ADU Electrical Service

Electrical service rarely stops an ADU outright, but it regularly changes the budget and sometimes the design. The usual surprise is a panel that looked fine for the house and has no room left once a second household is added. Here is how to think about it, and where city and utility rules come in.

Load, in plain terms

Your service is the capacity of the wires and equipment that bring power from the utility to your main panel. Load is how much the home can demand at once. An electrician works out whether an ADU fits by doing a load calculation: adding up the house's existing demand and the ADU's new demand, using the method the electrical code sets out, and comparing the result with the service.

The big loads are the ones that heat or move a lot of energy:

• Heating and cooling, especially electric resistance heat.
• Water heating.
• An electric range, oven and clothes dryer.
• Car chargers, hot tubs and shop equipment already on the house.

Design choices matter here. Efficient heat pumps, a heat pump water heater and right-sized appliances can keep the ADU's added load down, while using gas for some appliances moves load off the electrical service. That is one reason we look at the panel before the ADU's equipment is chosen.

Three common outcomes

• The panel has room.A feeder runs from the main panel to a subpanel in the ADU. For a detached unit that feeder goes in a trench, often shared with other utilities.
• The panel is full, but the service is large enough.The main panel may need replacing or reorganizing to make room for the feeder.
• The service is too small.A service upgrade is needed: a larger service from the utility, often with a new meter base and main panel. This involves the utility and usually adds time and cost.

Meters: usually one for both homes

Many homeowners assume an ADU gets its own electric meter. In most of these cities it cannot. Millcreek, Murray, South Salt Lake, Holladay, Sandy and Cottonwood Heights prohibit separate utility meters for an ADU. Murray's building checklist asks outright whether the house has only one address and one utility meter. There are a few nuances:

• Sandy may allow private meters installed behind the primary meters and inside the dwelling.
• Millcreek's ADU page says secondary meters may be allowed to monitor utility usage.
• Unincorporated Salt Lake County allows a detached ADU to be separately metered if the owner is billed for both.

A private submeter can help split costs with a tenant, but it does not change the fact that the service and panel are sized for both homes together. See ADU utility connections for the full meter table.

Code points that apply to ADUs

Utah's construction code adopts the 2023 edition of the National Electrical Code (Utah Code 15A-2-103). The state has also amended the residential code for ADUs: an occupant of an accessory dwelling unit is not required to have access to the disconnect serving the unit they live in (15A-3-206). That amendment sits within a state definition of ADU that covers units created within the existing footprint of an owner-occupied home, so for a detached unit, confirm the approach with your building department.

Local rules can add to this. Holladay requires that the ADU tenant have unobstructed access to utility connections. Salt Lake City's ADU handbook lists separate, accessible power disconnects for each unit among its building code items. Where local guidance and the state amendment read differently, the building department's reading is the one that counts.

Working with the utility

Rocky Mountain Power, the power provider listed on Holladay's utilities page, publishes a six-step process for new construction service: apply online or through its New Construction Hotline at 1-800-469-3981; the local office contacts you within two business days to decide whether a site meeting with an estimator is needed; the estimator reviews the house location, load data and site plans; the design is completed and paperwork sent; the applicant completes items such as the electrical inspection, trench approval, contract and any easement documents; and the job is scheduled once the last item is received. Its 2022 Electric Service Requirements Manual (Rev 1) applies to installations made after January 1, 2025. Ask Rocky Mountain Power directly how it handles an upgrade on an existing home.

Murray City Power is a publicly owned municipal electric utility created in 1913. If your property is in Murray, check whether Murray City Power serves your address and ask about its requirements for an upgrade before design is final.

What to gather first

Photograph the main panel label and the meter, note where the service enters the house, and list the large electric appliances already in use. With that, an electrician or our team can tell early whether an upgrade is likely. Start with the order of questions in Can I build an ADU?, and read ADU sewer and water for the other half of the utility picture. Our ADU feasibility review flags service capacity as a cost driver before design. To start, See What Your Property Can Support.

Questions about ADU electrical service