Disconnect Sizing

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Alwayslearningelec

Senior Member
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NJ
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Estimator
What's the calculation for sizing disconnect switch in this scenario?

250A MCB feeding disconnect. The disconnect then feeds a 25AMCB panel.
Curious about the calculation but also asking as, I believe 250A is not standard disconnect size...200 then 400A
 
You are correct, after 200, 400 is the next standard size.
A 400 amp non-fuse could be used or a 400 amp with 250 amp fuses if desired.
 
horsegoer said:
What's the calculation for sizing disconnect switch in this scenario?

250A MCB feeding disconnect. The disconnect then feeds a 25AMCB panel.
Curious about the calculation but also asking as, I believe 250A is not standard disconnect size...200 then 400A
Click to expand...
Did you mean 250A panel?
 
augie47 said:
You are correct, after 200, 400 is the next standard size.
A 400 amp non-fuse could be used or a 400 amp with 250 amp fuses if desired.
Click to expand...
I would go with the 400A disconnect fused at 250A option. If the fault current information is available and you know what the upstream circuit breaker is, it is possible that it will series rate. If you use class R or J fuses, the disconnect switch will be rated to 200KA and you don't have to worry about it. Typically, non-fused disconnect switches have limited withstanding rating, so you have to be careful where you place them.
 
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