Casita and ADU Electrical in Arizona: Panel Capacity, Subpanels, and Trenching

You have the plans. Maybe it is a mother-in-law casita off the back patio, maybe it is turning the third garage bay into a rental, maybe it is a 600 square foot backyard unit you have been sketching since the law changed. The builder has given you a number. Then somebody asks the question that stalls the whole thing: is the panel on the house going to carry it?

That question has a real answer, and it is a lot cheaper to get before the slab is poured than after. Here is how the electrical side of an Arizona casita actually works, in the order you will run into it.

Why Everyone in the Valley Is Suddenly Building One

Arizona rewrote the rules. House Bill 2720, the bill most people call the Casita Bill, took effect January 1, 2025 and required every Arizona city over 75,000 residents to allow accessory dwelling units on single-family lots. That covers Phoenix, Mesa, Scottsdale, Chandler, Gilbert, Tempe, and most of the rest of the Valley. House Bill 2928 extended nearly the same framework to counties starting January 1, 2026, which pulled in unincorporated Maricopa County and the smaller towns around us.

So the zoning fight mostly went away. The electrical limits did not. Your house was wired for a house, not for a house plus a second dwelling with its own kitchen, its own laundry, and its own cooling.

Step One: Does Your Existing Service Have Room?

Everything downstream depends on this. A casita is a whole block of load: a mini-split or heat pump, a range or cooktop, a water heater, a dryer, a dishwasher, lighting, and receptacles that somebody is going to run a space heater and a hair dryer on at the same time.

The way that gets answered is a load calculation, not a guess. We add up the connected load of the house as it sits, add the casita as designed, and compare it against the rating of your service. A few patterns show up over and over in the Valley:

• Homes from the mid 1990s forward usually have a 200 amp service, and quite often there is real room in it, especially if the house runs gas for the range, dryer, and water heater.
• Homes from the 1970s and early 1980s frequently have a 100 or 125 amp service with a full panel and no space left. These almost always need the service addressed first.
• Anything with a panel that is already at capacity, has double-tapped breakers, or is an obsolete brand gets solved at the panel before anybody talks about the casita.

If the service does need to come up, that is a like-for-like 200 amp replacement in most cases, generally $5,200 to $6,500, or $6,400 to $8,750 when the meter base and panel are combined into a new all-in-one and the whole service gets replaced. Our panel upgrade cost breakdown walks through what moves that number, and the panels service page covers what the work involves.

This is usually money you were going to spend eventually. A casita just forces the timeline.

Step Two: Sizing the Feed to the Casita

The casita gets its own subpanel. That is not optional and it is not a place to economize, because every circuit inside the unit lands there and everything about the build after that point is downstream of how it was sized.

What drives the size:

• 60 amps is common for a smaller unit with a mini-split, a kitchenette, a microwave, and no electric range or dryer.
• 100 amps is the realistic number once you add an electric range or cooktop, an electric water heater, or a washer and dryer. Most full casitas built to be rented land here.
• 125 amps or more shows up on larger units, or when somebody wants an EV charger at the casita too. If a charger is even a maybe, say so now. Pulling a bigger feeder during the trench costs a fraction of what a second trench costs later.

Size it for what the unit will be in five years, not what is on the plan today. The feeder is the one part of this job nobody wants to redo.

Step Three: The Trench Nobody Budgets For

Here is where casita quotes go sideways. The panel is on one side of the house. The casita is in the back corner of the lot. Between them is 80 feet of yard, and in a Valley backyard that 80 feet usually contains some combination of pavers, a pool, a pool equipment pad, a block wall, irrigation, and a slab patio.

The underground feeder runs in conduit at a required burial depth, and the path matters more than the distance. Straight across dirt is one price. Cutting a pool deck, boring under a driveway, or working around an existing pool and its bonding grid is another, and that is a conversation to have while the landscaping is already torn up rather than after it has been restored.

Two more things worth knowing about the feeder itself. First, long runs need to be sized for voltage drop, not just for amps. A 100 amp feeder that is correct at 40 feet can be undersized at 150 feet, and the symptom is subtle: lights that dim in the casita every time the AC starts. Second, feeders on a run like this are frequently multi-strand aluminum, which is completely normal and safe modern practice. That is not the same thing as the single strand aluminum branch wiring in some 1960s and 1970s Valley homes, which is the actual hazard people are thinking of.

One Meter or Two?

Most casitas in the Valley run off the existing service on one meter. It is simpler, it is far less expensive, and it works fine when the unit is family, a guest house, or a long term rental where utilities are folded into the rent.

A second meter means a second service, which means coordination with APS or SRP and a much larger scope of work. It only makes sense when you genuinely need the tenant billed separately by the utility. Decide this early, because it changes the design of everything upstream.

What Goes Inside the Casita

Once the subpanel is set, a casita is wired like a small home, and code treats it like a dwelling unit rather than a shed with a bed in it. A typical build includes:

• A dedicated circuit for the mini-split or heat pump, plus the disconnect at the outdoor unit
• Two 20 amp small appliance circuits if there is any real kitchen, and a dedicated circuit for the refrigerator
• A dedicated 240 volt circuit for a range or cooktop, and another for an electric water heater
• Laundry circuits if there is a washer and dryer
• GFCI protection at the kitchen, bath, and exterior receptacles, and arc fault protection where required
• Interconnected smoke and carbon monoxide alarms
• Interior lighting, exterior lighting at the entry, and at least one exterior receptacle

On the cooling side, most casitas in the Valley end up on a mini-split, and the electrical requirements for those are specific enough that we wrote them up separately in our guide to mini-split electrical requirements in Arizona. If you are still deciding what equipment to put in the unit, our sister company The Cooling Guy HVAC (opens in a new tab) (Arizona ROC 364481) sizes these for casitas regularly, and getting the equipment picked before we rough in the wiring saves everybody a change order.

Garage Conversions Are a Different Animal

Converting a garage is usually the cheapest path to an ADU, and the electrical is usually the messiest part of it.

Garages are typically fed by one or two general purpose circuits that also serve the laundry, the exterior receptacles, and sometimes half the back of the house. The second that space becomes a dwelling unit, that wiring no longer covers it. You need dedicated circuits, correct receptacle spacing, GFCI and arc fault protection, and in most cases a proper subpanel rather than trying to extend what is there.

The other thing we find in garage conversions is existing work somebody did years ago without an electrician: an added receptacle on a lighting circuit, a 240 volt outlet for a compressor, a subpanel fed with the wrong conductors. All of it has to be sorted out before drywall goes up, which is one more reason to have the electrical walked before the framer starts.

When to Bring the Electrician In

Earlier than most people do. The right sequence looks like this:

1. Before you finalize the design. A load calculation and a look at your panel tells you whether a service upgrade is part of this project. That is a real number and it belongs in the budget from day one.
2. Before the trench is dug. Feeder size, conduit size, route, and depth all get decided together, and any future EV charger or pool equipment on that side of the yard gets accounted for.
3. At rough-in, before insulation and drywall. Everything is cheap while the walls are open and expensive after they close.
4. At trim out, when devices, fixtures, and the subpanel get finished and tested.

The projects that go badly are almost always the ones where the electrical got treated as a finish trade instead of the thing the whole schedule hangs on.

When to Call Us

Call us if you are pricing a casita or ADU and need to know whether your service can carry it, if a builder has told you the panel needs to be upgraded and you want that verified and quoted independently, if you are converting a garage and want the existing wiring evaluated before the work starts, or if you already have a unit going up and need the feeder, subpanel, and interior wiring handled by an electrician rather than a general crew.

We would rather look at your panel for free now than get called out after somebody buried the wrong conduit under a new paver patio.

At The Wire Guy Electric (Arizona ROC 365306), we do this work as a family-owned electrical contractor, which means the person who sizes your feeder is the same outfit that shows up at trim out.

We serve Mesa, Scottsdale, Gilbert, Chandler, Tempe, Queen Creek, Paradise Valley, and Fountain Hills. Request a free estimate and we will tell you exactly what the electrical side of your casita takes.