Subpanel vs Panel Upgrade: Which Does Your Phoenix Home Need?

Subpanel vs panel upgrade is one of the most common questions we get from Phoenix homeowners who are adding something: a garage workshop, a casita, a pool heater, an EV charger, a second AC unit. The panel has no open spaces, and somebody has told them they need a new one. Sometimes that's true. Often it isn't. The answer comes down to one distinction most people never hear explained.

Spaces vs capacity: the one thing to understand

Your panel has two separate limits. The first is spaces: the physical slots where breakers go. The second is capacity: how many amps the service can deliver, set by the main breaker, the service wires and the utility's equipment.

A subpanel fixes a space problem. It is a smaller panel fed by one breaker in your main panel, and it gives you new slots for circuits, often right where you need them. What it can't do is add capacity. Every amp that flows through a subpanel still comes through your main breaker. If the house is already short on capacity, a subpanel just moves the problem a few feet away.

A panel or service upgrade is what fixes a capacity problem. That means a larger main breaker and panel, usually new service conductors, and coordination with SRP or APS. It's a bigger job, with a permit and a city inspection, and it's the only thing that actually gives the house more power.

When a subpanel is the right call

A subpanel makes sense when the load calculation shows the service has room, but the panel doesn't. Common Phoenix examples:

• A detached garage or workshop. One feeder from the house to a small panel in the garage is usually cleaner than running several long circuits.
• A casita or guest house. A subpanel fed from the house is the most common way to power one, as long as the house service can carry it. Our casita electrical guide compares it with a second meter.
• Pool equipment far from the house. A small panel at the equipment pad keeps the pump, heater and lighting circuits together.
• A full panel in a house with modest loads. Gas range, gas water heater, one AC unit and no EV. Plenty of capacity, just no slots left.

When a subpanel is the wrong answer

A subpanel is a poor fix, and sometimes an unsafe one, in these situations:

• The load calculation doesn't pass. If the house can't carry the new load, more slots won't help. The main breaker will trip on hot afternoons, or worse, the service equipment will run hot for months.
• The main panel is a known problem brand, such as Federal Pacific Stab-Lok or Zinsco. Adding onto it keeps the weakest part of the system in service. Replace it first.
• The panel is damaged. Rust from a roof leak, scorched bus bars, melted breakers or a warm cover mean the panel needs attention before anything gets added to it.
• The service is 100 or 125 amps and you're adding a big load. An EV charger, a heat pump with heat strips or a second AC unit on a small service usually ends in an upgrade anyway. Doing the subpanel first means paying twice.

How the load calculation decides it

The load calculation is the method electricians and plan reviewers use to size a service. It counts the home's square footage, kitchen and laundry circuits, fixed appliances, and heating or cooling at full value, then adds the new load. In Phoenix the AC is almost always the biggest number. Our post on how much load AC puts on a panel walks through it in plain terms.

If the total fits the service, you're choosing between a subpanel and rearranging the existing panel. If it doesn't, you're choosing between a service upgrade and a load management device that limits the new load, such as an EV charger that backs off when the house is busy. Breaker sizes alone never answer the question. Neither does a quick look at the panel.

What a subpanel install involves

• A feeder breaker in the main panel, sized for the subpanel.
• Feeder wire sized for the load, the distance and the heat. Long runs in Phoenix sun and hot attics need extra attention to wire size.
• Separate neutral and ground in the subpanel. They're bonded together only at the main. Getting this wrong is one of the most common subpanel mistakes we find.
• Grounding electrodes for a subpanel in a detached building.
• A permit and inspection. A subpanel is new electrical equipment, not a handyman job.

Subpanels on Fountain Hills hillside lots

In Fountain Hills, we see both sides of this question. Homes from the town's early decades are often still on their original service, so a new pool heater or EV charger can push them into an upgrade. Newer hillside homes tend to have bigger services, but the meter, the main panel, the pool pad and the casita are often far apart and at different elevations. There, a subpanel near the load is usually the better design, as long as the feeder is sized for the distance and the heat. Fountain Hills has its own building department, separate from Scottsdale, and we pull the permit there.

Not sure which one you need?

Send us a photo of your main panel with the cover on, a photo of the label inside the door, and a list of what you're adding. We'll tell you whether it's a subpanel job or an upgrade, and give you a free written estimate either way. See our panel upgrade services, or the panel upgrade cost guide for what drives the price. B and D Electric LLC is licensed and bonded, Arizona ROC 361086, and our two owners have more than 40 years of combined experience. Call or text (480) 828-1181, or request an estimate.