Is this right for a 100 amp subpanel in a detached shop?


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Old 05-17-18, 10:46 AM
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Is this right for a 100 amp subpanel in a detached shop?

I previously got information from some fine folks on this forum about this and am finally getting around to putting into action. From that and information on other forums, I think I have everything figured out, but just wanted to bounce it off you somebody before I start buying stuff.

I would like to install a 100 amp subpanel in my detached shop building. Here is what I have come up with, I will list the components in two parts, existing and proposed.

Existing:
1. 225 amp main disconnect next to meter box outside feeding existing breaker panel in attached garage
2. 42 space Eaton CH panel with open space for a 220 breaker. No main breaker in this panel, only branch circuit breakers for lights, dishwasher, etc.

Proposed:
3. 100 amp Eaton CH breaker into existing panel in #2 above.
4. SER 1/0-1/0-1/0-2 ALU cable from 100 amp breaker, run up interior wall to attic, run across attached garage in un-insulated attic space and then run down un-insulated exterior wall and exit into exterior workbox. (Total run from panel to workbox is less than 60 feet)
5. Exterior workbox will have a distribution block to convert from SER to URD/XHHW. (I would use MHF, but haven't been able to find it in 1/0-1/0-1/0-2 in my area for a reasonable price.)
5a. URD is 1/0-1/0-2 for the hots and ground and XHHW is 1/0 for the neutral.
6. Run URD/XHHW in conduit from workbox to subpanel in shop. Conduit will be buried 18"-24" deep. (Total run from workbox to subpanel is ~40 feet)
7. Install Eaton CH 100 amp panel with 100 amp main breaker (as a local disconnect) in shop with unbonded ground/neutral.
8. Install ground rod at shop for the subpanel connected with #2 ALU.

I am located in Texas, which is why I chose the larger 1/0 for the attic space due to the ambient temperature. I am not subject to any local inspection authority but the nearest municipality uses NEC 2011 if that is important.

Any additional guidance is greatly appreciated!
 
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Old 05-17-18, 12:31 PM
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For 100A you only need #1 Al, not #1/0. For the ground rod conductor use #6 copper. Aluminum connection under ground will rot away. Also should use 2 rods driven 6ft apart from each other. Al SER size needed is 1-1-1-3. Use same sizes in XHHW in conduit for under ground part and into subpanel. Straight type URD is not to enter structure.
 
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Old 05-17-18, 04:13 PM
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This is only a sub panel.... one ground rod should be sufficient. There should be at least one at the main panel.

I would not recommend installing ANY cable in conduit. Pull separate conductors.
Pulling underground cable thru conduit is miserable.
 
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Old 05-17-18, 05:40 PM
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Being a sub panel does not diminish the need for for good electrode grounding at a separate building. The second ground rod will not cost you that much more to install and I highly recommended it.
 
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Old 05-17-18, 08:02 PM
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I believe I will be going with the XHHW to make the pull easier. Thanks for the tip.
 
 

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