FHB Logo Facebook LinkedIn Email Pinterest Twitter Instagram Plus Icon Close Icon Navigation Search Icon Navigation Search Icon Arrow Down Icon Video Guide Icon Article Guide Icon Modal Close Icon Guide Search Icon Skip to content
Subscribe

In every issue you'll find...

  • Expert insights on techniques and principles
  • Unbiased tool reviews
  • Step-by-step details to master the job
  • Field-tested advice and know-how
Subscribe Now!
Subscribe
  • How-To
  • Design
  • Tools & Materials
  • Videos
  • Blogs
  • Forum
  • Magazine
  • Members
  • FHB House
  • Podcast
  • Join
  • Log In
Subscribe

Fine Homebuilding Project Guides

Wiring

Guide Home
Chapter
  • Working with Electricity
  • Tools and Materials
  • Receptacles and Switches
  • Lights and Fans
  • Rough-In
  • Outdoor Wiring
  • Panels and Subpanels
Editor's Notepad

Wiring a Subpanel

Learn how to safely and successfully install a garage subpanel.

By Andy Engel

When I was a builder, my partner was a licensed electrician. We did a lot of the work ourselves on the houses we built, and I got a pretty good apprenticeship in house wiring. Consequently, I’ve wired a number of panels and service connections on new houses, but until building my own garage recently, I had never done a subpanel. Like any electrical work, this is something I recommend you tackle only if you have a thorough understanding of the process, and that you pull the proper permits.

Subpanels have both similarities and differences with main panels. Both require properly sized entrance cable, from the main disconnect in the case of main panels, and from the main panel in the case of a subpanel. The conductor size depends on the amperage the subpanel’s main breaker can handle and the distance from the main panel. There are any number of online references to find that information. I had to buy about 60 ft. of cable, and considered both copper and aluminum conductors. Because copper is a better conductor than aluminum, a smaller diameter wire can be used, which makes it a lot easier to pull through conduit. However, even though I had to buy a larger gauge of aluminum conductor, it was substantially cheaper than the copper. One critical thing when using aluminum wires is to coat their ends thoroughly with Noalox to keep the aluminum from corroding where it ties to the panel. Corroded aluminum doesn’t conduct electricity well, and the extra resistance creates heat, which has caused fires.

Because the subpanel is fed from a new 100 amp breaker in the main panel, no other disconnect was required in my case, so tying in was a simple matter of clamping the wire ends to the lugs. The biggest difference between a subpanel and a main panel is that the ground and neutral buses on a subpanel have to be separated. Most panels come with a bar joining the two, which is easily removed. Code requires subpanels to have a ground connection that’s independent of the main panel’s. In my town, two 8 ft. ground rods separated by at least 6 ft. are required. With our rocky soil, they are a challenge to drive. One tip that’s served me well is to chuck the rods into a hammer drill and spin them into the ground. The combination of rotary motion and impacts seems to move the rod around most obstructions, and will even set a rod in frozen ground. And since the rods are 8 ft. long, I drive them at a 45 degree angle to avoid having to stand on a ladder initially.

Because the ground and neutral bars are separate, all the grounding conductors have to go the grounding bus and all the neutral conductors to the neutral bus. Beyond that, there’s no difference between wiring from a subpanel and a main panel. One thing I find very helpful in either case is to mark the incoming cables with the name of the circuit they serve. That helps not only in labeling the panel, but in balancing the load between the two hot busses.

Sign up for eletters today and get the latest how-to from Fine Homebuilding, plus special offers.

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Previous: How to Wire an Instagram-Worthy Electrical Subpanel Next: Subpanel Installation: Fitting and Fastening the Box

Guide

Wiring

Chapter

Panels and Subpanels

View Comments

  1. melli | Mar 09, 2015 10:42pm | #1

    Just adding a consideration on sub panel. Aluminum is a lot cheaper, especially for long runs, but the size of wire needed for a typical 100 amp sub panel in garage or shed may exceed what the sub panel can handle. That is, as an example, the lug in sub panel can handle #1-4 wire, whereas you may need bring in a 1/0 wire due to the fact you have a long run.

  2. ddivito | Mar 17, 2015 07:17am | #2

    If the sub panel is located in the same building, then 4 service wires have to go back to the main panel to complete the grounding.

  3. HWG | Mar 17, 2015 08:23am | #3

    I take exception to the statement that a subpanel requires its own ground rod. In fact it is only permitted in very limited cases in detached buildings, and only where there is no metallic connection back to the main building (telephone wire, copper water line, etc. ) Additionally, good grounding practice for lightning protection requires there be only one ground source for the entire electrical system. Having two grounding rods, even as close as six to eight feet apart, can direct current from a nearby lightning strike into the structure through one rod, run through the entire electrical system, and exit through the second rod. This can do enormous damage.

  4. sbandyk | Jun 29, 2017 06:10pm | #4

    I can confirm that my local Electrical Code (DuPage County, just outside Chicago.. possibly an addendum from my town) requires two grounding points.
    I pulled a permit for a panel upgrade and the inspector let me know he'd be happy pass me as soon as made a couple more additions, one of those being a 2nd grounding point (in addition to the line bonded to steel water line where it enters the basement). He told me all new construction in my area requires two grounds.
    I was a little surprised, not because it occurred to me lightening might be a problem, but because I'm familiar with multiple ground points occasionally introducing noise into the electrical infrastructure through a ground loop.

    Also now required by me, an emergency shut-off where my service enters. That's a pretty good idea, especially if you're a fireman who might have to slog through a water-soaked house with live power. If only 200A external cut-off switches weren't so expensive. :-(

Log in or create an account to post a comment.

Sign up Log in

Become a member and get full access to FineHomebuilding.com

Wiring

Wiring

Trusted, code-compliant guidance from the pros for working safely on any wiring project

View Project Guide

View All Project Guides »

Become a member and get unlimited site access, including the Wiring Project Guide.

Start Free Trial

Working with Electricity
  • Basics
  • Safety
  • Troubleshooting and Upgrades
Tools and Materials
  • Electrical Tools
  • Electrical Boxes
  • Cables and Wires
Receptacles and Switches
  • Materials
  • Wiring Receptacles
  • Wiring Switches
Lights and Fans
  • Lighting Design
  • Installing Lights
  • Fans
Rough-In
  • Planning
  • Installing Electrical Boxes
  • Running Cable
  • Retrofit Work
  • Making Connections in Boxes
  • Electrical Conduit
Outdoor Wiring
  • Basics
  • Outlets
  • Installing Lighting
Panels and Subpanels
  • Understanding Panels
  • Circuits
  • Installing a Subpanel

Fine Homebuilding

Follow

  • twitter
  • facebook
  • instagram
  • pinterest

Newsletter

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Membership & Magazine

  • Online Archive
  • Start Free Trial
  • Magazine Subscription
  • Magazine Renewal
  • Gift a Subscription
  • Customer Support
  • Privacy Preferences

Taunton Network

  • Green Building Advisor
  • Fine Woodworking
  • Fine Gardening
  • Threads
  • About
  • Contact
  • Advertise
  • Careers
  • Copyright
  • Terms of Use
  • Site Map
  • Do not sell or share my information
  • Privacy Policy
  • Accessibility
  • California Privacy Rights

© 2023 The Taunton Press, Inc. All rights reserved.

X
X
This is a dialog window which overlays the main content of the page. The modal window is a 'site map' of the most critical areas of the site. Pressing the Escape (ESC) button will close the modal and bring you back to where you were on the page.

Main Menu

  • How-To
  • Design
  • Tools & Materials
  • Video
  • Blogs
  • Forum
  • Reader Projects
  • Magazine
  • Members
  • FHB House

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Account

  • Log In
  • Join

Podcasts

  • FHB Podcast
  • ProTalk

Webinars

  • Upcoming and On-Demand

Popular Topics

  • Kitchens
  • Business
  • Bedrooms
  • Roofs
  • Architecture and Design
  • Green Building
  • Decks
  • Framing
  • Safety
  • Remodeling
  • Bathrooms
  • Windows
  • Tilework
  • Ceilings
  • HVAC

Magazine

  • Current Issue
  • Past Issues
  • Magazine Index
  • Subscribe
  • Online Archive
  • Author Guidelines

All Access

  • Member Home
  • Start Free Trial
  • Gift Membership

Shop the Store

  • Books
  • DVDs
  • Taunton Workshops

More

  • FHB Ambassadors
  • Reader Projects
  • Podcast
  • Customer Support

Account

  • Log In
  • Join

Newsletter

Get home building tips, offers, and expert advice in your inbox

Signing you up...

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
See all newsletters
See all newsletters

Follow

  • twitter
  • facebook
  • instagram
  • pinterest

Join All Access

Become a member and get instant access to thousands of videos, how-tos, tool reviews, and design features.

Start Your Free Trial

Subscribe

FHB Magazine

Start your subscription today and save up to 70%

Subscribe

We hope you’ve enjoyed your free articles. To keep reading, become a member today.

Get complete site access to expert advice, how-to videos, Code Check, and more, plus the print magazine.

Start your FREE trial

Already a member? Log in