Wire & Circuit Sizing
Wire & Circuit Sizing calculators — each with the formula, a worked example and a reference table. Results are estimates for planning and education; verify all sizing with a licensed electrician and your AHJ.
Voltage Drop
Enter the conductor metal, AWG size, current, one-way run length and system voltage to get…Wire Size
Enter the load current, one-way distance, system voltage and your maximum voltage-drop tar…Ampacity
Pick the AWG, metal and temperature column to read the NEC 310.16 base ampacity, then appl…Ohm's Law
Enter any two of voltage, current, resistance and power (leave the unknowns at 0) and the …Conduit Fill
Choose the EMT trade size, the THHN conductor gauge and how many you plan to pull. The too…Breaker / OCPD Size
Enter the load current and whether it is continuous (on for three hours or more). The tool…Sizing a circuit is two checks, not one
Every branch circuit and feeder has to pass two independent tests, and the conductor you install is whichever one demands the larger wire. First is ampacity: the conductor must carry the current continuously without exceeding its temperature rating, using the allowable values in NEC Table 310.16 after any derating for ambient heat and for bundling several current-carrying conductors together. Second is voltage drop: because copper and aluminum have real resistance, a long run loses voltage along the way, and the industry guideline is to hold that loss at or below 3% on a branch circuit. A wire that easily passes the ampacity test can still fail the voltage-drop test on a 100-foot run — and the reverse is true on a very short, heavily loaded circuit.
The tools here let you run both checks and reconcile them. Start with the Wire Size calculator, which returns the smallest AWG that satisfies both ampacity and your voltage-drop target, then confirm the loss with the Voltage Drop calculator. Check the raw carrying capacity with the Ampacity tool, and remember the small-conductor rule of NEC 240.4(D): even where the table reads higher, 14, 12 and 10 AWG are capped at 15, 20 and 30 A overcurrent devices. The Breaker / OCPD Size calculator applies the 125% continuous-load factor, the Conduit Fill calculator keeps you under the 40% raceway fill limit, and the Ohm's Law calculator ties voltage, current and resistance together for any circuit.
New to this? Read how to size electrical wire for amps and distance for the full worked method, or the 3% voltage-drop rule explained. Sizing here is an estimate: the installed conductor, termination temperature rating and all derating must comply with the NEC edition your jurisdiction has adopted and be verified by a licensed electrician.