Subpanel installation cost: the five things that decide it, and why no honest page prints a figure

Someone asking what a subpanel installation costs is close to a decision. What follows is not a price, because none published here would be measured, but the five things that move the number, so that two quotes can be read side by side.

1. The distance and the route of the feeder

The feeder is the heaviest cable in the project, and it is priced by the foot in material and by the obstacle in labor. A subpanel on the other side of a basement wall is a short run through open joists. A subpanel in a detached garage means leaving the house, a trench, a conduit, and re-entering another building. The route, more than the straight-line distance, is what separates a modest job from a large one. The same reasoning for a charger circuit is set out under location and cable run.

2. The rating

A larger feeder means a larger breaker, heavier conductors and often a larger conduit. Choosing the rating by calculation, rather than by the round figure people name, is the single decision that most directly trims material. A rating chosen for a planned future is money well spent; one chosen by reflex is not.

The 100-amp version of the question gets the same answer: the rating changes the conductor and the breaker, but the trench, the walls and the state of the main panel still weigh more. A 100 A subpanel ten feet from the main panel can cost less than a much smaller one at the end of a long driveway.

3. What the main panel can spare

The feeder breaker needs two free slots in the main panel and room in the service. If the load calculation shows the service is short, the project is no longer a subpanel but a panel or service upgrade, and the cost belongs to a different category entirely. That is why the calculation comes before any quote. What drives the cost of an upgrade is described in what an electrical panel upgrade costs.

4. The building at the far end

An attached garage shares the house grounding system. A detached one needs its own grounding electrode, its own disconnect, and a feeder that crosses open ground. Finished walls at either end add the cost of opening and closing them. Outdoor or damp locations call for enclosures rated for them.

5. Permit, inspection and the utility

Permit fees are set by the jurisdiction and vary widely, which is one more reason no fixed figure holds from one town to the next. A subpanel alone rarely involves the utility; a project that turns into a service upgrade nearly always does. How permits and inspections are sequenced is described under permits and project planning.

Why the cost calculators do not help

Online calculators take a rating and a distance and return a number. They cannot see the route, the walls, the trench, the state of the main panel or the jurisdiction, which are the five items above. The figure they print is an average of projects that are not yours. A quote built on a site visit, or on photos and measurements, is the only number that means anything.

How to compare two quotes

Check that both name the feeder size and the breaker rating, the route and how it is made, the grounding at the far end, the number of spaces in the subpanel, the permit and who pulls it, and whether the charging circuit is included. A quote that is far lower usually leaves one of these out. Send us the photos and the distance, and we set out in writing what the job includes, so that quotes are compared on the same scope.