Garage Subpanel - Check My Plans, Please?

johnliu_gwMay 12, 2011

Hi guys, would you please let me know if you see any obvious problems in my plans here?

The project is to put a 60A subpanel in my detached garage.

- Garage currently has a single 15A 120V circuit, which I will disconnect.

- I want to get 60A for 120V and 240V circuits. Loads in the garage will be lights, garage door, and one power tool at a time, the largest being a 240V 9A table saw.

- The garage is detached, separated from the the nearest exterior wall of the house by a 4 foot wide breezeway. That happens to be the house wall that the electric meter is on. House and garage are wood frame.

- The breezeway between garage and house is concrete, running cable underground will be hard. So I would like to go overhead. The garage eave is 8 feet above ground.

- The main panel is 125A, located in the unfinished basement, directly below the meter location. It has four slots unused, so I can fit a 240V breaker.

Basically, I believe I need to:

- Add 240V 60A breaker to main panel, to feed subpanel.

- Use four THHN wires (#4 black/hot, #4 black/hot, #4 white/neutral, #6 bare/ground). In the main panel, hot wires to the breaker, neutral wire to the neutral bar, ground wire to the ground bar.

- Connect PVC conduit to top of main panel with coupler/locknut. Run conduit up the wall, exit house, run conduit up exterior of house to a level just below the garage eave. Is schedule 80 PVC conduit, 1.5 inch okay? Total length of the run from main panel to subpanel will be about 40 feet, including all the bends.

- Connect house to garage with a boxed horizontal wood member, 8 feet above the ground - I'll call it a "header". I'll put a gate under that header, so that it looks like there is a reason for it to be there, I'd like to gate off that walkway anyway. I've asked electrical inspector if there's a problem connecting the two structures in that way - we'll see if I get an answer.

- Run conduit inside header, where it is concealed and protected. Can I use 90-degree access fittings to make the bend from exterior/vertical to in-header/horizontal - easier to conceal than a standard 90-degree bend - or is that a water intrusion point? This will put the conduit 8 feet above the ground. Is there a minimum height, code or common sense, for overhead electrical in a conduit/attached to structure? I know there are requirements for overhead spans of hanging cable, but don't know if these apply to conduit.

- Inside garage, install panel with a ground bar. Any suggestions on type/model? Main panel is a Square D, I was going to use the same type for the subpanel. Chosen wall location has 30 inches horizontal clearance, and 3 feet clear space in front. Any other location requirements?

- If city requires - I've asked inspector, waiting on his answer - drive ground rods into the ground by the garage. One or two rods? Copper bonded, 8 feet, with bolt-type connector? Connect rod to ground bar in subpanel.

- Inside the subpanel, make sure neutral and ground bars are not bonded. Install 60A main breaker. Hot wires to the main breaker, neutral to the unbonded neutral bar, ground to the ground bar. Ground wires from the ground rods, if any, also to the ground bar.

- Disconnect existing circuit to garage.

- Hook up my 240V and 120V circuits normally. The garage interior is finished, so use PVC conduit for all the circuits schedule 40 okay here?.

You can probably tell, I've not installed a service panel before. What do you think - what am I missing, is there anything else I should ask the inspector?

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Oof, I realized the #4 wire I mentioned is overkill, and probably too large for the conduit. #6 then?

Do inspectors usually make you show a load demand calculation?

Finally, someone told me connecting the buildings is a bad idea, because they move. He says I should use two masts. That seems hella ugly to me?

    Bookmark   May 12, 2011 at 8:34PM
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Oof, I realized the #4 wire I mentioned is overkill, and probably too large for the conduit. #6 then?

Do inspectors usually make you show a load demand calculation?

Finally, someone told me connecting the buildings is a bad idea, because they move. He says I should use two masts. That seems hella ugly to me?

    Bookmark   May 12, 2011 at 9:41PM
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I'd definitely go underground.

You may be able to avoid headaches with your concrete breezeway pad by routing your underground feed out a different side of the house and into a different side of the garage.

    Bookmark   May 13, 2011 at 12:29AM
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The garage is entirely surrounded by concrete - driveway, breezeway, back patio. The only side of the garage not guarded by concrete is facing my neighbor, as the garage is built very close to the lot line (I guess there were no setback rules 100 years ago). I cannot go underground without breaking out concrete, and there are no handy joints to separate old and new concrete. I'd have to just cut a narrow trench and patch it, which would look awful. I'd rather use masts, even though I think that will look bad too.

It is really too bad that the person who remodeled the house didn't plan ahead and install a really sizable chase/conduit path. The circuit to the garage is NM cable buried in concrete.

By the way, do I actually need to disconnect the existing circuit to the garage? Somehow I recall reading that would be required, it is unsafe/not code to serve a garage with a subpanel and a branch circuit from the main panel. I don't know where I read that. If not, then it would be more convenient to leave the existing garage door and 2 gang outlet on the existing circuit.

On the ground rods, I'm reading I need 2 rods, at least 8 feet apart, with #6 bare wire.

    Bookmark   May 13, 2011 at 1:07AM
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Well, I got some information from the electrical inspector.

I will need two ground rods for the garage, since it is a detached structure. Connecting the garage to house with my proposed header doesn't change the detached nature of the garage, he says.

The inspector said if I enclose the wires in conduit supported by a permanent structure connected to house and garage, then the 12-foot height from grade and other clearance rules of article 225 won't apply.

I read article 225.30 and now I understand why I must disconnect the existing branch circuit to the garage.

Still trying to educate myself to do this properly.

    Bookmark   May 13, 2011 at 1:39PM
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Sorry for all these questions.

On the conductor type - I am now thinking that since part of the conduit will be outdoors, I need to treat the whole conduit run as a ''wet'' location. If that is right, then I think I need wet-rated conductors?. I thought single conductors would be easier to pull through conduit, so that means THWN?

Since I will only have three current-carrying conductors (plus neutral) from main panel to subpanel, do I have to worry about de-rating?

I would also appreciate some help on the conduit type and size. My impression is that 1'' schedule 80 PVC conduit can hold up to six #4 or nine #6 conductors, so if I use 1.5'' conduit that would be more than okay for filling and easier for pulling, and PVC is okay for exterior use?

    Bookmark   May 13, 2011 at 2:43PM
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Ron Natalie

You need wet rated, most of the stuff you'll find today is rated THHN/THWN so you'll be OK.

No derating (that's three current carrying plus ground by the way).

PVC will be fine but if it's exposed you need to make sure it's rated for sunlight.

    Bookmark   May 13, 2011 at 3:41PM
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Duh - ground not neutral.

Going to start this weekend. Thanks for looking over my posts.

    Bookmark   May 13, 2011 at 6:10PM
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