How Do You Install an Electrical Subpanel in a Garage?

Adding power to your garage can feel overwhelming, but with a clear plan and the right tools, it’s a project you can handle.

We will cover planning your circuit needs, selecting the correct panel and wire, running conduit safely, making the electrical connections, and passing the final inspection.

I’ve wired several garage subpanels over my years as a residential garage pro, always prioritizing code compliance and safety above all else.

Do You Actually Need a Subpanel in Your Garage?

Let’s answer that first big question. A subpanel isn’t a requirement for every garage, but it’s often the smartest solution for a modern space. Think of your main electrical panel as the city’s main water line. A subpanel is like adding a dedicated, high-capacity branch line just for your garage. It organizes your power and provides room to grow.

You should seriously consider a subpanel if your garage plans include any of these:

• An electric vehicle charger (a Level 2 charger often needs a dedicated 50-amp circuit).
• A serious workshop where you might run a dust collector, air compressor, and a table saw at the same time.
• A home gym with treadmills, heaters, or loud sound systems.
• A detached garage of any significant size.
• You look at your main panel and there are no more empty breaker slots left.

For a detached garage, a subpanel isn’t just a good idea, it’s almost always required by the National Electrical Code. Running multiple individual circuits all the way from your main house panel is inefficient and against code. A subpanel allows you to run one appropriately sized feeder cable to the garage, then distribute power safely from a central point there. I’ve seen too many setups where homeowners tried to daisy-chain outlets from a single circuit, leading to constant tripping and a real fire hazard. Understanding garage electrical code requirements is essential for a safe, compliant setup. In the next steps, we’ll highlight the key code requirements to keep your installation on track.

Adding a new circuit to your main panel seems easier, but it has clear limits. Your main panel has a finite capacity, measured in amps (like 100A, 200A). Every new circuit eats into that total. A subpanel, fed by its own large breaker in the main panel, dedicates a block of power just for the garage. It protects your home’s power from a garage overload and gives you a clean slate to organize your garage circuits. It’s the difference between plugging a power strip into a power strip (dangerous and messy) and having a proper, grounded outlet installed right where you need it.

Safety, Permits, and Playing by the Rules

Installing a subpanel is serious electrical work. This isn’t swapping out a light fixture or installing a new outlet. I treat every panel job with the highest level of respect for the power involved.

Before you touch a single wire, gear up with the right safety equipment. This is non-negotiable in my shop. You need safety glasses to protect your eyes from debris inside the panel. You need a reliable voltage tester. I personally trust and use a Fluke T+ Pro because it gives a clear, unambiguous readout. A non-contact voltage pen is also great for a quick double-check before you work. Every screwdriver and wrench you use should have insulated handles. Treat every wire as if it’s live until you prove to yourself, with your tools, that it is not.

The golden rule is simple: turn off the main breaker to your entire house. This cuts power at the source. Then, you must verify the power is off. I use a digital multimeter set to AC voltage to test between the main lugs in the panel and from each lug to the ground bus. Seeing “0.0 V” on that display is the only signal that it’s safe to proceed. Never skip this step.

You must pull an electrical permit from your local building department for this work. I know it feels like extra hassle and cost, but it’s your protection. The permit ensures your work will be reviewed by a licensed inspector against your local building codes. This inspection protects your life, your property, and your home’s resale value. An unpermitted electrical job can void your homeowner’s insurance if a fire starts and can be a huge red flag for future buyers. It’s worth doing right.

Your local codes will dictate the specifics of your installation. They will tell you what type of conduit (like rigid metal or PVC) you must use to protect the feeder wires, especially for a detached garage. They specify the exact gauge of copper wire (like #2 or #1/0) needed based on the distance and amperage. They even have rules for how high off the floor the panel must be mounted. A quick call to your building department can answer these questions before you buy a single foot of wire. Doing this homework upfront prevents failed inspections and costly rework. If you’re planning to install a garage electrical outlet, map out its location and circuit requirements early. That planning helps ensure the outlet meets code and is wired safely.

Choosing the Right Subpanel: Size and Amperage Explained

Figuring out how many amp service for garage you need is the most critical step. Get this wrong, and you’ll trip breakers the first time you run your compressor and welder together. You don’t just guess. You calculate. This is where garage workshop conversion planning comes in.

Start by listing every electrical item you plan to run. Add up their wattage. Most tools list amps, not watts. The formula is simple: Amps x 120 Volts = Watts. A 15-amp table saw uses 1,800 watts. A 20-amp air compressor uses 2,400 watts.

Here’s a real-world walkthrough for a basic, functional workshop:

• LED Shop Lights (4 banks): 200 watts
• Garage Door Opener: 600 watts
• 15-amp Table Saw: 1,800 watts
• 13-amp Miter Saw: 1,560 watts
• 20-amp Air Compressor: 2,400 watts
• Battery Chargers, Radio, Misc. Outlets: 500 watts
• Future EV Charger (add this now): 9,600 watts (40-amp circuit)

Add those up, and you get roughly 16,660 watts. To find the total amps at 240 volts (which your subpanel uses), divide by 240. That’s about 70 amps. This is why a 60-amp panel often feels tight. For a modern garage with any future plans, I always recommend a minimum 100-amp subpanel. It costs a little more upfront but saves you from a full rewire later.

You also need to think about panel “spaces” versus total “amps.” A 12-space, 100-amp panel is my go-to recommendation. The 100 amps is your power budget. The 12 spaces are the slots for individual breakers for your circuits. This gives you room for lights, many outlets, dedicated tool circuits, and that EV charger without needing a sub-subpanel.

The Gearhead’s Checklist for a Subpanel Install

Do not start this job without the right tools and materials. Using the wrong size cable or a flimsy connector is a fire hazard. This is my shop-tested list.

Tools:

• Cordless Drill/Driver: A Milwaukee M18 Fuel or similar high-torque model. You’ll be driving large screws into framing.
• 3/4″ Spade Bit or Hole Saw: For boring clean holes through studs to pass cable.
• Wire Strippers: Klein Tools auto-adjusting strippers are foolproof for the different cable sizes.
• Torque Screwdriver: This is non-negotiable for the main lugs. Under-tightened connections fail. Over-tightened ones break. A simple model from Klein or Ideal works.
• Fish Tape: For pulling wire through conduit.
• 24″ Level: To mount the panel perfectly plumb.
• Safety Gear: Voltage tester, safety glasses, and heavy-duty leather gloves for handling cable.

Materials:

• Subpanel: I trust Square D QO or Eaton BR series. Their breakers snap in solidly and the construction is robust.
• Feeder Cable: This depends on your amperage and distance. For a 100-amp subpanel under 50 feet, 2-2-2-4 Aluminum SER cable is common and cost-effective. For a 60-amp run, you might use 6-6-6-8 Aluminum SER. Your local code is the final authority here.
• Breakers: One double-pole breaker for your main panel (to feed the subpanel) and assorted single-pole/dual-pole breakers for your new garage circuits.
• Conduit: For any exposed or buried runs. I prefer schedule 80 PVC for underground; it’s tough and doesn’t corrode.
• Connectors & Fittings: Proper cable clamps and conduit connectors to secure everything at the panels.
• Grounding: For a detached garage, you’ll need a separate ground rod kit (copper rod, clamp, and #6 copper wire).

DIY Difficulty & Time Estimate

I rate this job an 8 out of 10 for a homeowner. This is advanced electrical work that requires meticulous attention to code and safety. If you’re not completely comfortable working inside your main service panel, hire a professional.

For a skilled DIYer who has done basic circuit wiring, plan for 8 to 12 hours of active work. This usually spreads over a full weekend. Pulling the heavy feeder cable through conduit or an attic is a major task. You absolutely need a second person for this; trying to manhandle a 50-foot run of stiff cable alone is frustrating and unsafe.

Remember, this project requires a permit and inspection in nearly all areas. Factor in waiting time for the inspector to visit after you’ve run the cable and wired the panel, but before you close up any walls. You cannot energize the panel until it passes inspection.

Phase 1: Mounting the Panel and Running the Feeder

With your materials on hand, the physical work begins. Start by choosing your panel location. Pick a spot on a sturdy, accessible wall. The National Electrical Code requires a 30-inch wide by 36-inch deep clear space in front of the panel—no shelves, water heaters, or parked cars can block it. Garage fire code safety requirements also emphasize keeping the area around electrical panels clear to prevent hazards.

Hold the panel against the wall, use your level to make it perfectly plumb, and mark the mounting holes. You must secure it directly to the wall studs. I use long, 3-inch deck screws for a rock-solid hold. Drywall anchors will not support the weight.

Running the feeder cable is the hardest part. The method depends entirely on your garage type.

For an Attached Garage: The run is often through interior walls, a crawlspace, or the attic. You’ll need to drill through top or bottom plates and carefully snake the cable. Use your fish tape to pull it through tight spaces. Take into account whether you’re working in an attached or a detached garage, since routing and access points differ by garage type.

For a Detached Garage: You must bury the cable. This means digging a trench (typically 18 inches deep) and running electrical wiring through a conduit. You run schedule 80 PVC conduit in the trench, then pull your feeder cable through it. A pro tip for long conduit runs: tie a small plastic bag to a string, insert it at one end, and use a shop vac on the other end to suck it through. Then use that string to pull your pull rope, and finally the cable.

Whether digging or fishing, measure twice and add a few extra feet of cable. Running short is a costly mistake. Get the cable from your main panel to the new subpanel location, leaving about 8 feet of slack coiled neatly at each end for connections.

Phase 2: Making the Critical Connections

At the Main Panel: Tapping Into Power

Now for the moment of truth. With the feeder cable run and your main breaker off, you can open the main panel. Work slowly and deliberately here, taking extra care to keep your tools and hands away from the live bus bars at the top of the panel where the main breaker connects. Your job is to install a new two-pole breaker for your subpanel.

I usually use Square D Homeline or Eaton BR breakers because they’re reliable and easy to find. Here’s the process I follow:

1. Snap the new two-pole breaker into an empty slot at the bottom of the panel. It should click firmly into place on the hot bus bar.
2. Land your feeder wires. The two black (hot) wires go under the screws on the new breaker. I use a square-drive screwdriver, not a Phillips, to get a much tighter connection without stripping the screw head.
3. The white (neutral) wire gets connected to the main neutral bar, which is bonded to the panel box.
4. The bare copper (ground) wire gets connected to the main ground bar.

This is where “making up” the panel matters: dress your wires neatly along the sides, leave a gentle service loop of slack, and ensure no stray wire strands are touching other terminals. A messy panel is a confusing and potentially dangerous panel.

At the Subpanel: The Most Important Rule

This is the step where DIYers most often get it wrong. In your main panel, the ground and neutral bars are connected. In a subpanel, they must be completely separate. Mixing grounds and neutrals in a subpanel can create a parallel path for current, making metal parts electrified and creating a major shock hazard.

Think of it like the brake and fuel lines in your car. They’re both critical systems, but connecting them directly would cause a catastrophic failure.

First, check your subpanel. Many come with a factory-installed bonding screw or strap linking the neutral bar to the box. You must remove it. If your panel only has one bar, you need to add an isolated neutral bar kit. I’ve used kits from Eaton and Siemens; they just bolt right into the predrilled holes in the panel’s plastic interior.

Now, connect your feeder wires from the main panel:

• The two hot (black) wires go to the main lugs, usually labeled “Line 1” and “Line 2.”
• The white (neutral) wire goes to the new, isolated neutral bar.
• The bare copper (ground) wire goes to the ground bar, which is bonded directly to the metal subpanel box.

Keep these wires completely separate on their own bars. This separation is non-negotiable for safety.

Adding Your First Branch Circuits

To make your new subpanel functional, let’s add a simple 20-amp general outlet circuit. This powers your garage lights and basic outlets. Adding this first circuit lets you test everything is working before you close up the walls.

I grab a roll of 12/2 NM-B Romex wire and a single-pole 20-amp breaker for the subpanel. Run the cable from a new knockout on the subpanel to your first outlet box. Leave about 18 inches of wire hanging out of the panel.

Inside the subpanel, strip the cable sheathing, land the black wire on the new breaker, the white wire on the neutral bar, and the bare wire on the ground bar. I always coil several feet of extra wire neatly at the bottom of the subpanel; this gives you slack for future circuits and is a standard pro practice. Now your subpanel is alive and ready to distribute power.

Final Steps: Testing, Troubleshooting, and Knowing Your Limits

Before you even think about turning anything on, run through this final checklist. I tape it to my toolbox:

• Every connection is tight and secure.
• No stray copper strands are touching anything they shouldn’t.
• The bonding screw is REMOVED from the subpanel.
• All breakers in the new subpanel are in the OFF position.

Now, follow this sequence for the “big turn-on”:

1. At the main panel, turn ON the new two-pole breaker feeding the subpanel.
2. At the subpanel, turn ON the main breaker.
3. Turn ON the new 20-amp branch circuit breaker you installed.

Go to your new outlet and plug in a simple plug-in circuit tester. You want to see the correct three-lights pattern indicating a proper connection. For total confidence, I use a multimeter. At the subpanel, you should read about 240 volts between the two main lugs and 120 volts between any hot lug and the neutral bar. My Klein Tools MM600 has never let me down for this.

When NOT to Try This: Call a Professional

Your safety and your home’s safety are paramount. Garages pose unique electrical safety hazards—moisture, overloaded outlets, and risky use of power tools. If any of these red flags apply to your project, put down the tools and call a licensed electrician.

• Your main panel is a Federal Pacific (FPE) or Zinsco panel. These are known fire hazards and should only be worked on by a pro.
• Your main panel has no empty slots for a new two-pole breaker.
• Your home’s main electrical service is less than 100 amps. Adding a subpanel might overload your entire system.
• The wiring path from your main panel to the garage involves drilling through multiple fire blocks, running through finished living spaces, or seems overwhelmingly complex.

The cost to install a subpanel in a garage can range from a few hundred to over a thousand dollars depending on complexity. Paying a professional is an investment in a code-compliant, safe installation and your own peace of mind—especially when DIY projects might come with hidden costs in time and materials. Knowing when a job is beyond your skill set isn’t a failure; it’s the mark of a responsible homeowner.

Garage Subpanel Installation: Your Expert FAQ

How do I verify my main panel has the capacity for a subpanel before I start?

Check the amperage rating on your main breaker (e.g., 200A). Then, add up the ratings of all existing breakers. If that total is close to or exceeds the main rating, your panel is near capacity. For a definitive answer, especially in older homes, consult a licensed electrician for a load calculation. It’s the only way to be sure you’re not overloading your home’s main system.

What’s the single most critical wiring mistake to avoid in the subpanel?

Failing to isolate the neutral and ground bars. In the subpanel, they must be separate, with the bonding screw or strap removed. Connecting them creates a dangerous parallel path for current, which can energize the panel box and cause severe shock risk. Always double-check this before making any feeder connections.

I see you recommend aluminum feeder cable. What’s the maintenance catch?

Aluminum (like SER cable) requires proper termination. You must use anti-oxidant paste on the wire ends and torque the lugs to the manufacturer’s exact specification-a torque screwdriver is non-negotiable. Loose connections cause heat and failure. Inspect these terminals during your annual garage electrical check for any signs of discoloration or warmth. Apply the same care to the wire garage door opener sensors to ensure reliable sensor operation. Secure sensor wiring is vital for the safety features to function correctly.

How should I plan the panel layout for circuits I might add in the future?

Install a panel with more spaces than you currently need-a 12 or 16-space panel is a wise investment. When you run your first branch circuit, coil several extra feet of wire neatly at the bottom of the enclosure. This “service loop” provides slack so you can easily reach new breaker positions later without pulling new cable from the outlet.

After passing inspection and powering up, what’s the first operational check I should do?

Load-test the panel. Don’t just turn the lights on. Simultaneously run your highest-draw tools, like the compressor and table saw, while monitoring the panel. Listen for hums or feel for excessive heat at the main lugs and breaker connections. Any unusual smell, sound, or warmth indicates a problem that needs immediate professional attention.

Securing Your Garage’s Electrical Future

From my years on the job, the one piece of advice I always come back to is this: treat every wire as if it’s live until you’ve personally verified with a trusted tester that the power is off.

• Always use a torque screwdriver, like my go-to Klein Tools 69411, to fasten lugs to the exact inch-pounds listed in the panel’s manual to prevent dangerous loose connections.
• Label every single circuit breaker clearly and permanently right after installation; I use a Brady BMP21 label maker for this because smudged tape leads to confusion.
• Before closing up, do a final visual sweep for any stray wire clippings or tools left in the panel-it’s a simple habit that prevents shorts and fires.