What Is an ADU Electrical Panel?

Key takeaways

• An ADU electrical panel is the breaker box that feeds the circuits inside the unit.
• Most ADUs get a subpanel fed from the house. Some get their own service, meter and main panel.
• Whether the house can carry the ADU is decided by a load calculation, not a rule of thumb.
• California uses the 2025 California Electrical Code, in force since 1 January 2026.
• Electrical work is permitted and inspected. A licensed professional designs and installs it.

What it means, in plain English

A panel is the metal box of breakers you flip when a circuit trips. Each breaker protects one circuit, such as the kitchen outlets. The ADU needs its own panel so its circuits can be shut off and protected inside the unit.

A subpanel is a smaller panel fed by a cable from the main house panel. A service is the power line from the utility, plus its meter. The service has a rating in amps, which is the most it is built to carry.

A load calculation adds up what the house and the ADU will draw, using set code rules. It tells you if the service rating is big enough for both.

Why it matters to you

The panel itself is rarely the big cost. The big cost comes when the load calculation says the house service is too small. Then the fix can mean a larger service, a new main panel and work by the utility.

It also shapes your design choices. Heat pumps, induction cooking and EV charging all add load. Knowing your headroom early lets you plan around it, instead of finding out at plan check.

Finally, it ties to billing. If the ADU gets its own service, it gets its own meter and bill. If it shares, you pay one bill for both homes.

Subpanel or separate service

The guide below matches common situations to the route that usually follows. A load calculation by a licensed electrician decides it.

Most ADUs get power one of two ways. In the first, a feeder cable runs from the main house panel to a subpanel in the ADU. The ADU shares the house's utility service and meter, and the house panel must have room and capacity for the feeder.

In the second, the utility installs a new service with its own meter, and the ADU has its own main panel. That route is tied to the metering decision covered on the separate utility meter page.

Either way the panel itself is a modest line item. The expensive surprise is when the calculation shows the existing service to the property is too small, which can mean a bigger service, a new main panel at the house, and utility work at the pole or transformer.

How capacity is checked

This is the sequence a licensed electrician or engineer follows. It is a description of the process, not instructions; electrical work requires a permit and a licensed professional.

1. List the existing loads in the house: lighting and receptacles by floor area, appliances, heating and cooling, water heating, EV charging.
2. Add the ADU's planned loads the same way.
3. Apply the calculation method in Article 220 of the electrical code, which allows demand factors because not everything runs at once.
4. Compare the calculated load with the rating of the existing service and main panel.
5. If it fits, design a feeder and subpanel. If it does not, choose between a service upgrade, a separate service, or reducing planned loads.

The electrical service upgrade research explains the four inputs that drive the result.

What pushes a property toward an upgrade

California context

California adopts the National Electrical Code as the California Electrical Code, Part 3 of Title 24. The 2025 edition took effect on 1 January 2026, and the State Fire Marshal's adoption documents list Article 220 among the adopted articles and name the 2023 NEC as the base code ^{1 2}.

State ADU law does not set electrical capacity. It does say an ADU follows the local building code for detached dwellings and does not by itself trigger a Group R occupancy change, unless the building official makes a written finding of a specific adverse health and safety impact (Gov. Code § 66314(d)(8)) ³.

Utility-side work, such as a new service drop, is governed by the utility's own tariff rules ⁴.

Sources: [1] [2] [3]

How the panel choice ties to the meter

The panel route and the meter route go together. A subpanel shares the house meter. A separate main panel sits behind its own meter, set by the utility.

Utilities write their own metering rules. In PG&E territory, Electric Rule 18 says PG&E will furnish and meter electricity to each individual residential dwelling unit, with listed exceptions. The same rule says a junior ADU is not required to be separately metered ⁵.

So ask the utility early how it will serve your ADU. Its answer can decide whether a subpanel is an option at all, before the electrical plan is drawn.

Sources: [5]

Rules of thumb that do not hold

Myth "Every ADU needs a 100-amp subpanel."

Reality Panel and feeder size come from the calculated load of the specific design, not a fixed number.

Myth "A 200-amp house can always add an ADU."

Reality It depends on what the house and the ADU already draw. The calculation decides.

Myth "A separate meter avoids the load question."

Reality A separate service moves the question to the utility's capacity and design rules; it does not remove it.

Subpanel and separate service compared

Where you will see it

• The electrical plan. Part of the permit set. It shows the panel, feeder and circuits.
• The load calculation sheet. Many building departments ask for it with the plans.
• A contractor bid. Look for the panel, feeder, trenching and any service upgrade as line items.
• A utility work order. If the service changes, the utility has its own steps. In PG&E territory, service extensions follow Electric Rule 16 ⁴.
• Inspection records. The building inspector signs off the work before it is covered or used.

Benefits and limits of each route

What to ask your electrician or designer

• Have you done a load calculation for the house plus the ADU?
• What is the rating of my existing service and main panel?
• Does the plan fit without a service upgrade? If not, what are the options?
• Would a separate service be simpler than an upgrade at the house?
• How do planned heat pumps, EV chargers or solar change the result?
• Are you licensed for this work, and who pulls the permit?

Checklist for your project

Planning steps for owners. None of these is electrical work; a licensed professional does that.

1. Find the rating printed on your main panel or ask your electrician to confirm it.
2. List the big appliances you plan for the ADU.
3. Ask for a load calculation before the design is final.
4. Decide between a subpanel and a separate service.
5. If the utility is involved, contact it early.
6. Check the state license of any contractor you hire.
7. Keep the permit and final inspection records.

Outside California

Plan the budget

• ADU construction hub
• ADU solar requirements
• ADU cost calculator
• Find a licensed professional

Related terms

Separate Utility Meter

A separate utility meter is a meter the utility installs and reads for the accessory dwelling unit alone, so the ADU's electricity, gas or water is billed on its own account instead of through the main house's meter.

ADU Utility Connection

An ADU utility connection is the way an accessory dwelling unit gets water, sewer, electricity and, where used, gas: either as a branch off the main house's existing service, or as a new, separate connection running directly from the utility to the unit.

ADU Hard Costs

ADU hard costs are the money spent physically building an accessory dwelling unit: site preparation, foundation, framing, roofing, windows, mechanical, electrical and plumbing systems, finishes, and the trenching that brings utilities to the unit, together with the contractor’s labor, overhead and profit.

ADU Contractor

An ADU contractor is a licensed construction business that contracts to build an accessory dwelling unit, either doing the work itself or running the subcontractors who do the trades.

ADU HVAC

ADU HVAC is the heating, ventilation and air-conditioning system of an accessory dwelling unit.

California Residential Code

The California Residential Code (CRC) is Part 2.5 of Title 24 of the California Code of Regulations: the state building code for detached one- and two-family dwellings and townhouses, which is the code most ADUs in California are designed under.

Also referenced from: ADU Kitchen, ADU Utility Connection, Separate Utility Meter, ADU HVAC