Free Online NEC Calculators
Calculators built for working electricians
Quick, accurate answers for the calculations electricians do every day — voltage drop, conduit fill, ampacity with derating, box fill, and motor branch-circuit sizing. All based on the current NEC, all free, no signup.
- NEC 2023
- Code references included
- Mobile-friendly
Featured
Voltage Drop Calculator
Single-phase: VD = (2 · K · I · L) / CM Three-phase: VD = (1.732 · K · I · L) / CM K = 12.9 (Cu) | 21.2 (Al)
Drop
2.34%
Limit
3.00%
Status
PASS
Every calculator, ready when you need it
Each tool cites the NEC sections it applies, shows the formula it uses, and explains the result so you can verify it on the job.
Voltage Drop Calculator
Calculate voltage drop in volts and percentage for single-phase or three-phase circuits. Sized against NEC 3% / 5% recommendations.
Formula
VD = (2 · K · I · L) / CM
Conduit Fill Calculator
Check whether your conductor combination fits within NEC fill limits. Supports same-size and mixed conductor fills.
Formula
53% / 31% / 40% fill
Ampacity Calculator
Look up conductor ampacity from NEC Table 310.16 with ambient-temperature and conductor-count adjustments applied automatically.
Formula
Adjusted = Base · TempCF · CountCF
Box Fill Calculator
Calculate the required cubic-inch volume for a junction or device box from your conductor counts, devices, and clamps.
Formula
Σ (count · volume per AWG)
Motor Branch-Circuit Sizing
Size motor branch conductors, OCPDs, and overload protection for single- and three-phase motors per NEC 430.
Formula
Conductor = 125% FLC
These calculators are educational aids. Always verify against the current National Electrical Code and consult a licensed engineer for production designs.
Test what you just calculated
Lock in the math with practice questions. Every quiz is graded with explanations and NEC citations.