← Back to Voltage Drop Calculator · All Guides
NEC Voltage Drop Requirements
Understand NEC voltage drop limits, code references, and how to size wires for residential and commercial circuits
NEC Voltage Drop Limits
The National Electrical Code (NEC, NFPA 70) provides voltage drop recommendations in informational notes to Sections 210.19(A) and 215.2(A). While technically advisory, these limits are enforced by most inspectors and authorities having jurisdiction (AHJ) as practical requirements.
| Circuit Type | Maximum Drop | Code Reference |
|---|---|---|
| Branch circuit | 3% | NEC 210.19(A) Informational Note No. 4 |
| Feeder | 2% (so branch + feeder ≤ 5%) | NEC 215.2(A) Informational Note No. 2 |
| Total (Feeder + Branch) | 5% | NEC 210.19(A) Informational Note No. 4 |
Common NEC Scenarios
These pre-filled examples cover the most common residential and commercial circuits. Click any link to open the calculator with those values already loaded.
Calculate →
Calculate →
Calculate →
Calculate →
Calculate →
NEC Wire Sizing Tips
The NEC uses the K-factor formula to calculate voltage drop: VD = (2 × K × I × L) / CM, where K is the conductor resistivity (12.9 for copper, 21.2 for aluminum at 75°C), I is the current in amps, L is the one-way length in feet, and CM is the wire's circular mil area.
- Derating factors from NEC Table 310.15(C)(1) for temperature and Table 310.15(C)(2)(a) for conduit fill reduce the wire's ampacity. Always check both ampacity and voltage drop.
- When in doubt, size up one wire gauge. The cost difference is small compared to the safety margin and reduced voltage drop you gain.
Checking an Existing Installation
Use Voltage Drop mode to verify whether existing wiring meets NEC recommendations. Enter your circuit parameters and the installed wire size to check compliance.
Calculate →
Calculate →