What Are the Electrical Requirements for ADUs?

By Buz Artiano, BuildX | October 2025

We get this question pretty much every week. Someone is already pretty deep into planning their ADU, with floor plan and everything already picked out, and then they start delving into all the questions they have about the electrical. Doesn't take long before they realize that it gets pretty complicated.

When it comes to planning an ADU, there are a lot more questions that come up about the electrical setup than people realize at first. It's way more than little things like where to put outlets. You have to think about everything like service amps, groundwork for future solar and EV chargers, panel placement, ventilation systems, and code-compliant disconnects. It's a bit overwhelming, we get it, because if you get one of these wrong, it could cost you thousands down the road.

At BuildX, we've been through this with all our projects and we know what happens when electrical planning is rushed. So we have a special process we go through to make sure small issues don't become expensive problems later.

What Service Panel Size Does an ADU Need?

Panel sizing is where a lot of these ADU projects get tripped up. The code requirement is at least a 100-amp subpanel for a 900 square foot ADU, but there's way more to it than that.

In Massachusetts an all-electric ADU running a heat pump, a heat pump water heater, an electric stove, electric dryer, and other standard appliances pulls around 120 to 130 amps of electricity with everything on. When you add the required 60-amp EV charger wiring that can easily push you over the 100 amps.

So we strongly recommend you install a separate 200-amp service for most ADUs. There's barely any difference in cost between a 150-amp and 200-amp service, and you get plenty of extra amperage to make sure you never run out of power. You'll never have to worry about tripping breakers when there's too many things running at once.

Here are the typical electricity draws for the average ADU:

Why Is a 60-Amp EV Charger Circuit Required?

The code has recently changed in Massachusetts. The new code requires all new construction, including ADUs, to pre-wire for electric vehicle charging. That means you have to have a 60-amp dedicated circuit installed during construction.

We install a blank plate on the outside of the ADU, where a vehicle would normally park. The wiring and breaker are already in place, so when you decide to add an EV charger later, you just remove the plate and install your charger of choice (Tesla, Ford, or any other brand), and then turn on the breaker. The 60-amp circuit actually supports Level 2 charger, which is the fast charger you're gonna want.

The EV charger requirement is why 100-amp is really never enough. Once you account for that 60-amps, you have only 40 amps left for everything else in the ADU, which will barely run one or two appliances at the same time.

What Solar Future-Proofing Does Code Require?

The new code also requires ADUs to be ready for future solar installation. To do this, we have to run conduit from the crawl space (or basement) up to the roof during construction.

The conduit serves two purposes. First, it provides an easy route for all the solar wiring you'll need without having to drill through anything or put conduit on the outside of your house. Second, it keeps the ADU looking nice and neat on the outside when solar panels are eventually installed.

The conduit size we use is 1.5 to 2 inches, depending on how big of a system you think you might install. We run all the conduit during construction, so it barely adds anything to the cost.

Where Should the Electrical Panel Be Located?

Panel placement is important because when you are working with limited space, every inch counts. Electrical code requires three feet of clearance in front of any electrical panel. So in a smaller ADU, that requirement can take up too much square footage.

The question becomes whether to install it on the inside or the outside.

Interior Placement: The panel is more accessible but it takes up three feet of space that can't be used for furniture or storage. In a small ADU they usually end up at the back of a hallway or utility area. One good thing about putting them on the inside is that it is easier to flip a breaker that has tripped, and with modern AFCI breakers being more sensitive than the old breaker, they might trip more often. If the panel is upstairs or inside somewhere, resetting a tripped breaker only takes a second and you don't have to go outside to do it.

Exterior Placement: Weatherproof panels can be mounted on the outside of your ADU as well. This frees up extra space inside your house and it makes it simple to pass code inspections. Outside panels can easily be painted to match the siding so you barely even notice them.

There's really no standard correct answer, so we discuss everything while still in the design phase and we talk about what will work best for you based on how you will be using the space.

What Exterior Disconnect Does Code Require?

Every ADU requires an outside utility disconnect. The code mandates a 60-amp knife-style disconnect switch that is mounted on the outside of your ADU. They're usually about 12 inches wide by 18 inches tall.

The reason is simple. Utility workers have to be able to cut the power to the ADU without having to go inside. Code requires it, it's not optional, and the disconnect has to be easily accessible and clearly labeled.

We coordinate all this while keeping things looking tidy and meeting all code requirements.

Why Are ERV and HRV Systems Required Instead of Bathroom Fans?

Bathroom exhaust fans alone do not meet current ventilation code for new ADU construction in Massachusetts. The code says you have to have mechanical ventilation systems that provide constant and controlled air flow. Either from an ERV (Energy Recovery Ventilator) or an HRV (Heat Recovery Ventilator).

The difference between these systems matters based on what kind of heating system you have:

ERV (Energy Recovery Ventilator): Retains around 50% of moisture from the air that's going out. It's the better choice if you're using heat pumps without any kind of humidification because it helps keep the humidity at a comfortable level during the winter time.

HRV (Heat Recovery Ventilator): Removes moisture by condensation. It works better if you have a heating system with a humidifier and need the air to be dryer inside.

Both systems recover heat from exhaust air and transfer it to incoming fresh air, which is great for energy efficiency. We install whatever unit you pick in the conditioned crawl space whenever we can. It keeps ductwork short and helps you get better performance from the unit.

Why Does Equipment Placement in Conditioned Space Matter?

The placement of all your electrical and HVAC equipment affects how efficiently it runs, the amount of maintenance required, and the long-term performance. We much prefer installing equipment in conditioned crawl spaces over the attic.

That's because an unconditioned attic in Massachusetts can get up 120 degrees in summer and get way below freezing in the winter. Equipment that has to operate in extreme temperatures has to work harder, but performs worse. Which means it will fail a lot sooner. Besides that, nobody wants to crawl into a 120 degree attic just to change a filter.

A conditioned crawl space stays at mild temperatures all year-round. Equipment runs more efficiently, usually about 20% to 30% better than when installed in an attic. Plus the maintenance is easier and the equipment lasts longer.

We design our crawl spaces with proper insulation and easy access from the beginning. We treat them as an official utility space and not just an afterthought.

How Does the Electrical Planning Process Work?

We follow a very specific process when it comes to planning out the electrical. Our method prevents surprises and makes sure you have the most control.

Step 1: Full Electrical Plan. We put together a complete electrical plan showing where the panel will go, the circuit routing, where outlets will go, lighting, and all the required pre-wiring for future solar and EV charging.

Step 2: Boxing. Before any wires are run, we put in all the boxes. There are outlet boxes, switch boxes, light fixture boxes, and others. The term for this is actually just called boxing.

Step 3: Electrical Walkthrough. We walk through the space with you and an electrician while there's still only the boxes up. This is when you stand in the bedroom and decide exactly where the outlet next to the bed should go, or if you want to add an outlet that wasn't in the original plan. We do it this way because moving a box at this stage takes minutes. If you wait until after all the wires are run, changes get very expensive.

Step 4: Wire Run. Once you approve the layout, then we run all the wiring. At that point, the electrical plan is pretty much set in stone.

This process takes a little longer in the short run, but you don't want to later wish that you had added more outlets in certain places, but you didn't because you were rushed.

ADU Electrical Code Compliance Checklist

Before your ADU will pass inspection, make sure you've met all these requirements:

• Subpanel sized appropriately for load calculation (we recommend 200-amp service)
• Exterior disconnect switch installed and labeled
• Solar conduit run from panel area to roof (1.5 to 2 inches)
• EV charger 240V circuit is all pre-wired (60-amp minimum)
• All necessary GFCIs are installed
• AFCI breakers that are up to code
• Grounding and bonding per NEC requirements
• ERV or HRV system installed and wired
• Smoke and CO detectors all put in
• Emergency lighting where required

Is BuildX Right for Your ADU Electrical Planning?

We install 200-amp service with all the future-proofing of solar readiness and EV chargers in every ADU we build. For most Massachusetts ADUs this gives you all the flexibility you need for now and as needs grow in the future.

But if you plan on building a smaller ADU with lighter electrical needs and you have no plans for EV charging, and no interest in solar, then 100 amps might be sufficient. We can talk about your specific situation and help you decide what's best.

It's important to consider working with a builder who coordinates all the electrical planning from the start. These decisions affect your floor plans, equipment placement, and long-term use of your ADU. It's always much more expensive to retrofit and add things later than it is to just do it upfront.

Ready to Plan Your ADU's Electrical System?

Electrical requirements can get confusing and overwhelming, but they don't have to stall your ADU project. Just make sure you work with a team that understands Massachusetts code requirements and coordinates electrical planning with your specific design.