What panel size does an ADU need?

The honest answer is: whatever a formal NEC Article 220 load calculation says — not a guess. That said, most California ADUs land in a predictable range, and understanding the drivers helps you plan service and budget before the calc is run.

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 compared with retrofitting later, and the extra breaker space accommodates future EV chargers, batteries, or a hot tub.

Why a rule of thumb isn't enough

It's tempting to pick a number based on square footage, but the load calculation routinely surprises people. 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. Square footage drives only the general lighting load (3 VA per SF); the appliances drive everything else, and that's where the variation lives.

What pushes the number up

All-electric mandates are the single biggest driver. When you can no longer use gas, the panel has to carry loads that used to be served by a gas line:

• 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 dedicated circuit on its own.
• Washer/dryer: a heat-pump or ventless electric dryer adds another dedicated circuit.

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

The catch with existing service

The ADU subpanel doesn't exist in a vacuum — it's fed from your main house. If the property has a 100-amp main service, a load calculation that includes both the existing house and the new ADU often shows the service can't carry both, triggering a main service upgrade. A 200-amp main with available capacity frequently can host a 125A ADU subpanel, but only the calculation confirms it for your specific existing loads.

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.