430.62 (Motor Feeder SCGF) Hypothetical Question

[wwhitney]

Say you have a feeder supplying just 2 motors, each with an FLC of 40A. Then 430.24 tells you that the feeder conductors need an ampacity of 1.25 * 40 + 40 = 90A. And with 90A feeder conductors, if the feeder OCPD is an inverse time breaker, 430.62 tells you that the OCPD may be up to 2.5 * 40 + 40 = 140A. [Which is not a standard size, and 430.62 does not permit the next size up, so you'd be limited to 125A.]

But nothing in Article 430 overrides 408.36, which has no exceptions for Article 430 applications, right?. So if a 125A feeder OCPD is used, an MLO panelboard that has houses the branch circuit breakers would need a bus rating of 125A, even though the feeder conductors supplying it only need a 90A ampacity? Somehow panelboard busbars are more delicate than wire-type conductors (which you could just tap twice for two separate disconnects with OCPD), or this is just a case of the NEC not covering every corner case?

[I guess in this example, if a 100A OCPD is used, then the downstream OCPD could possibly be omitted, turning the feeder into a branch circuit, under 430.53(B).]

Cheers, Wayne

 

[petersonra]

Say you have a feeder supplying just 2 motors, each with an FLC of 40A. Then 430.24 tells you that the feeder conductors need an ampacity of 1.25 * 40 + 40 = 90A. And with 90A feeder conductors, if the feeder OCPD is an inverse time breaker, 430.62 tells you that the OCPD may be up to 2.5 * 40 + 40 = 140A. [Which is not a standard size, and 430.62 does not permit the next size up, so you'd be limited to 125A.]

But nothing in Article 430 overrides 408.36, which has no exceptions for Article 430 applications, right?. So if a 125A feeder OCPD is used, an MLO panelboard that has houses the branch circuit breakers would need a bus rating of 125A, even though the feeder conductors supplying it only need a 90A ampacity? Somehow panelboard busbars are more delicate than wire-type conductors (which you could just tap twice for two separate disconnects with OCPD), or this is just a case of the NEC not covering every corner case?

[I guess in this example, if a 100A OCPD is used, then the downstream OCPD could possibly be omitted, turning the feeder into a branch circuit, under 430.53(B).]

Cheers, Wayne

I am not sure what your point is. 408.36 requires the PB bus be protected at its rating. Just because you have a motor in the circuit downstream does not change that requirement.

You to remember that the CB for the motor feeder is being protected from overload by the motor controller. There is no such protection for a typical PB, although in this case there would be, but not in the general case.

 

[wwhitney]

although in this case there would be, but not in the general case.

That's my point, that 408.36 could/should include an exception for this case. Unless there's some reason I'm missing that between the 125A feeder OCPD and the branch circuit OCPDs, busbar-type conductors should be required to have a higher rating than wire-type conductors?

Cheers, Wayne

 

[tortuga]

Somehow panelboard busbars are more delicate than wire-type conductors

Interesting
I might agree, that paneboards should be protected at their rating, wire type conductors ampacity table is slightly inflated from the get go, panelboard bussbars are sized by a UL table driven by manufacturers trying to save every last nickel.
Also panelboards busbars are a little more exposed to corrosion and subject to heat from bad connections.