#14 Cu on a 100A breaker

[wwhitney]

As I explained thats not the way I interpret it, but it will be interesting to see what happens with your PI, I think we all agree it could be more clearly addressed.

Not following you: Feeder supplies two motors of 40A FLC, nothing else. 430.24 says the conductors must have 90A ampacity. 430.62(A) says the OCPD may be up to 140A, so lets use 125A. The 125A OCPD provides SC/GF protection. The two downstream motor overloads provide overload protection. Protection is split.

Cheers, Wayne

 

[wwhitney]

For example, a 400A 120/208V 3ph panel board with a mixture of motors, AC equipment for freezers, freezer door heaters and maybe some forklift chargers. The largest motor is 150A (with 125%) and the other loads (continuous or otherwise) added up to 300A.

OK, other loads are 150A, total is 300A load.

IF you used 430.62(A) as the method (2.5* largest motor + everything else)

But 430.62(A) doesn't say that, it says 2.5 * largest motor plus all the other motors. It ignores everything else. That's why it's obvious that 430.62(A) doesn't apply when there is any non-motor load.

I wouldn't say you could 2x1/0 Cu or 2x2/0 and a 400A breaker for that panel. 400A OCPD meets 408.36 but the terminations at the load end could fail prior to that protecting the feeder.

Say for discussion the non-motor loads are 50A. If those non-motor loads are protected by a 50A breaker from overload, then the 300A conductors supplying this 400A panelboard are protected from overload by the combination of that 50A breaker and all the motor overloads.

I agree there is some concern when the non-motor loads do not have sharp aggregate overload protection like that, e.g. the 50A of non-motor loads is spread over (5) 20A branch circuit breakers. But it seems to me that if that arrangement requires a larger conductor size, then 430.24 needs changing: either to limit its scope to branch circuits and motor-only feeders, or base the contribution of non-motor loads on the aggregate downstream overload protection provided for them, or something like that.

Cheers, Wayne

 

[tortuga]

Since were pivoting to discussing the physics / theory of feeder overload protection, reading the old 1950 and 1953 sections I could see folding these sections into article 120 (Formerly 220) and 215, that is if there is a use case for split protection of a motor only feeder (which I dont see from where I sit). Keep 430 focused on motors and motor branch circuits. But to me a 'motor only feeder' seems a fragile thing, put in one 20A breaker for a GFCI and its no longer a 'motor only' feeder.

 

[wwhitney]

reading the old 1950 and 1953 sections

Where are you finding this? The oldest ROP information I find on nfpa.org/70 is for 1956.

Cheers, Wayne