Voltage Drop Quick-Check Calculator

Circular-mil voltage drop for temporary power feeders and long cord runs — bypass pumps, trailers, welders — checked against the customary 3%/5% guideline. Runs entirely offline; inputs persist in this browser via localStorage.

Use limits: This is a resistive (K/CM) estimate — good for sizing a temp run, silent on reactance (which matters on big conductors and long runs at low power factor), conductor ampacity, derating, and overcurrent protection. Voltage drop is a performance guideline; ampacity and protection are code requirements, and the electrician of record owns both. Motor starting draws ~6× FLA — a run that's fine running can still drop enough at start to trip contactors.
Circuit

Motor FLA reference table in the notes below.

How the Numbers Are Derived

The circular-mil method

Three phase: V_d = √3 × K × I × L / CM    Single phase: V_d = 2 × K × I × L / CM

K is conductor resistivity in Ω·cmil/ft (12.9 copper, 21.2 aluminum at operating temperature), L is one-way length in feet, CM is the conductor's circular-mil area. Parallel sets divide the current per conductor, equivalent to multiplying CM. This is the standard estimating method; it neglects reactance, which is conservative to ignore only at high power factor and modest conductor sizes.

The 3% / 5% guideline

The NEC's informational notes suggest limiting drop to 3% on a branch circuit or feeder and 5% total (source to load) for "reasonable efficiency of operation." It's a recommendation, not a code mandate — but motors, VFDs, and contactor coils get grumpy well before lights dim. Low voltage at a motor means higher current for the same load, which means more heat, which is how a marginal temp feeder cooks a rental pump motor over a week.

460 V three-phase motor FLA reference (NEC Table 430.250 values)
HPFLAHPFLA
12.12534
34.83040
57.64052
7.5115065
10147596
1521100124
2027125156

Table values (used for sizing), not nameplate — verify against the current NEC edition and the actual motor nameplate.

References

  1. NFPA 70 (NEC) — Art. 210/215 informational notes on voltage drop; Table 430.250 motor FLA; Chapter 9 Table 8 conductor properties. https://www.nfpa.org/
  2. The electrician/electrical engineer of record — ampacity, protection, and anything permanent.