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Convenience disconnect switch has insufficient rating?

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I

Isaiah

Senior Member
Location
Baton Rouge
Occupation
Electrical Inspector
Construction wants to add a 400A convenience disconnect switch between the switchgear the vendor supplied Control Panel. The CB at the SWGR side is rated 800A with a 500AT
And the control panel contains a 600A fused switch. I told construction the disconnect needed to be rated 600A. But I couldn’t cite the applicable Code article or section.


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I

Isaiah

Senior Member
Location
Baton Rouge
Occupation
Electrical Inspector
ptonsparky said:
Depends. What is the actual load calculation at this time?

As long as the conductors are sized appropriately, the 400 is fine. Consider it a tap.
Click to expand...

I thought it had to be 115% of the sum of loads? Since the upstream CB is set at 500A, wouldn’t you need a disconnect rated min 500x115%=575A?


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R

retirede

Senior Member
Location
Illinois
Isaiah said:
I thought it had to be 115% of the sum of loads? Since the upstream CB is set at 500A, wouldn’t you need a disconnect rated min 500x115%=575A?


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Click to expand...

You’re conflating the calculated load with the rating of the OCPD. They are not necessarily the same.
 
I

Isaiah

Senior Member
Location
Baton Rouge
Occupation
Electrical Inspector
retirede said:
You’re conflating the calculated load with the rating of the OCPD. They are not necessarily the same.
Click to expand...

I reviewed the loads - the 500AT is accurate
I’ll go with 115% x 500 per 430.110(C)(1)


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J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Isaiah said:
I reviewed the loads - the 500AT is accurate
I’ll go with 115% x 500 per 430.110(C)(1)


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Click to expand...
Why not use the 400A fuse as your starting point?

In general I would have a the switch rated the same as the circuit conductors.
 
I

Isaiah

Senior Member
Location
Baton Rouge
Occupation
Electrical Inspector
jim dungar said:
Why not use the 400A fuse as your starting point?

In general I would have a the switch rated the same as the circuit conductors.
Click to expand...

There is no such 400A fuse -
The circuit conductors are sized to 600A


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E

Elect117

Senior Member
Location
California
Occupation
Engineer E.E. P.E.
Isaiah said:
I reviewed the loads - the 500AT is accurate
I’ll go with 115% x 500 per 430.110(C)(1)


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Click to expand...

You could also argue that their protection is not coordinated. Depends on the application. 240.12. That section is mostly meant for situations that would be considered emergency loads for emergency systems / legally required stand by systems.

You might be able to argue that the lower rated disconnect switch would create a personnel hazard when all other OCPD is rated higher. As in, the conductors are not being protected at their ampacity and it does not meet the definition of a tap.

Also, I would ask them who choose the 500AT setting on the 800A MCC. Did they higher an engineer? Like who puts a 500AT setting with a 600A fused switch for the load.
 
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