Electrical Calculators

When electricity travels through wire, some voltage is lost along the way. The longer the run or the thinner the wire, the more you lose. NEC recommends keeping this loss under 3% for branch circuits. This calculator shows exactly how much voltage is lost and whether your wire size is big enough.

Good to use when running a circuit to a detached garage, barn, or outdoor panel — anything more than about 100 feet from your main panel.

A conduit can only hold a certain amount of wire before it gets too crowded. Overfilled conduit causes heat buildup and makes pulling wire very hard. The NEC limits fill to 40% of the conduit's inside area when you have 3 or more conductors. This tool calculates the total wire area and tells you if you're within that limit.

If your fill comes out over 40%, either go up one conduit size or remove a conductor run.

Every wire has a maximum current (amps) it can carry safely. Run too much current and the wire overheats. The NEC requires adjustments for high-temperature areas and bundled wires — both of which reduce how much current a wire can handle. This tool applies those corrections and finds the smallest wire that works for your load.

If the load runs for 3 hours or more at a time (like HVAC or water heaters), it counts as a continuous load — select Yes to apply the required 125% sizing factor.

Motors pull 5 to 7 times their normal current when they first start up, so you cannot size a motor circuit the same way as a regular circuit. NEC Article 430 has its own rules. This tool looks up the motor's full-load amps from the NEC tables, then calculates the minimum wire size and the maximum allowed breaker size.

Always use the NEC table FLA value, not the motor nameplate FLA, for branch circuit sizing. The nameplate value is used for overload protection only.

Every outlet box, switch box, and junction box has a maximum cubic-inch volume that limits how many wires can go in it. If you go over that volume, you need a bigger box. The NEC assigns a volume allowance to each conductor, device, and clamp. This tool adds them up and tells you whether your box is big enough.

No matter how many ground wires you have, they all count as just one conductor of the largest size in the box.