Voltage Drop Calculator
Volts dropped and percent drop on copper or aluminum conductors, single- or three-phase. Flags the 3% / 5% NEC informational thresholds.
Voltage drop
—
— %
Voltage at load
—
V
Status (NEC informational)
—
Suggested upsize
—
// Ad slot — fills after AdSense approval
How voltage drop is calculated
Conductors have resistance. When current flows through them, some of the source voltage gets used up heating the wire instead of reaching the load. The classic formula uses K (resistivity expressed in ohms per circular-mil-foot) and circular-mil area:
Single phase: VD = (2 × K × I × L) / CM
Three phase: VD = (√3 × K × I × L) / CM
% drop = VD / V_source × 100
- K = 12.9 for copper, 21.2 for aluminum (at 75°C operating temp)
- I = current in amps
- L = one-way length in feet (the 2 and √3 handle the return path)
- CM = circular mils of the conductor (the area)
NEC informational guidance
Voltage drop isn't a code requirement in most cases — it's strongly recommended in two informational notes:
- NEC 210.19(A) IN 4: branch circuit conductor sized to limit voltage drop to 3%
- NEC 215.2(A)(1) IN 2: combined feeder + branch drop ≤ 5%
Exceptions where 5% / 3% becomes mandatory: sensitive electronics, fire pumps (NEC 695.7 — 5% max during starting), and some industrial control applications. For most residential and commercial work, treat 5% as the hard limit and 3% as the target.
Quick mental shortcuts
- Doubling the run length doubles the voltage drop.
- Doubling the current doubles the voltage drop.
- Each step up in wire size (12 → 10 → 8 → 6 → 4 …) cuts voltage drop roughly in half.
- 240 V circuits drop half the percentage of 120 V circuits for the same load and wire.
Frequently asked questions
What's the voltage drop formula?
VD = (2 × K × I × L) / CM single-phase, or (√3 × K × I × L) / CM three-phase. K = 12.9 Cu, 21.2 Al at 75°C.
What does the NEC require for voltage drop?
It's recommendation, not requirement: 3% on the branch, 5% combined feeder+branch. Some applications (fire pumps, sensitive electronics) make it a hard limit.
How far can 12 AWG go?
About 60 ft one-way for a 20 A 120 V load before hitting 3%. Past that, jump to 10 AWG.
Why does aluminum drop more?
Resistivity is ~1.6× copper. Aluminum compensates by being one or two sizes larger for the same ampacity.
One-way or round trip length?
One way. The 2 (single phase) and √3 (three phase) in the formula handle the return.