100A sub panel

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  #1  
Old 11-07-10, 07:25 AM
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100A sub panel

Hi All,
I'm installing a 100A sub panel in a new attached garage. The main panel is about 75' away. I was told I need #4awg copper hot wires to feed this sub panel. I prefer NOT to install conduit. The feed wiring can run thru the attic space all the way to the new garage. Is NM 4-3w/ ground available? I can't find any such product online.
Thanks!
 
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Old 11-07-10, 08:51 AM
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#4 is defintely too small, regardless of using conduit, for 100 amps because this is not service entrance wiring. In my opinion, if you are using NM-B cable (romex), you'll need 2-3 w/ground. That would be 3 #2 copper conductors and a ground. Have you considered using aluminum SER cable?

Southwire - CU-NM-B
 
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Old 11-07-10, 04:21 PM
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25mm {#4 AWG } is way too small for 100 amp breaker.

Just wondering who told you to run this size ?? { I just hope not one of the big box guys ( they don't always give a correct infomation at all )}

It cheaper to get a SER quadplex cable than straight copper and it more common found in few big box store but of course electrical supply centre will have them in stock and to run this size you will need 50mm { #1 AWG } alum conductor with two pole breaker.

If some reason if they stock the 35mm {#2 AWG } SER you can use that but you will have to get a 90 amp breaker for this useage.


Merci.
Marc
 
  #4  
Old 11-07-10, 05:31 PM
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Originally Posted by french277V View Post
25mm {#4 AWG } is way too small for 100 amp breaker.

Just wondering who told you to run this size ??
An electrical inspector gave me the wire size from Table 5 Chapter 9 of the NEC (2008) - copper - #4 AWG THW for hots #6 for Neutral and #8 for ground.
aluminum - #2 for hots, #4 neutral, # 6 ground
If these sizes are not correct please indicate the proper sizes and quote a code source.
I discovered the "big box" store carries NM 4/3 w/ground in copper. If AL is significantly cheaper I may use it but I suspect the smaller copper is easier to work with for neat make-up.
Thanks again!
 
  #5  
Old 11-07-10, 06:18 PM
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Here is the wire chart from article 310. Normally you would use the 60C column for NM-b. The 90C column is usually used for derating purposes. Houston Wire & Cable Company - NEC Table 310.16
 
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Old 11-07-10, 08:07 PM
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senor-mouse .,

I was going to reply but Ray did provide the link and that is the correct answer and by the way UF and NM are rated at 60C { do not use the 75 or 90C rating that is not used in this purpose }

The THHN/THWN are rated at 75C per chart rating what Ray posted here. { you have to click on Ray's link to get that one }

Merci.
Marc
 
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Old 11-07-10, 08:12 PM
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Let me add it is very unlikely you would fail an inspection because you used wire larger then the inspector thought was minimum necessary.
 
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Old 11-08-10, 04:44 AM
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Go do a price comparision of 3-3 NM in copper, that would be 3 #3 wires with a #6 ground i would imagine, and compare that to 2-2-2-4 SER cable, in AL. i can almost gaurantee that the SER cable is cheaper by at least a dollar per foot.
 
  #9  
Old 11-08-10, 07:10 AM
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Great advice from everyone and I bookmarked the link to the chart.
Thank you!
 
  #10  
Old 11-08-10, 08:35 AM
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Originally Posted by senor-mouse View Post
An electrical inspector gave me the wire size from Table 5 Chapter 9 of the NEC (2008) - copper - #4 AWG THW for hots #6 for Neutral and #8 for ground.
The values the inspector gave you would be for a main service coming into the house, but not for a subpanel. Some jurisidictions still allow this method of sizing for subpanels, so #4 copper may be legal in your area. In most places it is not legal anymore.

The table for main service wire size is 310.15(B)(6); the table for everything else including subpanels is 310.16.
 
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