Sub Panel Feeder Sizing

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csoc64

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If I add a sub panel fed by a 100A breaker, am I correct that I need to feed with #1 thwn copper feeders? I am assuming 75 degree terminals and factoring in continuous load. 100 * 1.25 = 125A continuous load. From 310.15(B)(16), #2 copper is good for 115A, so I need to go up to the next size, #1 good for 130A. Am I correct here?
 
If you really have a 100 amps of continuous load you will need a panel with a higher rating.

What makes you feel the entire 100 amps of load is continuous?

Do you even have a 100 amps of load at all?
 
I agree with Bob,

You need to perform a load calculation to determine the feeder conductor size.

FWIW 210.20 (A) requires the breaker to be sized at 125% of the continuous load in addition to the feeder conductors, so if you actually have 100 amps of continuous load you would be required to have a 125 amp breaker.

Chris
 
iwire said:
If you really have a 100 amps of continuous load you will need a panel with a higher rating.

What makes you feel the entire 100 amps of load is continuous?

Do you even have a 100 amps of load at all?
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Sounds like I'm thinking backwards here. Bear with me, I don't do much of this. What I am doing is relocating a 40A 120/208 volt 3 phase beer cooler circuit to a dedicated mlo panel which only has three spaces, therefore there will be no additional loads possible. I have to feed with a 100A breaker because that is the minimum size breaker I can use for the panel frame. Assuming the cooler is a continuous load then I would only need to size for 40 X * 1.25 = 50A. Bus since I am limited to my 100A breaker feeding the mlo, looks like I'm good at #2. Is that correct?
 
Unless your feeder can qualify under one of the 240.21 tap rules, it will need to be sized to match the overcurrent protection.
There is no obvious need to use conductors with an ampacity greater than the 100 amp breaker so a #3 cu would be sufficient.
 
Maybe I am missing something but it sounds like he has a panel that has a rates busing of 100 amps and the op is assuming that you need to put a 100 amp breaker in it? My question is why don't you just run #6 copper and out it on a 60?
 
rhovee said:
Maybe I am missing something but it sounds like he has a panel that has a rates busing of 100 amps and the op is assuming that you need to put a 100 amp breaker in it? My question is why don't you just run #6 copper and out it on a 60?
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OR maybe the panel he is feeding the subpanel from won't hold any breaker smaller than 100A?
 
What about using the 100 amp breaker as a disconnect and landing the wire on a 60? Not sure how far it is but might be cheaper in wire and conduit size and the pulling factor.
 
rhovee said:
What about using the 100 amp breaker as a disconnect and landing the wire on a 60? Not sure how far it is but might be cheaper in wire and conduit size and the pulling factor.
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You can't run #6 just because it's landing on a 60 at the end of the circuit. The proper size OCPD has to be at the beginning of the circuit. Like fmtjfw said, that would make it a tap.
 
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