Electrical
Voltage Drop Calculator
Size conductors so voltage drop stays inside NEC recommendations. Handles AC and DC, copper or aluminum, single- or three-phase, with parallel runs.
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Inputs
Power factor rules of thumb: solar PV output = 1.0 (unity, inverters target this) · resistive loads (water heater, incandescent, electric heat) = 1.0 · motors / HVAC = 0.85 typical · mixed residential service = 0.85-0.90 · LED lighting / electronics = usually 0.9+. When unsure, 0.85 is the conservative dwelling default.
Calculations use NEC Chapter 9 Table 9 AC impedance values, with one-way distance doubled for round-trip and adjusted by power factor (cos θ for resistance, sin θ for reactance).
Voltage drop
1.73
%4.14 V drop on a 240 V system
Within recommendation
Comfortably below the 3% target.
About this calculator
NEC-compliant. Free to use. No watermarks.
Voltage drop isn't a code violation. The NEC's 3% / 5% numbers are recommendations in informational notes. But they're recommendations for a real reason: too much drop means motors underperform, lights flicker, and equipment runs hot.
The math uses Chapter 9 Table 9 (AC) or Table 8 (DC) for conductor resistance, factored against current and one-way distance. This calculator runs both paths, picks the right formula for the system type, and recommends the next wire size up if you're outside the threshold.
NEC references
- NEC Article 210.19(A) Informational Note 4: 3% Branch Circuit Recommendation
- NEC Article 215.2(A) Informational Note 2: 5% Total (Feeder + Branch) Recommendation
- NEC Chapter 9, Table 8: Conductor Properties
- NEC Chapter 9, Table 9: AC Resistance and Reactance for 600V Cables
The same calculations, running live as you design.
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