Sizing Motot Branch-Circuit Sc & GFP device according to 430.53B

[duck_huntr85]

Hey all,
This is my first post of many im sure. Its great having a forum full of fellow electricians and people of related fields. So heres the issue
MY boss and I were sizing out the Branch-circuit Short-circuit and Ground-fault protective device (which i will from now on abbreviate BC SC & GFP). WE have several 1hp 240v fan motors. I ran into "430.53 Several Motors or Loads on One Branch-Circuit". 430.53B states, verbatim:

If the bc sc &gfp device is selected not to exceed that allowed by 430.52 for the smallest rated motor, two or more motors or one or more motors and other loads, with each having individual overload protection, shall be permitted to be connected to a branch circuit where it can be determined that the bc sc & gfp device will not open under the most severe normal conditions of service that might be encountered.

There is no need to get into the math as i understand that. My problem is it says you cant exceed the rating of the bc sc & gfp device of the smallest motor sized according to 430.52 Not table 430.52 just 430.52.
WE are going to use an inverse time breaker. Which is 250% of your FLC according to Table 430.52. That number allows us to have two motors on one 20 amp breaker, assuming it can handle the inrush of two motors starting at once. BUt there is an exception to table 430.52 which states the rating of an inverse time breaker can increase to but not exceed 400% of the FLC. Using that excepiton we could fit one more motor on a single circuit (assuming ofcoarse that the breaker could handle the inrush of all the motors starting). Could we use that exception based on how 430.53B is worded? And i mean in general not just for my specific instance.
Thanks in advance for your replies

 

[augie47]

1ST welcome to the Forum
2nd I have a problem with your setup. 1 hp 240v per 430.250 is 4.2 FLA, 250% would be 10.5, so a 15 amp breaker would be you max.

3rd The exception to exceed 250% is for the starting current of "the motor" (singular), so you could not apply it for a group of motors, IMO.

 

[duck_huntr85]

single phase not three phase

single phase not three phase

Im sorry i didnt specify it was a single phase motor. The FLA for a single phase 1 hp motor is 8 amp according to table 430.248

And to your other comment, 430.53B states it cant exceed the sc&gfp device of the smallest motor that exception would apply when sizing the sc&gfp device of the smallest motor. IMO

 

[duck_huntr85]

Single phase not 3 phase

Single phase not 3 phase

I dont know if i didnt wait long enough for my first reply to post so this might be a repeat but. it is a single phase motor and according to table 430.248 its 8 FLA

And 430.53B says it must not exceed the smallest motors sc&gfp device sized according to 230.52, why wouldnt that motor apply to the exception? And the exception says nothing about why your allowed to bump up to 400%, only that you can.

 

[duck_huntr85]

disregard the last part of that message i read the exception all the way through and you were right about it only being if it wasnt sufficient to start the motor

 

[augie47]

duck hunter,
The question has come up a few times in actual inspections and each of those was 3 phase so my senile ole mind automatically went to three phase...sorry about that, but, as you note, the larger breaker would not meet NEC in either situation.