Busbar "sizing"

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dennis5150

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Does busbar have to be derated similar to conductors, in that they shall have an allowable ampacity of 125% of the continuous load, or is the derating for insulated "conductor" only?
 
I would say that a busbar is a conductor.

What context are you asking this question, is this in regards to busbars as part of panels, or busbars that are part of a busway?

Chris
 
dennis5150 said:
Does busbar have to be derated similar to conductors, in that they shall have an allowable ampacity of 125% of the continuous load, or is the derating for insulated "conductor" only?
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If you look at 210.20.(A), its the OC device that has the 125% factor and the conductor is chosen to match.
 
Take a look at 368.17 for overcurrent protection for busways.

368.17(D) States that a busway used as a branch circuit shall be protected against overcurrent in accordance with 210.20 which in turn will require that the busway be sized for the noncontinious load plus 125% of the continious load.

Chris
 
Busbar can continuously carry the rated current of the equipment.
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So can an insulated wire. The reason for the 125% for continious load is due to the fact that most overcurrent devices aren't rated to carry 100% of the load continiously.

Chris
 
raider1 said:
So can an insulated wire. The reason for the 125% for continious load is due to the fact that most overcurrent devices aren't rated to carry 100% of the load continiously.

Chris
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Understood, but the OP asked whether the busbar had to be derated, and it does not.

Also, the overcurrent devices are capable of continuously carrying 100% of the rated current as well, provided they are installed in the manner in which they are listed. Unfortunately, many equipment enclosures, especially panelboards, cannot disipate the heat and therefore you can only load the devices to 80%, essentially derating for heating effect.

Reviewing the trip curve for a breaker will reveal it's capability to carry fully rated current, the error is on the plus side of the curve.
 
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