To use or not use conduit for subpanel

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  #1  
Old 03-09-07, 10:06 AM
jdu
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To use or not use conduit for subpanel

Hello,
I am eventually going to install a 100A subpanel that will be located 40 ft away from the main panel. The subpanel wiring will be run under the house and I have been told that #4AWG wire will be fine for this. Can I use 4-3 Romex (with sheething) wire and run it under the house without conduit or do I need to run #4 wires in something like schedule 40 (no sheething)? I am hoping to run romex if possible. Thanks,
Jason
 
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  #2  
Old 03-09-07, 10:33 AM
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It depends on the location. Is the cable always out of the way (not exposed) and is the area dry?
 
  #3  
Old 03-09-07, 11:11 AM
jdu
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Yes. The cable is not visible unless you go under the house into the crawl space and the area is dry.
Jason
 
  #4  
Old 03-09-07, 11:22 AM
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No conduit needed if using Romex. But #4 is not large enough. You need #2 if running Romex, #3 if running individual wires in conduit. #4 is only good for 100 amps when used as the main line to the house (which is why some people might mistakenly think it's okay for you).
 
  #5  
Old 03-09-07, 11:45 AM
jdu
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John,
Thanks for your help. I was only planning a 100A subpanel since I thought the #4 would work. If I want to run 4-3 Romex under the house at a 40ft run length, what max size subpanel can I have?

If the answer is a 70A subpanel, how many 20A circuits can I add to this?

Jason
 
  #6  
Old 03-09-07, 11:52 AM
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70-amps is the maximum size breaker you can use in the main panel to protect this feeder.

The subpanel itself can be rated at anything 70 amps and above.

You can have as many 20-amp circuits as you want in the subpanel (or any other circuits of any size you want), limited only by the NEC maximum of 42 in one panel.
 
  #7  
Old 03-09-07, 12:02 PM
jdu
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John,
Thanks again. So I understand that I'll install a 70A breaker in the main panel that will feed this subpanel. Are you saying that I can make the subpanel 70A or even 100A for that matter? This is confusing to me as I would think that if the main panel breaker to the sub is 70A, so the subpanel main breaker would also be 70A. What would be a reason to use something larger than 70A in the subpanel? Thanks for your advice, I really appreciate it.
Jason
 
  #8  
Old 03-09-07, 12:08 PM
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> Are you saying that I can make the subpanel 70A or even 100A for
> that matter?

Yes. A main breaker is not even required in the subpanel provided it is located in the same building as the main panel.
 
  #9  
Old 03-09-07, 12:12 PM
jdu
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Thanks ibpooks. Also, I am saying the word "Romex" but do I really mean 4-3 Appliance cable (SER) or 4-3 NM.
 
  #10  
Old 03-09-07, 12:18 PM
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> I am saying the word "Romex" but do I really mean 4-3 Appliance
> cable (SER) or 4-3 NM.

Well, #4 comes in many types and each type has a different ampacity. Have you already bought the cable or are you trying to decide which one to buy?
 
  #11  
Old 03-09-07, 12:26 PM
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Just for your understanding...

The 70-amp breaker in the main limits already limits the total current in the feeder and in the subpanel to 70 amps. Putting a 100-amp breaker in the subpanel (optional) doesn't allow 100 amps because the upstream breaker will never allow more than 70 amps to get to it. So any main breaker (at or above 70 amps) that you might put in the subpanel isn't going to provide any additional protection. It would only serve as an on/off switch.

As just said, if the two panels are in the same residential structure, a main breaker in the subpanel is optional.

The rating of a panel is merely the maximum it can handle. You can always use a 100-amp-rated panel in an application requiring less.
 
  #12  
Old 03-09-07, 02:17 PM
jdu
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John, thanks, I get it know.

ibpooks,
I am currently trying to decide which wiring to buy for the 70A subpanel.

Jason
 
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