Subpanel installation cost

Installing a subpanel costs $600 to $2,500, typically around $1,200. A subpanel is a second breaker box fed from the main one, and it exists to put circuits where they are needed without running a dozen long cables back to the house panel.

The two things that move the price are the distance from the main panel and whether that panel has room to feed it.

What it costs

When a subpanel is the right answer

A detached garage, workshop or shed. One feeder out to a subpanel is far cheaper and tidier than several individual circuits.

A finished basement or attic conversion, where a dozen new circuits would otherwise each need a home run to the main panel.

A main panel that is physically full but has adequate capacity. A subpanel adds breaker spaces without replacing the service.

An addition, for the same reason.

A garage EV charger plus a workshop, where you want local disconnects and room to grow.

When it is not

If the main panel is at its capacity rather than merely out of spaces, a subpanel does not help. A 100 amp service that is already loaded cannot feed a 100 amp subpanel usefully; that is a service upgrade at $1,500 to $6,000.

The distinction matters and it is worth asking directly: is the panel full, or is the service undersized? Those are different problems with different prices, and a subpanel solves only the first.

A load calculation is what answers it, and any electrician quoting a subpanel should have done one.

The four-wire rule

This is the detail that separates a correct subpanel from a common and dangerous mistake.

In a subpanel, the neutral and the ground must be separated. The neutral bar is isolated from the enclosure; the ground bar is bonded to it. They are joined only once in the whole system, at the main panel.

Bonding them again at the subpanel puts normal return current onto the grounding conductors and onto any metal they touch: conduit, pipes, appliance cases. It is invisible, it is common in DIY work, and it is exactly the situation grounding exists to prevent.

So the feeder is four wires: two hots, a neutral and a ground. Three-wire feeders to subpanels were permitted in some older situations and are not now.

If you look in a subpanel and the white wires and the bare wires land on the same bar, that is the fault.

Detached buildings

A subpanel in a detached garage or shed adds requirements:

• Its own grounding electrode, usually a rod or two driven at the building
• A disconnect at the building
• Burial depth for the feeder, 18 to 24 inches depending on the wiring method
• Conductor sizing for voltage drop over the run, which is why a long feeder is a bigger cable than the amperage alone suggests

That last one is a real cost. A 100 amp feeder 150 feet to a workshop is a substantially larger and more expensive cable than the same feeder at 20 feet.

Frequently asked questions

How many circuits can a subpanel hold?

Physically, whatever the box is rated for, commonly 12 to 24 spaces. Electrically, whatever the feeder and the load calculation support. A 60 amp subpanel with 20 spaces is perfectly normal; the spaces are for convenience, not capacity.

Can I install one myself?

In some jurisdictions a homeowner may, with a permit and an inspection. The neutral-ground separation, the correct feeder sizing and the grounding at an outbuilding are the parts most often got wrong, and the inspection exists precisely for that.

Does a subpanel let me add an EV charger?

If the service has the capacity, yes, and it is often the tidy way to do it. If the service does not, no. Ask for the load calculation.

Do I need a permit?

Yes, essentially everywhere. It is a change to the distribution system and the inspection is the only independent check that the neutral and ground were separated.

Can a subpanel be fed from another subpanel?

Physically yes and it is usually a sign the design should be reconsidered. Each stage adds voltage drop and complexity, and it makes the system harder to understand for whoever works on it next.