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Subpanel Feeder Sizing

A subpanel feeder is sized from the load it will carry, not from the bus bars stamped on the can. Work load → conductor → drop → raceway → breaker, and treat neutral and equipment grounding as separate decisions.
Code basis
215.2(A)(1), 310.16
Also check
Voltage drop, 250.122
Start from
Feeder load, not bus rating
Common miss
Outbuilding voltage drop

The Procedure

5 steps, in the order the code applies them.

  1. Step 1

    Establish the feeder load

    Calculate or measure the load the subpanel will supply. The subpanel bus rating is a maximum, not the design current.

  2. Step 2

    Size ungrounded conductors

    Run ampacity with ambient, bundling and termination inputs for the feeder material you intend to pull.

  3. Step 3

    Check voltage drop for the route length

    Garage and ADU feeders often upsize here. If drop governs, enlarge the ungrounded conductors and revisit grounding conductor proportion.

  4. Step 4

    Size the raceway for the final conductors

    After any drop-driven upsize, recalculate conduit fill. Do not lock EMT size from the first ampacity pass.

  5. Step 5

    Select the feeder breaker and landings

    Confirm the supply-end breaker protects the feeder conductors and that both panels accept the conductor material and size.

Work from load to installation

Start with the feeder load, not the panel's maximum bus rating. Then size the conductor, check the drop and check the raceway.

If voltage drop upsizes the ungrounded conductors, the equipment grounding conductor needs proportional review.

Separate neutral and grounding decisions

Feeder neutrals and equipment grounding conductors are governed by different rules.

Bonding details and service-equipment decisions need project-specific review beyond feeder ampacity.

Decision Guide

The choices that change the answer, and how to make them.

  • Bus rating ≠ feeder size

    A 125 A subpanel can be fed by a smaller feeder when the calculated load is smaller and the installation allows it. Conversely, a large bus does not authorize an undersized feeder.

  • Neutral and EGC are different rules

    Feeder neutrals and equipment grounding conductors follow different sections. Upsizing hots for drop can require EGC review under 250.122.

  • Outbuildings reopen bonding questions

    Separate structures have grounding and bonding requirements beyond feeder ampacity. Resolve them as part of the design, not as punch-list work.

FAQs

How do I size a feeder to a subpanel?
Start from the feeder load, size conductors for ampacity and conditions of use, check voltage drop for the route length, size the raceway, then select a breaker that protects those conductors.
Can I feed a 125 amp subpanel with a 60 amp breaker?
Sometimes, when the calculated feeder load and conductor ampacity support it and the equipment instructions allow. The subpanel bus rating is a ceiling, not a mandatory feeder size.
Does voltage drop matter on subpanel feeders?
Yes, especially on outbuilding and long garage runs. Ampacity can pass while drop still forces a larger feeder.

Calculated against the NEC 2023 edition using 215.2(A)(1), 310.16, 250.122. Confirm the edition adopted by your AHJ, which may differ and may carry local amendments.

Not covered by this tool

  • Motor, air-conditioning and welder circuits, which are sized under their own articles.
  • Conductors in free air and photovoltaic circuits.
  • Parallel conductor sets, taps and busbars.
  • Local amendments, which can be stricter than the published code.

Data last verified