40 Amp Subpanel: Is It Enough for a Garage or Workshop?

Most people who ask about a 40 amp subpanel are trying to answer one question: will it run everything they plan to plug in out in the garage, shed or shop? For lighting, a handful of outlets, a garage door opener, a small air compressor and one power tool at a time, 40 amps is usually plenty. Once you add a welder, a table saw on a 240-volt circuit, an electric heater, a mini-split and an EV charger, it starts to feel tight fast.

The honest answer depends on what runs at the same time, not the total list of equipment you own. A 40 amp feeder at 240 volts delivers roughly 9,600 watts at its rating, and a sensible planning target for loads that run for hours is about 80 percent of that, or close to 7,700 watts. If your realistic “everything on at once” moment stays under that number, a 40 amp setup works. If it does not, plan for 60 or 100 amps now, because upsizing later usually means pulling a new feeder.

This is a sizing and planning article. Installing a subpanel is licensed electrician work that needs a permit and an inspection in nearly every US jurisdiction, so treat everything here as groundwork for a better conversation with your electrician, not as installation instructions.

How Much Can a 40 Amp Subpanel Actually Power?

Start with the math, then apply common sense. A 40 amp subpanel fed at 240 volts gives you two 120-volt legs, and each leg can carry up to 40 amps. That split matters: you can run several 120-volt circuits on each side, plus a 240-volt tool that draws from both legs together.

Here is what a typical detached garage or hobby shop might draw while working:

• LED shop lighting: 200 to 500 watts for a two-car garage, often less.
• Garage door opener: short bursts of 500 to 900 watts.
• Portable air compressor (120 volt): 1,500 to 1,800 watts while running, with a higher startup surge.
• Table saw or miter saw: 1,800 watts on 120 volts, or 3,000 to 3,700 watts for a 240-volt cabinet saw.
• Dust collector: 1,200 to 2,400 watts depending on size.
• Small 240-volt wall heater: 2,000 to 5,000 watts.
• Refrigerator or chest freezer: 100 to 800 watts, cycling on and off.

A woodworker who runs lights, a saw and a dust collector together sits comfortably under the 40 amp ceiling. Add a 5,000-watt heater running all winter and the margin disappears. The pattern holds for most shops: tools rarely run together for long, but heat, cooling and vehicle charging run for hours and eat capacity.

Where a 40 amp panel gets tight

Three loads tend to push people past 40 amps. A Level 2 EV charger on its own circuit commonly draws 32 to 40 amps continuously, which can consume nearly the whole feeder. A stick or MIG welder on 240 volts can pull 30 to 50 amps under load. And electric heat, whether a baseboard, wall unit or garage heater, draws steady current for hours on cold days. If any one of these is on your list, bring it up with your electrician early.

40 vs 60 vs 100 Amp Subpanels for a Garage or Workshop

Choosing a size is mostly about planning for the next five to ten years. Pulling a larger feeder once costs far less than running a second one later, and the price difference in the panel itself is small. The larger costs come from wire size, trenching distance and any service upgrade at the main panel.

• 40 amp: A good fit for a detached garage with lights, general outlets, a door opener, a freezer and occasional power tools. Also common for a shed or small studio with a window air conditioner.
• 60 amp: A strong middle option for a serious hobby shop with a 240-volt saw, dust collection and a small heater, or a garage that adds a modest EV charger with careful load planning.
• 100 amp: Worth considering for a full workshop with a welder, a compressor, heat or a mini-split, and a Level 2 charger all on the same building, or for a garage apartment or accessory space.

Many homeowners searching for a 50 amp panel box are really describing the same middle ground. A 50 amp feeder is an in-between size that some electricians use when the main panel has limited spare capacity. The panel enclosure itself is often rated higher than the feeder breaker that protects it, so the feeder and breaker size, not the box, sets how much power you actually get.

Feeder and Breaker Basics to Discuss With Your Electrician

You do not need to design the circuit, but understanding a few terms helps you ask good questions and compare quotes fairly.

The feeder

The feeder is the cable or set of conductors that runs from your main panel to the subpanel. Its size is set by the breaker rating, the wire material (copper or aluminum), the distance and the installation method. Long runs to a detached building may need a larger conductor to limit voltage drop, so a 40 amp feeder over 150 feet might be sized up. Your electrician will choose the conductor, conduit and burial depth to match local code.

The feeder breaker

A two-pole breaker in the main panel protects the feeder. Its rating is what people mean by a 40 amp subpanel. The main panel has to have physical space for that breaker and enough calculated capacity to support the new load. On an older 100 amp service with an electric range, dryer and central air, a load calculation may show there is not much room left, which can steer the choice toward a smaller subpanel or a service upgrade.

Grounding and the detached building

Subpanels keep grounds and neutrals separated, and detached buildings usually need their own grounding electrode system. These details are exactly why the work belongs with a licensed electrician: they are invisible once finished, and mistakes create shock and fire hazards that an inspector is trained to catch.

Branch circuits inside the subpanel

Plan the circuits you want before the electrician arrives. A typical 40 amp garage layout might include a lighting circuit, two or three 20 amp outlet circuits, a dedicated circuit for a freezer or compressor and one 240-volt circuit for a larger tool. Garage and outdoor outlets need GFCI protection, and many areas require AFCI protection on some circuits as well. Count spaces generously: a panel with 8 to 12 spaces leaves room to grow even if the feeder stays at 40 amps.

Planning Your Garage Electric Panel: A Step-by-Step Sizing Checklist

This checklist is for planning and conversation only. It does not involve opening, touching or working on any electrical equipment.

1. List every electrical load you plan to use in the space. Include future items such as an EV charger, a heater, a mini-split or a second freezer.
2. Find the wattage or amperage on each nameplate or manual. Motors list running amps; heaters and chargers list watts or amps directly.
3. Mark which loads run together. A saw and dust collector often run at once; a welder and a heater might; a charger may run overnight while nothing else does.
4. Add up your worst realistic moment. Compare that number with about 7,700 watts for a 40 amp feeder, about 11,500 watts for 60 amps and about 19,200 watts for 100 amps.
5. Measure the distance. Pace off the route from the main panel location to the new building and note obstacles such as driveways, patios or trees.
6. Note your main service size. The rating is usually printed on the main breaker handle or the panel label, which you can read without removing any cover.
7. Get two or three quotes. Ask each electrician to include the load calculation, feeder size, trenching, permit and inspection in writing.

If your worst-case number lands within about 10 percent of the 40 amp limit, most electricians will suggest the next size up. That extra headroom is cheap insurance for a garage electric panel you plan to keep for decades.

Safety: What You Should and Should Not Do Yourself

Homeowners are often curious about what is inside an electric panel, and plenty of online videos show open panels. Please do not follow them. A panel stays energized on the incoming side even when the main breaker is off, and the service conductors feeding it cannot be switched off by the homeowner at all. Never remove a panel cover, and never attempt to add, move or wire a subpanel yourself.

Tasks that are reasonable for a homeowner are limited to the load planning in the sizing checklist, resetting a tripped breaker by its handle, and testing GFCI outlets with their test and reset buttons. If you ever replace a light fixture or switch on an existing circuit, turn off the breaker and confirm power is off with a non-contact voltage tester before touching anything. Never test or measure voltage on a live circuit. New circuits, new outlets, feeders and subpanels are all licensed electrician work.

A few habits keep a garage safer once the subpanel is in:

• Plug heaters, compressors and freezers directly into a wall outlet, not an extension cord or power strip.
• Keep at least 3 feet of clear working space in front of the panel so an electrician or firefighter can reach it.
• Label every breaker clearly so anyone can shut off a circuit fast.
• Keep gasoline, solvents and oily rags away from the panel and from any heater.

Troubleshooting Common Subpanel Sizing Problems

Breakers trip when two tools run at once. That is usually a branch circuit problem, not the feeder. Two large tools on one 20 amp outlet circuit will trip it. Ask your electrician about splitting tools onto separate circuits.

The feeder breaker trips on cold days. A heater plus tools is likely exceeding 40 amps. Options include a smaller heater, better insulation, staggering use, or upsizing the feeder.

Lights dim when the compressor or saw starts. Motor startup surges cause brief dips, especially on long feeder runs. Persistent or heavy dimming is worth mentioning to your electrician, who can check voltage drop and connections.

The main panel has no space for a new breaker. Your electrician may recommend rearranging circuits, a larger main panel or a service upgrade. Avoid any workaround that stacks loads beyond what the load calculation allows.

What a Subpanel Typically Costs

Costs vary widely with region, distance and site conditions, so treat these as rough ranges. An attached garage with a short run from the main panel often lands in the low four figures for a 40 amp subpanel, including the panel, breakers, feeder, permit and labor. A detached garage or workshop with a trench of 50 to 150 feet can run from the mid to high four figures, and hard digging through concrete or rocky soil pushes that higher. Stepping up from 40 to 60 or 100 amps adds mostly for larger wire and conduit. A main service upgrade, if the load calculation calls for one, is a separate and larger project.

Permit fees are usually modest compared with the job total. Skipping the permit can cause problems with insurance claims and home sales, so a quote that leaves it out is a red flag.

When to Call a Licensed Professional

Call a licensed electrician for any subpanel, feeder, new circuit or new outlet. The electrician should perform a load calculation on your main service, pull the permit, choose the feeder and grounding method, install GFCI and AFCI protection where required, and schedule the inspection. If you are running a trench, the electrician or excavator should call 811 before digging to locate buried utilities.

Bring your load list, your distance measurement and a note of any future plans, such as an EV charger or heat. With that information, a good electrician can tell you in one visit whether a 40 amp subpanel covers your garage or workshop, or whether 60 or 100 amps is the smarter long-term investment.