How it works
Single-phase: VD = 2 × I × R × L / 1000. Three-phase: VD = √3 × I × R × L / 1000
I is the load current in amperes, R is the conductor DC resistance in ohms per 1000 ft from NEC Chapter 9, Table 8 (75°C, uncoated stranded), and L is the one-way circuit length in feet. The single-phase formula doubles the length to account for the return conductor; the three-phase formula uses √3 because the currents in a balanced system are 120° apart. Parallel sets divide the effective resistance (R ÷ sets), so two parallel runs halve the drop. The 3% recommendation applies to a branch circuit (210.19(A)) and to a feeder (215.2(A)) separately; the combined feeder + branch drop should stay within 5%. The "min size for 3%" hint solves for voltage drop only — always confirm the suggested conductor also carries the load current on the wire size calculator, since a short low-current run can meet 3% on a wire whose ampacity is below the load. If the drop pushes you to a larger conductor, also check that the new size still meets the NEC conduit fill limits for its raceway.
Code references
- Conductor resistance values NEC 2023, Chapter 9, Table 8 — Conductor Properties
- Recommended 3% branch-circuit drop NEC 2023, 210.19(A) Informational Note No. 4
- Recommended 3% feeder / 5% combined drop NEC 2023, 215.2(A) Informational Note No. 2
- Informational Notes are advisory, not enforceable NEC 2023, 90.5(C)
- Conductors in parallel NEC 2023, 310.10(G)
FAQ
What is an acceptable voltage drop?
The NEC does not mandate a maximum, but Informational Note No. 4 to 210.19(A) recommends keeping branch-circuit drop within 3% and the combined feeder-plus-branch drop within 5%. Some local codes and energy standards make these limits mandatory.
Is 3% voltage drop actually required by the NEC?
No. The 3% branch recommendation (210.19(A) IN No. 4) and the 3% feeder / 5% combined recommendation (215.2(A) IN No. 2) live in Informational Notes, and NEC 90.5(C) states that Informational Notes are explanatory only and are not enforceable as code requirements. That said, many local jurisdictions adopt these limits, energy codes like ASHRAE 90.1 enforce them, and most inspectors reference them — so treat 3%/5% as a strong design target rather than an optional one.
What is the difference between the 3% and 5% limits?
The 3% figure is the target for each segment on its own — a branch circuit (210.19(A)) or a feeder (215.2(A)). The 5% figure is the ceiling for the whole path: feeder plus branch combined. Use the Feeder + Branch mode to enter the drop already used on the upstream feeder; the tool then checks that the feeder and this branch together stay within 5%.
Should I enter the one-way length or the total wire length?
Enter the one-way distance from the source to the load. The formula already accounts for the return path (factor 2 for single-phase, √3 for three-phase).
Why does the calculator use DC resistance instead of AC impedance?
For conductors up to roughly 4/0 AWG at 60 Hz, AC reactance is negligible and the DC resistance from Table 8 is the standard field method. For very large conductors or long high-current feeders, an impedance-based calculation per NEC Chapter 9, Table 9 gives a more precise figure.
Does temperature affect the result?
Yes. Table 8 values are listed at 75°C conductor temperature. A cooler conductor has slightly lower resistance, so the real-world drop is usually a little lower than calculated — the result is conservative.
This calculator is provided for estimation purposes. Always verify results against the current NEC edition and local amendments with a licensed electrician or electrical engineer before sizing conductors.