Voltage Drop Calculator
Estimate how much voltage is lost along a wire run. Choose the wire, enter the one-way length and the load current.
How it works
Single-phase: VD = 2 × L × R × I ÷ 1,000. Three-phase: VD = 1.732 × L × R × I ÷ 1,000. L = one-way length in feet, R = conductor resistance in ohms per 1,000 ft, I = load current in amps. Percent drop = VD ÷ source voltage × 100. Resistance values are DC resistance at 75 °C from NEC Chapter 9, Table 8 (stranded unless marked solid).
Worked example: City of Chino: 2 × 210 ft × 0.78 ohm/kft × 3.167 A ÷ 1,000 = 1.038 V = 0.865% at 120 V. A 100 ft run of 12 AWG stranded copper (1.98) at 16 A drops 6.34 V, 5.28% of 120 V.
Assumptions
- DC resistance only. For AC runs, especially large wires or steel conduit, reactance adds to the drop; NEC Chapter 9, Table 9 gives AC values. You can enter a custom resistance (for example from Table 9 or the cable maker).
- Conductor temperature 75 °C as in Table 8; cooler wire has slightly less resistance.
- Load current is constant and all at the far end of the run.
- General information: NEC informational notes (210.19(A), 215.2(A)) describe about 3% drop on a branch circuit or feeder and 5% total as giving reasonable efficiency. EC&M explains informational notes are explanatory, not enforceable rules.
- This is an estimate for information only, not a wiring design. Wire sizing, breakers and installation are governed by your local electrical code; ask a licensed electrician.
Frequently asked questions
How do you calculate voltage drop?
For a single-phase circuit, VD = 2 × one-way length (ft) × resistance (ohm per 1,000 ft) × current (A) ÷ 1,000. The City of Chino's example: 2 × 210 ft × 0.78 × 3.167 A ÷ 1,000 = 1.038 V, which is 0.865% of 120 V.
What is the resistance of 12 AWG copper wire?
NEC Chapter 9, Table 8 lists 1.98 ohm per 1,000 ft for stranded uncoated copper and 1.93 for solid, at 75 °C.
Is 3% voltage drop required by the NEC?
No. The 3% and 5% figures are in informational notes, which EC&M explains are explanatory material, not requirements. Local rules or project specs may still set limits; check with a licensed electrician.
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Sources
Formulas and reference values are from these sources. Page last reviewed 2026-10-19.
- NFPA 70 (NEC) 2017, Chapter 9, Table 8: Conductor properties (copy hosted for a construction management course)
- City of Chino, CA: Voltage drop calculations (worked examples, July 2025)
- NCCER: Electrical and instrumentation formula sheet (voltage drop, K = 12.9 copper, 21.2 aluminum)
- EC&M: Insights from NEC Article 90 (informational notes are not requirements)