sub panel and 310.15(B)(7)2

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dm9289

Industrial Maintenance Electrician
Location
Pennsylvania
Occupation
Industrial process repair/ maintenance Electrician
For a sub panel will the 83% rule of the above reference apply. 310.15(B)(7)2

Example 200 amp main panel feeding 100amp sub panel.
Would you put an 100amp breaker in the main panel feed the sub panel with wire 100amp x .83= 83amp wire
 
dm9289 said:
For a sub panel will the 83% rule of the above reference apply. 310.15(B)(7)2

Example 200 amp main panel feeding 100amp sub panel.
Would you put an 100amp breaker in the main panel feed the sub panel with wire 100amp x .83= 83amp wire
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No. The 100A subpanel feed must have 100A wire. 310.15(B)(7)(2) would only apply to a feeder supplying the ENTIRE load associated with a dwelling unit.
 
david luchini said:
No. The 100A subpanel feed must have 100A wire. 310.15(B)(7)(2) would only apply to a feeder supplying the ENTIRE load associated with a dwelling unit.
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Thank you that's what i thought just trying to straighten myself out
 
Or with a 200A service, 310.15(B)(7)(3) would apply to any other 200A feeder on the premises, whether it is carrying the entire dwelling load or not. It would also allow a 175A feeder with an ampacity of only 200*.83 = 166A.

Cheers, Wayne
 
wwhitney said:
Or with a 200A service, 310.15(B)(7)(3) would apply to any other 200A feeder on the premises, whether it is carrying the entire dwelling load or not. It would also allow a 175A feeder with an ampacity of only 200*.83 = 166A.

Cheers, Wayne
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Good point. That would be if the supply of the the 200 amp service didn't use the 83% rule in the first place. If the service used #3/0 then you would need to use #3/0 for the 200 amp sub-panel. :)
 
infinity said:
Good point. That would be if the supply of the the 200 amp service didn't use the 83% rule in the first place. If the service used #3/0 then you would need to use #3/0 for the 200 amp sub-panel. :)
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I don't believe that the details of the actually installed 200 amp service conductors matter. 310.15(B)(7)(3) refers to the ampacity "specified in 310.15(B)(7)(1) or (2)." So even if you used 200A conductors for the 200A service, you could use 166A conductors for the 200A feeder.

Cheers, Wayne
 
wwhitney said:
Or with a 200A service, 310.15(B)(7)(3) would apply to any other 200A feeder on the premises, whether it is carrying the entire dwelling load or not. It would also allow a 175A feeder with an ampacity of only 200*.83 = 166A.

Cheers, Wayne
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Where my stuff gets odd is 200amp service feeding 100amp subpanel Underground to detached garage.
Local supply house jumps from #2 aluminum to 2/0 in sizes of UG mobil home feeder cable. They insist #2 Aluminum would be good for 100 amp feed. But its rated for 100amp in 90C column not 75C column that where more confusion came in. The cut sheets simply list it at 100 amp ampacity with no temp options I'm assuming it is only good for 90 amp at 75C feel free to blow up my logic
 
You are correct, the 83% factor doesn't apply to a 100A feeder on a 200A residential service. #2 Al would require protection at 90A (assuming no temperature or CCC derating).

Cheers, Wayne
 
dm9289 said:
Where my stuff gets odd is 200amp service feeding 100amp subpanel Underground to detached garage.
Local supply house jumps from #2 aluminum to 2/0 in sizes of UG mobil home feeder cable. They insist #2 Aluminum would be good for 100 amp feed. But its rated for 100amp in 90C column not 75C column that where more confusion came in. The cut sheets simply list it at 100 amp ampacity with no temp options I'm assuming it is only good for 90 amp at 75C feel free to blow up my logic
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90A is correct. You can install a 90A breaker in the main panel for your feeder and be compliant.
 
retirede said:
90A is correct. You can install a 90A breaker in the main panel for your feeder and be compliant.
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Mobil home feeder Its a popular UG option here in PA. I think where the warehouse get confused is I'm using it as sub feed many times its used as service entrance for trailers where the 83% can apply.
 
retirede said:
90A is correct. You can install a 90A breaker in the main panel for your feeder and be compliant.
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Plus i was going to get a 125amp sub to make sure terminals are 75C. 110.14 c wants me to assume 100 amp and under equipment 60C
 
dm9289 said:
Mobil home feeder Its a popular UG option here in PA. I think where the warehouse get confused is I'm using it as sub feed many times its used as service entrance for trailers where the 83% can apply.
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They're probably just getting the ampacity from a specification sheet. For instance, Southwire lists the ampacity of #2Al mobile home feeder as 100.

Just for informational purposes, look at Annex B, Table B.310.15(B)(2)(9).
 
david luchini said:
They're probably just getting the ampacity from a specification sheet. For instance, Southwire lists the ampacity of #2Al mobile home feeder as 100.

Just for informational purposes, look at Annex B, Table B.310.15(B)(2)(9).
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That is very interesting cause there it says 119amp at 60c
 
david luchini said:
They're probably just getting the ampacity from a specification sheet. For instance, Southwire lists the ampacity of #2Al mobile home feeder as 100.

Just for informational purposes, look at Annex B, Table B.310.15(B)(2)(9).
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oh whoops the 119 amps is directly buried in earth im assuming that means not in conduit.
 
dm9289 said:
That is very interesting cause there it says 119amp at 60c
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Yes, but that's based on a soil temperature of 20C instead of an air temperature of 30C, and a soil resistivity of 90.

Different soil temperature and resistivity would give a different ampacity.
 
david luchini said:
Yes, I assumed direct buried since you were talking about a direct buried type cable.

Try Table B.310.15(B)(2)(7) for conduit.
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I think i need detail 7 for triplexed . i think Table B.310.15(B)(2)(7) refers to the ducts having one wire and being separated by dirt.

Whoops again you can have 3 wire per duct so prbably 115 for Aluminum condition 1
 
david luchini said:
Yes, I assumed direct buried since you were talking about a direct buried type cable.

Try Table B.310.15(B)(2)(7) for conduit.
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I have no idea what the RHO 60-90-12 or LF 50-100 mean im going to have to look up
 
dm9289 said:
I have no idea what the RHO 60-90-12 or LF 50-100 mean im going to have to look up
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Don't forget that the Annex isn't actually part of the Code, it's just there for informational purposes.

I think you would need an engineer to calculate actual ampacity for the soil conditions of your installation.
 
david luchini said:
Don't forget that the Annex isn't actually part of the Code, it's just there for informational purposes.

I think you would need an engineer to calculate actual ampacity for the soil conditions of your installation.
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yes after looking more into it RHO LF i see what your saying
 
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