An electrical subpanel installation adds a secondary breaker panel fed from your main service panel through a dedicated breaker and feeder wires, giving you more circuit slots or putting distribution closer to a distant load like a detached garage. A professional residential job typically runs between $400 and $1,750, with the final number driven by panel size, feeder length, and whether the subpanel serves a separate structure. The work almost always requires a permit, an inspection, and strict compliance with the National Electrical Code on sizing, grounding, and clearance.
When You Actually Need One
Two situations drive most residential subpanel projects. The first is running out of breaker slots in the main panel. Older homes were wired for far fewer circuits than modern kitchens, home offices, and finished basements demand, and once every slot is full you can’t add circuits without adding capacity. A subpanel gives you a fresh set of slots fed by a single breaker in the main panel, which is cheaper than upgrading the entire service.
The second is voltage drop over long runs. Electricity loses voltage as it travels through wire, so a workshop 150 feet from the house will see noticeably lower voltage at the end of the run than at the main panel. Motors and compressors overheat or underperform when they’re starved for voltage. Installing a subpanel in or near the detached building moves the distribution point closer to the load and keeps voltage within acceptable limits.
Checking Whether Your Main Service Can Handle It
Before you buy anything, confirm the main panel has spare capacity to feed a subpanel. This is where many projects stall, and it’s the first thing the building department will want on the permit application.
The NEC provides an optional load-calculation method for dwelling units. In broad terms, you multiply square footage by 3 volt-amperes for general lighting and receptacles, add 1,500 volt-amperes for each required small-appliance and laundry circuit, add the nameplate ratings of major appliances, then apply demand factors: the first 10,000 volt-amperes at 100 percent and everything above that at 40 percent. Heating and cooling loads are compared, the larger one wins (cooling at 100 percent of nameplate, heating at 65 percent), and the final total divided by 240 gives the amperage your service needs to carry.
If that calculated load plus the new subpanel demand exceeds your main service rating (typically 100 or 200 amps), a service upgrade comes first. An electrician or your local building department can run the numbers if the math is a stretch.
Sizing the Panel and Feeder Wire
Residential subpanels are most commonly rated at 60 amps or 100 amps. A 60-amp panel handles a modest addition with general lighting, receptacles, and one or two dedicated appliance circuits. A 100-amp panel suits a detached workshop, a large addition, or any space with heavy equipment such as welders, EV chargers, or electric heat.
The feeder breaker in the main panel has to match the subpanel’s amperage rating, and the wire between the two panels must safely carry that current. At standard 75°C copper ampacity, a 60-amp feeder needs at least 6 AWG copper (rated for 65 amps), and a 100-amp feeder needs at least 3 AWG copper (rated for 100 amps). Aluminum is acceptable in larger gauges. Long runs may require upsizing to control voltage drop even when the smaller gauge meets ampacity on paper.
Feeder conductors must also handle continuous loads at 125 percent of their rated value. If any circuits on the subpanel will run for three hours or more at a stretch, the feeder has to accommodate that extra demand.
Why Neutrals and Grounds Must Be Separated
This is the single most common mistake in subpanel installations, and it’s dangerous enough to call out on its own. In the main panel, the neutral bus and the grounding bus are bonded together, because the main panel is the one point where the grounding system connects to the neutral conductor. In a subpanel, that bond must not exist.
If neutral and ground are bonded in a subpanel, normal return current flowing on the neutral will also travel through the equipment grounding conductor, the metal enclosure, conduit, and any other grounded metal in the system. That energizes parts that should never carry current and creates a shock hazard. It can also keep ground-fault breakers from detecting faults, because the return current splits across two paths.
In practice, this means installing a separate ground bar in the subpanel (most panels include one or offer a kit). The neutral bus must float, connected only to neutral conductors and never to the enclosure. If the subpanel ships with a green bonding screw tying the neutral bar to the box, that screw has to come out. Every subpanel requires a four-wire feeder: two hots, one neutral, and one equipment grounding conductor. Three-wire feeders have not been code-compliant since the 2008 NEC.
Clearance and Location Requirements
Where the subpanel can go is governed by NEC 110.26, which sets minimum working space for safe access.1Mike Holt Enterprises. Working Clearances, Based on the 2020 NEC
- At least 30 inches of width, or the width of the equipment if wider.
- At least 36 inches of clear depth in front of the panel, measured from the enclosure face.
- Headroom from the floor to at least 6½ feet, or to the top of the equipment if taller.
That space can’t double as storage. Shelves, water heaters, furnaces, and stacked boxes all violate the requirement even if they could be moved. The clearance has to stay clear at all times.
The center grip of the highest breaker handle can’t sit higher than 6 feet 7 inches above the floor, which in practice means mounting the panel low enough to reach every breaker without a step stool. Overcurrent devices can’t be installed in residential bathrooms, and because the code prohibits placing them where someone would have to stand on stairs to operate them, panels effectively can’t go over stairways. Garages, utility rooms, and basements are the usual locations, as long as the clearance dimensions check out.
Extra Rules for Detached Buildings
Running a subpanel to a detached garage, workshop, or guest house adds requirements beyond what applies inside the main dwelling, and these are the rules that catch people off guard.
A Disconnecting Means at the Building
Each detached building fed by a feeder needs its own way to shut off all power at the building itself. That can be a main breaker in the subpanel (many panels are sold with one integrated), a separate disconnect switch nearby, or up to six grouped switches or breakers that together kill all circuits. The disconnect has to sit at or near the point where the feeder enters the building.
Its Own Grounding Electrode System
A detached building served by a feeder generally needs its own grounding electrode system, usually one or two ground rods driven into the earth at the building and connected to the subpanel’s ground bar with a properly sized grounding electrode conductor. That’s in addition to the equipment grounding conductor running back to the main panel through the feeder. Conductor size is set by NEC Table 250.66, based on the largest feeder conductor.
Neutral-ground separation matters even more here. Bonding them at both panels creates parallel paths for neutral current through the earth itself, which is both a shock hazard and a code violation.
Four-Wire Feeder
The feeder to a detached structure must carry four conductors: two hots, one neutral, one equipment grounding conductor. Older installations sometimes used three wires with the neutral doing double duty as the ground path. That practice came out of the NEC in 2008, so bringing an older three-wire feeder up to current code means pulling new wire.
Required Circuit Protection
A subpanel feeding dwelling-unit circuits has to meet the same circuit-protection rules as the main panel, and those rules have expanded in recent code cycles.
All 120-volt, single-phase, 15- and 20-amp branch circuits supplying outlets in dwelling units require arc-fault circuit interrupter protection. Under current NEC, that covers nearly every room in the house: bedrooms, kitchens, living rooms, dining rooms, family rooms, hallways, closets, laundry areas, recreation rooms, libraries, dens, sunrooms, and similar spaces. AFCI breakers catch the kind of dangerous arcing that starts fires inside walls.
Ground-fault protection is required for receptacles anywhere water and electricity are likely to meet: bathrooms, kitchens within six feet of a sink, garages, outdoors, unfinished basements, crawl spaces, laundry areas, and near pools or hot tubs. GFCI protection can come from a breaker in the subpanel or from GFCI receptacles at individual locations.
Since the 2020 NEC, every dwelling-unit service must include a Type 1 or Type 2 surge-protective device. The device normally lives at the main service panel, but the code lets it sit at the next downstream level of distribution equipment instead, so the subpanel can house it if the main panel doesn’t already.2Electrical License Renewal. 230.67 Surge Protection When service equipment is replaced, surge protection becomes mandatory regardless of what was there before.
Permits and Inspections
A subpanel installation requires an electrical permit in virtually every jurisdiction. The application typically asks for a load calculation showing the service has capacity for the new circuits, a list of the circuits the subpanel will serve, and a basic installation diagram. Permit fees usually run $50 to $300 for a residential project.
Skipping the permit carries real risk. Unpermitted electrical work can trigger fines, and some jurisdictions set penalties at several times the original permit cost. The bigger exposure is insurance: if a fire starts in an unpermitted subpanel, your homeowner’s insurer may deny the claim outright.
After the wiring is finished but before the circuits are energized or the walls close up, an inspector verifies the work. Inspections cover wire terminations, lug torque, conduit sizing, grounding and bonding, and breaker ratings. A passed inspection produces a signed card or sticker that documents compliance, which matters at resale and at claim time.
What It Costs
Total cost for a professional residential subpanel installation generally lands between $400 and $1,750. A straightforward 60-amp subpanel mounted on the back side of the wall from the main panel sits at the low end. A 100-amp subpanel in a detached garage 100 feet away, with trenching, conduit, ground rods, and a disconnect, pushes the top of the range or past it.
The main cost components break down roughly as follows:
- Subpanel unit: a 100-amp main-lug panel starts around $100 to $250 at retail, with integrated main-breaker panels (needed for detached structures) costing more.
- Feeder wire: priced per foot and varying with gauge. A 100-foot run of 3 AWG copper for a 100-amp feeder can run $300 to $500 in materials alone.
- Electrician labor: typically $75 to $100 per hour, with a full range of $50 to $140 depending on area and experience.
- Permit and inspection: budget $50 to $300 for the permit, plus any separate inspection fees.
Trenching for an underground feeder to a detached building adds substantial cost for both the digging and the required conduit and burial depth. And if a service upgrade is needed to support the subpanel in the first place, that’s its own project at roughly $1,500 to $4,000.