Sub-panel sizing calculator

NEC Article 220 — size feeders for detached garages, shops, and additions

Enter your loads and get the correct panel amperage, feeder wire gauge, and main breaker size. Includes shop loads like welders and compressors, distance-based wire upsizing, and copper or aluminum feeders.

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What this calculator does

This calculator sizes a sub-panel (feeder panel) for a detached garage, workshop, barn, or addition fed from your main service. It applies NEC Article 220 demand calculations with a 125% continuous load factor to recommend panel amperage, feeder wire gauge, and main breaker size.

NEC rule to knowSub-panel feeders are sized to 125% of continuous loads plus 100% of non-continuous loads per NEC 215.2. The calculator applies this automatically.

What each input means

Voltage — Most shops use 240V single-phase from a residential service.
Distance — Runs over 100 feet often need wire upsized one size for voltage drop.
Wire material — Copper carries more current per gauge; aluminum is acceptable for feeders when properly terminated.
Loads — Check equipment you plan to run; include everything that might operate at the same time.

Frequently asked questions

Do I need a permit to add a sub-panel?

Yes in almost every jurisdiction. Sub-panel work is inspected electrical work.

Can I put a sub-panel in a detached garage?

Yes. You need a disconnect at the structure (main breaker in the sub-panel) and a grounding electrode system — typically two ground rods.

What's the difference between a sub-panel and a main panel?

A main panel connects to the utility. A sub-panel is fed from the main panel via a feeder and has its own main breaker.

Sub-panel sizing calculator