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Besides being NEC non conforming to the smaller than 1/0 sizing, what problems could/would occur if one paralleled the conductors color for color in a # 10 3 wire (black, red, white) plus bare ground UF cable? Would this increase an allowable ampacity to 50 amps if each end of the phase wires were pigtaled into a # 6 THHN and properly polarized and phase connected into a 50 amp locked breaker. Could this arrangement provide a feeder for a 50 amp subpanel @ 240 VAC since it would have a neutral and separate ground. Thank you.

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You're rationalizing. You want to do it, and you are trivializing the many reasons why this is a bad idea, and trying to boil it down to one reason: that it's not allowed for wires smaller than 1/0. And then your logic is "well that's a silly rule, so I can just ignore it".

One of the wires breaks

The first problem is that if one of your wires (hot or neutral) has a simple wire break, now, ALL the current is flowing on the surviving wire. That is why you do not just split wires like that.

"Oh, well, I'll just use two 30A breakers then, so every hot has a 30A breaker". What about neutral? Neutrals don't have breakers.

Special equipment

Go read the paralleling rules. There are a bunch of requirements, but one of them is that paralleling must use custom equipment specifically designed for paralleling. I have such equipment in my shop. I have six 1000 kcmil conductors going up to a busbar, 2 wires per bar. The origin equipment is a disconnect switch with six 400A fuses. ONE PER WIRE. See how that nicely solves the "broken wire" problem?

So you too would need "special equipment" at the origin of your paralleling rig, and that equipment would need to give neutral a breaker. If I were out to violate code and wing-ding that, I'd use a 3-phase service panel, treating neutral as a phase, and fill it with 6 x 30A breakers. But I'd still respect the rest of the paralleling rules.

You'll be at least $120 into that. Why did you say you wanted to do this? To save money?

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  • Thank you. In this case the 10 - 3 w g UF cables are already burried in place each fed with separate 20 amp single phase breakers. The owner was simply inquiring about Apr 28, 2020 at 12:11
  • Thank you. In this case the 10 - 3 w g UF cables are already buried in place each fed with separate 20 amp locked single phase breakers. The likely hood of one of the wires breaking is minimal but your point about one feed now carrying a heavier load, including a broken neutral is well taken. The owner was simply inquiring about increasing a feeder ampacity, installing a subpanel for circuit breaker protection in the outbuilding by paralleling the wires and down rating the combined wires to 50 amp. Wouldn't the same situation be true for 1/0 wires paralleled if one broken? Just asking. Apr 28, 2020 at 12:27
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    ...and here's another example of why direct burial wire is a bad idea .vs. conduit. If you (or "the owner") already had conduit buried, rather that having buried two UF wires directly, you could just pull the wires you needed, up to the size of the conduit. Of course, if "the owner" is not "you" then even contemplating doing this is a Very Very bad idea, rather than simply being a Very bad idea. Sins against code in the line of work are much more costly than sins against code on your own time.
    – Ecnerwal
    Apr 28, 2020 at 12:49
  • @Badertscher001 The chance of ANY wire failing is minimal, so by that logic we don't need any circuit breakers at all. Or GFCI or any of that jazz. The reason it works with 1/0 is you are using special equipment at the source which solves that problem i.e. fuses every wire including neutral. I told you how to improvise that. Apr 28, 2020 at 17:14
  • Good point Ecnerwal, citizens are only allowed to work on homes they own AND occupy. All else must be done by a licensed electrician, who would never ask this. Apr 28, 2020 at 17:15
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My understanding is the minimum size dealt with production size variances in small wire that would result in unequal resistance and current between the parallel conductors.

I personally would be concerned about physical damage causing resistance variance. A minor shovel or rock impact might cause less damage (and less damage by percentage of size) against the surface area of a larger, more flexible stranded conductor over a smaller solid conductor. A lot of concerns start to develop about what is the manufactured safety margin of the capacity of the grounding conductor and such, to the point that I have to sometimes stop and recognize that if I wanted to argue engineering logic I should have gone to college to be an electrical engineer instead of entering a building trade.

The Code has revision proposal procedures, this is one section that nobody has been able to present a proposal to change that has satisfied all of the concerns of the Code Making Panels. The final production of the NEC doesn't included all the documents and discussions of the Code Making Panels, 800 pages is enough, more pages would just create more arguments. (Other times you just got to look at the customer and say "you give the government the permission to regulate and you get to pay for the regulation, and this isn't hill I'm willing to stage a battle from".)

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