Counting conductors in conduit

Status
Not open for further replies.

naplespete57

Member
Location
Eastern Kentucky
Occupation
Project Manager in Conveyor Industry
Ran into a situation recently where I had a two speed motor. Six conductors and a ground were feeding motor from starter cabinet. It is mechanically impossible for all six conductors to be carrying current at the same time. How many conductors do I count in conduit? Do I need to derate the conductors or not? Any code references would be helpful.

Thanks!

Pete
 

tkb

Senior Member
Location
MA
You need to count all of the conductors in the conduit for fill of the conduit, and only the current carring ones for derating.
 

naplespete57

Member
Location
Eastern Kentucky
Occupation
Project Manager in Conveyor Industry
In the context of appliance feeder calculations

In the context of appliance feeder calculations

The NEC has a rule on noncoincident loads, only use the largest for your calculations see 220.60


At first glance this code reference makes sense. But it is in the middle of appliance feeder calculations and not derating of conductors.

Am I missing something?
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
No, you're not missing anything. You are right; those are unrelated sections. It's just another reason to have confidence that you don't have to count more than three. The real basis for not derating is that you only count current-carrying conductors. If all six can't carry current at the same time, you only count three.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
Well would these conductors not carry current?
Three of them would. I don't say this as a general concept. But in this specific installation, there is an interlock to prevent more than three of the conductors from carrying current. The presence of the others is a conduit fill issue, as has already been mentioned. But that is the only issue.
 

iwire

Moderator
Staff member
Location
Massachusetts
The CMP sees it as Charlie does. Check out this proposal from our own George Stolz

6-50 Log #1405 NEC-P06 Final Action: Reject
(310.15(B)(2)(a), Exception No. 6 (New))

____________________________________________________________
Submitter: George Stolz, II, Pierce, CO

Recommendation: Add an Exception to read:
Exception No. 6: Of those conductors that are switched cable or raceway
installations, only the maximum number of conductors capable of being
simultaneously energized need to be derated.

Substantiation: In most threeway and fourway switching methods, the load is
alternated between travelers, eliminating the need to include both travelers in
derating.

Panel Meeting Action: Reject

Panel Statement: The proposed exception is not necessary. The present
language of 310.15(B)(2) already permits what the submitter is proposing.

Number Eligible to Vote: 11

Ballot Results: Affirmative: 11
________________________________
 

iwire

Moderator
Staff member
Location
Massachusetts
I just don't see anything in the code that supports the "panel statement".
This would have been a good revision, reading what is not there sucks.

310.15(B)(2)(a) supports the panel statement along with commonsense, conductors not in use do not add heat to the group.
 

iwire

Moderator
Staff member
Location
Massachusetts
It is more about what it does not say.

But if you do not accept the CMPs comments I would say your mind is already made up and anything I can add is pointless.
 

raider1

Senior Member
Staff member
Location
Logan, Utah
My mind is not made up.......
It says "More than 3 Current-Carrying Conductors in a Raceway". What it does not say is anything about simultaneous use.

In the original posters question how many current carrying conductors do you think there are?

I see 6 conductors and only 3 current carring conductors.

Chris
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Related (I hope) question:

Let's say you have an 8/3 NM cable entering a panel, strip the cable as usual, route the conductors unbroken through the panel and into a nipple to a box with a 3p contactor, and then three more conductors back through the nipple and onto the 3p breaker.

How does one size the stripped NM conductors for the purpose of sizing the nipple?
 
310.15(B)(2)(a) supports the panel statement along with commonsense, conductors not in use do not add heat to the group.

You right, but a lawyer will argue that it should have said "at any given time" or "simultaneously".:D

In the case of closed transition all 6 conductors will carry current for the time limited by the transition timer. In general the heating effect would take hours to be detrimental, but it is of course just in this case. In other cases - which are not as common as the OP - the number of current carrying conductors may not be subject to the same limitations, so the Code would need to be generic enough to allow or deny allowance for such possibilities.

IMO the language is insufficient and the proposal did have merit.
 
Status
Not open for further replies.
Top