Estimates only. These results may be wrong for your specific installation, and they are not engineering or code-compliance advice. Check every result against the code that applies to you and with your local inspector (AHJ) before you install anything. Terms of Service

Copper conductor, breaker, and equipment ground sizing, with optional voltage drop. Results update as you type. Conductor reference

Size wire for a load Check a wire size

Circuit

Load current A System Where is the wire? Panel wiring table Temperature column Defaults: 75°C in the panel, 60°C in the field. Parallel sets

Motor or continuous load Sizes the wire and breaker at 125% of the load.

Derating

Current-carrying conductors together

Voltage drop optional

One-way distance ft System voltage V

Enter a distance to check voltage drop.

How the wire and breaker calculator works

Enter a load current and the calculator suggests a copper conductor size, a breaker size and an equipment grounding conductor size. Add a one-way distance and system voltage and it also estimates voltage drop. It works in your browser, so nothing you type is uploaded.

Steps

Enter the load current in amps, or switch to Check a wire size to test a wire you already have.
Choose three-phase or single-phase, and whether the wire is inside a panel or field wiring. This picks the wire table and the default 75°C or 60°C column.
Tick Motor or continuous load if it applies. The wire and breaker are then sized at 125% of the load.
If conditions call for it, set the ambient temperature, the number of current-carrying conductors and the number of parallel sets.
Add distance and voltage to see voltage drop. Open How this was worked out to see each step behind the answer.

Example

With the default settings (inside a panel, 75°C column, three-phase), a 40 A load gives 8 AWG copper (rated 50 A in the panel wiring table), a 40 A breaker, and a 10 AWG equipment ground.

Questions

Is the result code compliant?: No. It is an estimate to speed up your thinking. Conditions of use, terminal ratings, local amendments and the equipment's own instructions can change the answer, so check it against the code edition your inspector enforces.
Which tables does it use?: Copper conductor tables of the kind published in the NEC (for example the 60°C and 75°C columns of the conductor ampacity table in Article 310) and a separate panel wiring table for conductors inside a panel. Check them against your own copy of the code.
Why does it add 125%?: Continuous loads and motor circuits are commonly sized at 125% for the conductor and the breaker. Tick the box only when it applies to your load.
Does it work for aluminum?: This calculator is copper only. The Conductor reference tool also lists aluminum and copper-clad aluminum.
What does voltage drop tell me?: It shows how much voltage is lost along the run for the load, distance and system voltage you entered. Limits are set by your specification, the equipment, or your own standards, so compare it to the number you are aiming for.

The code references on this page are pointers, not a substitute for the code. NEC® and National Electrical Code® are registered trademarks of the National Fire Protection Association (NFPA). WireAndWidgets is not affiliated with or endorsed by NFPA. Rules differ by edition and by local amendment, so check the code your inspector (AHJ) enforces before you build or install anything.

Wire and breaker sizing