NEC calculators
Free, no sign-up, and everything runs in your browser — what you type is never sent anywhere. Each result comes with the arithmetic behind it and the article it rests on, so you can check the work instead of trusting it.
These are the same engines that ship in the AmpRule iPhone app, validated against the same test suite.
Voltage drop calculator
Minimum wire size, maximum run length, or drop percentage — with the working shown.
OpenWire ampacity chart — NEC Table 310.16
Ampacity by size, material and temperature column — for whichever edition your AHJ enforces.
Open
Conduit fill, box fill, bending and motors
Those live in the iPhone app rather than here. What is on this page is the same set that is free in the app — we are not putting the paid calculations on the web and charging for them on the phone. If you want the reasoning without the app, the guides work through box fill and conduit fill by hand, worked examples included.
Read up on the code
- What actually changed in the NEC 2026
- How to calculate box fill, item by item
- Where the 40% conduit fill rule comes from, and when it is not 40%
These calculators are a reference aid, not a substitute for the code book or for the judgement of a licensed electrician. Verify every result against the NEC edition your AHJ has adopted before you install anything.
AmpRule is not affiliated with or endorsed by NFPA. NEC and National Electrical Code are trademarks of the National Fire Protection Association. Always verify against the edition adopted by your AHJ.