Calculate Feeder Size

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mstrlucky74

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I have a 3P 80A breaker from one panel feeding Panel B right next to it. What would be the correct way to calculate the feeder size? I'm assuming not off the 3P 80a breaker. Should I look at the connected loads of panel B. See attached. Thanks.
 

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For your estimating purposes, matching the feeder to the 80A breaker is sufficient. CU or AL and specific wiring methods according to whatever specifications you received.
 
mstrlucky74 said:
I have a 3P 80A breaker from one panel feeding Panel B right next to it. What would be the correct way to calculate the feeder size? I'm assuming not off the 3P 80a breaker. Should I look at the connected loads of panel B. See attached. Thanks.
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I don't quite understand why you will feed a 100A panel with an 80A breaker! Am I missing something here?
 
topgone said:
I don't quite understand why you will feed a 100A panel with an 80A breaker! Am I missing something here?
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Most commercial style panels are 100 amp minimum so as Digger stated if your calculated load is only 80 amps (or less) then your feeder only needs to be 80 amps. We've installed 100 amp panels with 60 amp feeders.
 
GoldDigger said:
To allow use of smaller wire if the calculated load on the panel is well below 80A?

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Given they are immediately adjacent to each other the two or three dollars difference in Copper cost isn't going to make any real difference. I would not even worry about it and make it one hundred amp circuit and be done with it. You never know what you might want to hook up there later. Some decisions are not worth spending more than about 20 seconds making.

However if you insist on going through the exercise you need to know what the calculated load is. The problem is you've already selected an 80 amp circuit breaker. I don't recall if 80 amps is a so-called standard size breaker rating or not but if it is you might have a little leeway in the ampacity of the conductors but for such a short distance it's just not worth the time to to even look at it especially since you already selected the 80 amp circuit breaker
 
infinity said:
Most commercial style panels are 100 amp minimum so as Digger stated if your calculated load is only 80 amps (or less) then your feeder only needs to be 80 amps. We've installed 100 amp panels with 60 amp feeders.
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If you are observant enough, you would see the calculated load (from the load schedule) is above 80 amps. Why choose a breaker with lower rating?:happysad:
 
topgone said:
If you are observant enough, you would see the calculated load (from the load schedule) is above 80 amps. Why choose a breaker with lower rating?:happysad:
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I cannot read his attachment so show me.
 
Max per phase in load schedule appears to be about 5.6kVA plus the unevaluated existing load on some circuits. Doesn't look like more than 80A to me unless you add the three phase numbers.

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GoldDigger said:
Max per phase in load schedule appears to be about 5.6kVA plus the unevaluated existing load on some circuits. Doesn't look like more than 80A to me unless you add the three phase numbers.

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IDK how you do things but granting you have 5.6kVA per phase, you should arrive at a line current of 80.89A! We don't choose 80A breaker for that.
 
topgone said:
IDK how you do things but granting you have 5.6kVA per phase, you should arrive at a line current of 80.89A! We don't choose 80A breaker for that.
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The panel schedule lists line to neutral loads.
5600 divided by 120 equals 47.
How do you do it?

P.S. All breakers are single pole, therefore all loads are line to neutral.

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