Voltage Drop Calculator
Copper or aluminum, 14 AWG through 750 kcmil, single- or three-phase. NEC 3% branch and 5% total recommendations checked live. When the drop is too high, we suggest the next conductor size up that solves it.
Circuit details
Frequently asked questions
What voltage drop does the NEC allow?
NEC 210.19(A) Informational Note 4 and NEC 215.2(A) Informational Note 2 recommend keeping branch-circuit voltage drop at or below 3% and total combined feeder + branch-circuit drop at or below 5%. These are recommendations, not enforceable code requirements, but exceeding them affects equipment performance and energy efficiency.
How is voltage drop calculated?
For single-phase circuits: VD = (2 × K × I × L) / CM. For three-phase: VD = (√3 × K × I × L) / CM. K is the resistivity constant (12.9 for copper, 21.2 for aluminum at 75 °C). I is current in amps, L is one-way length in feet, CM is the conductor area in circular mils from NEC Chapter 9 Table 8.
Should I use copper or aluminum?
Aluminum has roughly 1.6× the resistance of copper for the same gauge, so it typically requires upsizing two AWG steps to match the same drop. Aluminum is materially cheaper and lighter, which is why utilities use it for service-entrance conductors. For most NJ residential branch circuits, copper is standard. Larger feeders and service-entrance runs are commonly aluminum.
Why does my EV charger pull more current than the breaker rating?
It does not — but it draws the full continuous current for hours, which is why NEC 625.42 requires sizing the circuit at 125% of the charger nameplate. A 48 A charger needs a 60 A breaker and 6 AWG copper minimum, often upsized to 4 AWG for long runs to keep voltage drop under 3%.