Paralleling Conductors

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derf48

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What are the problems with paralleling conductors smaller than 1/0, especially as small as #6. Other than a code issue I need to explain to non qualified employees the hazards, if any, of this type of an installation. Thank you.

Fred Bender
 
Now come on, Steve, it is a good question. And in fact I do not know the answer. Here are a few guesses (all of which may be nonsense, but let’s get the conversation going):

  • Perhaps because it is not necessary. For example, if you had in mind to parallel a couple of #12 conductors, why don’t you just use a #8 instead?

  • [*]Perhaps because the smaller conductors are more fragile.
    [*]Perhaps because there are fittings that make it easy to connect a large conductor to another large conductor, and the only fittings that work for smaller conductors are wire caps, and they aren’t secure or permanent enough to trust them in this application.
    [*]Perhaps because the smaller wires are stranded, and if just a few strands are broken or are not firmly connected, that small difference in impedance can make a big difference in the split of current between the multiple conductors.
OK, my brain is now empty. Someone else want to hazard a guess? Someone else actually know the answer?
 
I like all of Charles' answers, but mostly because it is not neccesary. Using the right conductor for the job is better than parallel conductors. Using parrallel conductors in larger sizes is a concession to practicality. It is usually much more practical to install two 500 kcmil conductors rather than one 1000 kcmil conductor. The difference in installing 2 #12 vs 1 #8, as in Charles' example, is negligable. (I miss spell check!)
 
I doubt the code would prohibit something on the basis of 'no practical need' although that's what the handbook implies. I believe Charlie's points regarding damage and load sharing issues at the <1/0 size increases the risk of conductor overheating.
 
From a technical standpoint, there is no reason you could not, however, as pointed out it is not very practical. You can do a few hand calcs to determine very quickly that it will never be possible to get a higher current capability by parallelling over using a single conductor. It's a case of diminishing returns.

Equipment terminations would need to be modified to take multiple conductors, and the poor inspectors would be pulling their hair out trying to figure out what is installed.
 
If you can provide separate heat sinking for the individual conductors, then you certainly get much higher current handling capacity per unit copper with smaller conductors.

Take table 310.16 and table 8 and calculate the 'circular mils per amp', 12ga conductors at 20A is operating at 327 circular mils per amp, and a 1/0 conductor at 125A is at 845 circular mils per amp. In terms of total copper, a 1/0 conductor is the equivalent of 16 #12 conductors in parallel.

The reason, of course, is that the surface area for dissipating heat of the 16 #12 conductors is much greater than that of the single 1/0 conductor. If you were to compare 3 #1/0 conductors in a conduit with 48 #12 conductors in the same conduit, where the #12 conductors are bundled together and 310.15(B)(2)(a) applies, then you find that the net ampacity of the two sets is pretty much the same.

My _hunch_ is that in addition to the points that Charlie mentions, current balance is a significant issue. Code requires that all conductors used in parallel be matched in terms of cross section, material, and length. This is an _impossible_ requirement; the best you can do is use conductors of the same gage which measure out to the same length...but wire gage has allowed tolerance, length measurement has error, and when you install the cables they will be distorted in different ways. There will _always_ be some difference between two conductors used in parallel. As long as this difference is 'small enough', then the resulting current imbalance will not be significant. My hunch is that the significance of these errors is bigger with smaller conductors.

-Jon
 
How about the magnetic forces applied during fault situations? The bundled #12 surely performs differently than the single 1/0. Are the forces strong enough to damage the multiple #12 terminations/wire? Over time, probably. The 1/0 would be less likely to suffer fault damge
 
I asked the question because "the code says so" doesn't always work. For example, I ran into a situation where 2 #6 Cu conductors are paralleled, for voltage drop, and they followed all the requirements of 310.4 ex. #2, except the load restrictions, in other words they used it to serve other than control power. The #6 conductors are contained in a cable direct buried. I don't want to bore you with details like it is 240 volt and runs approx. 2 miles.
 
derf48 said:
I asked the question because "the code says so" doesn't always work. For example, I ran into a situation where 2 #6 Cu conductors are paralleled, for voltage drop, and they followed all the requirements of 310.4 ex. #2, except the load restrictions, in other words they used it to serve other than control power. The #6 conductors are contained in a cable direct buried. I don't want to bore you with details like it is 240 volt and runs approx. 2 miles.
Click to expand...

If the wiring is not for control, Exception #2 doesn't apply, so the installation would be a violation. I'm sure the previous speculation probably hit on at least some of the issues, most importantly "because Code says you can't".

Jim T
 
derf48 said:
I asked the question because "the code says so" doesn't always work.
Click to expand...

I agree. It seems to many people blindly follow the NEC without having an understanding as to why it says what it does. Keeping in mind the Code is minimum requirements, means that as an engineer I can engineer a system that, if proved by calculation is better then the minimum, then it has to be accepted. Unfortunately, in many cases budgets don't allow for such thorough evaluation and therefore many engineers simply accept the Code and never question why.

A good engineer or designer when asked why they did what they did should have a legitimate reason. The answer better not be, "because that's the way we've always done it".
 
IMHO if the installation is such that, if all current were flowing on a single 'branch' of the parallel conductor set, that conductor would be below its ampacity, and if the OCPD were correctly sized for a single 'branch' of the parallel conductor set, then it is a _safe_ installation.

If this is in fact the case, then the only answer is 'the code say so', perhaps combined with 'the writers of the code did not anticipate this situation, and in an attempt to prevent real hazards they inadvertently prohibited what should be a safe installation.'

See http://www.mikeholt.com/code_forum/showpost.php?p=642483&postcount=31 for an interesting take on 310.4

How about this for a _permissive_ analysis of 310.4:

Generally 310.4 is read as prohibiting conductors smaller than 1/0 from being used in parallel. However there is in fact no explicit prohibition from using smaller conductors in parallel, but instead 310.4 _permits_ certain benefits from being derived from conductors in parallel.

Using this interpretation, if we didn't have 310.4, there would be no prohibition against using conductors in parallel, however there would be no capacity benefit to such use. All of the code rules applying to conductors would have to apply to the _individual_ branches of the parallel set.

For example, consider overcurrent protection. 240.4 requires conductors to be protected by overcurrent device in accordance with their ampacity as specified in 310.15, with various refinements. Nothing in 310.15 says anything about the amapcity of conductors in parallel; it just tells me things like 'the ampacity of a #6 conductor with 60C insulation is 55A'. So 240.4 requires that I protect that conductor with no more than a 60A breaker, and feed a load of no more than 55A. If I put two #6 conductors in parallel, I would still be limited by 240.4 to putting this set on no more than a 60A breaker.

310.4 then _permits_ us to use conductors in parallel as single conductors. A single #1/0 conductor has an ampacity of 125A, but because of 310.4, two of these conductors in parallel would be permitted to be used at an aggregate ampacity of 250A, with corresponding OCPD.

I grant that this interpretation makes some of the exceptions to 310.4 redundant, and does not jive with the standard interpretation of the code. However it does fit with the general requirement that the code be permissive (if it doesn't say you can't do something, then you can do it). Also it makes the example installation (parallel #6 conductors for voltage drop), which seems like a reasonable and safe installation, legal.

-Jon
 
Jon,
Using this interpretation, if we didn't have 310.4, there would be no prohibition against using conductors in parallel,
Click to expand...
While the CMP does not agree, there is no way that the words "shall be permitted" act as a prohibiton. There is no rule in the code that actually prohibits the the installation of conductors smaller than 1/0 in parallel. Yes, that is the intent of 310.4, but according to the NEC manual of style, the words "shall be permitted" are to be used like an exception...to permit an altenate installation method. That would mean there would have to be a rule in the NEC that says conductors must be not be multiple wires. I am not aware of any such rule.
Don
 
Just to be clear, I want to emphasize that I am playing with this idea, not stating a firm position on the code. Based upon the various exceptions to 310.4, it is clear that the CMP doesn't follow this position :)

Don, the use of "shall be permitted" to create an exception is exactly what I am trying to twist 310.4 into :) While there is nothing in the code that explicitly prohibits putting small conductors in parallel, all of the requirements for conductor protection and calculating ampacity speak about _individual_ conductors. As far as 310.15 is concerned, a 6ga 60C conductor in normal conditions has an ampacity of 55A, and must be protected by no more than a 60A breaker (with exceptions). If you put 10 #6 conductors together, fed a 200A load, and protected them with a 200A breaker, you would violate 240.4, because a 55A conductor was being protected with a 200A breaker.

Thus the "shall be permitted" 310.4 creates an exception, permitting conductors to be tied together at both ends and treated as an aggregate conductor, and permitting 125A conductors to be protected by OCPD much greater than 125A. Nothing prohibits smaller conductors from being tied together at both ends...but if they are smaller than 1/0 your OCPD and maximum loading would be set by the ampacity of the _individual_ conductors.

Just playing with words :)

-Jon
 
Continued playing with the words:

310.4 doesn't say anything about the parallel conductors being treated as a single conductor. It just says "shall be permitted to be connected in parallel (electrically joined at both ends). No where does it say anything about using the aggregate ampacity or provide permission to use other OCPD values. Clearly in my previous posts on the topic, I was imagining a statement in 310.4 about 'treated as a single conductor'. 310.4 simply says that you can join some conductors together at both ends.

240.4 has no exceptions for parallel conductors. 240.4 says that conductors _shall_ be protected at their ampacity as specified in 310.15. If it isn't in 310.15, and isn't in the exceptions to 240.4, then you _shall_ use the methods of 310.15 to pick your OCPD.

310.15 does not provide any method for calculating the ampacity of parallel conductors.

The implication: you can put any number of any size conductors in parallel. But it doesn't get you anything. If you put 10 #1/0 conductors in parallel, you must still provide 125A OCPD in order to protect the conductors as required by 240.4. 310.4 is simply redundant, because it says 'shall be permitted' without any prohibition that it is an exception to. All of those large services installed with parallel conductors: code violations :)

-Jon
 
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