Voltage drop calculator. Real NEC math.
Configure inputs and press Run to compute a recommended size.
No calculation yet. Press Run Calculation to evaluate compliance.
How the calculation works
Voltage drop is computed with the standard simplified formula Vd = factor × K × I × L × PF ÷ CM. The factor is 2 for single phase and √3 for three phase. K is the specific resistance of the conductor material, corrected from the 75°C reference for the ambient temperature you enter. I is load current, L is one-way length in feet, PF is power factor, and CM is the conductor circular mils from NEC Chapter 9, Table 8.
Percentage drop is that result divided by the system voltage. A conduit adjustment factor accounts for the difference between raceway types. Conductor ampacity is compared against the load using the 75°C column of NEC 310.16, which is the terminal rating most equipment is listed for.
The recommended conductor is the smallest size that satisfies both ampacity and the 3% voltage drop target from the NEC 215.2(A)(1)(b) Informational Note. When the two constraints disagree, the advisory tells you which one is driving the recommendation.
What this tool does not do
- Does not apply ampacity adjustment factors for more than three current-carrying conductors in a raceway (NEC 310.15(C)(1)).
- Does not apply continuous-load factors or check termination temperature limitations under NEC 110.14(C).
- Does not perform short-circuit, arc-flash, or coordination studies.
- Uses a simplified resistance-only model; it does not account for conductor reactance, which becomes significant on long runs of large conductors.
- Conduit trade size shown is an envelope estimate — use the conduit fill calculator for an actual fill check.
Results are for planning purposes and must be reviewed by a qualified professional before use on a real installation. Read the full disclaimer.
Common questions
- What is an acceptable voltage drop per the NEC?
- NEC 215.2(A)(1)(b) Informational Note recommends feeders be sized so voltage drop does not exceed 3%, with combined feeder and branch-circuit drop not exceeding 5% per 210.19(A) Informational Note No. 4. These are recommendations rather than enforceable requirements, so the responsible designer sets the project criteria.
- How is voltage drop calculated?
- This calculator uses the simplified formula Vd = factor x K x I x L x PF / CM, where the factor is 2 for single phase and the square root of 3 for three phase, K is the specific resistance of the conductor material corrected for temperature, I is load current, L is one-way length in feet, PF is power factor, and CM is the conductor circular mils from NEC Chapter 9 Table 8.
- Does the calculator check ampacity as well as voltage drop?
- Yes. Conductor ampacity is compared against the load current using the 75°C column of NEC 310.16, and the recommended size is the smallest conductor that satisfies both ampacity and the 3% voltage drop target.
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