Article 310-4

[electrk]

Nec article 310-4 states that all Parallel Feeders must be the same length. I recently worked a job rewiring a process oven using 4 350 MCM run in parallel. The Foreman did not allow me dress (trim) the cabels at points of termination. This resulted in an unsightly and unmanagable bundle of cables. The Contractor I am working for asked me to return to the job site and trim and reterminate to clean up the appearance.

What's the right thing to do???

 

[pierre]

Re: Article 310-4

Hello electrk

Who signs your checks, if you catch my drift?

Pierre

 

[tom baker]

Re: Article 310-4

Explain to the forman why the cables have to be the same length. Whos going to see it when done? I see his point, but the length is more important than the neatness...

 

[don_resqcapt19]

Re: Article 310-4

If the terminations on both ends of a parallel cable installation look neat, there is a code violation. You can't have both neatness and the same length.
Don

 

[charlie b]

Re: Article 310-4

Originally posted by don_resqcapt19: If the terminations on both ends of a parallel cable installation look neat, there is a code violation. You can't have both neatness and the same length.

I think these statements are a bit harsh. NEC 310.4 does not provide a ?standard of precision.? It does not say cables must be the same length to the nearest inch, or nearest cubit, or nearest furlong, or nearest 5% of length. I don?t believe that the appropriate standard to use should be based on the concept of ?You can?t have your cake and eat it too.? I prefer the ?Drop-in-a-Cup? standard, also known as the ?Let?s Be Sane About This? standard: If the difference in length between two parallel conductors makes as little difference to total impedance of each conductor as is made to the level of coffee in a nearly-full cup by adding a single drop to that cup, then the installation fulfills the requirements of 310.4.

If you go directly from one conduit stub-up point directly to the bus bar, but wrap a second parallel cable around the floor seven times before bringing to the bus bar, you have probably violated the ?Drop-in-a-Cup? standard. If one cable takes 4 feet to reach the bus bar and the second parallel cable takes 4.5 feet, and if the total length of the run is 100 feet, then I?d call it in compliance with 310.4. I don?t think ?engineering supervision? is needed to treat <1% as a ?negligible difference.?

But how do we create a practical, useful standard? My answer is to take no actual measurements of cable length. Rather, make the installation look as neat as possible, while keeping the cables close enough the same length such that an observer would not see any blatantly obvious visual difference in length. My rule would be, ?If they look close to the same length, it is close enough.?

Hey "Other Charlie," how do I get this rule into the Code?

 

[charlie]

Re: Article 310-4

Charlie, this is my last response on another one that applies here.

Be sure to put your proposal in very concise Code language and be very clear with your reasoning. Lastly, make sure you have any substantuation included.

One thing that turns off a panel very quickly is a proposal that is made just because the submitter thinks it is a good idea. It will be rejected in a heartbeat due to lack of substantuation. You substantion should include any problems this is or has caused in the field and any reports or studies that were done.

 

[don_resqcapt19]

Re: Article 310-4

charlie b,
While you are correct that 1% difference in length should not cause any problems with excessive current in the shorter conductor, it is not uncommon for short runs of parallel cables to have 5%+ difference in length. With two conductors in parallel, a 1% difference will result in 49.5% of the total current on the longer conductor and 50.5% on the shorter conductor.
A rule that specified the maximum permitted difference in length as a percent of the total length would make this section better. As is stands now, the code requires the same length. It doesn't say the same length to the nearest foot or any other value. It only says the "same length" without any qualifiers, and to me, that means the code requirement is that the conductors be matched exactly. Is this reasonable, no it is not, but it is the code wording.
Don

 

[charlie b]

Re: Article 310-4

The code wording is ?the same length,? not ?exactly the same length.? I do see a difference between these two phrases. The second implies that a degree of importance is being placed on the precision of measurement. The first does not. But the difference is not significant in the practical world.

One problem that we face is that any Inspector can, by literal interpretation of the phrase ?the same length,? measure two cables with a micrometer, and fail the installation if one cable is 1 mm longer than the other. The opposite problem is that any Electrician can leave one paralleled cable five feet longer than the other, and call it ?close enough.? Both are nonsense, but where do we draw the lines between non-compliance, compliance, and malicious compliance?

My answer remains: In order for an Inspector to fail an installation for unequal cable lengths, the call must be made on the basis of visual observation only (no measurements with yardsticks, rulers, or micrometers). It?s the same standard of care used in the National Football League to overturn a ?ruling on the field?: Undisputable visual evidence.

 

[jim dungar]

Re: Article 310-4

Charlie B,
I agree with you concerning the visual length difference at the terminations.

However, an area of bigger concern for "the same length" is in the conduit run itself. "Nice" looking terminations can hide a large problem. A poorly laid out run with many sets of conduit can result in very large radii at the turns causing several feet difference in length (imagine 4 runs of 3" in a U shape). In a case like this, if the terminations are dressed up the length can not be "the same".

 

[pierre]

Re: Article 310-4

Sometimes the semantics are hard to deal with.

-------------------------
-------------------------

Are these lines the 'same length'? or are they
'exactly the same length'?

Is she 'pregnant'? or is she 'really pregnant'?

Pierre