Sub-Panel for Detached Garage or ADU in Massachusetts

A sub-panel for a detached garage or ADU in Massachusetts runs roughly $1,500 to $4,500 installed for a 60-amp or 100-amp panel with a buried feeder under 100 feet, and $5,000 to $9,000-plus when the run is long, ledge gets in the way, or your main panel has to be upgraded first to feed it. Those are the soft ranges Massachusetts electricians quote, not a price list, and the spread is almost entirely about the trench, the feeder, and what your existing service can spare. The thing the cost calculators leave out, and the thing every Massachusetts Inspector of Wires cares about, is the rule under Massachusetts Electrical Code 527 CMR 12.00: a sub-panel in a separate building needs a 4-wire feeder, neutral isolated from ground at the sub-panel, and its own grounding electrode system at the detached structure (NEC 250.32). And you only get one feeder to that building (NEC 225.30), which is why "I'll just run another cable later for the EV charger" is the most expensive mistake on this page.

Most people land here for one of two reasons: they're wiring a detached garage for a workshop, mini-split, or EV charger, or they're building an ADU now that Massachusetts allows them by-right statewide. Both jobs share the same hardware (a sub-panel, a feeder, a trench) and the same code rules, but the load math is very different. This guide gives you the honest installed cost, the feeder and amperage decision, the Massachusetts-specific code wrinkles, and the financing reality. For the bigger picture on services and main panels, start at the electrical hub, and for the main-house 100A-to-200A heavy-up question, see electrical panel upgrade cost in Massachusetts.

How much does a sub-panel in a detached garage or ADU cost in Massachusetts?

The sub-panel itself is cheap, $150 to $400 for the box and breakers. The price you pay is the feeder, the trench, and whatever your main panel can or can't accept. Here's the spread that comes up in real MA quotes, by job scope.

Job scope	Typical installed range (MA)	What it includes
60A sub-panel in attached garage or basement	$800 – $1,800	Panel + breakers + short interior feeder; no trench, no Inspector-of-Wires for the trench
60A or 100A sub-panel in detached garage, short run	$1,500 – $3,500	Panel, 4-wire feeder, trench under ~50 ft, grounding electrode at the garage, permit
100A sub-panel in detached garage, long run or ledge	$3,500 – $6,000	Same as above, plus 75–150 ft trench, possible conduit + rock cutting
100A or 125A sub-panel for an ADU, sharing the main service	$4,000 – $7,500	ADU sub-panel + dedicated feeder + a separate disconnect; may require a main-panel load redistribution
Separate 200A service to the ADU with its own meter	$8,000 – $15,000+	New service entrance, new meter, utility coordination; effectively a full new service, not a sub-panel job

Two cost realities that move these numbers more than anything else. First, the trench. Boston-metro lots with old utility runs, ledge under the lawn, or paved driveways between house and garage push the trench cost up fast, sometimes more than the electrical itself. Central and western MA on softer soil is the easier end. Second, your existing main panel. If your house has a 100A panel that's already near full on a load calculation, a 100A sub-panel cannot be fed from it. The conversation pivots from "let's wire the garage" to "let's upgrade the house first," which is a different project.

60 amps vs. 100 amps, which sub-panel do you actually need?

Short answer: 60 amps is plenty for a workshop or a hobby garage with lighting, outlets, a small heater, and one EV charger you don't run at the same time as a welder. 100 amps is the right pick when the garage is also part workshop, part EV-charging, part future mini-split, or when it's an ADU. Sizing up from 60A to 100A at the install adds a few hundred dollars in wire and panel cost. Sizing up later means a whole new feeder, which is the expensive way.

Honest demand for a Massachusetts detached garage:

• LED lighting + receptacles + occasional power tools: about 15–25A
• Cold-climate ductless mini-split (one head): about 15–20A
• 240V workshop tools (table saw, welder, dust collector, not simultaneously): 20–30A
• Level 2 EV charger at 32A continuous: 40A breaker (see EV charger installation cost in Massachusetts for the charger side of the same project)
• Electric vehicle plus a heated/insulated workshop running at the same time: this is where 100A starts to matter

For an ADU, the math changes. A fully electric ADU in Massachusetts (heat pump, heat pump water heater, induction range, electric dryer, lighting, receptacles, and the EV pre-wire that's now common in MA new construction) easily lands at 120 to 130 amps of connected load with everything on, even though demand-factored load on a real calculation comes in much lower. That's why a 100A sub-panel for an all-electric ADU is workable, but only if the main house's service can carry both. A 200A house panel with a gas-heated main dwelling can often feed a 100A ADU sub-panel comfortably. A 100A or 150A panel with electric heat in the main house usually can't, and the right move is either a heavy-up on the main service or a separate 200A service for the ADU. The heat-pump piece of that load calculation is covered in does a heat pump need a panel upgrade in Massachusetts.

The feeder, NEC 250.32, and the rule almost everyone gets wrong

A sub-panel in a separate building gets a 4-wire feeder: two ungrounded (hot) conductors, one grounded (neutral) conductor, and one equipment grounding conductor. At the sub-panel, the neutral bar and the ground bar must be isolated from each other (you remove or never install the main bonding jumper in the sub-panel). The detached building also needs its own grounding electrode system, typically two driven ground rods spaced at least 6 feet apart, bonded together. That's the rule under the Massachusetts Electrical Code, which adopts the NEC (NEC 250.32). Massachusetts is operating on the 2023 NEC now, with amendments based on the 2026 NEC effective April 24, 2026.

Most national cost articles either skip this or get it half wrong. The two failure modes:

• Treating the sub-panel like a main panel and bonding neutral-to-ground at the detached building. This creates parallel return paths on the ground wire and is a guaranteed inspection failure.
• Driving ground rods at the garage but not installing them, "no neutral-ground bond at the sub-panel" and "grounding electrode at the separate building" are the SAME requirement, not alternatives.

Wire sizing tracks the breaker. A 60-amp sub-panel takes #6 AWG copper or #4 AWG aluminum. A 100-amp sub-panel takes #4 AWG copper or #2 AWG aluminum. Long runs add another layer: at distances over about 100 feet you start losing meaningful voltage, and the electrician will upsize the conductors to keep voltage drop under roughly 3% on the feeder. Aluminum is cheaper per foot and is fine for direct-burial feeders, copper is smaller diameter and easier in tight conduit. Either is code-compliant when sized correctly.

Only one feeder per building (NEC 225.30), and why "I'll add an EV charger later" is a code problem

Here's the rule that catches Massachusetts homeowners off guard. Under NEC 225.30, a building or structure on the load side of the main service can only be supplied by one feeder, with narrow exceptions (fire pumps, EV power-transfer systems in some configurations, multi-occupancy buildings). You cannot run a second cable from the main house to the same garage later just because you've added a Level 2 EV charger and the original 60A sub-panel is tight.

What that means in practice. If there's any chance you'll add an EV charger, a mini-split, or a workshop circuit in the next decade, size the sub-panel and the feeder for the future load now, not the current load. Going from a 60A sub-panel + 60A feeder to a 100A sub-panel + 100A feeder is a few hundred dollars at install. Going back later to upsize the feeder is a re-trench, a new wire pull, and a permit, easily $2,000 to $4,000 of wasted work. Massachusetts inspectors enforce 225.30; "I'll just run another wire" is what fails. The same rule is why an ADU usually wants a single 100A or 125A feeder sized for the full load up front, not a smaller feeder you intend to supplement.

Trench depth, conduit, and Massachusetts ground

Burial depth depends on what's in the trench. Per NEC 300.5, the residential minimums are 24 inches for direct-burial UF cable, 18 inches for conductors run in PVC conduit, and 12 inches for a GFCI-protected 120V circuit at 20 amps or less (the last one rarely applies on a feeder). Massachusetts inspectors hold to those minimums. The trench gets inspected by the Inspector of Wires before you back-fill, so do not cover it until the sign-off.

Two MA-specific realities. First, ledge. From the North Shore down through the South Shore and out into the Berkshires, hitting rock 12 inches down is normal. Cutting or saw-cutting through ledge can add $20–$60 per foot on a trench, and the electrician's estimate may have assumed clear digging. Ask for the ledge contingency in writing. Second, frost. Even though the feeder doesn't care about frost depth (it's not a water line), you do care about scheduling: trenching from November through March in Massachusetts is expensive and slow because the top foot of soil is frozen. Plan the work for April through October when you can.

For the wire itself, direct-burial UF cable at 24 inches is the cheap option. PVC conduit at 18 inches is the easier-to-pull, easier-to-upgrade option (you can pull a new wire later through the existing conduit), and it's the standard on most pro installs in MA. Use sand or pea-gravel bedding under and over the cable or conduit in any trench that hit rocks or roots, and a buried-warning tape 12 inches above the line so the next person who digs the lawn knows it's there.

ADUs under the Affordable Homes Act, separate meter or shared service?

Since February 2, 2025, Massachusetts allows accessory dwelling units (ADUs) by-right on single-family lots statewide, under the Affordable Homes Act and the implementing regulation 760 CMR 71.00. An ADU is capped at 900 square feet or 50% of the principal dwelling, whichever is smaller, no owner-occupancy mandate, and parking requirements are limited within a half-mile of transit. The zoning piece is now standard. The electrical piece is where homeowners get stuck.

The 760 CMR 71.00 rule says a municipality may NOT require a separate utility connection for the ADU; the utility itself or state law still can. In practice that means:

• If the existing main service has the headroom on a real load calculation, you can feed the ADU off a 100A or 125A sub-panel from the main house. One meter, one electric bill, no separate utility account. Simpler and cheaper.
• If you want to bill the ADU separately (a rental tenant), or the main service can't carry the full ADU load, you install a second meter and a second service. That's a full new 200A service to the ADU, not a sub-panel, and it lands in the $8,000–$15,000+ tier with utility coordination by Eversource, National Grid, or Unitil.

A useful rule of thumb. If the ADU is going to be all-electric (heat pump, heat pump water heater, induction range, electric dryer), and you also want to keep a healthy main panel for the primary house, plan on the ADU getting its own 200A service from the utility. If the ADU is small, mixed gas + electric, or being built into an existing structure (an over-garage conversion, a basement unit), a 100A sub-panel from the main house often works. Have the electrician run a load calculation under NEC Article 220 on the existing main service before you commit to either path.

Mass Save HEAT Loan, federal credits, and the math on financing the feeder

Massachusetts does not have a direct rebate for installing a sub-panel. What it does have is the Mass Save HEAT Loan: 0% interest financing, up to $25,000, with terms up to 7 years, through the Mass Save sponsor utilities (Eversource, National Grid, Unitil, Cape Light Compact, Berkshire Gas, Liberty Utilities). The HEAT Loan covers heat-pump installs, weatherization, ENERGY STAR equipment, and, importantly for this guide, the panel and feeder work needed to support a heat pump. If you're putting a heat pump in the ADU or in the detached structure, the sub-panel and feeder cost is HEAT-Loan-eligible. That's the most under-used financing lever for this kind of project.

A few federal credits to know about (or not lean on):

• The 25C Energy Efficient Home Improvement Credit, which used to cover 200-amp-and-up panelboards at $600, ended for property placed in service after December 31, 2025. A 2026 sub-panel install does not qualify. If a contractor's quote relies on it, the math is stale.
• The 30C EV-charger credit, 30% of the equipment, capped at $1,000 for residential, runs only through June 30, 2026 and only in eligible census tracts. It covers the charger, not the sub-panel or the feeder. Worth checking before you commit to a 2026 install date.
• Mass Save offers no separate sub-panel rebate, and there is no Massachusetts state credit for the panel itself. The financing leverage is the HEAT Loan, not a rebate.

MLP-town caveat. If your electricity comes from a municipal light plant (about 40 Massachusetts towns), you are not on Mass Save and the HEAT Loan does not apply. Your light department may have its own electrification incentives, but they vary widely, so ask them directly.

Permit, Inspector of Wires, and what a fair sub-panel quote looks like

A sub-panel in a separate building requires a licensed electrician, an electrical permit, and an inspection by your town's Inspector of Wires before the system is energized. The trench gets its own inspection before back-fill. This is non-negotiable in Massachusetts and is the part the cheap quote often glosses over.

A fair quote names the parts that drive the price:

• Sub-panel amperage and brand, and feeder conductor size and material (copper vs. aluminum). If the quote just says "sub-panel," ask.
• Feeder length and burial method (direct-burial UF or conductors in PVC conduit), and what the trench depth and bedding will be.
• Whether the bid includes the ledge or rock contingency. In MA, this is what blows up otherwise-tight quotes.
• Permit and Inspector-of-Wires inspections included, both the energization inspection and the trench inspection.
• Grounding electrode at the detached building (two ground rods, bonded), with neutral-to-ground isolation at the sub-panel explicitly called out.
• Load calculation on the main service if the new sub-panel is being fed from an existing 100A or 150A panel. Skipping this is the most common error.

The contractor who explains the 4-wire feeder and the 225.30 one-feeder rule without you asking is the one to hire. The contractor who promises a 60A garage sub-panel for $700 with no trench detail and no permit is selling you a future failed inspection.

FAQ

How much does it cost to install a sub-panel in a detached garage in Massachusetts? Roughly $1,500 to $3,500 installed for a 60A or 100A sub-panel with a buried feeder under 50 feet, including the trench, the grounding electrode at the garage, and the permit. Longer runs, ledge, or main-panel upgrades push that to $4,000–$6,000. ADU sub-panels run $4,000–$7,500 when the main service can feed them, and $8,000–$15,000+ when the ADU needs its own meter and a full new service from the utility.

60 amps or 100 amps, which sub-panel size do I need for a detached garage? 60 amps handles a workshop with lighting, receptacles, a mini-split, and one Level 2 EV charger that is not running at the same time as 240V power tools. 100 amps is the right call for a garage that is also part workshop and part EV-charging hub, for an ADU, or whenever there is any realistic chance of adding load later. Sizing up at install costs a few hundred dollars; sizing up later means a new trench and a new feeder under NEC 225.30, which is the expensive way.

Do I need a separate ground rod at the detached garage, and can I run a second wire later for an EV charger? Yes on the grounding, and no on the second wire. Per NEC 250.32 (as adopted in Massachusetts Electrical Code 527 CMR 12.00), a sub-panel in a separate building requires a grounding electrode system at that building, typically two driven ground rods spaced at least 6 feet apart and bonded together, and the neutral bar at the sub-panel must be isolated from the ground bar. Treating it like a main panel by bonding neutral to ground at the detached building is a code violation. And under NEC 225.30, you only get one feeder to a separate building (with narrow exceptions). That is why the practical move is to size the sub-panel and feeder for your future load now, not to plan on running a second cable later.

Does an ADU in Massachusetts need its own electric meter? Not by law. Since February 2, 2025, under the by-right ADU rules in 760 CMR 71.00, a municipality cannot require a separate utility connection for the ADU. A 100A or 125A sub-panel off the main house service is fine if the main service can carry the load. A separate 200A service with its own meter is the cleaner answer when you want to bill a tenant separately or when the main service does not have headroom for an all-electric ADU.

Is a sub-panel covered by the Mass Save HEAT Loan? Yes, when the sub-panel and feeder are installed in connection with a heat-pump electrification project. The Mass Save HEAT Loan is 0% interest, up to $25,000, with terms up to 7 years, and it covers the panel and feeder work needed to support a heat pump. It is not available in municipal light plant towns. There is no direct sub-panel rebate in Massachusetts, and the federal 25C credit that covered 200A panelboards ended for property placed in service after December 31, 2025.

Ready to wire the garage or feed the ADU? Get vetted Massachusetts electricians who handle sub-panel and feeder work, including the trench, the permit, and the Inspector of Wires sequencing. Start a quote at /get-estimate.