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Old 04-18-2011, 08:05 AM   #1
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Sub-panel off of sub-panel?


So I'm rewiring my house in phases, as part of the overall plan I'm connecting new circuits (so far I have new circuits for the kitchen and second floo bedrooms) on a 200 amp panel which I am using as a sub-panel off our main panel. The plan is eventually to upgrade to 200 amp service and relocate the service entrance to this 200 amp panel.

I want to run a 100 amp sub-panel in the garage, and while we're having other plumbing replaced under the driveway, I'd like to dig the trench to run the wiring to the garage and do the part of the gas line that is underground.

I have enough information that the gas line can share a trench and will have the plumber take care of that.

My question is if anyone could please confirm if it's allowable to run the garage subpanel off of the subpanel in the house?
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Old 04-18-2011, 08:27 AM   #2
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Sub-panel off of sub-panel?


While I'm at it, am I correctly understanding that 2 gauge aluminum USE-2 can carry 100 amps either direct buried or in raceway if I'm using that for going from the 200 amp panel I mentioned to the 100 amp panel in the garage? And the 100 amp panel in the garage would require a ground rod, and I don't run a ground conductor in the trench?
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Old 04-18-2011, 08:32 AM   #3
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Sub-panel off of sub-panel?


Yes, you can run sub-panels off of sub-panels, with the proper size wires, breakers, ground rods for detatched bldgs, etc.
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Old 04-18-2011, 08:43 AM   #4
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Sub-panel off of sub-panel?


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Originally Posted by WillK View Post
I correctly understanding that 2 gauge aluminum USE-2 can carry 100 amps either direct buried or in raceway
USE-2 is rated for 90 amps when used to feed a sub panel.
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Old 04-18-2011, 08:45 AM   #5
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Sub-panel off of sub-panel?


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While I'm at it, am I correctly understanding that 2 gauge aluminum USE-2 can carry 100 amps either direct buried or in raceway if I'm using that for going from the 200 amp panel I mentioned to the 100 amp panel in the garage? And the 100 amp panel in the garage would require a ground rod, and I don't run a ground conductor in the trench?
The #2 Al will only be good to 90A, but that should be more than sufficient for a garage. You will need three #2 conductors PLUS a ground. If you use what is referred to as "mobile home feeder", you will have three #2 and one #4 already twisted together. I would also plan to put it in conduit in the trench. And yes, you will also need a ground rod (or two) at the garage if it is detached.
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Old 04-18-2011, 08:43 PM   #6
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Sub-panel off of sub-panel?


While it's allowable to run a sub panel off of a sub panel....you still need to keep in mind the service capacity of the main panel....it's not infinite. Some load measurments and calculations may be in order. What is the capacity of your service main?
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Old 04-18-2011, 09:39 PM   #7
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Sub-panel off of sub-panel?


It's a 100 amp main. The 100 amp panel in the garage is going to be running off a subpanel which is running off a 60 amp breaker on the main, for the time being, but the subpanel in the house will eventually become the main at 200 amps. So I'm using a wire that will for now be bigger than needed, but sized based on the long-term plan.

And for the time being, I'll probably only be running lights and basic power tools (table saw etc) in the garage. I won't be running anything where I need more than 20 amps until the point where the service move and upgrade to a 200 amp main panel is done.

So I believe HD has 2/0 aluminum mobile home feeder wire, so I'll plan on that. That brings me to the subject of conduit then.

For conduit, is RMC appropriate for burial? The car restoration guy in me just thinks steel, even if it's galvanized, is only doomed to rust to nothing if you bury it... But I know that my AHJ doesn't seem to care for PVC because it isn't for mechanical protection. He did accept EMT, but am I correct in that EMT would be too thin for burial?

ERg... $53.57 for 10' on RMC vs $16.24 for EMT??? Isn't there something in NEC about schedule 40 PVC being okay for burial, then you just go to metal for the riser? So if you do that, can you get a watertight joint or do you do something to provide drainage?
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Last edited by WillK; 04-18-2011 at 09:49 PM.
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Old 04-18-2011, 09:47 PM   #8
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Sub-panel off of sub-panel?


So you currently have a 100 amp main? Hmm. And what wire size from the entrance head to the panel? All of these need to be considered. What you plan to use isn't as important as what it is fused for. The wire must meet the requirements of the circuitry. Just "running lights and basic power tools" isn't enough precaution. Overloading your entrance feeders can be catastrophic. As long as your service is adequate, and your entrance cabling is adequate you will be alright....but this you need to verify before you start adding loads.
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Old 04-18-2011, 11:19 PM   #9
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Sub-panel off of sub-panel?


2 gage aluminum.

I'd really appreciate an answer about my conduit questions.
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Old 04-19-2011, 08:33 AM   #10
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Sub-panel off of sub-panel?


You can use Sch40 for the buried sections and transition to Sch80 for the above ground sections. Use 2" pipe. It's a little overkill, but for a DIYer, 2" will be easier to pull through.
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