Parallel Feeder Conduits

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infinity

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Someone is trying to convince me that EMT runs for parallel feeders have to be physically next to each other (they're on two different tiers of a conduit rack). The conductor length for each conduit is the same. Of course they're insistent without any code backup.
 

mwm1752

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I would think the better spacing between would help heat loss -- Though I would wonder if you took non similar paths if it would be compliant being it it a single circuit.
 

GoldDigger

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If the lengths are the same, then no code that I know of requires it.
But if the paths are different enough, the inspector may call foul because it is too hard to verify that the lengths are the same.

And if you go from a box on the wall to either the top or bottom tier of the rack and then back down to a box on wall, the path using the upper tier will necessarily be longer unless you somehow "waste" some space with the conduit that follows the lower path.
If you connect from a box at one end of the run located below the rack to one at the other end of the run located above the rack, then the two path lengths will be the same, to first order.

If the difference works out to be 12" is a 200' run, it then comes down to how exactly the lengths are required to match. :)
 

infinity

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But if the paths are different enough, the inspector may call foul because it is too hard to verify that the lengths are the same.

And if you go from a box on the wall to either the top or bottom tier of the rack and then back down to a box on wall, the path using the upper tier will necessarily be longer unless you somehow "waste" some space with the conduit that follows the lower path.
If you connect from a box at one end of the run located below the rack to one at the other end of the run located above the rack, then the two path lengths will be the same, to first order.

If the difference works out to be 12" is a 200' run, it then comes down to how exactly the lengths are required to match. :)

For arguments sake the conductors lengths are exactly the same. You could have a few conduits on the same rack and end up with several feet of difference in the conductor length. The question is are they're required to be next to each other on the same rack with nothing in the NEC to require it.
 

GoldDigger

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For arguments sake the conductors lengths are exactly the same. You could have a few conduits on the same rack and end up with several feet of difference in the conductor length. The question is are they're required to be next to each other on the same rack with nothing in the NEC to require it.
If, for arguments sake, the conductor lengths are the same, then the NEC does not add any additional restrictions. (There is an unwritten presumption that the paths are otherwise comparable, e.g. not one underground and one exposed on a hot rooftop, but even that is not explicit in [2014].)
But putting them on the same conduit tier makes it marginally easier to convince the inspector that you made the lengths equal. If he trusts you, then no problem either way. ;)
 

Carultch

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Someone is trying to convince me that EMT runs for parallel feeders have to be physically next to each other (they're on two different tiers of a conduit rack). The conductor length for each conduit is the same. Of course they're insistent without any code backup.

As long as each conduit contains all phase and neutral conductors, including the EGC, you can route them along any path you need. Provided that you follow all other parallel set rules.

You can route them 30 feet apart from one another, as long as they end up being the same length. Although that construction would stick out as an example of poor planning.

In the case of isolated phase installations, which are much more restricted, you need to be careful not to magnetize the vicinity of the conduits. And therefore route the isolated phase conduits as closely as possible to one another. In my opinion, I don't see the value in doing an isolated phase installation. To me it makes much more sense to balance the magnetic field in the conduit, and not even think about it outside the conduit.


Parallel feeders need to be constructed in such a way that no one set becomes a preferred path for current. All paths must be set up to share the current equally.
1. At least 1/0 or larger, for the current-carrying conductors.
2. Same length. In a practical sense, the obvious question is how rigid is this rule?
3. Terminated in the same manner.
4. Be of the same conductive material with the same type of insulation.
5. Routed in the same size and type of conduit, or within the same conduit.
6. Routed through the same ambient temperature and environment.
7. Routed with the same quantity of current-carrying conductors in each conduit.

#2 is an interesting rule. Consider a 500A feeder, built with two sets of 250 kcmil Cu in separate conduits. Suppose you constructed it such that one set ended up being 45 ft, and the other set ended up being 48 ft. If you calculate the current in each set in the case of a full 500A load, you get 242A in the 48 ft set, and 258A in the 45 ft set. Assuming wire resistance dominates over other ohms in the system. On paper, this is a problem, because the wire is rated for 255A. I see this as a very realistic example, and understand exactly how it could happen, even for an installer who knows and understands the intent of this rule. Unfortunately, the NEC doesn't quantify just how different the lengths can be in practice.

I'd also like to speak to #7. Suppose you have three sets, and you think you can group two of them in the same conduit. Think again. Because now the remaining third set isn't heated as much due to the number of active wires in the conduit. The remaining third set becomes a preferred path for more current, because it isn't operating as hot.
 
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Carultch

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Massachusetts
But if the paths are different enough, the inspector may call foul because it is too hard to verify that the lengths are the same.

Is there any good rule to determine just how different the path lengths may be in practice?

I can see this as being a problem, when feeder set #1 has the inside track of all the bends, and feeder set #6 is a couple feet longer because it is on the outside.
 

electricalist

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dallas tx
0b04a9d0db50a24e84b53efc604b4e2c.jpg
4715dcc5db210bcb5e4c468580e27958.jpg

Show them this and tell them it's break time.
 

infinity

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What would an inspector expect you to do? Put a couple of loops of extra wire in that pull box, to make up for the couple of feet difference in the sweeps,

The NEC would require some process like the one you've mentioned to get the lengths to be equal.
 

jap

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Electrician
If one wanted to get really technical, I'd bet very few conductors of parrallel runs end up being exactly the same length.
If one were to cut each conductor exactly the same length and install them in parallel runs of conduit I think they would find they come out a little different in the end no matter how hard you try.

JAP>
 
I metered a set of parallel conductors once. I measured, cut and installed them and I know they were all super dead nuts, like all +/- a quarter inch. They showed a not insignificant difference with the current clamp - IIRC between 5 and 10 amps. Not sure if that was inconsistency with my clamp (good fluke 337) or actual differences in impedance. Just got my wondering how meticulous one needs to be with the length. Anyone else tried this?
 

ActionDave

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I have never gone back with an ampmeter and checked, now you have me curious.
I do know two things,
1. My boss is pretty adamant about only trimming off the longest longest conductor enough to land it and then using that trimmed piece as a measuring stick to trim all the others so the lengths are as close to the same as possible.

2. Nobody else in town seems to try to come this close. They just cut 'em and land 'em.
 

infinity

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I have never seen anyone make an attempt to ensure that the conductor lengths were equal.
 

jap

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Electrician
I have never seen anyone make an attempt to ensure that the conductor lengths were equal.

Nor I.

The physical layout of the bussbars your terminating to compared to where the entry of the conduit comes into the enclosure would make that fairly tough unless you pulled all the conductors out and cut them the exact same length before you installed them in the conduit and didn't trim any off at all when you went to terminate them..... which would look like crap.

JAP>
 

ActionDave

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Nor I.

The physical layout of the bussbars your terminating to compared to where the entry of the conduit comes into the enclosure would make that fairly tough unless you pulled all the conductors out and cut them the exact same length before you installed them in the conduit and didn't trim any off at all when you went to terminate them..... which would look like crap.

JAP>
It is tough, it does look like crap, but they are supposed to be the same length, no exception in the code allowing for tidiness.
 

don_resqcapt19

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Nor I.

The physical layout of the bussbars your terminating to compared to where the entry of the conduit comes into the enclosure would make that fairly tough unless you pulled all the conductors out and cut them the exact same length before you installed them in the conduit and didn't trim any off at all when you went to terminate them..... which would look like crap.

JAP>
If the connections for parallel system don't "look like crap" I take a closer look at the lengths. It can be a big deal on short high current runs. On an short run, you could easily have over a 5% difference in length if the the terminations look neat. If there are two cables and one is 5% longer than the other one, the short one will be carrying 5% more current than the longer one, assuming that every thing else is identical.
 

ron

Senior Member
If the connections for parallel system don't "look like crap" I take a closer look at the lengths. It can be a big deal on short high current runs. On an short run, you could easily have over a 5% difference in length if the the terminations look neat. If there are two cables and one is 5% longer than the other one, the short one will be carrying 5% more current than the longer one, assuming that every thing else is identical.

I do the same thing.

I also specify that the lengths be less than 5% different from longest to shortest, and then we have them load bank the feeder at the end of the project and show ampere measurements to reflect the same.
 

jap

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Electrician
I do the same thing.

I also specify that the lengths be less than 5% different from longest to shortest, and then we have them load bank the feeder at the end of the project and show ampere measurements to reflect the same.

You all originally said the conductors had to be the same length.
Now your talking percentages.
At those margins you could terminate, be code compliant, and not look like crap.
Good deal. :)

JAP>
 
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