feeder circuits

[j_braddock]

i do not understand one of the examples i am working with in regards to feeder sizing & protection.

any help would be greatly appreciated.

230 volt system, single phase, inverse time breaker for feeder.

the system invloves two motors 5 hp & 3 hp.
1. the motor full load currents are 28 for 5 hp & 17 for 3 hp. table 430.148
2. the branch circuit protection is 28 * 2.5 = 70 amps for the 5 hp & 17 * 2.5 = 42.5 (next size up) = 45 amps. based on 430.52(c)1, table 430.52, and 240.6(a).
3. feeder conductor is 28 * 1.25 + 17 = 52 amps. table 310.16 allows for a #6 rated 55 amps at 60 deg c.
4. feeder protection - 70 amps + 17 amps = 87 amps (next size down) = 80 amps. based on 430.62

here is my questions
#1 why is the feeder conductor based on 60 deg c rather than 75 deg c?
#2 why do you size down the feeder protection rather than going to the next size up?

thanks in advance,

j_braddock

[ March 12, 2004, 02:19 PM: Message edited by: j_braddock ]

 

[ryan_618]

Re: feeder circuits

#1: See 110.14(1)(a). Be sure that (4) doesn't apply, since we are talking about motors.

#2: I don't completley understand your scenario. You are using the term "feeder circuit". Is this a feeder circuit, meaning that you leave one breaker, then go to two other breakers? Or is this a branch circuit that goes from a breaker to feed two motors?

 

[j_braddock]

Re: feeder circuits

ryan,
i'm sorry for the confusion.

from the main circuit panel, we have a feeder circuit, say 100 ft long.
the feeder circuit then splits to feed the two motors.
each motor then has a circuit breaker, motor starter with overloads, and start/stop pushbutton station.
thanks for your interest and reply.

 

[ryan_618]

Re: feeder circuits

Let me start by saying that motors are not my strongpoint, so don't take my opinion as gospel.

I would imagine that you round down because by rounding up you would be setting the overcurrent protective device for the larger motor at more than 250%, which would be a violation of 430.52(C)

 

[bob]

Re: feeder circuits

J
Thats a great question. First this may be an old question and has not been brought up to date.
You may use the 75 degree rating if the terminations of the equipment being used has 75 degree ratings.
When you size a feeder for two or more motors the feeder OC device is equal to the largest breaker of the largest motor plus the FLA of the other motor. The "largest breaker" for one of the motors is sized from table 430.52 and maybe 250% of the FLA. Exception 1 of 430.52C allows this breaker to be sized up as you said.
If you did the same for the feeder you could be
going to the next size twice.

[ March 16, 2004, 02:38 PM: Message edited by: bob ]