Subpanel installation: capacity, feeder, and circuit requirements

A subpanel quote should begin with the loads it will serve and the feeder path from the existing service equipment. Panel size alone does not create capacity. Ask for the load calculation, feeder rating, panel make and listing, circuit schedule, grounding and neutral arrangement, route, permits, and finish work.

Eaton describes a main-lug-only loadcenter as equipment downstream of the service entrance panel and states that a subpanel needs an isolated neutral and a separate ground bar (Eaton load-center fundamentals). Eaton’s catalog repeats that sub-feed panels use a non-bonded neutral with ground conductors on a separate ground bar (Eaton loadcenter catalog). These manufacturer sources support the equipment distinction; the adopted code and local inspector control the project.

What the electrician is really pricing

Scope	Questions
Load and feeder	Which circuits move, what are their calculated loads, and what feeder protection and conductor route are proposed?
Panel equipment	Is the panel listed for the breakers, indoor/outdoor location, spaces, main lugs or main breaker, and future expansion?
Grounding and neutral	Where is the neutral isolated, where do equipment grounds terminate, and how is the feeder bonded at the source?
Circuit relocation	Are breakers, cables, labels, junction boxes, and AFCI/GFCI requirements included?
Closeout	Are permits, inspection, panel directory, cover, wall repair, and cleanup included?

The route can dominate the field work. A garage wall beside the main panel differs from a detached workshop requiring trenching, conduit, weather-rated equipment, and a grounding electrode system evaluated under local rules. A finished basement may require ceiling access and patching. Ask for a route sketch and access assumptions.

Do not fill a new panel with breakers because spaces are available. The feeder and service have to support the intended loads, including motors or heating equipment with starting or continuous demand. If the project is actually a service upgrade, compare it with electrical panel upgrade work.

Eaton’s installation guidance also shows that bonding hardware and neutral configuration vary by service versus non-service use. The exact panel instructions should be delivered with the quote and followed by the installer. Do not open a live panel or move breakers to test capacity. A qualified electrician should perform the load review and inspection closeout.

Ask how circuits will be labeled during the move and whether any shared-neutral, AFCI, or GFCI circuit needs a compatible breaker after relocation. Confirm the panel’s working clearance, mounting height, enclosure rating, and access for future service. A large panel in a bad location can be less useful than a smaller panel placed along a sensible feeder route.

For a detached building, ask whether the feeder route crosses a driveway, whether conduit is exposed to weather or impact, and whether the structure’s grounding and disconnect equipment are included. For an interior addition, ask who restores drywall and insulation after the feeder and branch circuits are installed. These answers turn a capacity purchase into a maintainable installation.