grounding subpanel in garage (detached)


Old 01-16-05, 11:50 PM
sparks n arcs
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grounding subpanel in garage (detached)

I want to install a 60a subpanel in garage 50ft from house. I intend to run 6awg THHN in PVC buried from the main service panel in my house. PVC will contain two hots and one neutral (no ground). Their is no metal plumbing or ductwork or anything else connecting the house and garage. I also intend to install an 8' grounding rod to be used exclusively by the subpanel (garage). In this scenario I believe I need to have the neutral bus, grounding bus, and the metal housing of the subpanel itself bonded togother in the same fashion that the main service panel (house) is done.


1. Safe?
2. Required depth of the PVC?
3. How does this differ from running a ground feeder through the PVC from the ground bus of main panel to the ground bus of the subpanel and isolating the neutral bus on the subpanel (and no additional grounding rod)?
4. Am I missing something?
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Old 01-17-05, 06:32 AM
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
The installation that you describe is almost correct; the requirements for grounding a subpanel in a _detached_ structure are the same as for grounding a main panel, which means either _two_ ground rods, _or_ testing a single ground rod for low resistance. It is generally easier (and cheaper) to simply install two ground rods.

The NEC permits _two_ different methods of grounding subpanels in detached structures. One is to install the subpanel in the conventional fashion, with separated neutral and grounds. The second method, permitted only if there are no metallic systems connecting the two structures, is to bond the ground and neutral, just like a main service panel. In both cases you need a complete ground electrode system.

When you say "no metal plumbing or ductwork or anything else connecting the house and garage", do you mean nothing at all; no chance for a telephone wire or computer cable? If there is any chance of ever wanting to run a phone cord or a light switch, then I'd recommend running a separate EGC and using a separate ground and neutral bus in the subpanel.

It is not clear which is actually safer. In most cases (and especially for shorter distances), it is probably safer to use the 'conventional subpanel approach' with separate ground and neutral. However for longer runs, my _guess_ is that using the 'service entrance' approach would give better grounding of the electrical system with respect to transient overvoltages. The difference in safety between these two approaches is small enough that the NEC permits either as a design decision, much like the 'ground pin up or down' issue.

One important issue: some people recommend feeding subpanels in detached structures with GFCI breakers. If you do this you _must_ use the 'conventional' approach with separated grounds and neutrals. The connection between the neutral and the grounding electrodes in the detached structure is an intentional connection to ground, but the GFCI has no way of knowing that it is intentional. It will simply trip on the imbalance current flowing through the parallel neutral path.

1. Yes, the install is safe; note the required correction to _two_ ground rods (or testing of a single ground rod, or another of the many possible grounding electrode systems)

2. I don't have the numbers available.

3. Running a separate equipment grounding conductor is generally recommended, and will permit you to run additional metallic systems in the future. In both cases you are required to have ground rods. Running a separate equipment ground will let you use GFCIs.

4. Almost certainly; that is why I always recommend reading more books on electrical system installation. *grin* The devil is in the details, and there are _many_ details.

Old 01-17-05, 06:44 AM
Join Date: Sep 2000
Location: United States
Posts: 18,497
2. 18" in most cases.

I also encourage you to run the grounding wire, even though not required.

Make sure the THHN is dual rated THWN. Most is.
Old 01-17-05, 06:51 AM
Join Date: May 2001
Location: Dry Side of Washington State
Posts: 738

Talk to your electrical inspector for your area. See if your plan is O.K. by him.
Some cities, counties, states, etc., require 4-wires to a detached structure.
That's the case here in Washington state.
Old 01-17-05, 09:27 AM
sparks n arcs
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Many thanks

Thank you for the quick and helpful responses.

As I may someday wish to run a three way switch from the house to the garage, I will run a ground conductor from the main panel to the subpanel and isolate the neutral bus on the subpanel. It would be just as easy (read easier) to do it this way.

To extend my question a little further.....with a little more information.
The house is 49 years old and the main panel has been updated from a fuse panel (I suspect 60a) to a Square D QO 20/20 breaker panel that is 100a. Few branch circuits have been updated though. The home is a one story with a basement and has roughly 1100 sq. ft. on the main floor. Gas range/oven, gas waterheater, gas furnace, and a gas clothes dryer are all in place and I plan to keep things this way. The question is, if my local power co. and my local building inspector will both allow use of the unlimited tap rule, does the meter housing need to be changed to allow double lugs on the load side of the meter? I don't suspect that you could connect into this service entrance cable anywhere else.

-If the unlimited tap rule is acceptable by all the local authorities, and I ultimately decide to go that way, will the method of grounding the subpanel (now I suppose it's technically a service entrance panel for the garage) change? Or is it required to have its own ground system (correct terminology?) and all continious metal that could be a conductor (pipes, wires, ect.) must not connect to both the house and the garage?

-If power is tapped from the meter base to the garage I do understand that I will NEED a main breaker/disconnect at the point of entrance inside the garage. My question is wether or not it NEEDS to be 100a. I suppose it would be the logical thing to do. I just don't know if it is necessary

--somewhat off subject, why is the Square D Homeline series so much cheaper than the QO series?? Just curious.

And I'd just like to add this is an incredibly helpful website. Thank You.

Learn the hard way and you'll never forget; Assuming you didn't fry your memory while learning

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