Electrical Safety & Insights

How Electricians Size an ADU Feeder on a Shared Electrical Service

November 5, 2026

Adding an accessory dwelling unit often raises a simple question: how large should the ADU feeder be? The answer depends on the calculated electrical load and the limits of the entire service—not just the amp rating printed on the main equipment.

Start with the complete load calculation

A licensed electrician reviews the existing dwelling loads and the planned ADU loads together. The calculation may include heating and air conditioning, cooking equipment, water heating, laundry equipment, general lighting, receptacles, and any planned EV charging. Applicable demand factors are then used to determine the expected load on the service.

Check every part of the power path

A feeder can only be as large as the equipment and conductors that support it. Before selecting a feeder size, the electrician verifies:

• The service and meter equipment ratings
• The distribution section and available breaker position
• The feeder breaker rating
• The feeder conductor size and installation method
• The ADU panel rating and available spaces
• Grounding, bonding, disconnect, and labeling requirements
• Utility and local permitting requirements

Why the main service rating is not enough

A property may have a large service while several units already use substantial capacity. A 400-amp service does not automatically mean that any additional feeder size will work. Existing loads, future equipment, the internal distribution design, and utility requirements all affect the final answer.

For example, changing a planned ADU feeder from 100 amps to 125 amps requires more than installing a larger breaker. The load calculation, panel bus rating, feeder conductors, raceway, terminations, and upstream equipment must all be suitable for the higher rating.

Plan for likely future loads

It is helpful to identify future electrical needs before permits and materials are finalized. An electric range, heat-pump water heater, HVAC system, laundry equipment, or EV charger can materially change the design. Planning early may prevent a costly redesign after construction begins.

Get the design verified before ordering equipment

A proper ADU feeder design should be based on approved plans, a documented load calculation, compatible listed equipment, and the requirements of the serving utility and local building department. Confirming these items before ordering panels or conductors helps keep the project moving through inspection and energization.