Short circuit

[cppoly]

When calculating available short circuit current, do fuses allow you to reduce the available short circuit on the load side using a fully rated system?

For example, if the utility short circuit is 100,000 and if calculating short circuit to be 80,000 for a panelboard, if you install a fused disconnect right before the panelboard which gives 22,000 on the load side let through current let's say, then can the panelboard be rated for 22,000 for a fully rated system OR is this considered series rated short circuit current?

 

[jim dungar]

... is this considered series rated short circuit current?

It is considered a series rated installation. Protective devices must be selected using the breaker manufacturer's published data.

 

[cppoly]

So under a fully rated system, you can't use a fuses let through current to reduce the available AIC seen by a panelboard?

 

[LarryFine]

So under a fully rated system, you can't use a fuses let through current to reduce the available AIC seen by a panelboard?

You can, but it needs to be a manufacturer-approved combination of equipment, such as a fuse type, and not just a fuse ampacity.

 

[jim dungar]

... it needs to be a manufacturer-approved combination of equipment....

More specifically, the series combination needs to come from the breaker manufacturer. It makes no difference what the fuse manufacturer says or what may be found in other manufacturer's equipment (e.g. transfer switches often have combinations that are different from those for general use).

 

[ron]

So under a fully rated system, you can't use a fuses let through current to reduce the available AIC seen by a panelboard?

It is easier to consider the answer no.
The fuses give you a series rated option to increase the rating of the OCPD with manufacturer guidance, not reduce the available fault current.

 

[don_resqcapt19]

Series rated systems must be listed and tested systems unless it is an existing system and you can get an registered professional engineer to sign off on a calculated system. 240.86