310.4 Conductors in Parallel

[doden]

I've looked everywhere and am having trouble finding an answer to this: why does the NEC require conductors in parallel to be 1/0 or larger? What issue is there in a smaller gauge wire if the resistance is the same between both runs?

 

[GoldDigger]

I've looked everywhere and am having trouble finding an answer to this: why does the NEC require conductors in parallel to be 1/0 or larger? What issue is there in a smaller gauge wire if the resistance is the same between both runs?

An accurate balance of resistance between members of a parallel group of wires is necessary to prevent an uneven distribution of current which could cause overheating in one of the wires of the group.
It is difficult to ensure this, and there is a readily available alternative for smaller gauges, namely a larger wire size.
Once you get above 1/0 you start to run out of room at the larger end because of availability, handling an termination problems.
Also at 1/0 and above the increasing role of skin effect on the resistance of a large wire makes use of several smaller conductors a more efficient way to carry the same current.

 

[Dennis Alwon]

I've looked everywhere and am having trouble finding an answer to this: why does the NEC require conductors in parallel to be 1/0 or larger? What issue is there in a smaller gauge wire if the resistance is the same between both runs?

My guess is,as stated above, that the availability of larger wires is an issue. I also think that parallelling conductors leaves room for error and obviously, we want to avoid this.

 

[petersonra]

It's a rule

 

[Carultch]

I've looked everywhere and am having trouble finding an answer to this: why does the NEC require conductors in parallel to be 1/0 or larger? What issue is there in a smaller gauge wire if the resistance is the same between both runs?

One reason I would suggest, would be that the relative manufacturing uncertainty is less in larger sizes.


As a hypothetical example to show how this works mathematically, suppose you can manufacture to the tolerance of +/- 1 kcmil. This is not necessarily a realistic example of manufacturing tolerance, it is just to show the math. A #10 wire is about 10.6 kcmil, so this is about a relative tolerance of +/- 10%. A #1/0 wire is 106 kcmil, so a tolerance of 1 kcmil means a relative tolerance of +/- 1%. For current to divide between parallel paths, it will only divide equally if the symmetry is maintained. If the parallel wires are at opposite ends of the tolerance window, a +/-10% tolerance means up to a 22% difference in current between the paths. By contrast, a 1% tolerance means up to a 2.2% difference in current between the paths, which is a significant improvement.

 

[tom baker]

The reason we parallel conductors is the ampacity is not proportional to size. Below 1/0 there is little to be gained from running in parallel.

 

[jumper]

The reason we parallel conductors is the ampacity is not proportional to size. Below 1/0 there is little to be gained from running in parallel.

True, but if it creates no harm or has no real potential to - why limit the size? Just because something is inefficient is not a reason to forbid it.

One could pipe a dwelling in RMC and then pull NM-B. Inefficient yes, but legal.

 

[MyCleveland]

The reason we parallel conductors is the ampacity is not proportional to size. Below 1/0 there is little to be gained from running in parallel.

Tom

Did a quick Current Divider circuit check.
Assuming I did the math right.
1000A - via 4 100' 500kCMIL, chopped the fourth wire by 3' and extra 6+/- amps through the shorter wire or carrying 2.4% over balance (250A).

100A - via 4 100' #10, chopped the fourth wire by 3' and extra 0.6+/- amps through the shorter wire or carrying 2.4% over balance (25A).
Not sure this means anything.

 

[JFletcher]

Welcome to the forum.

This may not be a reason that went in to the NEC rules, but smaller terminations are usually rated for just one wire. Even if the NEC would let you use 2 runs of 14/3 to a dryer vs one 10/3, you'd have nowhere to land the second set of wires on the receptacle or most breakers. Also, with just two small sets, if one failed the other would be definitely overloaded. There is no good reason to parallel small wires in most cases.

Dunno if the OP works in residential, but try getting AFCI breakers to hold if there is an inadvertently paralleled neutral. Then again, if one tied together neutrals of circuits on different legs, he's accidentally made a MWBC, and, short of AFCI/GFCI breakers, a better install.

Ring circuits for LV lighting are one of the other few places were parallel conductors work better despite being against the NEC rules.