What Is a Subpanel, and When Do You Need One?

What is a subpanel?

A subpanel is a second, smaller breaker panel fed from a breaker in your main panel through a set of wires called a feeder. It gives you more breaker spaces and puts them closer to the things they serve. What it doesn’t do is add power. Everything still comes through your main service.

That last point clears up most of the confusion around subpanels, including the cases where one is the wrong fix.

More room, same power: why the difference matters

Think of a power strip. It gives you six outlets where you had one, but it can’t pull more electricity out of the wall than that one outlet allows. A subpanel works the same way for your whole house. It solves a space problem: your main panel has no open slots, or the new loads are far from it.

It can’t solve a capacity problem. If your home’s service is already at its limit, a subpanel just spreads the same overloaded supply across more breakers.

So how do you know which problem you have? A load calculation. An electrician adds up what your home draws now plus what you plan to add, and compares it with what your service can carry. If there’s headroom, you need space, and a subpanel fits. If there isn’t, you need capacity, which means a service upgrade. Our post on when to upgrade a 100-amp panel to 200 amps covers the signs.

When a subpanel makes sense

June is when a lot of these projects start: pool equipment going in, a garage turning into a workshop. Common reasons for a subpanel include:

• your main panel is full
• a detached garage or workshop
• a home addition
• pool or spa equipment (see our pool and spa panel work)
• several new loads clustered far from the main panel
• a kitchen remodel

Say you’re adding a pool pump, a heater and some lighting out by the back fence, and your main panel is on the opposite side of the house. One feeder out to a subpanel near the equipment beats running a separate long circuit for each piece.

How a subpanel is sized and wired

Typical subpanel feeds are 60, 100 or 125 amps, chosen by the loads it will serve. The wiring has rules a homeowner won’t see but an inspector will check:

• A four-wire feeder: two hots, a neutral and a ground.
• Separate neutral and ground bars. In your main panel they’re joined. In a subpanel they’re kept apart, so normal current doesn’t travel on the ground wires.
• A detached building needs its own grounding electrode (ground rods) and a disconnect.
• Clear working space in front, about 30 inches wide and 3 feet deep.

A subpanel needs a permit and an inspection. This is panel work for a licensed electrician, start to finish.

What about tandem breakers?

Tandem breakers (sometimes called “twins”) fit two circuits into one slot, and they can add room to a full panel. But only if your panel is listed to accept them, and only in the slots its label allows. Not every panel is. An electrician can read the label and tell you whether tandems are an option before you plan a subpanel around a full box.

Planning your subpanel

Infiniti Electric is licensed and insured and does residential and commercial electrical panel work around Riverside and the Inland Empire. If you’re not sure whether you need space or capacity, send us a note with what you’re adding and where. We’ll start with the load calculation.

Subpanel questions homeowners ask