Installing a sub panel in a metal building


  #1  
Old 10-15-09, 09:46 AM
P
Member
Thread Starter
Join Date: Oct 2006
Location: USA
Posts: 46
Received 0 Votes on 0 Posts
Installing a sub panel in a metal building

I just put up a 10' X 12" metal building on a permanent wooden foundation. I don't know whether or not these details matter, but there they are. I don't need much power in the building, but I would like to install one interior light fixture and one exterior for security, and possibly a couple of outlets for convenience sake, although I have no plans to run any power tools or equipment in the building. Should I treat this like any other detached building install, or are there special considerations with a metal building? I thought I would do a 4-wire in buried PVC install off a 30 amp breaker at the main panel, to a small sub panel at the building. I have looked at the great drawings in this forum and I think I understand what needs to be done.
 
  #2  
Old 10-15-09, 11:09 AM
I
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,942
Received 44 Votes on 42 Posts
Originally Posted by pkoutoul View Post
Should I treat this like any other detached building install, or are there special considerations with a metal building?
If you use metal boxes and/or conduit fastened directly to the metal structure and ground them like normal then that is really all you need to do. If you want to make sure to get it good, bolt a #8 copper wire on a structural beam and run it to the ground bar of the subpanel.

I thought I would do a 4-wire in buried PVC install off a 30 amp breaker at the main panel, to a small sub panel at the building. I have looked at the great drawings in this forum and I think I understand what needs to be done.
That sounds like an okay plan to me. Based on your description I don't even think the subpanel is necessary though. You could easily do a 20A multiwire circuit with a simple disconnect switch instead of full panel. That's more than enough for a few lights and the occasional tool.
 
  #3  
Old 10-16-09, 10:12 AM
P
Member
Thread Starter
Join Date: Oct 2006
Location: USA
Posts: 46
Received 0 Votes on 0 Posts
Thanks for the help, Ben. I like the idea of eliminating the subpanel if that is not required. One clarification: what do you mean by "multiwire circuit"? Also, is direct burial wire a viable option? Thanks!
 
  #4  
Old 10-16-09, 11:14 AM
I
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,942
Received 44 Votes on 42 Posts
Yes direct burial wire is an option. You can use #12/3g UF-B cable -- it only requires conduit at the vertical entry and exit of the trench for protection. The horizontal run can be directly buried at 24" depth or 12" depth if you use a GFCI breaker at the house.

A multiwire circuit uses a double-pole 20A breaker and a four wire cable (hot-hot-neutral-ground). The neutral wire is shared between the two hots. At the outbuilding, you split the circuit into two "legs" which act just like individual 20A circuits. This method requires no subpanel or grounding rods, just a double pole disconnection switch like a DPST toggle or an air conditioner disconnect when the conduit first enters the outbuilding.

What is the distance from the main panel at the house to the outbuilding? The #12 cable is good for about 100-150', but you should probably upsize if the distance is longer.
 
  #5  
Old 10-18-09, 06:35 AM
P
Member
Thread Starter
Join Date: Oct 2006
Location: USA
Posts: 46
Received 0 Votes on 0 Posts
Ben, thanks again for your very helpful replies. The entire length of the run from main panel to outbuilding will be about 120', but the first 40' or so will be indoors, so I'll use regular #12/3g indoor romex for that. Or maybe upgrade that part of the run to #10 to be on the safe side? I figure I'll install a junction box at the point where the run will leave the house and go underground, and run the direct burial wire from that point on. I'll go with the GFCI breaker and bury the wire 12" as you suggest. I really appreciate the help.

Oh, one more quick question. Since I won't be using ground rods and since the metal building is insulated from ground by the wooden foundation, should I ground the building itself?
 
  #6  
Old 10-19-09, 11:07 AM
I
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,942
Received 44 Votes on 42 Posts
Originally Posted by pkoutoul View Post
I'll use regular #12/3g indoor romex for that. Or maybe upgrade that part of the run to #10 to be on the safe side?
Either option would be okay. I don't think an upsize to #10 is absolutely required for that distance, but if you wanted to future-proof a little bit go for the #10.

I figure I'll install a junction box at the point where the run will leave the house and go underground, and run the direct burial wire from that point on.
That sounds good to me. You just need conduit down to the bottom of the trench then use a 90 sweep and direct bury from there.

I'll go with the GFCI breaker and bury the wire 12" as you suggest.
That's a good idea. The 20A double-pole GFCI breaker is sometimes a special order item so you'll probably have to go to a supply house instead of a big box store. It can be a little pricey, but it's worth every cent compared to digging a 24" trench. You also won't need to buy any GFCI receptacles for the outbuilding because the breaker will automatically protect everything on that cable.

Since I won't be using ground rods and since the metal building is insulated from ground by the wooden foundation, should I ground the building itself?
I think it makes good sense to ground the metal building with a ground rod, but strictly speaking it is not required with the multiwire circuit.
 
  #7  
Old 10-19-09, 12:07 PM
P
Member
Thread Starter
Join Date: Oct 2006
Location: USA
Posts: 46
Received 0 Votes on 0 Posts
I think that covers everything. I'm ready to get at it. Thanks again for all the help!
 
  #8  
Old 10-19-09, 12:15 PM
I
Forum Topic Moderator
Join Date: Feb 2005
Location: Near Lansing, Michigan
Posts: 10,942
Received 44 Votes on 42 Posts
No problem. I hope it goes well.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: