ECN Forum Forums Code Related Discussion NEC & other Code issues wire ampacity less than OCP

I'd imagine that the situation I am about to explain is code compliant. However, since I can make no sense of it myself, I am hoping somebody can explain it to me.

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110A breaker, feeding #3 Thhn conductors, feeding 80A fuses, feeding an elevator

The #3 in only good for 100A (75 deg. terminals). I realize that OCP is upsized beyond the ampacity of the wire it feeds all the time for motor loads. However, since 80 amp fuses are installed downstream, how can that be the case here? Is there ever going to be a time when a smaller, 100A breaker would prove problematic? Aren't the 80A fuses going to let go long before the breaker? More to the point, does the code allow the breaker to be "oversized" in this case? If motor loading is the engineer's reason behind requiring a 110A breaker, then how will the 80A fuses work? It seems to me that if the 80A fuses are sized correctly for their load then the breaker upstream should be sized for the conductor ampacity. Is a person allowed to have an infinite number of undersized feeders and fused disconnects in series even if the last disconnect is actually fused at less than the ampacity of the conductors?

This type of situation occurs throughout the project I am on right now and I have been told it is done all the time. I’m not sure if I have explained my position very well so let me know if you don’t understand.

I am making the assumption that the first circuit breaker is sized for the motor circuit and is for short circuit and ground fault protection. Further, I assume the downstream fuses are dual-element time-delay and are set close to provide backup protection for the motor. The motor overloads will protect the motor in most cases. Fuses sized close will provide loss of phase protection for the motor.

In other words, if all the rules of Article 430 are followed, it will work well. Without having all the specifics, we can't give you any more than that.

To me as per definitions to the ist overcurrent device ,that is a feeder. Comes under the rules of a feeder. (430.62)
Then from there to motor (430.22)
The wire to the ist Breaker must be sized per over current device, from the motor over current device the wire 125% the over current device per 250.52

Where I work we size the fuses protecting the motor at 125% of RLA minimum, 150% preferred. The circuit breaker can be up to but not over 400%. The fuses are time delay to discourage nuisance tripping on inrush or temporary (seconds) locked rotor amps, while the breaker is fast acting. This gives an operational gap between the fuse and circuit breaker trip points.

It will allow the fuses to avoid trips that are momentary fluctuations but allow protection if the motor actually locks up and the spike jumps up to the level the circuit breaker is at.

Hope that helps.

Rad welcome to the forum, glad to have you posting.

I do want to make it clear what is allowed for breaker ratings.

400 % is the absolute maximum for FLAs of 100 amps or less and that is only allowed if the motor will not start.

In general standard breakers are allowed to be 150% to 250% of FLA maximum.



Bob



[This message has been edited by iwire (edited 05-01-2004).]

I know how to calculate feeders and breaker/fuse size for motors. I think I have done a poor job of explaining my misunderstanding. First of all, I am not hooking up a motor; I am hooking up a listed unit. Thus, I would assume I would follow the nameplate and/or instructions. I would expect those items to state a maximum OCP and a minimum circuit ampacity. I searched briefly and could find no such information. I simply followed the engineered drawings and now I am trying to understand them. The disconnect inside the elevator mechanical room has 80A fuses. I feed those 80A fuses with #3 Cu THHN. Those threes terminate in a 110A breaker. My question is: why can the threes be sized for motor load when fuses sized 30A less are installed downstream? Do you think the company that manufactures the elevator requires this set-up? It seems to me that the conductors from the fused disconnect to the elevator equipment could be sized based on the rules established for motors. However, any conductor before that should meet the requirements of 310.16 should it not? Also, I realize that fuses and breakers do not react the same. If a conductor-sized, 100A breaker were used (in place of the current 110) what type of circumstance would cause it to trip before 80A, time-delay fuses? Thanks

I thought Charlie explained it?