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how to install sub panel in detached garage

How to Install a Subpanel in Your Detached Garage: Step-by-Step Guide

Are extension cords running everywhere because the existing circuits aren't enough? Installing a subpanel is the best way to add more power safely. 

A subpanel gives your garage the dedicated feed it needs for vehicle charging, heavy-duty tools, shop equipment, and serious DIY projects. This guide will walk you through the subpanel installation process step-by-step. With the extra circuits a subpanel provides, you'll gain the electrical capacity your garage demands. Let's get started!

installing a sub panel

Key Takeaways

  • Choose the right size sub panel based on your garage's electrical needs - too small and you'll max out capacity; too big is costly. A 100 amp, 24-space subpanel is a good middle ground.

  • Use thick, quality copper feeder wires between the main panel and subpanel. Undersized wires cause problems. Follow the ampacity tables to select the proper wire size.

  • Properly ground the subpanel with ground rods and thick wire. Good grounding is critical for safety and preventing electrical hazards. Don't skip this step.

  • Carefully plan circuit layouts and labeling. An organized subpanel makes future troubleshooting much simpler. Take your time on the wiring.

An Overview of Subpanels for Detached Garages

Before we dig into the nitty-gritty of the installation process, let’s go over some subpanel basics. This background information will help you choose the right subpanel and wiring for your detached garage.

What is a Subpanel?

A subpanel is essentially a supplementary breaker box that allows you to distribute power from your main electrical panel to another structure. It works just like your main home panel, allowing multiple 220V and 110V circuits to be wired for lights, outlets, and appliances in the detached building.

Why Do You Need a Subpanel for a Detached Garage?

There are a few reasons you may need to install a subpanel in your detached garage:

  • Limited capacity on your main panel - If your main breaker panel is already full, a subpanel allows you to add more circuits without replacing the main panel.

  • Long distance from the main panel - Running very long wire lengths from your main panel can cause voltage drop issues. Localized circuits in a subpanel help maintain voltage.

  • Adding high-power devices - Welding, EV charging stations, and workshops with large tools often require 220V circuits that may exceed the capacity of your existing home electrical system.

  • Separation of circuits - A detached garage subpanel keeps those circuits separate from the home's electrical load. This also makes it easier to troubleshoot any issues.

  • Following building codes - In many cases, building codes require a separate subpanel in detached structures rather than tapping directly off the main panel.

subpanel vs main panel

Subpanel vs. New Main Panel

Depending on your needs, you may be able to get by with a subpanel or you may need to install an entirely new main panel in your garage.

A subpanel is the easiest and most affordable option for basic electricity needs like lighting, outlets, and smaller tools or appliances. If you only need to add a handful of new circuits, a subpanel is ideal.

However, if your detached garage requires a large number of 220V circuits for major tools and machinery, or you need more than 60-70 amps of total power, you may be better off installing a new main panel instead of just a sub-panel.

The decision depends on your specific electrical requirements. For help deciding if your detached garage would be better served by a subpanel or a completely new main panel, see our detached garage buyers guide.

Choosing the Right Size Subpanel

One of the first decisions you’ll need to make is choosing the amperage size for your detached garage sub panel. This determines how much total power the subpanel can provide for your garage circuits.

Here are some tips for selecting the proper sub-panel size:

  • For a small detached garage with just lighting and outlets, a 60-100 amp subpanel is usually sufficient. This covers basic electricity needs for storage or hobby garage uses.

  • For a workshop or garage with power tools, air compressors, EV chargers, and other medium-duty electrical loads, consider a 100-150 amp sub panel. This gives you room to grow.

  • For heavy-duty uses with high-power equipment like industrial tools or machinery, a 200 amp subpanel (or a full 200 amp main panel) is a better fit to handle the large power demands.

  • When in doubt, go a bit bigger. You want your subpanel to be able to handle future electrical upgrades down the road.

  • Consider checking with an electrician if you need specialized 220V circuits for large equipment. They can ensure your subpanel is properly sized.

The number of spaces for breakers and the amperage capacity are interrelated. Typically a 100 amp, 24-space subpanel is a good middle ground for most homeowners. This provides room for expansion. For quick guidance, refer to this table showing the recommended sub-panel size based on your garage's electrical requirements:

installing sub panel and detached garage

Key Steps for Installing a Subpanel in a Detached Garage

Once you settle on the right size subpanel, you're ready to tackle the installation. The following steps will take you through the complete process:

Step 1: Choose a Location for the Subpanel

Pick an interior garage wall where you want to mount the sub-panel. Some guidelines:

  • Install it in a clean, dry area protected from moisture and debris.

  • Leave ample workspace around it for access - at least 30 inches clearance in front.

  • Height should be 4-5 feet above the floor for visibility and usability.

  • Follow all local building codes for proper placement and clearance.

Also consider:

  • Distance from the main panel - closer proximity makes wiring easier.

  • Future garage expansion plans - allow room for extension.

Step 2: Mount and Connect the Subpanel Enclosure

With the location chosen, it's time to mount the subpanel enclosure:

  • Cut the wall opening if necessary and securely fasten the metal or plastic subpanel box. It must be level and plumb.

  • Use angle iron supports or plywood backing for a sturdy installation.

  • Use proper knockout holes for routing the incoming and outgoing wires.

  • Open the front panel and remove the inner covers to access the main lugs or bus bars.

Important: Do not connect any wiring until the feeder is run to the main panel!

Step 3: Determine the Correct Feeder Wire Size

The feeder wire carries power from your main home electrical panel to the new sub-panel. Choosing the proper wire size is critical.

Follow this process:

  • Add up the amp ratings of all circuits that will be powered from the sub-panel. For 220V circuits, use the full amp rating.

  • Select the proper feeder wire size based on the National Electrical Code ampacity tables. Bigger is better here.

  • For example, a 100 amp subpanel at 60 feet distance would need #1/0 AWG copper feeder wire (or #3/0 aluminum wire).

  • Increase wire size if long distance for voltage drop. Consult an electrician if unsure.

You want thick, quality copper THHN/THWN feeder wire for safety and performance. Don’t skimp on wire size - it must be rated to carry the full load current.

Step 4: Run the Feeder Wire to the Main Panel

Now for the big task - running the thick feeder cable between the detached garage sub panel and your main home electrical panel:

  • Carefully route the feeder wire. You can bury conduit underground or run overhead. Use weatherproof materials.

  • Use large PVC conduit - 1.5" to 2” diameter to fit the bulky cable. Run one conduit line for power and one for grounding.

  • Pull cables through the conduit using proper techniques. Tape wire ends to safely pull.

  • Leave extra slack wire at both ends - 2-3 feet extra.

Take precautions not to damage insulation when running the feeder cable. This can cause dangerous shorts.

Step 5: Install the Ground Rod and Connect the Grounding Wire

Proper grounding is extremely important for safety and preventing electrical hazards. Follow National Electrical Code requirements:

  • Drive one or more 5/8” ground rods 6-8 feet into the soil near the garage foundation.

  • Run #6 solid copper ground wire through buried PVC conduit to connect the ground rods to the sub-panel ground bus bar.

  • Bond the ground wire to any underground metal water pipes.

The earth-ground pathway gives electricity a safe route if a short occurs, preventing shocks. Never skip grounding!

Step 6: Connect Feeder Wires and Grounding to Subpanel

With the feeder cable run and grounding installed, now you can wire up the subpanel:

  • Connect the hot feeder wires to the main lugs or main breaker. Double-check polarity.

  • Connect the neutral and ground wires. The ground wire goes to the ground bar; the neutral attaches to the neutral bar. Keep them separate.

  • Ensure wires are stripped and tightened securely on the correct terminals. No loose connections!

Refer to the subpanel instruction manual for proper lug locations and torque specs on connections.

Step 7: Install Breakers and Connect Circuits

Now the fun part - wiring up the individual circuits the sub-panel will supply:

  • Install appropriately sized circuit breakers in the subpanel - 20 amp, 30 amp, etc. as needed. Match wire size to breaker amp rating.

  • Run the branch circuit wiring from breakers to new outlets, lights, switches, and appliances. Use code-approved methods.

  • Correctly size the wire for lighting circuits (#12 AWG), outlet circuits (#12 AWG), and higher voltage appliance circuits.

  • Label all wires clearly. Follow the electrical code for proper wire colors.

Take your time creating clean, organized circuit runs from the sub-panel. This improves safety and makes future troubleshooting much easier.

Step 8: Inspect, Test, and Connect the Subpanel

Before closing up the subpanel, it's crucial to validate all wiring:

  • Double-check that all connections are tight and properly terminated. Inspect all cables for damage.

  • Turn the main breaker on and test the operation of all circuits. Verify proper voltage and polarity.

  • Check ground continuity with a multimeter. Resistance should be very low - less than 1 ohm.

  • Inspect that neutral and ground buses remain isolated. Mixing them can be dangerous.

  • Once everything checks out, close and secure the subpanel covers.

A thorough inspection and testing help confirm safety and functionality before the subpanel goes live. Call an electrician immediately if you find any issues during testing.

Step 9: Connect the Subpanel to the Main Breaker Panel

The final step is to turn off the main power and connect the new subpanel feeder wires to the main house electrical panel:

  • Shut off the main breaker and verify power is off with a voltage tester. Lock out the panel.

  • Remove the cover and connect feeder wire terminals to available spaces on the main bus bar or lugs.

  • Carefully follow main panel instructions for proper feeder configuration and connections.

  • Ensure wires are torqued properly and terminals are tightened securely.

Once connected to the main panel, restore power and test the subpanel operation. Your new detached garage subpanel should now be ready to enjoy!

electrician configuring subpanel

Hiring an Electrician for Subpanel Installation

If handling complex electrical work makes you nervous, it may give you peace of mind to hire a professional electrician to install the sub panel.

Pros of hiring an electrician include:

  • Experience with local codes and best practices for safety

  • Ability to size and configure the subpanel correctly

  • Accuracy in running conduit and cables cleanly

  • Ensuring proper grounding and bonding

  • Advanced testing equipment to validate the installation

  • Certification that the work was done properly

Just be sure to hire a licensed electrician, get a permit if required, and check references to find someone reputable.

While paying an electrician adds to the total cost, electrical mistakes can potentially be dangerous and lead to fires or electrocution. For many homeowners, professional subpanel installation provides the necessary expertise and accountability. If you're wondering what it will cost to add a subpanel to your garage, use this table as a guide to the typical expenses for the project:

Task Estimated Cost
Subpanel equipment $100-300
Circuit breakers $5-50 per breaker
Feeder wires $1-5 per foot
Conduit $0.50-2 per foot
Grounding $50-150
Electrician labor $40-100 per hour
Permits $50-200
Total for a typical install $500-2000

Conclusion

Adding a subpanel to your detached garage is a cost-effective way to expand electrical capacity and circuits. This step-by-step guide covers crucial steps: sizing feeder wire and subpanel amperage, safe conduit installation, quality grounding, and thorough wiring inspections. While it requires effort, the benefits include enhanced power for hobbies, projects, EV charging, and workshop tools, all while ensuring safety.

FAQs

Do I need to insulate the subpanel in a detached garage?

It is not necessary to insulate the subpanel itself. However, it is important to ensure that the subpanel is located in a dry and protected area to prevent moisture and other environmental factors from damaging the electrical components.

Can I install a subpanel without a ground?

No, according to the NEC, a subpanel in a detached garage must have an equipment grounding conductor. This ensures proper grounding and safety.

How do I install circuit breakers in the subpanel?

To install circuit breakers in the subpanel, you will need to follow the manufacturer's instructions. Make sure to turn off the main power before working on any electrical panels.

Can I use a tandem breaker in the main panel for the subpanel circuit?

It depends on the specifications of your main circuit breaker panel. Some panels allow the use of tandem breakers, while others do not. Consult the manufacturer's instructions or a licensed electrician for guidance.

Can I install a subpanel in a detached garage myself?

Installing a subpanel requires knowledge of electrical wiring and safety guidelines. If you are not experienced or comfortable working with electricity, it is recommended to hire a licensed electrician to ensure the installation is done correctly.

Previous article The Best Foundation for a Detached Garage: A Complete Guide
Next article The Pros and Cons of Attached vs Detached Garages

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About The Author

Andy Wu - Resident Expert

Andy Wu - Resident Expert

Andy Wu is the resident backyard products expert and hails from Atlanta, Georgia. His passion for crafting outdoor retreats began in 2003.

As a fellow homeowner, he founded Backyard Oasis to provide top-quality furnishings and equipment, collaborating with leading manufacturers.

His main focus is on sheds and generators!

In his spare time he like to hike the tallest mountains in the world and travel with his family.

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