Adding a Subpanel to a Garage or Workshop

A garage or workshop is the most common place a subpanel earns its keep. The circuits are clustered in one area, they're often heavy, and running one feeder out beats running six branch circuits back to a main panel across a finished house.

The planning question is almost always the same: people size for what's in the garage today and regret it within a few years.

Sizing a garage or shop subpanel

Most home garages are well served by 60 A. The upgrade to 100 A is worth it when a welder, a dust collector and a car lift may run together — and it costs far less to install now than to redo later, because the conduit and trenching are the expensive parts, not the conductors.

What size subpanel does a garage need?

Garage loads grow predictably. The list of things that show up later:

• An EV charger — the single most common addition, and a substantial continuous load
• Electric heat — a unit heater or mini split, once you've spent a winter out there
• A compressor, a dust collector, a welder, a table saw
• A freezer or second refrigerator
• A hot tub or pool equipment, if it ends up nearby
• A well pump or sump pump, in some layouts

The incremental cost of a larger feeder at install time is small. The cost of redoing it is not. If the service can support it, take the bigger feeder.

Which brings up the constraint: run a whole-house load calculation first, because a subpanel adds circuit positions but not service capacity. Use the home electrical load calculator and see how to size a subpanel.

What drives the load

Motor loads — compressors, dust collectors, table saws — draw a large starting current well above their running current. The NEC handles this with specific motor rules, including factoring the largest motor. Nameplate addition doesn't capture it correctly.

Welders have their own calculation method based on duty cycle. A welder is not sized like a resistive load.

EV charging is a continuous load, sized at 125% of the charger's rated current. A charger rated for continuous output at a given amperage needs a circuit sized above that. Work it out with the Level 2 charger breaker size calculator, and see EV charger breaker sizing and how many amps does an EV charger need.

Electric heat is also continuous.

Everything else — lighting, general receptacles, the door opener — is minor by comparison.

Does a detached garage subpanel differ from an attached one?

Attached garage: just another area of the same building. Standard subpanel rules apply — four-wire feeder, isolated neutral bar, separate ground bar. See subpanel grounding and neutral separation.

Detached garage or shop: a separate structure, which brings additional requirements:

• A disconnecting means at the separate building
• A grounding electrode system at that building
• Voltage drop on the feeder run, which often drives conductor size more than ampacity does
• The feeder itself has to get there — trenched underground, or overhead, each with its own burial depth and clearance requirements

Full treatment in feeding a detached building.

What are the code requirements for garage circuits?

Garages have their own rules, and they've tightened over recent code cycles. Your jurisdiction's adopted NEC edition and local amendments govern — confirm the current requirements locally rather than working from an older rule.

GFCI protection. Receptacles in garages require GFCI protection. The scope of required GFCI protection has expanded across recent code cycles — covering more locations, and in recent editions extending to certain hard-wired equipment and larger circuits that previously weren't included. Check what your adopted edition requires. See GFCI outlet requirements by room and GFCI breaker vs GFCI outlet.

Required receptacles. The code requires at least one receptacle outlet in an attached garage and in each detached garage with electric power, and the requirements around how garage receptacle outlets are supplied have changed across editions.

Vehicle bay receptacles have placement requirements in recent editions.

Damp and wet locations. Receptacles in a damp or wet location need weather-resistant devices and appropriate covers. A garage that gets washed down or takes in weather is not a dry location.

Lighting. Not a code issue so much as a practical one — a shop needs far more light than a builder-grade garage has. Put it on its own circuit so a tripped tool circuit doesn't leave you in the dark, and switch it at the door.

Practical layout

• More receptacles than you think. Spacing them generously around the perimeter costs little at rough-in and saves extension cords later — and extension cords are the thing that causes problems. See extension cord safety.
• Dedicated circuits for the heavy tools. A compressor and a dust collector that run simultaneously on one circuit will trip it. See dedicated circuit requirements.
• 240V where you need it. Larger tools, a welder, a unit heater, an EV charger. Decide the outlet type early — see NEMA 14-50 vs 6-50 outlet and how to install a 240V outlet.
• A ceiling receptacle for the door opener, and one for a retractable cord reel if you use one.
• Panel location. It needs full working clearance — see electrical panel working clearance requirements. Don't put it where the car door swings or where shelving will end up in front of it.
• Label it properly at install time, while you still know what everything is. See how to label an electrical panel.

Do I need a subpanel for an EV charger?

If EV charging is the reason for the subpanel, a few things worth deciding up front:

• Hardwired or receptacle? See NEMA 14-50 vs hardwired.
• Charger amperage determines the circuit. Bigger isn't automatically better — see choosing a Level 2 EV charger.
• Two EVs eventually? Plan the capacity now, or plan for load management. See charging two EVs at home and EV charger load management.
• If the service is tight, load management may let you charge without a service upgrade — see EV charger without a panel upgrade and panel upgrade for EV charger.

Cost planning: home charger install cost calculator and EV charger installation cost.

Permits and who does the work

Permitted and inspected work in essentially every jurisdiction, and it involves landing a feeder breaker in an energized main panel. Detached structures add trenching or overhead work with their own inspection points.

This is licensed-electrician territory. See electrical work homeowners can legally do and panel upgrade permits and inspection.

The bottom line

Size the feeder for the garage you'll have in ten years, not the one you have now — EV charging and electric heat are the two additions that catch people out, and both are continuous loads sized at 125%. Check the whole-house calculation first, since a subpanel adds slots rather than amps. Expect GFCI protection on garage receptacles and confirm the current scope under your adopted code edition, put the heavy tools on dedicated circuits, and keep the panel location clear of where the car and the shelving will go.

Start with the home electrical load calculator.

Code reference

The requirements behind this guide, for looking up in your adopted edition:

• NEC 210.8(A) — GFCI protection for garage receptacles
• NEC 250.32 — where the subpanel is in a detached structure

Code editions and local amendments vary by jurisdiction. Confirm the adopted edition with your AHJ before relying on any specific article.