motor feeder calculation OCPD questions

[ericsherman37]

I'm studying (cramming actually) for my Washington administrator's license exam. I was hoping to schedule it for roughly 3 weeks from now, to give me ample time to brush up on my NEC and calculations and whatnot, but the only testing date I could get in the near future is this Wednesday the 20th. Eek!

So I'm practicing my motor calculations and came up with a couple questions.

First, 430.62(A) says that a feeder supplying a specific fixed motor load with conductors sized per 430.24 shall be provided with a protective device having a rating or setting not greater than the largest rating or setting of the branch circuit OCPD for any motor supplied by the feeder, plus the sum of the FLCs of the remaining motors. So my question is, does this mean that 240.4(B) might not apply? Is the NEC saying that if my calculated specific fixed motor feeder load does not correspond with a standard OCPD size, I can't bump it up to the next standard size like we are usually allowed? For example, if I calculated a specific fixed motor feeder load at say, 96 amps, then I couldn't put it on a 100 amp OCPD. Am I interpreting this correctly?

Second, if I'm doing a calculation for feeders to, say, a subpanel that supplies motors and other loads (basically, a normal feeder not supplying specific fixed motor loads only) would my feeder OCPD then be based on 430.63?

Third, if I'm doing a service calculation for a single-family dwelling unit, is 220.50 telling me that I need to figure out which is my largest motor supplied by the service, figure it at 125%, and then have the rest of my load calculation added to it? What if the largest motor was just like the garbage disposal or something? Wouldn't this hypothetical garbage disposal be one of my appliances fixed in place? If I calculated it as an appliance and as a motor, then it would be redundant; and if counting it as the largest motor screwed me out of the 4-or-more appliance 75% demand factor then that would suck. Could someone elucidate this for me?

Thank you in advance!

 

[LEO2854]

I'm studying (cramming actually) for my Washington administrator's license exam. I was hoping to schedule it for roughly 3 weeks from now, to give me ample time to brush up on my NEC and calculations and whatnot, but the only testing date I could get in the near future is this Wednesday the 20th. Eek!

So I'm practicing my motor calculations and came up with a couple questions.

First, 430.62(A) says that a feeder supplying a specific fixed motor load with conductors sized per 430.24 shall be provided with a protective device having a rating or setting not greater than the largest rating or setting of the branch circuit OCPD for any motor supplied by the feeder, plus the sum of the FLCs of the remaining motors. So my question is, does this mean that 240.4(B) might not apply? Is the NEC saying that if my calculated specific fixed motor feeder load does not correspond with a standard OCPD size, I can't bump it up to the next standard size like we are usually allowed? For example, if I calculated a specific fixed motor feeder load at say, 96 amps, then I couldn't put it on a 100 amp OCPD. Am I interpreting this correctly?

Second, if I'm doing a calculation for feeders to, say, a subpanel that supplies motors and other loads (basically, a normal feeder not supplying specific fixed motor loads only) would my feeder OCPD then be based on 430.63?

Third, if I'm doing a service calculation for a single-family dwelling unit, is 220.50 telling me that I need to figure out which is my largest motor supplied by the service, figure it at 125%, and then have the rest of my load calculation added to it? What if the largest motor was just like the garbage disposal or something? Wouldn't this hypothetical garbage disposal be one of my appliances fixed in place? If I calculated it as an appliance and as a motor, then it would be redundant; and if counting it as the largest motor screwed me out of the 4-or-more appliance 75% demand factor then that would suck. Could someone elucidate this for me?

Thank you in advance!

Read the commontery after240.4(B) in the 2008 hand book
Good luck on the test you will pass..:grin::grin:

 

[Smart $]

Third, if I'm doing a service calculation for a single-family dwelling unit, is 220.50 telling me that I need to figure out which is my largest motor supplied by the service, figure it at 125%, and then have the rest of my load calculation added to it? What if the largest motor was just like the garbage disposal or something? Wouldn't this hypothetical garbage disposal be one of my appliances fixed in place? If I calculated it as an appliance and as a motor, then it would be redundant; and if counting it as the largest motor screwed me out of the 4-or-more appliance 75% demand factor then that would suck. Could someone elucidate this for me?

Thank you in advance!

First, you are interpretting the 430.62(A) protective device rating correctly... IMO.

Second, yes... but your feeder OCPD would be based on 430.63(B) and the minimum feeder conductor ampacity is determined under Article 220 load calcs. So your feeder ampacity will be greater than determined by 430.24. I know, wording and arrangement uses circular logic, or should that be circular illogic.

Third, do not duplicate loads in your calcs. For the purpose of load calculation, appliances with motors are calculated as appliances... again IMO.

 

[jusme123]

First, 430.62(A) says that a feeder supplying a specific fixed motor load with conductors sized per 430.24 shall be provided with a protective device having a rating or setting not greater than the largest rating or setting of the branch circuit OCPD for any motor supplied by the feeder, plus the sum of the FLCs of the remaining motors. So my question is, does this mean that 240.4(B) might not apply? Is the NEC saying that if my calculated specific fixed motor feeder load does not correspond with a standard OCPD size, I can't bump it up to the next standard size like we are usually allowed? For example, if I calculated a specific fixed motor feeder load at say, 96 amps, then I couldn't put it on a 100 amp OCPD. Am I interpreting this correctly?

Second, if I'm doing a calculation for feeders to, say, a subpanel that supplies motors and other loads (basically, a normal feeder not supplying specific fixed motor loads only) would my feeder OCPD then be based on 430.63?

Third, if I'm doing a service calculation for a single-family dwelling unit, is 220.50 telling me that I need to figure out which is my largest motor supplied by the service, figure it at 125%, and then have the rest of my load calculation added to it? What if the largest motor was just like the garbage disposal or something? Wouldn't this hypothetical garbage disposal be one of my appliances fixed in place? If I calculated it as an appliance and as a motor, then it would be redundant; and if counting it as the largest motor screwed me out of the 4-or-more appliance 75% demand factor then that would suck. Could someone elucidate this for me?











Look over this example;

What size feeder conductor is required for two -7.5 hp, 230v, 3 phase motors, if the terminal are rated 75 degrees?

motor flc= 22a (table 430.250)
motor feeder conductor= (22A x 1.25) + 22
motor feeder conductor= 49.5a, #8 awg, rated 50a at 75 degrees(table 310.16)

the feeder overcurrent device (inverse time ckt breaker) must comply with 430.62 as follows:

step 1. determine the largest branch circuit overcurrent device rating(240.6(a), and 430.52(c)(1) Ex. 1
22a x 2.5= 55a, next size up 60a

step 2. size the feeder overcurrent devicein with 240.6(a) and 430.62

feeder inverse time breaker; 60a + 22a = 82a
next size down 80a

hope this helps