How far up the system to apply section 430?

Status
Not open for further replies.
D

DeanSoftware

Member
Location
Canada
Hello,

I am hoping one of the longtime NEC experts can stop me from burning more time trying to nail something down. I am working on a project involving the writing of software to help electrical designers work faster, but most of the subject matter experts are much more experienced with Canadian Electrical Code than NEC. I have spent literally 15 hours chewing on a question so far reading the NEC and many many electrical forum posts (although admittedly not all 10,000ish in this category!) Can some kind and wise soul help me out?

The question is how far does one apply the special handing of conductor and protection calculations for motors? I understand that the conductors and protection directly feeding motors need their conductors sized at 125% of largest motor FLA plus other motors' FLAs & that the protection equals largest protection in use among the motors plus remaining motors' FLAs. After that initial calculation and conductor/protection sizing, does one use that calculation any higher up the 'power tree'? For example if we have an MDP serving a four wing building each wing being served by a CDP that then services an MCC and three branch circuit panels in each wing. Does section 430 affect conductor and protection selection for all of these services to accommodate for inrush current or does section 430 stop applying after the motor's individual conductors and protection are selected...or one step up, like the MCC conductors/protection between each CDP and the MCC fed by it? How does the MCC to CDP and CDP to MDP and MDP to utility supply transformer accommodate for inrush current if the motor specialized calculations are not done all the way up?

My apologies in advance if this is a ridiculously simple question, I have just read so much from so many sources and I am yet to stumble across anything truly definitive :(

Thanks for the help,
Dean
 
I don't think it's a "ridiculously simple question". What I see in the field as "practice" differs from I think is Code. It appears to me the 430.24 would allow the "motor factor" to be taken into consideration all the way back to the service. In a system where the panels are "sub-panels' fed from other panels I do not see that applied in the field, perhaps because as you go up the line the motor contribution becomes less a value.
For example, if we have a 50 hp motor, the branch circuit is determined by 430.22. If the panel feeding that branch circuit has other loads, then it appears the feeder would be sized per 430.24. If that panel was fed from another, I would think 430.24 would still apply, although I also would like to here others opinion here.
Unless we are talking about a motor load that is a significant percentage of the total load, I think it has little impact on the feeders farther up the chain.
 
augie47 said:
I don't think it's a "ridiculously simple question". What I see in the field as "practice" differs from I think is Code. It appears to me the 430.24 would allow the "motor factor" to be taken into consideration all the way back to the service. In a system where the panels are "sub-panels' fed from other panels I do not see that applied in the field, perhaps because as you go up the line the motor contribution becomes less a value.
For example, if we have a 50 hp motor, the branch circuit is determined by 430.22. If the panel feeding that branch circuit has other loads, then it appears the feeder would be sized per 430.24. If that panel was fed from another, I would think 430.24 would still apply, although I also would like to here others opinion here.
Unless we are talking about a motor load that is a significant percentage of the total load, I think it has little impact on the feeders farther up the chain.
Click to expand...


As has been mentioned here many times before, I/we come here to read the opinions of others and learn.

I have never read or heard what Augie has mentioned in the portion I highlighted. It is an interesting thought and I too am wondering what others may have to say about the thought pattern.
 
Dean, welcome to the forum! :)


I believe the "125% of the largest motor FLA plus 100% of the remainder" rule applies here.
 
I guess my "reality vs NEC" is more directed at 430.63.
As an example, (3) 500 kcmil on a 1200 amp feeder to a distribution panel would normally raise eyebrows, however, if there was a significantly sized motor downstream of the distribution panel, applying 430.63 and 430.24 might justify the "undersized" feeder. Correct ?
 
At the appearance of being overly simple in my answer, I'm recommending a couple of old newsletters -

Part 1 and Part 2

from the archives.

From a NEC code start point, I'm going with 220.14(C)
 
Seeings my editting time has timed out, let's see if this is any better than the red "X" I received for clicking on the links -

Go to Mike Holt's opening page. Go to the left hand column, and scroll down to Newsletter "click it". Scroll down through the news letters to #1291 (Part 1) and 1289 (Part 2)
 
430.62(A) exception #2 points us to 430.94. Motor control centers shall be provided with overcurrent protection in accordance with parts I, II, and IX of article 240. Blah blah blah "This protection shall be provided by (1) an overcurrent protective device located ahead of the motor control center or (2) a main overcurrent protective device located within the motor control center.
So I may have confused myself if the OCPD is located in the MCC and sized as so how do we treat the feeder from here?? Not sure or do we use article 240 and disregard the motor loads?
 
IMO, 430.6(A)(1) tells you to use the tables listed for determining the ampacity of the

conductors, or amp rating of switches, branch circuit SC and GF protection, instesd of

what is marked on the motor nameplate. (A)(1) is for general motor applications, (B) and

(C) for other.
 
Getting there!

Getting there!

Thanks for the comments so far. I have reviewed the newsletters referred to by RockyD, and understand that they apply to the motor's conductors and short-circuit + ground-fault protection device. They seem, however, to be focused only on the branch circuit that directly feeds a motor or motors?

The area I need clarification is where to apply the multiple motor calculation rules and where to stop doing so?

I.e. the MCC typically has a number of 'branch circuit' motor loads, but I believe the conductors and protection from the MCC to the CDP serving it (given my example scenario in the original post) would qualify as a feeder?

Referring to another newsletter at:
http://www.mikeholt.com/newsletters.php?action=display&letterID=369

The feeder calculations do not mention any special handing about motor loads, other than classifying all loads as continuous or noncontinuous.

So I am still stuck on the original questions: A) how far beyond the branch circuit directly supplying a motor do we apply the motor specific special rules for conductor and protection sizing; and B) how is the inrush start-up current being accommodated for after we stop doing special motor calculations for conductor/protection sizing?

Thanks again for all the comments so far, I hope you all have the patience to help me to bring this to conclusion!

Regards,
Dean
 
Status
Not open for further replies.
Top