Running power to a Garage

flypaper00December 24, 2009

I am planning to build a small garage. I plan to install a sub panel with two 20amp breakers, one for outlets and one for lights and an outlet for a 120 small AC unit. I know I need a GFI for an outside outlet, but do I need to have the rest of the inside outlets ran through it? I have a 125amp main panel that is about 80ft away. My plan is to install a 40amp double pull in the main and run THWN #8 in 1 conduit to the garage. My thought is to choose the conduit because of the extra protections and option to pull new wire if anything goes wrong later. I think my angle limit is 360 and it is almost a straight shot so I should be fine there. I am machine trenching so depth isnÂt a concern.

I have extensive experience in septic systems using pumps, panels, and floats, but this is a bit different so I could use some helpful advice if anyone could help I would appreciate it.

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Is this garage doing to be attached to the house? If not, it changes the rules a bit.

    Bookmark   December 24, 2009 at 3:23PM
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There are a few local jurisdictions that require a minimum of 60-amp subpanels in detached buildings. That's not a national rule (in fact it's definitely the exception) but it's worth a phone call to your AHJ to confirm.

It's a tad unclear from your description (double pull?) whether you'll be pulling a 120/240V feed (i.e., two hots, one neutral and an equipment ground). If so, a two-pole 40-amp sub should easily meet your power requirements and allow for some expansion.

If, on the other hand, you were planning to pull only a 120V feed (one hot, one neutral and a ground), you could certainly power two 20-amp circuits that way, but it is far from the best way to accomplish that. (You'd be better off pulling a multi-wire branch circuit -- two #12 hots on opposite poles, one shared neutral and a ground.)

Otherwise, your plan sounds like it oughta work fine and, if properly executed, meet code.

Running THWN in conduit is a good choice, IMO, and your trench depth should generally be 18" or deeper.

About the GFCIs: Yes, all 15- and 20-amp receptacles in a garage must be GFCI protected. (There may be an exception or two depending on the code version your area is using, but that's the best current advice.)

    Bookmark   December 24, 2009 at 5:41PM
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Thank you. It is a detached garage.

Double pull meaning 240v feed. I just though it would be easier. I was planing to branch off 120v to each of the breakers in the garage. Also, I an not sure I need to do a ground rod since it will be grounded off the main panel...?

I didn't know about the 60 amp minimum. If that is the case, it might be better for me to run a 60 at the main and pull #6 wire. Would that allow me to do a 240v outlet? I hadn't planned on wiring for a welder, but that might be an advantage of having 60 amps out there. I could use a bit of advice on how to split the 60 into 120v and 240v at the sub panel. I could probably run the lights and outlets off from one 20amp and have a 30 or 40 amp 240v service.

Thanks also for the GFCI answer. Knowing that will dictate the direction I go when I wire the garage. I will go though the GFCI before the rest of the outlets.

    Bookmark   December 24, 2009 at 6:14PM
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"Double pull meaning 240v feed. I just though it would be easier."

Yup. Gives you more options in the future, too.

While it's not terribly likely that your area enforces the 60-amp subpanel minimum I mentioned, if you're even remotely thinking of adding a welder, then by all means bump up to #6 wire and make it a 60-amp panel. At least that's the way I'd be inclined to go.

Even with a 120/240V 40-amp panel, you could have a 240V outlet. The question is whether it'd be large enough to supply a welder. (A 240V AC unit, larger compressor or a big table saw? Sure.)

Yes, a subpanel in a detached building does require its own ground rod. In fact, two ground rods are usually required these days!

I'll try to sketch this out for you quickly. Your subpanel will have two hot busses, each of which is fed by one of the hots, which are on "opposite poles". You'll also have a neutral bar and a ground bar. (Because not all panels come with ground bars installed you may have to buy the ground bar separately and add it yourself.)

OK, here goes:

A 240V circuit is fed by two hots -- a black from one pole and a red from the other. When you install a two-pole ("double") breaker, that's what you get, since the bus stabs alternate by pole.

A 120V circuit is fed by one of the hots (doesn't really matter which one) and a neutral. When you install a single breaker, you get that one hot feed and the other wire -- the white -- lands on the neutral bar.

Some circuits ("120/240V") are fed by two hots and a neutral. That's so the appliance it feeds can use both 240V and 120v. For instance, the heating elements of an oven use 240V but the clock and the light may only need 120v.

Welders usually require 240V circuits (no neutral required).

The ground bar in a subpanel only serves equipment grounds (the bare or green wires that are for safety only).

The neutral bar in a subpanel only serves current carrying white wires and should not be connected ("bonded") to the ground bar. (That's slightly different that what you might see in your main panel, where neutrals and grounds may be connected.

Your ground rod should connect only to the subpanel's ground bar (use #6 copper wire for this regardless of whether your subpanel ends up being 40- or 60-amps).

BTW, when you pull your wires in conduit, you should pull a bare or green equipment ground that connects from your main panel ground bar to the subpanel ground bar. Whether your current carrying conductors are #6 or #8, the equipment ground can be #10. (That'll save you a couple of bucks.)

Feel free to stop back in if any of this is unclear.

    Bookmark   December 24, 2009 at 7:11PM
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I am curious. Can I run a panel like this one: Model # CESMPSC55GRHR Internet/Catalog # 100669936 at Home Depot and just run the rest of my electrical (lights and 120v outlets) off from the GFCI? The 240v is a 50amp and the 120v is a 20amp. It just looked like an inexpensive and convenient solution. Can the total amps in the sub panel be larger than the total amps of the breaker the main panel?

Any idea how deep the ground rods should be?

Thanks for your help.

    Bookmark   December 24, 2009 at 11:03PM
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"Can I run a panel like this one: Model # CESMPSC55GRHR Internet/Catalog # 100669936 at Home Depot and just run the rest of my electrical (lights and 120v outlets) off from the GFCI?"

I won't go so far as to say that you can't, but that sure wouldn't be my choice. It looks like it might be a "turnkey" solution to a particular problem: an RV that needs a 50A receptacle, a 120V outlet and one GFCI receptacle.

As I see it, there would be a couple of disadvantages to going that route:

1. At ~$110 you won't save much, if any, with this package deal compared to buying the parts you actually need.

2. You limit your flexibility (now and in the future).

3. Unless you'd be satisfied using extension cords and worklights all the time, I doubt you'll be happy with the way your lighting works. Keep in mind that permanent wiring -- like overhead lighting fixtures controlled by wall switches, etc. -- can't legally be "plug in".

Instead of this package deal, you can pick up, say, a Square D Homeline 6-slot sub panel for about $25. You'd need four circuit breakers (2 x 20A singles plus 1 x 50a double) for $20, a ground bar for about $5, a junction box for $2 and a GFCI receptacle for $15.

So, your "break even point" using inexpensive readily available components is roughly $70 versus $110 for the package. (Of course, I'm assuming DIY labor is free.) Beyond that, having a general purpose subpanel is all goodness -- you're not constrained by the drop-in "solution".

But maybe that's just me.

"Can the total amps in the sub panel be larger than the total amps of the breaker the main panel?"

Absolutely. The principle is called "diversity." That is, not everything draws full power at the same time. In fact, it is rare that any individual circuit draws approximately 100% of its ampacity and it is much rarer still that all the circuits in a panel draw their full rated capacity simultaneously.

And there may be something else that you may or may not be taking into account: If you have a 60-amp 120/240 panel, each of the two poles delivers 60 amps at 120V. Thus, if you had only 120V circuits, you could have six 20-amp circuits each drawing at or near their full ampacity (120 amps total!). The same is not true of 240V circuits since they draw from both poles simultaneously.

So, as an example only, a 60-amp subpanel could have, say, a welder drawing 40 amps and two 20-amp circuits each drawing 20 amps. Is that making sense?

But again, I'd emphasize that circuits seldom draw their full ampacity and, indeed, should generally be planned to draw no more than 80% if the load may be continuous.

I'm oversimplifying here, but that's the general idea. In your case, you should be fine with a 50-amp welding circuit and two 20-amp general purpose circuits even though that may add up to 70 amps total on each pole.

[I'll grant you that it is possible that one day you might be running a compressor drawing 19 amps and have the welder cranked up to its maximum amperage and trip the main, but that ain't likely to be a real problem.]

    Bookmark   December 25, 2009 at 12:46AM
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Not quite sure about Connecticut Electric, I'd more inclined to go with the big national brands (SquareD, Cutler-Hammer, Siemens). I'd think most here would recommend a Square D QO panel or Cutler-Hammer CH. For what you're talking about the different in price is negligible.

I'd also suggest running a 60 amp circuit for your subpanel. The cost difference isn't huge but it gives you plenty of room for expansion. Use a little bigger conduit that the wires you running would call for in case you need to change things down the road

The sum of the amperage ratings of the individual breakers will almost always add up to more than the main breaker or in your case, the breaker serving the sub panel. What's important to do is size it correctly for the load. I've seen a few "starter" panels in the home center where they throw in the breakers at a pretty fair price.

While you're at Home Depot look for a book called "Wiring Simplified." It's green and goes for about $6. In the electrical department near the wire nuts and other miscellaneous stuff. While it certainly won't answer every question overall it has a lot of good information.

    Bookmark   December 25, 2009 at 12:59AM
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Thank you for the information. I thought that panel was a bit limiting also.

I sent the panel info to my dad. He was a manufacturing plant electrician for a little over 20 years. He emailed me a CH panel suggestion and a diagram consistent with your suggestions. Apparently, this is a simple set up. He wasn't sure about the ground rods though. Even though he has been retired for about 15 years, he still remembers how to do some of this stuff.

Thanks again.

    Bookmark   December 25, 2009 at 2:16AM
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You're welcome.

"He wasn't sure about the ground rods though."

Whoops! Sorry that I forgot to address that part of your question.

To meet 2008 code requirements, the "generic recommendation" is that you'll need two rods driven to a depth of at least 8' and placed at least 6' apart, connected to one another and to the panel's ground bar by a single unspliced wire.

Unfortunately, though, generic answers aren't all they're cracked up to be. And so, I'm afraid a few footnotes are in order:

1. You can get by with only one ground rod if it measures 25 ohms resistance or less. That's not something that's easy for a DIYer to test and, in any event, many if not most ground rods flunk this test. (It depends on soil and moisture conditions as well as the distance to the POCO's transformer.)

2. Because soil conditions vary so much throughout the country, this is a code topic that's definitely subject to local variations. Some jurisdictions require 10' rods and a few specify even deeper penetrations (typically in very sandy soils). This is one of those "check with your local code enforcement office" items.

3. Since this is a new garage you'll be building, it's worth mentioning that it is becoming more and more common to achieve earthing (grounding) by embedding grounding wires in the concrete foundation and bonding with the structural rebar. In some locales, this is now a requirement for new buildings. Again, check with your AHJ or a knowledgable sparky in your area.

    Bookmark   December 25, 2009 at 11:16AM
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