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?
Sub Panel Feeder Sizing
- Thread starter csoc64
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- Location
- Massachusetts
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?
What makes you feel the entire 100 amps of load is continuous?
Do you even have a 100 amps of load at all?
- Location
- Logan, Utah
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
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
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.
There is no obvious need to use conductors with an ampacity greater than the 100 amp breaker so a #3 cu would be sufficient.
fmtjfw
Senior Member
- Location
- Fairmont, WV, USA
OR maybe the panel he is feeding the subpanel from won't hold any breaker smaller than 100A?
That's my take.
That is correct.
fmtjfw
Senior Member
- Location
- Fairmont, WV, USA
Only if it meets a tap rule.
Cow
Senior Member
- Location
- Eastern Oregon
- Occupation
- Electrician
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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