parallel conductors

[chris white]

I realize that 310.4 says that parallel conductors shall be the same length. But let's say that you have a run 300' long, 4" conduit, and 500 kcmil conductors, above the ceiling. You can't tell how much longer one of the pipes may be, with the spacing, various bends, etc., so you don't know exactly how much difference there is in wire length before you even get into the panels. How would you go about trying to keep the conductors the same length when you go to land them in the breakers? Is there an "allowance" you might use, where you figure the difference could be negligible? For instance, in this example, a foot difference would be .3333% of the total length. How much do you think that will matter when the current divides between the parallel branches?

 

[chris white]

Re: parallel conductors

Obviously, if I ran the pipe, I should know how long the pipes are, but in this example I didn't run the pipe, I can't see most of it, all I know is that the run is very long, with a lot of bends and offsets between the 2 enclosures.

 

[Nick]

Re: parallel conductors

Pull some true tape in the pipes and compare the difference.

 

[iwire]

Re: parallel conductors

Chris,

Don R. put up some calculations for this I just can not remember where, the results were small and nothing to worry about for the example he gave.

 

[iwire]

Re: parallel conductors

Found it.
Topic: Parallel Conductors

 

[bennie]

Re: parallel conductors

Only the same phase conductors need to be the same length.

The same phase conductors should be pulled from the same reel. Don't mix factory wire runs.

1/0 is the minimum size for paralleling, due to a slight difference in length will not have a great amount of change in impedance.

 

[chris white]

Re: parallel conductors

Bob, thanx, that was very helpful. Although I suspected that a short difference in length isn't that significant, I'm fighting between trying to apply a reasonable usage of the code rule and working with some people who blow the whole thing off as irrelevant.

Bennie, what do you mean by not mixing factory wire runs? That 2 different reels, maybe by 2 different manufacturers, would have different characteristics? We just installed 24 reels of cable, each marked 340'. It was convenient to purchase and install the wire in that manner. What would the problem be?

 

[bennie]

Re: parallel conductors

The forensic analysis of the First Interstate Bank building fire in Los Angeles, revealed the fire started by overheated parallel conductors.

The conductors were from different factory runs. There was a variation of impedance. The feeder was loaded to 90 percent or more of its ampacity.

Oxidation formed first on the lower impedance conductor terminals. This raised the impedance shifting the load to half or less of the total conductors.

The few conductors, carrying the entire load, overheated to the point of ignition. The protective devices never activated.

I am sure this situation has caused more fires with cause never discovered.

 

[Nick]

Re: parallel conductors

Bennie,
FEMA states the fire was believed to be electrical in origin but the precise cause of the fire was never found. Do you have a different source?

 

[bennie]

Re: parallel conductors

Nick: I was in the building during the clean-up and demolition phase, after the fire.

I have a source, it is un-official due to legal litigation at the time. The cause was established by tests on lugs, and buss bars, of same material.

There will probably be denial of my statements. There is a lot of people, and companies, involved in the liability.