What size electrical panel does an ADU need?

Whatever a formal NEC Article 220 load calculation says — there is no valid square-footage shortcut. That said, most California ADUs land in a predictable range, and knowing the drivers helps you plan service and budget before the calc is finalized.

Typical sizing by ADU profile

Industry practice favors 125 amps for a standard all-electric unit even when 100 amps technically passes the calc. The incremental cost of a larger panel and feeder is small next to retrofitting later, and the extra breaker spaces leave room for a future EV charger, battery, or hot tub.

Why square footage doesn't decide it

Floor area drives only the general lighting load — 3 VA per square foot. Everything else comes from the appliances. Two 750 SF ADUs can need very different panels: one with a gas range, gas water heater, and a small mini-split might calculate well under 100A, while an all-electric unit of the same size with induction cooking and a Level 2 charger can exceed it. That's where the variation lives.

What pushes the number up

• Induction range: 40–50A dedicated circuit.
• Heat pump water heater: 20–30A.
• Heat pump / mini-split HVAC: 15–30A depending on capacity.
• Level 2 EV charger: 40–50A on its own dedicated circuit.
• Electric or heat-pump dryer: another dedicated circuit.

Stack those and a fully electric ADU with EV charging can legitimately approach 150–200 amps of calculated demand even at modest square footage.

Don't forget the main service

The ADU subpanel is fed from your main house panel, so its size is constrained by what the existing service can spare. A 100A main often can't carry both the house and a new ADU subpanel; a 200A main with capacity frequently can host a 125A subpanel — but only the load calculation confirms it for your specific existing loads.

Breaker spaces matter as much as amperage

Panel selection isn't only about the main breaker rating — it's also about how many circuits you have to fit. An all-electric ADU can easily need a dozen or more breakers once you count two small-appliance circuits, laundry, bathroom, lighting, receptacles, range, water heater, HVAC, dishwasher, disposal, and an EV charger. A panel that's adequate on amperage but short on spaces forces tandem breakers or an early replacement. This is a second reason to favor a 125A panel with generous spaces over a minimally sized 100A one — the extra slots leave room for a future battery, hot tub, or workshop circuit without a rework.

Requirements vary by jurisdiction and utility — confirm with your local building department and serving utility.

We run the Article 220 calc and justify the panel size in every electrical plan set; you can order a single discipline from $995. For deeper background, see the ADU electrical guide.