100A main panel to sub panel quesiton

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Old 05-02-13, 11:11 AM
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100A main panel to sub panel question

Hi all, have a quick question just to confirm what my electrician told me.

My residential home's main panel to sub panel are interconnected with a THHN gauge 4. They are opposite sides of our garage wall, so the wire run should be less than a foot. Will this be able to handle 100A? My electrician said that yes, it will be able to handle it.

Thank you.
 

Last edited by Steve T MQ; 05-02-13 at 11:32 AM.
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Old 05-02-13, 11:45 AM
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4 AWG copper THHN (rated at 90 degrees C) used as a feeder is rated at 100 amps according to table 310.15(B)(7). 310.15(B)(16) shows it at 95 amps, but (B)(7) addresses feeders, which is how yours is being used, feeding from a main panel to a sub-panel.

Mod Note: (B)(7) cannot be used for subpanels unless it carries the entire load of the dwelling. 310.15(B)(16) needs to be used.
 

Last edited by pcboss; 05-02-13 at 01:16 PM.
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Old 05-02-13, 11:53 AM
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Sweet, thank you! Now I feel quite safe =)
 
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Old 05-02-13, 11:58 AM
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Welcome to the forums, Steve!

My residential home's main panel to sub panel are interconnected with a THHN gauge 4. They are opposite sides of our garage wall, so the wire run should be less than a foot. Will this be able to handle 100A? My electrician said that yes, it will be able to handle it.
Originally Posted by SolarEd
4 AWG copper THHN (rated at 90 degrees C) used as a feeder is rated at 100 amps according to table 310.15(B)(7). 310.15(B)(16) shows it at 95 amps, but (B)(7) addresses feeders, which is how yours is being used, feeding from a main panel to a sub-panel.
Well, son of a gun, so it does!

Steve. I would caution you that the lugs that the conductors terminate to also need to be rated at 90[SUP]o[/SUP]C.
 
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Old 05-02-13, 12:03 PM
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Not to get off subject...but, Nash...could you clarify something? Higher temp means a lower rating? Is that ambient temp (unlikely I guess) or temp of the conductors when in use? (I never really understood that) I always thought conductors flowed better when cold?
 
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Old 05-02-13, 12:27 PM
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Yes, it is the temp of the conductors and their terminals under load.

Just to stick with #4 copper as an example, you can load it up to 95A, which really works out to 100A because 95A fuses and breakers don't exist and you're allowed to go up to the next level of overcurrent protection that does exist - if the wire and everything it's connected to is rated to be able to stand 90[SUP]o[/SUP]C. If any component is only rated up to 75oC, #4 is limited to 85A. If any piece can't stand the heat above 60[SUP]o[/SUP]C then it's 70A tops.

For the metrically challenged, 90[SUP]o[/SUP]C = 194[SUP]o[/SUP]F, 75[SUP]o[/SUP]C = 167[SUP]o[/SUP]F and 60[SUP]o[/SUP]C = 140[SUP]o[/SUP]F.

And yes, if the pesky things didn't heat up when we put'em to work, they could carry more current. Or they could do that if we were willing to invest in what it would take to cool them down while they're under lead. There's a good ROI on that inside a computer. Between two panels in a garage, not so much.

Clear as mud now?
 
  #7  
Old 05-02-13, 12:32 PM
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Thanks Nash, so all I gotta do is check the markings on the subpanel box and make it has the 90'C marking?
 
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Old 05-02-13, 12:37 PM
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Lol...yep...got it. Basically...it's not just the wire. If I'd seen your edit, I wouldn't have even asked.
 
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Old 05-02-13, 01:13 PM
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Table 310.15(B)(7) cannot be used unless it carries the entire load of the dwelling. Since this is a subpanel the OP needs to use 310.16 for the correct ampacity. #3 copper THHN would be the correct size for 100 amps since the lugs in the panel will not be 90 degree rated.
 
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Old 05-02-13, 02:12 PM
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Thanks Nash, so all I gotta do is check the markings on the subpanel box and make it has the 90'C marking?
PC beat me to it but there is no need to look at the terminals; I can guarantee that they are NOT rated for 90 degrees C.
 
  #11  
Old 05-02-13, 07:14 PM
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You're right pcboss, I over looked the part about the feeder supplying power to dwellings.
 
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Old 05-03-13, 04:24 AM
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No problem Ed. We all try to help and catch things in a helpful manner.
 
  #13  
Old 05-15-13, 08:27 AM
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Thanks all for the clarification =)
 
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