over current protection devices

[Bullit]

i am a neca instructor and wondering if anyone can answer the following question~

"Using fusetron or low-peak dual-element, time delay fuses in the main service, determine the size of the main fuses and disconnect switch for a 480volt system with the following full load currents."
a. non continous non motor load 12A
b. continous non motor load 23A
C. motor #1-13A
D. motor #2-31A
E. motor #3-16A

 

[George Stolz]

Re: over current protection devices

"Using fusetron or low-peak dual-element, time delay fuses in the main service...

Wouldn't this only be relevant at the Branch Circuit level (per 430.52)?

a. non continous non motor load 12A
b. continous non motor load 23A
C. motor #1-13A
D. motor #2-31A
E. motor #3-16A

12 + (23 x 1.25) + 13 + 31 + 16 + (31 x .25)
12 + 28.75 + 13 + 31 + 16 + 7.75 = 108.5 amps

 

[Bullit]

Re: over current protection devices

George,
Please explain the mathimatical equation you used per article 430.52

 

[George Stolz]

Re: over current protection devices

I didn't make any allowance for 430.52 at all. Looking at just the title for Table 430.52, I come to the conclusion that this only applies to a motor branch circuit's short-circuit/ground-fault max OCPD percentage.

It looks like a smokescreen question to me; I've never seen a test question that was on the level that made statements about a "fusetron or dual element fuse" when quizzing about a load calc.

So, with that disregarded,

a. 12A @ 100%
b. 23A @ 125% since it's continuous
C. motor #1-13A @ 100%
D. motor #2-31A @ 125% per 430.24 (highest motor load @ 125%)
E. motor #3-16A @ 100%

Without better info about non-coincidence, lighting loads, etc.

 

[dillon3c]

Re: over current protection devices

Originally posted by Bullit:
i am a neca instructor and wondering if anyone can answer the following question~

"Using fusetron or low-peak dual-element, time delay fuses
a. non continous non motor load 12A
b. continous non motor load 23A
C. motor #1-13A
D. motor #2-31A
E. motor #3-16A

a)12ampere
b)23x1.25=28.75=29ampere
motors
c)13ampere
d)31x1.75=54.25= 50* ampere fuse
e)16ampere

d)50*
a)12
c)13
e)16
b)29
------
120 ampere

useing 430.63(3)/430.62 & 240.6
*fuse size 110 ampere

*disconnecting means rating 430.110
sum of the loads + 115%(no less than)

 

[George Stolz]

Re: over current protection devices

d)31x1.75=54.25= 50* ampere fuse

Where do you get 175% ? If it's from Table 430.52, we've got a serious lack of information to make such a leap. We don't know what type of motors we're dealing with.

useing 430.63(3)/430.62 & 240.6

I'm probably thick (awfully close to bedtime) but I fail to see what these sections are doing for us.

disconnecting means rating 430.110
sum of the loads + 115%(no less than)

But doesn't 430.110 relate to the "disconnecting means for motor circuits, not services?

 

[jwelectric]

Re: over current protection devices

430.62 Rating or Setting ? Motor Load.
(A) Specific Load. A feeder supplying a specific fixed motor load(s) and consisting of conductor sizes based on 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 short-circuit and ground-fault protective device for any motor supplied by the feeder [based on the maximum permitted value for the specific type of a protective device in accordance with 430.52, or 440.22(A) for hermetic refrigerant motor-compressors], plus the sum of the full-load currents of the other motors of the group.