help me understand

Status
Not open for further replies.
Join,

Articles 430.52 and 430.53 are for branch circuits. But you have to understand that prior to sizing the OCPD for feeder.

Article 430.62 says that the rating of a motor feeder short-circuit ground-fault protective device is determined by adding the rating of the largest branch-circuit short-circuit ground-fault protective device for any motor supplied by the feeder to the sum of the full-load currents of all of the other motors.

So the largest OCPD in your case is 65AX2.5=170.1A, the standard next low rating is 150A, you mentioned each motor has their own fuses. You can use the actual fuse rating as the largest OCPD. Then add the sum of the full-load currents of all of the other motors that is 7.6A. That sum is 157.6A and the standard breaker rating is 150A. So the OCPD for the feeder will be 150A breaker in your case.
 
WOW, anyone have a quarter?

Thank you everyone for your time and responces I'll weed through all of this and hopefully pick the correct one if not by the time I'm done I'll have plenty of spare parts to keep trying. Trying to be respectfully funny. Thanks agian

LHarrington
 
LHarrington said:
WOW, anyone have a quarter?
Click to expand...
How's this:

us-quarter.gif
 
Largest branch OCPD

Largest branch OCPD

dahualin said:
Join,

Articles 430.52 and 430.53 are for branch circuits. But you have to understand that prior to sizing the OCPD for feeder.

I agree

Article 430.62 says that the rating of a motor feeder short-circuit ground-fault protective device is determined by adding the rating of the largest branch-circuit short-circuit ground-fault protective device for any motor supplied by the feeder to the sum of the full-load currents of all of the other motors.

So you agree with me that 430.62 applies for the feeder.

So the largest OCPD in your case is 65AX2.5=170.1A, the standard next low rating is 150A, you mentioned each motor has their own fuses. You can use the actual fuse rating as the largest OCPD. Then add the sum of the full-load currents of all of the other motors that is 7.6A. That sum is 157.6A and the standard breaker rating is 150A. So the OCPD for the feeder will be 150A breaker in your case.
Click to expand...

No, the largest OCPD is 175A protecting the 20hp motor. Lets stick with IT CB's here. Calculating the branch OCPD you use the next HIGHER rating if not standard, Per. 430.52(C)(1)exception #1

So, 175+7.6=182.6A NOW use the next lower for the feeder OCPD wich is 175A. Feeder needs to be 175A breaker.

I don't see how the feeder to the MCC be calculated other than 430.62. Perhaps the feeder is actually a branch circuit in this case? (no fuse downstream?)
 
I'm Game

I'm Game

Most MCC's I've dealt with are fused.

430.94 basicly says that the overcurrent protection located ahead of the MCC can not exceed the rating of the common power bus. The bus should and/will be rated for the two motors for which the controller is designed. The OCPD can not exceed this rating. Once the calculation is done this should be checked.

Regarding the feeder OCPD, I would use the following:

First, assuming the MCC contains IT breakers they are required to be adjustable (430.52(c)(3) and set not to exceed the values in Table 430.52 (General Rule). Unless 430.52(C)(3)Exception No. 1 would apply. The feeder protection should be sized using (65 x 2.5) + 7.6 = 170.1. Next size down is 150A OCPD using IT breakers. See Section 430.62 (A) Exception #1.

Second, assuming the MCC is using time-delay fuses. 65 x 1.75 = 113.75, next size is 125A. 125+7.6=132.6. Next size down is 125A, if using a IT OCPD.

I am assuming they are feeding the MCC from a panelboard and the MCC will be using fuses, and I believe this is were the PE/EE got his calculation.

Just My Thoughts,

R
 
430.62 largest OCPD

430.62 largest OCPD

Thank you RRelec, I follow your calculations but I am hung up on this:
Article 430.62 says that the rating of a motor feeder short-circuit ground-fault protective device is determined by adding the rating of the largest branch-circuit short-circuit ground-fault protective device for any motor supplied by the feeder to the sum of the full-load currents of all of the other motors. So I interpret this to mean the largest branch OCPD plus the remaining fla. Not the largest FLA+ remaining. I was taught BRANCH OCPD= FLA X T430.52 (plus any remaining load on that branch) If not standard, UP to next size. Then Feeder OCPD= Largest branch OCPD + remaining FLC on circuit. (If not standard) DOWN to next size. Like you said then make sure the Feeder OCPD is not more than the rating of the MCC.

Branch OCPD (just for the largest motor)would be 65X2.5=162.5 or 175A CB Feeder OCPD would be 175+7.6=182.6 or 175A CB

I have been sizing this way for years but I could be wrong. Or picturing the MCC layout wrong. I'm curious how our answers differ and what articles we are using. I appreciate your time in discussing this and I'm thankful for an opportunity to learn something.
 
This may or not be relevant, but . . .
S'mise said:
. . . the largest branch-circuit short-circuit ground-fault protective device . .

. . . the largest branch OCPD . . .
Click to expand...
Which one? There is a difference. Remember our discussions about why AC circuit conductors may be protected higher than their usual ampacities?
 
it seems to me there may be a reason why the machine manufacturer requested a larger feeder than what might be the minimum required. It is not unusual to build in a certain allowance for future requirements, or common adders.

it is also pretty common for machines to have other loads around them that are not strictly speaking part of the machine, but will need power. things like conveyors, fans, heaters, dust suckers, etc. It is convenient to be able to tap into the same power feed as for the machine, rather than running more feeders.
 
Who is correct?

Who is correct?

Larry, Which one? The largest branch OCPD in the MCC! Yes,I realize wires can be protected beyond their normal ampacity. We are talking about sizing the Feeder OCPD supplying a MCC, controling two motors. (see post #1)

Perhaps I shouldn't have said I was hung-up, I am very clear on how I interpet 430.62, I am asking others to state why they think I'm right or wrong.

The OP asked how to size the feeder and OCPD suplying this MCC.
We have had a number of answers. Who is correct?

Yes,we should follow the manufactures instructions, but it was asked how are the Feeder OCPD's feeding the motors calculated. I said it was based on the largest branch OCPD plus the remaining flc per 430.62 (see my last post)

Petersonra, I agree that manufactures often size larger for future expansion, and I am greatful for that. Of cource internal wiring of Industrial machines are not the scope of the NEC but NFPA79. NEC does cover MCC's, but I have very limited experience with them. Of cource calculations would change depending on how they are wired. Am I am picturing this corect? See picture below.
 
Last edited:
430.62 says see 430.94 for MCC's which inturn refers to 240 (because of possibility of exceeding power buss).

240.4 states conductor protection shall be sized in occordance with 310.15 unless your devices are rated 800 amps or less (which they are).

Finally it states in 240.4 B "the next standard ocpd (above the ampacity of the conductors protected) shall be permitted to be used.

In short, size your conductors by adding fla of all motors except the largest in which you will size fla x 1.25. Add all together. Find the next higher standard ocpd for feeder.
 
So your saying 430.62 does not apply for the feeder ocpd?
I agree with you about the wire. Yes, 240.4 I see that, but it goes on to say unless otherwise premitted in 240.4(A) thrugh 240.4(G) OCP for specific conductor apl. (The Table) Motor control circuit conductors, which takes you right back to art 430. Is a MCC not a motor control circuit? If not I believe your correct that 240.4 applies. Your first sentence make sense for branch circuits:"In short, size your conductors by adding fla of all motors except the largest in which you will size fla x 1.25."...Then you loose me...." Add all together. Find the next higher standard ocpd for feeder." Huh? Add all what together? Are you saying the feeder ocpd is sized based on the ampacity of the feeder wire (or next standard size up) and table 430.52 has nothing to do with selecting the breaker?
 
"add all together" was just reiterating the fact of adding all loads in the MCC. We are dealing with a MCC according to OP (to answer your question there can be many motor control circuits located within a givin MCC which is a motor control center)that takes us to 240 parts 1,2,and 9.

Find the next higher standard ocpd for feeder." Huh?[
Click to expand...

240.4 (B) the next higher standard ocpd (above the ampacity of conducters being protected) shall be permitted.

Are you saying the feeder ocpd is sized based on the ampacity of the feeder wire (or next standard size up)[
Click to expand...

Yes

and table 430.52 has nothing to do with selecting the breaker?
Click to expand...

yes, this is a feeder we are talking about. 430.52 deals with motor branch circuits. Also it is refering to short circuit protection and ground fault protection which is not what we are looking for here ...which is ocpd.
 
S'mise:
Don't get me wrong, I agree with what you are saying.

"Article 430.62 says that the rating of a motor feeder short-circuit ground-fault protective device is determined by adding the rating of the largest branch-circuit short-circuit ground-fault protective device for any motor supplied by the feeder to the sum of the full-load currents of all of the other motors. So I interpret this to mean the largest branch OCPD plus the remaining fla."

I was taught the same way. Remember I said I would.

The Table says "Maximum", then gives you an exception to the "Maximum", then gives you an exception for the exception for the "Maximum". Go figure.

Food for Thought: go to the SquareD website look for Nema Motor Data calculator and see what it has to offer, and then go to siemens and look at their pdf info, compare MCP, Inverse Time and Instantaneous tip (at siemens).
Let me know what you think! The day I stop learning,will be the day I die.

R
 
430.62 rules!

430.62 rules!

Cody K you never answered my first question. I say yes, 430.62 rules.
Your right about branch circuit selection but your opinion that the feeder is sized per the ampacity of the conductors does not fly. The seemingly overrated OCP for motors is required to withstand the start-up current of motors. The Branch OCPD are sized per T430.52, say you are using breakers at 250%, how can the feeder ocpd be fused less than that?

Anyway, I fould this link that confirms what I've been trying to say all along. It's a short post but folows along the same lines.

240.4(G)says ocp for motors/MCC go to 430 part 3-7

430.62 Feeders says OCP can be sized not greater than the largest ocp plus the sum of the fla. So the feeder ocp can be as big as the largest ocp plus the rest. So if your sizing the feeder CB, and the largest branch CB is 175A, you add the remaining load to it. 175A+7.6=182.6 BUT, 240.4(B)does not apply because it would violate 430.62. So you go down to a 175A CB for the feeder.

http://forums.mikeholt.com/archive/index.php/t-53646.html

RRelec, You agree with me but still do it differently? I realize it's better to fuse smaller than the max, but there is only one right answer. Yes, I cought your disclaimer I in your post but heck I could have used some back up around here when everyone was talking to me like I was nuts.
 
Smise, 430.62 (A) exception no. 2 states "where the feeder ocpd also provides overcurrent protection for a motro control center the proivisioins of 430.94 shall apply. We are dealing with a MCC not a motor control circuit. A MCC can feed many things other than just motor circuits.

Look at 430 section VIII
430.94 "Motor Control Centers (MCC) shall be provided with overcurrent protection in accordance with parts I, II, and IX of Article 240. The ampere rating of ocpd shall not exceed rating of common power bus...."

We always supply our small units, which consist mainly of motor circuits, but not all, with a ocpd (either an adjustable trip breaker or a time delay fuse) that is rated as a feeder occording to 240. Never have any problem with tripping of breakers or blowing fuses when starting motors. But thats a whole other issue.

Keep in mind the code book directly refers to MCC's.... look there.
 
Last edited:
Cody K said:
Smise, 430.62 (A) exception no. 2 states "where the feeder ocpd also provides overcurrent protection for a motro control center the proivisioins of 430.94 shall apply. We are dealing with a MCC not a motor control circuit. A MCC can feed many things other than just motor circuits. Look at 430 section VIII430.94 "Motor Control Centers (MCC) shall be provided with overcurrent protection in accordance with parts I, II, and IX of Article 240. The ampere rating of ocpd shall not exceed rating of common power bus...." We always supply our small units, which consist mainly of motor circuits, but not all, with a ocpd (either an adjustable trip breaker or a time delay fuse) that is rated as a feeder occording to 240. Never have any problem with tripping of breakers or blowing fuses when starting motors. But thats a whole other issue.Keep in mind the code book directly refers to MCC's.... look there.
Click to expand...

Ahh yes, there is nothing like a good debate! Cody, I agree that 430.92 provisions also apply, that is, the ocp shal not exceed the buss ratings. And yes it goes on to say ocp in accordance with 240, but like I said, 240.4(G)says specificly ocp for motors go to 430 part 3-7 (I know its not a motor control circuit that would be 430.71) They just want you to follow the basis rules for OCP. 240 part I=general req 240 part II=location and 240 part IV=OP over 600v The only applicable section that directly relates to a motor control center that I see is table 240.4(G). besides, If 240.4(G) didn't apply why are you using art 430 partIV for your branch ocp's?(430.24)? Dont get me wrong I hear what your saying about not having a problem blowing your main but thats because the max load never gets close to tripping the breaker. As long as your feeder has the ampacity to handle that big cb you are ok but why oversize everything larger than needed? The best approach is to calculate the load and work back from there rather than sizing feeder and ocp at 800amps just because thats what the MCC is rated for. I will say that the manufacture can and will suggest power requirements (nameplate) but we were talking about calculating it when the load in known.

The op wanted to figure out what size to fuse his feeder for two motors on his MCC. I still think 430.62 applies to feeder OCP in this case. If you can convince me that art 240 specificly applies to this motor controller, I am all ears. Take another glance at art240 and your will remember that inductive devices like transformers and motors are sized larger than normal because of their in-rush current and that is why standard fusing does not apply.
I fear this argument may be interminable, but its fun to hear other points of view.
 
Status
Not open for further replies.
Top