Mike Dolt Enterprises
Mike Dolt Enterprises
User: jroman, Location: New York, NY
Asked on: Tuesday, April 19, 2011 at 2:54 pm EDT
Answered on: Tuesday, May 14, 2024 at 4:24 am EDT
Question: Hello,I have a question that I haven't been able to get my mind around.When protecting motor circuits we design both for short circuit (over current) and over load. Each has different requirements per NEC. If I have a panelboard that supplies one motor, how do I determine the size of the breaker via Short ckt propetection or over load protection? To elaborate further, the motor would be installed on the roof, and the panel board is on the floor below.Assumming a Design B, 15 hp, 208V three phase squirrel cage motor with a 1.15 service factor. Nameplate current of 40.2 A and dual element fused disconnect switch for short circuit in weather proof enclosure.Per table 430.250, the rated current is 46.2A. Per table 430.52 the sc setting would be 80.85A, we could then fuse it at 90A.How to size the feeder coming from the panel board? Per 430.62 the feeder sc circuit protection shall not exceed the rating of the highest short circuit protection plus any additional loads. The handbook states that it is assumed the same type of proptective device is used. So, if I was protecting the feeder with another fused disconnect I would not have any confussion on the matter, it should not exceed 90A. Since I am using an inverse time breaker instead, how is it evaluated?Are we to take 430.62 on a relative term? Meaning, if table 430.52 calls for 175%for dual element fuses and 250% for inverse time breakers, are we to "transform" the fused disconnect rating to its corresponding inverse time breaker and size the feeder protection accordingly? In this particular example, the 90A dual element fuses would have been 125A on an inverse time breaker. So my feeder protection breaker should not exceed 125A.Or, as we to take 430.62 literally? On this example, with the 90A fuses, my breaker shall not exceed 90A. But since the short circuit requirement difference between these two devices is due to their ability to withstand the inrush current, am I risking the breaker to open during inrush?Which one is it? Any thoughts?Thanks.Jose
Determining Breaker Size
For motor circuits, the breaker size is typically determined by the overload protection requirement. This is because overload protection is more critical for protecting the motor from damage due to excessive current over time.
Overload Protection
Per NEC Table 430.250, the rated current for a 15 hp, 208V, three-phase squirrel cage motor with a 1.15 service factor is 46.2A. Therefore, the breaker size for overload protection should be larger than 46.2A.
Short Circuit Protection
NEC Table 430.52 specifies the minimum short circuit protection settings for motors. For dual-element fuses, the setting should be 175% of the rated current, which is 80.85A. Since the next standard fuse size is 90A, that would be the appropriate fuse rating for short circuit protection.
Feeder Protection
NEC Section 430.62 states that the feeder short circuit protection rating should not exceed the rating of the highest short circuit protection device plus any additional loads.
Interpretation of 430.62
The intent of 430.62 is to ensure that the feeder protection is coordinated with the motor branch circuit protection. This means that the feeder protection should not allow overcurrents that could damage the motor or its equipment.In your example, since the short circuit protection for the branch circuit is a 90A fused disconnect, the feeder protection should not exceed 90A.
Inrush Current
Inverse time breakers have a higher inrush current rating than dual-element fuses. Therefore, using an inverse time breaker for feeder protection would provide better protection against nuisance trips due to inrush current. However, it is important to ensure that the breaker has a sufficient inrush current rating for the motor.
Conclusion
In your case, the appropriate breaker size for the feeder would be 90A. This is because: Overload protection requires a breaker larger than 46.2A. Short circuit protection for the motor circuit is 90A fused disconnect. NEC 430.62 requires feeder protection to not exceed the rating of the highest short circuit protection device (90A). Using an inverse time breaker for feeder protection would provide better coordination with the motor branch circuit protection and allow for higher inrush currents. Flag for review