Calculated loads & circuit breaker sizing

[SCOOBY73]

Hello All, I am struggling with some math calculations that seem to contradict each other. I have a retail building that has a 490A connected load. Of the 490A, 360A is continuous and 130A will be non-continuous. See my math below.

[FONT=&quot]Circuit Breaker Sizing[/FONT]
[FONT=&quot]360A X 125% = 450A + 130A @ 100% = 580A[/FONT]
[FONT=&quot] [/FONT]
[FONT=&quot]Conductor Sizing[/FONT]
[FONT=&quot]580A X 125% = 725A = (3) 250MCM = 765A[/FONT]
[FONT=&quot] (2) 500MCM = 760A[/FONT]


By this math, I can use a 600A main service disconnect but when I apply the 80% rule to this thermal magnetic breaker I only have 480A to use. As you can see in my initial explanation, I have a 490A load. Should I just be concerned with the continuous load when applying my calculations? I know in the real world a 600A service will be fine but I am concerned with an inspector applying the 80% rule and saying that the service isn't large enough.

Any help would be appreciated. Thanks

 

[jumper]

Circuit Breaker Sizing
360A X 125% = 450A + 130A @ 100% = 580A

Correct

Conductor Sizing
580A X 125% = 725A = (3) 250MCM = 765A

Why are applying 125% here? There is no all encompassing 80% rule per se.

 

[charlie b]

I thought at first that you were double-counting the 125% factor. Now I think you are triple-counting it. You only need to apply it once. Your first calculation, with 580 as the result, is all you need. You can size the conductor for 580 amps and use a 600 amp breaker.

As to what you are calling the 80% factor, the one that tells you that 80% of 600 amps is lower than your connected load of 490, that is irrelevant. The breaker and the conductor are sized for 125% continuous and 100% non-continuous, and that is all that is needed. You arrive at the 80% restriction only when all of the load is continuous. If all the load is non-continuous, you can take it right up to the breaker's rating of 600 amps.

 

[SCOOBY73]

Excellent, thank you very much. I was just concerned that a site visit might cause some concern if an inspector takes the breaker value at 80%. I now understand that it has more to do with the continuous load than the non-continuous load. Thanks again.