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#15488 - 10/18/02 10:44 PM Parallel Conductors  
Nick  Offline
Member
Joined: Aug 2001
Posts: 599
Riverside, CA
In this picture the two banks of 4" towards the back are utility secondaries. I am taking the picture at about the corner of where the transformer will be. Obvious is the difference in length of the two duct banks. One of the requirements for this utility on a service of this size is a six foot separation between ductbanks. The reason is for heat dissipation. I initially had the conduits running together because it is such a short run it was impractical to obtain the required 6ft separation but was made to do this by the big boss at the engineering Dept. Maybe they don't understand how parallel conductors operate. (Granted they will probably only pull conductors in three of them anyway) [Linked Image]
Do other utilities have this requirement around the country? They must not have problems with it or they wouldn't keep up this practice.

[Linked Image]


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#15489 - 10/18/02 11:33 PM Re: Parallel Conductors  
Bjarney  Offline
Moderator
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
May seem obvious, but a foot difference in a 10-foot conductor run will disrupt equal division of current to a much greater degree than a foot difference in a 100-foot run.




[This message has been edited by Bjarney (edited 10-18-2002).]


#15490 - 10/19/02 10:00 AM Re: Parallel Conductors  
Roger  Offline
Member
Joined: May 2002
Posts: 1,716
N.C.
Nick, maybe they expect you to ball or wad the excess condutor length of the short runs in your gear. [Linked Image]

Roger


#15491 - 10/19/02 10:30 AM Re: Parallel Conductors  
Joe Tedesco  Offline
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Joined: Oct 2000
Posts: 2,749
Boston, Massachusetts USA
Review 310.4 and look at this:

"Where run in separate raceways or cables, the raceways or cables shall have the same physical characteristics.

Conductors of one phase, neutral, or grounded circuit conductor shall not be required to have the same physical characteristics as those of another phase, neutral, or grounded circuit conductor to achieve balance."


Joe Tedesco, NEC Consultant

#15492 - 10/19/02 11:30 AM Re: Parallel Conductors  
Roger  Offline
Member
Joined: May 2002
Posts: 1,716
N.C.
Joe, this would mean their intent is to run say,

A phase in two of the conduits on the right, B phase in two of the conduits on the right,
C phase in two of the conduits on the left,
Nuetral in two of yhe conduits on the left.

Not neccassarily this exact combo.

So, if heat dissipation is the issue,it doesn't make sense. (besides being buried)

Roger


#15493 - 10/19/02 12:03 PM Re: Parallel Conductors  
Joe Tedesco  Offline
Member
Joined: Oct 2000
Posts: 2,749
Boston, Massachusetts USA
310.4 -- "1. Be the same length,"

All phases are not required to be all of the same length, for example, All of Phase A can be 15 feet, all of Phase B can be 17 feet, all of Phase C can be 19 feet, and all of the neutrals can be 23 feet.

Lengths for these SEC's all depends on the overall length of the run --- to the service point at the overhead or lateral.


Joe Tedesco, NEC Consultant

#15494 - 10/19/02 01:11 PM Re: Parallel Conductors  
Roger  Offline
Member
Joined: May 2002
Posts: 1,716
N.C.
Joe, I agree, being that these are non metalic conduits this would be the case, ie idividual phase conductors and neutals in their own conduits. I still don't get the heat dissipation angle.

Roger


#15495 - 10/19/02 06:57 PM Re: Parallel Conductors  
Nick  Offline
Member
Joined: Aug 2001
Posts: 599
Riverside, CA
Joe,
I know and understand this rule but that is not how they install the conductors. They always instal per the usual A,B,C,N per pipe.


#15496 - 10/19/02 09:55 PM Re: Parallel Conductors  
Joe Tedesco  Offline
Member
Joined: Oct 2000
Posts: 2,749
Boston, Massachusetts USA
Nick:

I was thinking about that when I replied, so each phase, in each raceway would be the same length.

The code rule that allows all of each phase in each raceway is found in 300.5(I), Exception No. 2, and can only be applied for underground installations.

[This message has been edited by Joe Tedesco (edited 10-19-2002).]


Joe Tedesco, NEC Consultant


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