240 and 310.15

[richie921]

I believe the issue of 'presumptive' loads/temps/ratings etc were part of the estimating process vs test taking process - the worst-case scenario took precedence in this case.

Agreed

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[Dale001289]

If the vendor panel's supply is via a 30A breaker and #10 Cu it does not matter that the load side #10 is protected via a 32A breaker as it is protected by the 30A supply breaker.

That would stand to reason if we're considering overcurrent protection only. But what about short-circuit and coordination? I'm sure we'd want the 32A CB to trip before the 30A CB...in your example, you're basically saying the 32A doesn't exist.

 

[Smart $]

That would stand to reason if we're considering overcurrent protection only. But what about short-circuit and coordination? I'm sure we'd want the 32A CB to trip before the 30A CB...in your example, you're basically saying the 32A doesn't exist.

In my "example" I am only considering Code compliance. If you want selective coordination then, by all means, replace the vendor panel breaker with one of a lower value. If you replace it with an NRTL listed standard 30A you will have to compare TCC's to determine which would trip first. If you replace it with an IEC 30A breaker, chances are the supply breaker will still trip first.

 

[Dale001289]

Changing the vendor supplied CB will require us to jump through too many hoops and red tape. I believe, in a perfect world we change out the 30A CB in the existing power panel and install a 40A CB and feed with #8Awg cable. We're only talking about 150 feet or so.


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[Dennis Alwon]

If the unit is fed with a 30 amp breaker then it is compliant as the 30 amp device will protect the wire

Perhaps, but the cable on load side of the 32A CB is also rated 30A - don't think this is 'kosher'.

It doesn;t matter because it is still protected by a 30 amp overcurrent protective device.

 

[Dale001289]

'Protected' and good design are not always one in the same.


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[Dale001289]

I don't think this client will ever approve of simply disregarding the vendor supplied CB...there are too many grey areas involved


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[jim dungar]

Changing the vendor supplied CB will require us to jump through too many hoops and red tape. I believe, in a perfect world we change out the 30A CB in the existing power panel and install a 40A CB and feed with #8Awg cable.

There ain't no way you will ever get a 32A supplementary thermal-magnetic breaker to selectively coordinate with a 40A feeder thermal-magnetic breaker.
For many types of protective devices, a good rule of thumb is 'the upstream device needs to be at least 1 'frame size' bigger than the downstream device. Another common rule is 'a minimum of 2x difference in the ratings'.

For actual selective coordination, you need to know the available fault current as well as the time-current curves.

 

[Dale001289]

Thanks for the heads up Jim. We'll run it through Etap


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