ampacity check; a long run passes ampacity and still fails on drop. The rule of thumb that follows from the formula: a 120 volt, 20 amp circuit on 12 AWG copper reaches about 45 feet one way before it passes 3 percent; a 240 volt circuit on the same wire goes twice as far, because the same volts lost are a smaller share of a higher voltage. When the calculator suggests a larger wire, keep the breaker sized to the load and the ampacity of the smaller wire the circuit needs, and use the larger wire for the run.

When to go beyond this calculator

Detached garages, well pumps, EV chargers at the far end of a driveway, barn feeders and solar inverters are the classic long runs where voltage drop decides the wire size and the budget. For a feeder above 100 amps, a run above a few hundred feet, three-phase motors or any installation that will be inspected against a specification, have the design checked with the Table 9 method and the manufacturer's data for the conductor. The Code governs: a permit and an inspection are required for most new circuits, and the local authority may apply stricter rules than the informational notes.