What panel size does an ADU need in California?

The honest answer is: whatever a formal NEC Article 220 load calculation says — not a guess based on square footage. That said, most California ADUs land in a predictable range, and understanding 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 compared with 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

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

• 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.

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. For deeper background, see the ADU electrical guide.