How to Size a Wire for a Subpanel

Adding a subpanel - for a garage, workshop, addition, or detached structure - starts with correctly sizing the feeder wire that supplies it. Undersized feeders overheat and fail inspection; oversized feeders waste money. This guide walks through the checks that determine minimum wire size.

Step 1: Determine the subpanel's total load

Add up the continuous and non-continuous loads the subpanel will serve - lighting circuits, receptacles, any dedicated appliance circuits (like a shop tool or EV charger). For a simple garage or workshop subpanel, many electricians size for the panel's full bus rating (e.g. a 60A or 100A subpanel) rather than a detailed connected-load tally, especially when future expansion is likely.

Step 2: Check ampacity against the feeder breaker

The feeder conductor's ampacity (corrected for ambient temperature and any conductor bundling - see the Wire Ampacity Calculator) must be at least equal to the feeder breaker size at the main panel. A 60A subpanel feeder breaker requires conductors rated for at least 60A after all correction factors are applied.

Step 3: Check voltage drop over the feeder run

Long feeder runs - common for detached garages or workshops - are frequently limited by voltage drop rather than ampacity. Use the Wire Size Calculator with your subpanel's expected load current, the one-way distance to the subpanel, and a 3% maximum drop target, then compare the result against the ampacity-driven minimum from Step 2 and use whichever is larger.

Step 4: Don't forget the grounding and neutral conductors

A subpanel fed from the same structure typically requires an insulated (not bonded) neutral and a separate equipment grounding conductor run alongside the feeders - bonding neutral and ground at a subpanel, as is done at the main panel, is a common and dangerous code violation that creates parallel neutral return paths.

Frequently Asked Questions

Can I use the subpanel's rated bus size instead of calculating actual load?

Yes, this is common practice for garages, workshops and additions where future load growth is likely - it's a conservative approach that avoids undersizing if more circuits are added later.

Does the subpanel breaker size need to match the panel's main breaker?

No - the feeder breaker at the main panel just needs to protect the feeder conductors and not exceed the subpanel's bus rating; it's often smaller than the subpanel's full rated capacity.

What wire size is typical for a 100A subpanel feeder?

A common minimum is #4 AWG copper or #2 AWG aluminum at short distances - but always verify with the Wire Size Calculator using your actual distance, since longer runs often require larger conductors for voltage drop.

Related Calculators

Wire Size Calculator Wire Ampacity Calculator Voltage Drop Calculator

Last reviewed: 2026-09-12