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#55721 - 09/04/05 12:06 PM Parallel Feeders  
Cinner  Offline
Member
Joined: Dec 2004
Posts: 78
Kelowna, B.C., Canada
Does any one know if it is imparative that parallel feeders be the exact same length, within 1 inch. Something about line resistance can cause unequal loads per line if uneven lengths.


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#55722 - 09/04/05 12:14 PM Re: Parallel Feeders  
renosteinke  Offline
Cat Servant
Member
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
See the "photos submited for discussion" forum. Go back a page to "Multple Feeds, One Service" and you will see this discussed.

Simple answer- make the feeders as alike as possible.


#55723 - 09/04/05 12:25 PM Re: Parallel Feeders  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
I agree with Reno make them as alike as possible.

The code says the paralleled conductors in each phase, neutral, or grounded circuit conductor shall be the same length. Now that is virtually imposable to make them 'exactly' the same length so all you can do is try to make the runs as equal as possible.

The longer the total length the less critical this issue becomes.

IMO a 6" difference on a 200' run is no big deal, a 6" difference on a 24" run is unacceptable.

Bob (not recommending a 6" difference)

[This message has been edited by iwire (edited 09-04-2005).]


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#55724 - 09/04/05 12:26 PM Re: Parallel Feeders  
Redsy  Offline
Member
Joined: Mar 2001
Posts: 2,056
Bucks County PA
See 2002 NEC 310.4 (1)
No tolerances given.
You could see for yourself if theres any imbalance by calculating the resistances, but depending on the size of the conductors, I don't see that 1" could matter.


[This message has been edited by Redsy (edited 09-04-2005).]


#55725 - 09/04/05 12:33 PM Re: Parallel Feeders  
iwire  Offline
Moderator
Joined: Jan 2003
Posts: 4,391
North Attleboro, MA USA
Quote
See 2002 NEC 310.4 (1)
No tolerances given.


Exactly the problem. [Linked Image]

What tolerance is acceptable, 1", 1/2", .00025"?

It is imposable for one conductor to be exactly the same length as another, there has to be a tolerance.

And to say one specific tolerance applies makes no sense. It would make more sense to say the tolerance is a certain percentage of the total length.


Anyway thats my two cents. [Linked Image]

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts

#55726 - 09/04/05 01:11 PM Re: Parallel Feeders  
Gregtaylor  Offline
Member
Joined: Mar 2005
Posts: 212
Boise, Idaho, USA
When this issue was addressed during my apprenticeship days, (a long time ago) it was made clear to us that EXACT LENGTH was not the real issue, rather that running the parallel conduits by the same route and being as near as practically possible were the goals of the code. There are situations where it could be tempting to run the conduits by different routes to the same point. If you do that the conductors cannot be judged to be the same length.
When the wire is cut for you at the wholesale supplier, how well do you trust the length anyway? Do you unroll them all and measure? Do you use an electronic device to measure?


#55727 - 09/04/05 08:23 PM Re: Parallel Feeders  
renosteinke  Offline
Cat Servant
Member
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
Let's look at this from a different poit of view....

Any imbalance, caused by the differing resistances of the wires, will be expressed in heat.

I suppose that resistance measurements could be taken- but there is still the mattter of tolerances.

One thing that does not change is the temperature rating of the terminations. I suppose it could be argued that if the circuit can run three hours at full load, without exceeding 75 degrees C at the terminations, you're 'within tolerance.'


#55728 - 09/04/05 08:53 PM Re: Parallel Feeders  
resqcapt19  Offline
Member
Joined: Nov 2000
Posts: 2,148
IL
The current will divide in an inverse relationship to the lengths. If you have a 49' run and a 51' run, the 49' run will carry 51% of the total load and the 51' run will carry 49% of the total load. In most cases a difference of 5% or even more in length will not cause any problems, but as others have said there is no tolerance in the code and would be a violation.
Don


Don(resqcapt19)

#55729 - 09/05/05 02:09 AM Re: Parallel Feeders  
georgestolz  Offline
Member
Joined: Jun 2005
Posts: 91
Fort Collins, CO, US
Quote
renosteinke wrote:
See the "photos submited for discussion" forum. Go back a page to "Multple Feeds, One Service" and you will see this discussed.


Or click here . [Linked Image]

It's remarkable how much how little a difference can do! [Linked Image]


-George

#55730 - 09/05/05 08:13 AM Re: Parallel Feeders  
mvpmaintman  Offline
Member
Joined: Sep 2003
Posts: 123
Manhattan, Kansas, USA
Reno, I've seen the aftermath of a gross difference in distance of parallel feeders (about 5 feet on 4/0) one raceway got so hot the paint all burned off of the wall behind it. Wish I'd had a camera then, melted globs of insulation running out of the end of the conduit. One of the guys I was working with at the time said that since ti was AC power the Eddy currents contribute to the heat as well.


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