Section 240.86(C) motor contributions

[piccard1898]

I interpret part 2 of 240.86(C) as:
the lower rated circuit breaker cannot have a load that exceeds 1% of that circuit breaker's AIC rating
(So I consider the "sum of the motor full-load currents" as a piece of equipment with multiple motors being fed with 1 circuit breaker).
Examples:
The system in these examples series rates to 65KAIC @ 277/480V (Available utility fault current is 30KAIC)
The downstream lower-rated circuit breaker is 14KAIC

Example1:
200A, 3P C/B in distribution section feeds panel A.
Panel A has:
(3) 80A, 3P circuit breakers that each feed a 30HP motor @ 40FLA = 120FLA
(1) 80A, 3P circuit breaker that feeds a piece of equipment with multiple motors for and 50FLA
So panel 'A' has a total motor load of 170FLA.

Example 2:
400A C/B in distribution section feeds panel B.
Panel 'B' has:
(1) 250A, 3P subfeed circuit breaker feeding a piece of equipment with multiple motors totaling @ 160FLA.
(For this example, the 250A subfeed circuit breaker is rated @ 14KAIC)

My interpretation of 240.86(C) is that example 1 is OK because each individual load on the downstream circuit breaker is less than 1% of its AIC rating.
Example 2 would violate the section because the load on the downstream circuit breaker exceeds the 1% of its AIC.

Can someone please clarify this for me.
If I am correct, how can I argue this to the planchecker????
Thank you.

 

[jim dungar]

This section is dealing with series combination ratings of circuit breakers, it is concerned with motor fault contribution that is not seen by the upstream device.

I consider the full motor load at the downstream location, not the motor load through a single device.

 

[Julius Right]

If the available fault current will be higher than downstream breaker rated then will be locked closed and the protection will be transferred to the upstream breaker. Then no damage will occur to the downstream breaker. Nevertheless, another condition is imposed:
The sum of rated motors currents supplied after upstream breaker and before downstream breaker will be less than 1% of the downstream breaker rated. That means if the upstream breaker is already open, the source of motors is still present and-as downstream breaker is still locked- will supply- for a 1/2 wave ?the fault .Then this article limited -only up to 5-8% of breaker rating- the fault current. An induction motor will supply for 1/2 period the locked rotor current, but its breaker will not open in this short time, so will reach the fault point.
It is not so clear how your diagram is.
In the first case, the upstream breaker is 65kAIC rated and the downstream is 14kAIC, I guess.
The sum of 4 rated current of motors-connected at load side of the upstream breaker and at line side of the downstream breaker- is 3*80+170=290 A.
290 A represents approx... 2% [290*100/14000=2.07%] that means it is not permitted.
In the second case, if the subfeed Panel B it is another downstream breaker and all the induction motors are connected at the load side of this circuit breaker then these motors are not supplying the fault through this circuit breaker-they are not important.

 

[jim dungar]

If the available fault current will be higher than downstream breaker rated then will be locked closed

Series-combination ratings, of molded case breakers, have nothing to do with the sequence in which the protective devices operate. In fact if is extremely likely that both devices operate, followed by the down stream device operating and the upstream device remaining closed. I believe it is fairly rare to have the upstream device open while the downstream one stays closed.

 

[piccard1898]

Thank You for the responses

Thank You for the responses

Thank you all for the responses.
I have since found a better diagram and description that shows the multiple motor loads.
I can now understand how the motor load contribution is figured into the short-circuit current for the system.
I think that the poor example/illustration given in the 2005 NEC handbook (exhibit 240.14) led to my confusion.
Then again, I have enough people telling me what a dumbazz I can be without adding to the argument myself.
Thanks again and have a great day.