The electrical plan for an ADU, workshop or detached garage should be settled before walls, trench routes and equipment locations are fixed. The core questions are how much load the new structure adds, whether the existing service can carry it, how power will reach the building, and which circuits need to be planned from the start.
A large subpanel does not create additional utility capacity. The first step is to list the equipment the new structure will actually use. An ADU with electric cooking, laundry, heat-pump HVAC, water heating and EV charging can be a much larger electrical project than a simple detached garage used for lights and receptacles.
Cooking, HVAC, water heating, laundry, general receptacles, lighting, EV charging and any future equipment should be identified before the feeder and service strategy is finalized.
Lighting and receptacles may be modest, but welders, compressors, lifts, HVAC, EV charging or other 240V equipment can materially change feeder and service requirements.
An attached conversion may be close to the existing panel. A detached structure can require a longer feeder route, underground conduit, trench coordination, a dedicated subpanel and grounding provisions at the separate building. Distance and route matter because the project has to account for conductor sizing, physical protection, other underground utilities and the location of electrical equipment.
The City of Sacramento lists detached garages among project types requiring plans, while Sacramento County directs property owners to Building Permits and Inspections for ADU and garage-conversion permit requirements. The exact process depends on jurisdiction and project scope. See City of Sacramento electronic plan-check guidance and Sacramento County planning FAQ.
| Strategy | What it means electrically | Planning trigger |
|---|---|---|
| Existing service → feeder → subpanel | The new structure shares the existing utility service. | Calculated combined load fits and the feeder route is practical. |
| Existing service with load management | Selected large loads are controlled to reduce simultaneous demand. | The service is close to capacity and approved management can solve a specific constraint. |
| Service upgrade | Utility/service equipment capacity is increased. | The combined calculated demand requires more capacity or the project has another service-related reason. |
| Separate utility arrangement | The new structure has a utility/metering strategy separate from a simple shared feeder. | Project design, ownership, metering or utility requirements support it. |
SMUD’s meter-and-service process recommends beginning with the local agency and coordinating projects that require meter or service work. For ADUs specifically, SMUD has advised owners to involve the utility early because the building-permit process does not itself determine how electric service will be brought to the new unit. Early coordination can expose conflicts with service location, overhead facilities or metering before construction documents are locked.
Use SMUD Meter and Service for current utility process information.
The subpanel needs enough spaces and rating for the intended circuits, but planning also includes feeder size, route, disconnecting means where applicable, grounding and bonding, working clearances and future circuits. A detached building’s electrical design is not just “run a cable to a panel.”
If the main service itself may be the constraint, compare IBE’s 100A vs. 200A guide and panel vs. service upgrade guide.
Equipment choices can change branch-circuit requirements. A workshop may need dedicated circuits for tools. An ADU may need dedicated circuits for cooking, laundry, HVAC or water heating. EV charging can become a major additional load. IBE’s dedicated-circuit guide explains why nameplate requirements and equipment instructions matter, while the EV panel-readiness guide covers charger capacity planning.
No. Some projects can be supplied from the existing service through a feeder and subpanel; others may use a different utility arrangement. The load, site and utility/project requirements decide.
No. The existing house and proposed ADU should be evaluated together. The calculated load determines whether the current service remains workable.
A feeder and subpanel is a common architecture for detached structures, but the actual design must match the intended loads, route, grounding/bonding requirements and permit scope.
Before final equipment selections, trench routes and construction documents are locked. Early electrical planning reduces redesign risk.
I Build Energy can review the existing service, proposed loads, feeder route and circuit needs so the electrical scope is defined before construction gets expensive to change.