Voltage Drop Calculator
Enter the circuit and get the volts dropped, the % drop, the voltage at the load and the smallest wire that keeps you under your limit.
Result
Based on NFPA 70 (NEC) 2023 tables. An estimating aid, not an engineering design: confirm sizes against the code edition your authority having jurisdiction enforces, local amendments and manufacturer data.
Why voltage drop matters
Every conductor has resistance, so the voltage at the end of a long run is lower than at the panel. Motors run hot and stall on low voltage, LED drivers flicker, and the lost volts are paid for as heat. On long branch circuits the wire size is usually set by voltage drop, not ampacity — size for both with the wire size calculator.
Frequently asked questions
What is the voltage drop formula?
VD = (2 × K × I × L) ÷ CM for single-phase and DC, and VD = (1.732 × K × I × L) ÷ CM for three-phase. K is the conductor resistivity (12.9 Ω·cmil/ft for copper, 21.2 for aluminum at 75 °C), I the current in amps, L the one-way length in feet and CM the conductor area in circular mils from NEC Chapter 9 Table 8.
What voltage drop does the NEC allow?
The NEC recommends — in informational notes, not as a requirement — no more than 3 % on a branch circuit or feeder and 5 % combined from the service to the farthest outlet (210.19 and 215.2). Some energy codes and specifications make 2–3 % mandatory.
How much voltage drop is there on 12 AWG wire at 100 feet?
At 16 A on a 120 V single-phase circuit, 12 AWG copper drops about 6.3 V over 100 ft one way — 5.3 %, above the 3 % target. 10 AWG brings it to 4.0 V (3.3 %), and 8 AWG to 2.5 V (2.1 %).
Is the length one-way or round trip?
One-way: the distance from the source to the load. The formula’s factor of 2 (single-phase) or 1.732 (three-phase) accounts for the return path.
How accurate is this for large feeders?
The K-factor method uses DC resistance and ignores reactance and power factor. That is standard for branch circuits and small feeders. For conductors around 250 kcmil and larger on AC — especially in steel conduit — check with the effective impedance method in NEC Chapter 9 Table 9.
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