yesterlectric
Senior Member
- Location
- PA
- Occupation
- Electrician
I know this has been discussed but in searches it seems it hasn't been answered. The difference between table 430.7(B) and 430.251(B). I want to state what I think is the intention and then state what I think they should do to the NEC to clarify.
One confusing thing is that the table has a range of given KVA per HP, but you are always supposed to select the max. However, 430.7B (not the table) states that the motor locked current code shall comply with the table. So it seems that the NEC is making a construction specification for motors. If this is what they are doing, it would seem they should move that to a separate section for construction specifications. For wiring methods, that is always at the end of an article.
Secondly, there's the general rule in 430.52 stating that the branch circuit short circuit ground fault device should be capable of carrying the starting current of the motor. Since table 430.251B says it's ONLY for use with 430.110 (disconnects not short circuit ground fault devices) then one would need to use table 430.7 for guidance. If you want to size an inverse time breaker less than 250% of the FLA, then you might want to look at that table (as well as the circuit breaker tcc curve) to determine if it could be a problem. I suggest a change to 430.52 as well to clarify this. I think they should put an Informational Note below 430.52 stating that table 430.7 has information on locked rotor current of motors. Putting it an an IN would help keep the 430.52 as a general performance rule (the device must be capable of carrying the starting current). This way you won't have an inspector coming and failing an inspection because the breaker wasn't sized per 430.7; and also you wouldn't have someone sizing a breaker per what they think would work based on 430.7, have a tripping breaker, and saying "well it was code compliant."
In summary, I think they should do 2 things:
1. They should create a construction specifications section at the end of article 430 like they do everywhere, and move table 430.7B to that.
2. They should put an Informational Note at section 430.52 to state that table 430.7 has information on locked rotor current.
One confusing thing is that the table has a range of given KVA per HP, but you are always supposed to select the max. However, 430.7B (not the table) states that the motor locked current code shall comply with the table. So it seems that the NEC is making a construction specification for motors. If this is what they are doing, it would seem they should move that to a separate section for construction specifications. For wiring methods, that is always at the end of an article.
Secondly, there's the general rule in 430.52 stating that the branch circuit short circuit ground fault device should be capable of carrying the starting current of the motor. Since table 430.251B says it's ONLY for use with 430.110 (disconnects not short circuit ground fault devices) then one would need to use table 430.7 for guidance. If you want to size an inverse time breaker less than 250% of the FLA, then you might want to look at that table (as well as the circuit breaker tcc curve) to determine if it could be a problem. I suggest a change to 430.52 as well to clarify this. I think they should put an Informational Note below 430.52 stating that table 430.7 has information on locked rotor current of motors. Putting it an an IN would help keep the 430.52 as a general performance rule (the device must be capable of carrying the starting current). This way you won't have an inspector coming and failing an inspection because the breaker wasn't sized per 430.7; and also you wouldn't have someone sizing a breaker per what they think would work based on 430.7, have a tripping breaker, and saying "well it was code compliant."
In summary, I think they should do 2 things:
1. They should create a construction specifications section at the end of article 430 like they do everywhere, and move table 430.7B to that.
2. They should put an Informational Note at section 430.52 to state that table 430.7 has information on locked rotor current.
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