NEC 440.4 B) Exception No. 3

[fifty60]

Can someone please give me some insight as to why this exception exists? "Multimotor and combination load equipment used in one and two family dwellings, cord and attachment plug connected equipment, or equipment supplied from a branch circuit protected at 60A or less shall not be required to be marked with a short circuit current rating"

Is there generally not enough let-through current to be of a concern to components in this type of equipment?

 

[beanland]

Cynical Answer

Cynical Answer

It was probably onerous for the equipment manufacturers to provide labeling so they had the code changed to add the exception. "Follow the money!"

 

[fifty60]

There are not any technical reasons behind these exceptions?

 

[beanland]

Short circuit current rating

Short circuit current rating

The short-circuit current rating would be the interrupting rating of the overcurrent device in the equipment. The assumption is that in Exception 3 situations, the available fault current is always going to be below the interrupting rating of the overcurrent device. That is probably valid for residential and/or cord connected equipment, the small conductor would lead to low available fault current. The 60A MCB exception is a risk because the AFC on the output terminals of a 60A breaker in a large service could be substantial; the assumption is that the AC is not at the panel but some distance away and is fed through small conductors that lower the AFC.

It is also meaningless to the residential installer to know the interrupter rating because they have no clue what the AFC is. Having to work through those calculations for every window-AC unit installation would be onerous.