Can I connect Romex in Parallel and up the breaker size?

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JoeNorm

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I think I know the answer to this question but I'm curious anyway.

Let's say i had a 40 amp 240v load I want to power. I need 8-2 wire but don't have it on the van. I do have 12-2-2. What prevents me from using two of the #12's in that cable per line. I'd be paralleling conductors of the single Romex cable.

Is there a code preventing this?
 
No you cannot do this with 12/2/2 or any other NM cable for that matter because NM cable cannot be larger than #2 AWG. Parallel conductors cannot be smaller than #1/0. {310.10(H)(1)}
 
They used to allow that. It was common to see two runs of 10-3 w/o gr. in parallel supplying a range/oven circuit.
 
I wonder why its against code to parallel anything under 1/0?
It would certainly work just dandy.
Maybe its because breakers under 150 amp (especially smaller ones) are only designed for one wire and they don't want double tapping.
 
powerpete69 said:
I wonder why its against code to parallel anything under 1/0?
It would certainly work just dandy.
Maybe its because breakers under 150 amp (especially smaller ones) are only designed for one wire and they don't want double tapping.
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I think it is another of those design choices they made for you that the code is not supposed to make.
 
powerpete69 said:
I wonder why its against code to parallel anything under 1/0?
It would certainly work just dandy.
Maybe its because breakers under 150 amp (especially smaller ones) are only designed for one wire and they don't want double tapping.
Click to expand...
Don't know for certain, but my guess is there is more risk of unequal current division and potential overheating of one conductor of the set when compared to limiting it to somewhat larger conductors.

1/0 the smallest conductor allowed to be paralleled, but quite often you won't see parallel conductors until you get to 400 amp circuits or larger. For one thing equipment just won't have lugs designed for parallel conductors until it gets to this size or larger.
 
kwired said:
... quite often you won't see parallel conductors until you get to 400 amp circuits or larger. For one thing equipment just won't have lugs designed for parallel conductors until it gets to this size or larger.
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that may be related to not allowing for conductors under 1/0 to be paralleled though.

my experience with paralleling conductors is that two 1/0 conductors are the cheapest copper you can get with the most ampacity. I have been known to use (2) 1/0's in lieu of a single 4/0 just because of the cost factor. plus it is easier to work with. Usually (2) 1/0's and a single 4/0 are about the same price. The connectors are usually smaller too.
 
powerpete69 said:
I wonder why its against code to parallel anything under 1/0?
It would certainly work just dandy.
Maybe its because breakers under 150 amp (especially smaller ones) are only designed for one wire and they don't want double tapping.
Click to expand...
I'm pretty sure it's because the smaller the wire the more difference in resistance the same difference in length makes, i.e. a lower error tolerance. No two wires are ever exactly the same length.
 
petersonra said:
that may be related to not allowing for conductors under 1/0 to be paralleled though.

my experience with paralleling conductors is that two 1/0 conductors are the cheapest copper you can get with the most ampacity. I have been known to use (2) 1/0's in lieu of a single 4/0 just because of the cost factor. plus it is easier to work with. Usually (2) 1/0's and a single 4/0 are about the same price. The connectors are usually smaller too.
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Yes I'm sure it mostly is the reason, but even for your 1/0's paralleled instead of using single 4/0, now you will often need to find equipment with lug for two conductors, change lugs, or use other method to adapt for terminations, that can lessen any savings.

Over 250 often conductor prices begin to become fairly significant as well as raceways over 2 inch can become more significant cost factor as well. Can sometimes run two 2 inch raceways for similar or even less than a single 2.5 or 3 inch will cost. One raceway with parallel conductors inside also means ampacity adjustment and often increased conductor size.
 
ggunn said:
I'm pretty sure it's because the smaller the wire the more difference in resistance the same difference in length makes, i.e. a lower error tolerance. No two wires are ever exactly the same length.
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That's true of the difference in resistances between two lengths of the same wire, since the resistance per unit length is higher with smaller diameters. But the ratio of resistances should always be the ratio of lengths. Can you provide an example where the controlling factor for problems is the difference in resistances, rather than the ratio of resistances?

Thanks,
Wayne
 
I pretty much assumed that the reason was that for smaller wire sizes, a larger gauge delivering twice or more the ampacity is readily available and would not be excessively more difficult to pull, bend, and terminate.

Sent from my XT1585 using Tapatalk
 
kwired said:
... Don't know for certain, but my guess is there is more risk of unequal current division and potential overheating of one conductor of the set ...
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I doubt that very much. Paralleled conductors for a single circuit -- same origin and same destination -- are unlikely to be sufficiently different in length to cause a problem. If it were, the whole concept of paralleled conductors would be problematic, regardless of size.
Even paralleled conductors of different sizes wouldn't be problematic -- the current will divide according to the impedance of each conductor, keeping each conductor within limits if the whole assembly is. (Do the Math to confirm or deny)

As for the original question ... It would work. It would even be reasonably safe if well-implemented. I would even condone it if were necessary to mitigate an emergency. But nobody in the Western World would ever approve it as a permanent installation and nobody in the trade would respect you in the morning.
 
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