Does an ADU require an electrical panel or service upgrade in Sacramento?

Does an ADU actually require a panel or service upgrade?

The trigger for an upgrade is not the ADU itself. It is whether the electrical load you are adding pushes your home past the capacity of the service you already have. That question gets answered by a load calculation, not by a rule of thumb, and the answer is often no.

Here is the sequence a licensed electrician follows. They run a load calculation under the California Electrical Code (Title 24, Part 3, which adopts the National Electrical Code) to size your existing demand, then add the ADU's demand on top. If your main service still has headroom after that math, the ADU gets fed from a new subpanel and nothing on the utility side changes. If the calculation shows you are over, you upgrade the main service, usually to 200 amps, before the ADU load goes on.

What has changed the odds is electrification. A 100-amp service that comfortably ran a gas-appliance house for decades can come up short the moment you add an all-electric ADU with a heat-pump HVAC system, a heat-pump water heater, an induction range, and an EV charger in the driveway. The ADU is small, but its electrical demand is not trivial, and that is where a lot of older Sacramento homes get caught.

One detail that saves homeowners money: the current NEC adopted into California's code lets an electrician use twelve months of actual metered peak demand to evaluate an existing service instead of the worst-case theoretical numbers. SMUD can supply that interval data. If your real-world peak is low, that demand study can justify keeping a smaller service that a textbook calculation would have condemned.

See also:Detached ADU builds how we scope the electrical from day one

100-amp vs 200-amp service: what the number on your panel means

The amperage stamped on your main breaker is the ceiling for everything the house and the ADU can draw at once. Older Sacramento homes were built around gas heat, gas water heating, and gas cooking, so a 100-amp service was plenty. Today's all-electric ADU shifts several of those loads to the panel, which is why the existing number matters so much.

The busbar rating inside the panel matters as much as the main breaker. Even if the main is 200 amps, the bus and the available breaker spaces have to accept the ADU feeder and any future solar backfeed. An electrician checks all three: main breaker size, busbar rating, and physical space. The table below is general guidance for what different service sizes typically mean for an ADU in this region. It is not a substitute for a calculation on your specific panel.

Subpanel vs. a new service drop: three ways to power an ADU

There are three practical ways to get power to an ADU, and they climb in cost and complexity. The load calculation and how you plan to bill the unit decide which one you land on.

Option A, a subpanel off your existing main. This is the cheapest and most common path. A feeder runs from the main panel to a subpanel inside the ADU, and everything shares one meter. It only works if the calculation shows your service has spare capacity, which is why a 200-amp house is usually a straightforward subpanel job.

Option B, upgrade the main service, then subpanel. When the calculation shows a shortfall, you replace the panel and service (commonly 100A to 200A) first, then feed the ADU subpanel off the larger main. You are still on one shared meter, you have just made room.

Option C, a separate service and second SMUD meter. Here the ADU gets its own service drop and meter, fully independent of the house. This is the path when you want to bill a tenant directly for their own electricity, or when the parcel genuinely cannot share capacity. It is the most expensive option because it adds a utility service drop, a second meter, more trenching, and SMUD interconnection coordination.

The load calculation: why a licensed electrician, not a guess

A real load calculation is a document, not an opinion. The electrician uses either the standard method or the optional dwelling method (NEC Article 220, including the optional calculation many electricians use for a whole dwelling) to total your general lighting and receptacle loads, fixed appliances, HVAC, water heating, cooking, and any EV charging, then applies the code's demand factors. The metered-demand approach mentioned earlier is a third, often-cheaper route when your utility history supports it.

This is the step that separates a scoped ADU builder from a handyman. A contractor who glances at your panel and announces you need a 200-amp upgrade has not done a calculation, and you may be paying for an upgrade you do not need. Just as often, the opposite happens: someone taps an ADU subpanel off a main that is already near its limit, and the homeowner lives with nuisance trips every time the heat pump and the oven run together.

The person doing this work has to be licensed. In California that is a C-10 electrical contractor, or a licensed B general contractor running licensed electricians, and the work has to be permitted and inspected through your City or county building department. You can and should verify any contractor's license status directly with the CSLB before signing. Unpermitted panel or service work is a common reason an ADU fails its final inspection and stalls at the finish line.

See also:Estimate your ADU budget including the electrical scope

SMUD interconnection and metering: shared vs. separate meter

In the City of Sacramento and across most of Sacramento County, your electric utility is SMUD, and SMUD sets the rules for service upgrades and new connections. Before you plan anything, confirm which utility actually serves your address. Foothill parcels in Placer and El Dorado counties (Auburn, Loomis, Granite Bay, Cameron Park, Placerville, El Dorado Hills) are frequently served by PG&E instead, and PG&E runs its own service-planning process.

A shared meter keeps things simple. The ADU rides on your existing SMUD account through a subpanel, you get one bill, and there is no second monthly service charge. The tradeoff is that you cannot cleanly separate a tenant's electricity use without installing a private submeter and reading it yourself.

A separate meter gives the ADU its own SMUD service and its own bill, which makes tenant billing clean. The cost is real: SMUD has to set a service drop and meter, you add trenching and interconnection work, and the ADU carries its own monthly service charge going forward.

There is a legal nuance worth knowing. State ADU law limits when a utility can require a new or separate connection and a related capacity charge: for a junior ADU or an ADU created within existing space, a utility generally cannot force a separate connection, while for a new detached ADU it may. Whether that applies to your build, and what any interconnection work costs, is SMUD's call. Do not budget off a number you read online. Confirm the current process and charges with SMUD's construction services before you commit, because a service upgrade also requires SMUD to schedule a disconnect and reconnect, and may involve mast or overhead-to-underground work.

See also:Sacramento ADU builds we coordinate SMUD for you

What a panel or service upgrade costs in 2026 (Sacramento estimates)

The figures below are 2026 Sacramento-region market estimates, not quotes and not utility fees. Two variables move them the most: how far the ADU sits from the panel (trench length) and how much utility-side work SMUD requires. A subpanel on an attached ADU a few feet from the main is a different job than a separate service to a detached unit at the back of a deep Land Park lot.

Sacramento wiring realities by neighborhood and county

Where your house sits in the region tells you a lot about what you will find behind the panel cover, and that is what makes an ADU electrical scope local rather than generic.

The pre-war central grid (Boulevard Park, Poverty Ridge, parts of Midtown, and older East Sacramento) has the oldest services in the area. Expect 60 to 100 amps, occasional fused panels, and sometimes knob-and-tube remnants that an inspector will want addressed. On narrow central-grid infill lots, a garage-conversion ADU shares a wall and a short feeder run, which keeps the electrical simpler even when the panel needs upsizing.

Land Park and Pocket-Greenhaven run deeper, alley-loaded lots. A detached ADU at the rear means a long trench from the house panel to the unit, and that trench footage is a real line item, so the underground feeder or a rear-of-lot service can outweigh the panel work itself.

Mid-century suburbs (Curtis Park's later blocks, plus Carmichael, Fair Oaks, and Orangevale, which permit through Sacramento County Building Permits and Inspection) are the classic 100-amp upgrade candidates. Newer Natomas and North Natomas homes usually already have 200-amp service, which favors a simple subpanel, but Natomas is a levee-protected historic floodplain, so equipment elevation and meter height can come into play; confirm those details with the City.

The foothills (Auburn, Placerville, Cameron Park, Loomis, Granite Bay, El Dorado Hills) often mean PG&E rather than SMUD, longer rural service runs, and more overhead work, with El Dorado County Planning and Building or Placer County Building as the permit authority. West Sacramento sits across the river in Yolo County, permits through City of West Sacramento Community Development, and shares the same levee-area considerations as Natomas.

See also:Sacramento garage-conversion ADUs short feeder runs, simpler electrical

What some contractors get wrong, and shortcuts to refuse

This is where corners get cut, so know what an honest electrical scope looks like before you sign.

• Quoting a 200-amp upgrade reflexively, with no load calculation. You may not need it, and the calc is what proves it either way.
• Undersizing the feeder to the ADU to save on wire and trench, which shows up later as voltage drop and tripped breakers on a long run.
• Tapping the ADU subpanel off a main that is already near capacity, so the heat pump and the oven fight for the same headroom.
• Ignoring future EV and heat-pump load, leaving the panel full on day one with no room to grow.
• Doing panel or service work without a permit and without SMUD coordination, which fails inspection and can hold up the ADU's final.
• Overlooking the solar backfeed limit (NEC 705.12, the '120% rule'): a panel sized for the ADU load alone may still need an upgrade or a line-side tap once solar is added, which ties directly into Title 24 solar planning.
• Skipping the grounding and bonding upgrade to current code when the panel is opened, which an inspector will catch.
• Using an unlicensed handyman for the panel. Verify a C-10 or B license at CSLB and insist the work is permitted.

How Upside sequences the electrical scope

We treat the electrical as an engineering question we answer before framing, not a surprise at inspection. On every build we run the load calculation through a licensed electrician first, so we know honestly whether your service can carry the ADU or not. When it can, we feed a subpanel and leave your service alone. When it cannot, we scope the 200-amp upgrade and price it up front rather than as a change order.

Because permits and engineering are handled in-house, we pull the electrical permit with the City of Sacramento or your county building department, coordinate SMUD's interconnection and metering (shared or separate, depending on how you plan to use the unit), and schedule the SMUD disconnect and reconnect so it lands between framing and finish instead of stalling the job. As a licensed California contractor, we keep the panel, grounding, and feeder work to code and inspection-ready, and we plan for solar and EV load if they are anywhere on your roadmap.

If you want the panel question answered for your specific lot before you commit to a design, that is the right first conversation to have.

See also:Get a scoped electrical plan load calc and SMUD coordination included