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Old 05-05-2009, 01:39 AM   #1
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Subpanel


I am going to be putting a subpanel in my garage. I was wondering if 2 AWG aluminum will be big enough. My main service on the house is 200A. I will be using a 100A dual breaker in my main to service the sub in the garage. I will be running the wire underground in PVC conduit. The total distance from the main to the sub will about 90'. Any advice will help. Thanx

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Old 05-05-2009, 07:20 AM   #2
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You are on the right track.
1. You need three runs of #2 XHHW (L1, L2, N) and one run of #4 XHHW (G).
2. You need to drive a ground rod at the garage and run that ground lead into the subpanel also.
3. You do not bond ground to neutral in the garage subpanel. They stay seperate beyond the main panel.

One additional thing you might want to consider. That being a non-fused disconnect switch (also 100A) on the outside wall of the garage. That way you have a "local" way of completely shutting down the power to that building with one switch should you have some type of emergency there.

So your conduit would come up the outside of the garage into the disconnect. Then use another piece of PVC or a steel nipple to go through the wall into the load center.

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Old 05-06-2009, 12:41 AM   #3
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I forgot to add that I was using a main breaker box and not a lug. Thanks....
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Old 05-06-2009, 01:10 AM   #4
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I forgot to add that I was using a main breaker box and not a lug. Thanks....
Well then you're good to go. The disconnect IS NECESSARY to meet the national code because it's a separate building.
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Old 05-06-2009, 06:37 PM   #5
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Originally Posted by Gonzo1 View Post
.... I was wondering if 2 AWG aluminum will be big enough..... I will be using a 100A dual breaker...
No, #2 aluminum conductors is not big enough for a 100 Amp panel. You will need to install #1 Aluminum for 100 Amps.

Now, if you are really bent on using that #2, then you could feed it from a 90 Amp breaker in your house, and that would be sufficient. You could still use a 100 Amp main breaker panel in the garage, as long as it was fed from a 90 Amp breaker in the house.

Of course, as long as your calculated load is 90 Amps or less.

For such a job as you described, we would utilize (2) #1 hots, a #2 neutral, and a #6 equipment ground wire. A 1" PVC conduit accommodates this nicely.
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Old 05-06-2009, 06:41 PM   #6
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You are on the right track.
1. You need three runs of #2 XHHW (L1, L2, N) and one run of #4 XHHW (G)....
I disagree. You are recommending too small a conductor for the hots, and too large for the ground. While upsizing the ground is not a violation, too small a conductor for the hots is.
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Old 05-07-2009, 12:53 AM   #7
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Thanx. i just want to use the right conductor and i was throwing the ? out there because at Lowes they have the 2-2-2-4 AL listed as a 100A conductor.
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Old 05-07-2009, 06:44 AM   #8
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That 2-2-2-4 quadplex "URD" cable assembly is made for mobile home feeders. IF you happen to have a mobile home that needs a 100 Amp feeder, then table 310.15(B)(6) allows its use for underground feeders.

But since you are installing a sub-feeder for a garage, that exemption does not apply, and you have to either upsize your feeder conductor to a #1, or downsize your overcurrent protection to a 90 Amp breaker.

See table 310.16 for the ratings of those conductors under general use conditions. Use the 75 degree column for type "XHHW" conductors.
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Old 05-07-2009, 06:46 AM   #9
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Last edited by FIA Ranch; 05-07-2009 at 07:03 AM.
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Old 05-07-2009, 11:34 AM   #10
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Originally Posted by Gonzo1 View Post
Thanx. i just want to use the right conductor and i was throwing the ? out there because at Lowes they have the 2-2-2-4 AL listed as a 100A conductor.
Quote:
Originally Posted by kbsparky View Post
No, #2 aluminum conductors is not big enough for a 100 Amp panel. You will need to install #1 Aluminum for 100 Amps.

Now, if you are really bent on using that #2, then you could feed it from a 90 Amp breaker in your house, and that would be sufficient. You could still use a 100 Amp main breaker panel in the garage, as long as it was fed from a 90 Amp breaker in the house.

Of course, as long as your calculated load is 90 Amps or less.
.
As I recall, NEC also says if you can't match ampacity to breaker, then use the "next recognizable size breaker" (which assumes 5-10 amps difference), or words to that effect, which, in this case would be a 100 amp. So if the supplier of the panel you are using makes a 90 amp, then you can use that, if the manf. doesn't, you can either upsize the wire, which is a cost issue, or use a 100 amp breaker, both of which should pass inspection.

And unless you are doing something in your "garage" using monster motors or ??? that draw huge constant load current, then I don't see an issue. Even at 50 amps constant load (12KW or about 16 HP worth of motors), with 70-80 amp burst load, with a 100 foot run, you are going have less than a 2% voltage drop, which IMHO is a good cost/value relationship.
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Old 05-07-2009, 12:42 PM   #11
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But 90A is one of the listed sizes of a fuse or circuit breaker, so you cannot upsize from there.
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Old 05-07-2009, 03:25 PM   #12
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But 90A is one of the listed sizes of a fuse or circuit breaker, so you cannot upsize from there.
Persactly
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Old 05-07-2009, 09:58 PM   #13
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You are on the right track.
2. You need to drive a ground rod at the garage and run that ground lead into the subpanel also.
For my own education, why does he need a ground rod at the garage? I thought that the ground at the main covered all sub panels.
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Old 05-07-2009, 10:05 PM   #14
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A detached building needs it's own ground rods
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Old 05-08-2009, 02:21 AM   #15
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But 90A is one of the listed sizes of a fuse or circuit breaker, so you cannot upsize from there.
Yep ya got it right.

The NEC code did change on that one by changing the tempture rating on conductors that why it affect the rating.

Merci,Marc

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