Is doubling smaller copper sub-panel feeders more economical?

[Eric Commarato]

I had an electrical engineer tell me one time it is sometimes more economical to double up conductors to feed a sub-panel of a lower gauge wire in lieu of running one larger gauge. For example, 6-3 with a ground costs $1.28 a linear foot from my local supply house. Running double 6-3 with a ground would $2.56 a linear foot. 4-3 with a ground runs $2.85 a foot. If this is done would this be compliant with the NEC? Would I gain any additional amperage capacity by doing this? Is this better than running aluminum feeder wire?

Thanks,

Eric

[Lee Schierer]

The ampacity of #6 wire is 55 amps whereas the ampacity of #4 is 70 amps. From a mathematical standpoint two number 6 wires would carry 110 amps, so they would mathematically have more capacity. However, I can't answer if codes allow you do do this. One problem I could see is whether you could securely connect the two wires in a lug designed to hold one conductor.

[Anthony Whitesell]

That's similiar to my worry. If one conductor or connection of the pair fails the other conductor would be carrying the full load and probably burnout instantly if not worse.

As an EE, the initial assumption is correct, providing the resistance of both wires is the same.

[Eric Commarato]

I've done a little research concerning wire gauges/sizes. A #6 wire has a diameter of .162". Doubling the wire and theoretically doubling the area, the diameter would be .324, or about the diameter of a No. 0 wire. I think the problem with running double conductors as some of you have stated is terminating at the lugs on the breaker and subpanel lugs. My engineer did say though this was not a "normal practice" it wasn't a code violation per the NEC.

[Tom Veatch]

...Doubling the wire and theoretically doubling the area, the diameter would be .324, or about the diameter of a No. 0 wire...

I believe the math may be off a little. If you double the area of a .162 dia wire, the theoretical new diameter would be 1.41(.162) = .229.

Doubling the diameter gives 4 times the area.

edit: Oops, Sorry. Misread the intent. You're right, doubling the wire would effectively double the diameter as far as what the connector had to accommodate.

[Anthony Whitesell]

But unfortunately current capacity doesn't work that way. A 6ga wire can only carry so much current (55A), therefore two 6ga wire can carry a total of 110A. I don't know how much current a 0ga wire can carry, but two 6ga wires are limited to a total of 110A no more.

[Anthony Whitesell]

You have also made an error in the assumptions. Current capacity is based on surface area not diameter. If 6ga is 0.162" diameter or .0206 sq in. therefore two 6ga wire is 0.0412 sq in. If the 0ga wire is 0.324" in diameter (twice diameter of 6ga wire) then the surface area is .0824 sq in. Almost four times larger than the 6ga wire and therefore also 4 times as much material.

Two wire 0.162" diameter is not equivalent to one wire 0.324" in diameter.

Again, I would still be nervous about running two in parallel. But putting my EE circuit design back on and thinking about it further, I wonder if you could put two circuit breakers in the panel and "fuse" each line to 55A and tie them together at the other end (of course you'd have to pick the right two slots in the breaker panel). If one line or the connections to one line failed, then when the other attemped to supply the full load (110A), the circuit breaker would trip (55A).

I know I can build circuits this way for other types of equipment not covered by the NEC, but I don't know if this would be NEC legal for use in a house. Thoughts? Feedback? Questions?

[Lee Schierer]

Again, I would still be nervous about running two in parallel. But putting my EE circuit design back on and thinking about it further, I wonder if you could put two circuit breakers in the panel and "fuse" each line to 55A and tie them together at the other end (of course you'd have to pick the right two slots in the breaker panel). If one line or the connections to one line failed, then when the other attemped to supply the full load (110A), the circuit breaker would trip (55A).



As I recall the NEC frowns upon dual power sources.....to a single device, that is why the two legs of a 240 volt circuit are tied together at the breaker so one side tripping automatically trips the other side.

[John Baranowski]

Actually, if the two conductors are run together, you have to derate them, so you would end up with less than 110A of capacity. I don't have an NEC in front of me, so I don't know the exact numbers, but it would be less.

[Rollie Meyers]

Forget about paralleling those conductors as the minimum size allowed by the NEC is 1/0, see 310.4 2005 NEC, and there are other conditions that must be met.

[ROY DICK]

Forget about paralleling those conductors as the minimum size allowed by the NEC is 1/0, see 310.4 2005 NEC, and there are other conditions that must be met.

Bingo! The lugs must also be rated for duel conductors.

Roy

[Tommy Curtiss]

NEC 2008 Handbook 310.4 Conductors in Parallel
(A) General. Aluminum, copper-clad aluminum, or copper
conductors of size 1/0 AWG and larger, comprising each
phase, polarity, neutral, or grounded circuit conductor shall
be permitted to be connected in parallel (electrically joined at both ends).


this means nothing SMALLER than a 1/0 can be used as a parallel conductors

everybody is willing to spend 10 grand for tools in the shop,but not a few hundred bucks for the shop electrical,,safety starts at the beginning,,,not half way thru,,spend good money on the electrical system ONCE,,and if you are in dought,,consult a good electrician,,a six pack in thier direction will go a long ways,,,trust me:p

[Wayne Cannon]

I'm not an electrician (I am an electrical engineer), but I can't see any way you can legally parallel multiple wires to accomplish greater current capacity.

The code required that "conductors", not circuits, be protected by a circuit-breaker (or equivalent) of an ampacity based on the conductor size. Therefore, your circuit-breaker would have to limit you to the lower capacity of a single conductor and you would lose the benefit of doubling up.

This makes sense and resolves Anthony's concern -- the breaker would have to be small enough to handle the single-conductor case were the other conductor to fail. By the way, that is a plausible scenario, since one conductor could easily have slightly more resistance at a connection than the other conductor. The higher resistance connection would warm more, increasing the resistance, causing it to warm even more, which could lead to a run-away condition burning out the connection for that one conductor, leaving you with only one conductor.

In addition, circuit-breaker lugs are limited to a single conductor, so your split from one to two conductors would have to occur separately from the circuit breaker's lug.

[Rollie Meyers]

It is common for larger services to use parallel conductors where one can have 2,4,6,8 .... conductors per phase,& when done correctly is a safe, reliable & code compliant installation,the idea invisioned by the OP is simply not a code compliant way to do things.

[Richard Cordery]

Your an engineer and you have never heard of parallel runs hmmm. The biggest flaw in his plan is that he is thinking of doing this himself when his house burns down my insurance will go up. all because he wants to save a few cents a foot.

[Eric Commarato]

Your an engineer and you have never heard of parallel runs hmmm. The biggest flaw in his plan is that he is thinking of doing this himself when his house burns down my insurance will go up. all because he wants to save a few cents a foot.

If you are talking about me? I never said I planned to do this or was going to do this my self. I am simply posing the question, trying to become educated about this. Isn't that what these forums are intended for? Keep your criticisms to yourself about whether I am trying to save a few bucks here or there, and how it might impact your insurance. :mad:

[Tommy Curtiss]

Please don't misunderstand me,,I am not implying anything about anyone,,I am here to help:) I am not here tell you you can't do what you are describing,,I am just saying it isn't legal by the NEC according to 310.4,anything smaller than 1/0 cannot be run as a parrallel conductor,,service entrance or circuit conductor,,doesn't matter,,but let's say that you could!!!

We are going to run 2 #6 to a panel per phase,,and we do,just for the math sakes we are going to buy #6thhn for say .50 per ft
the breaker you are coming out of your main panel is rated only for 1 conductor,,so we also have to buy 2 ft of #4 at say .80 a ft to splice those # 6's to,,plus the 2 splitbolt kurney's at about 6 bucks, a roll of rubber tape at 3 bucks and a roll of good electrical tape to complete the work,now we have one phase complete,,you got one more phase,and one neutral left,,but you can use #6 for the ground,,I think you get the point:eek:

so what have you got now,,panel feeders with multiple splices that may over time come loose,and by the way,,you arn't supposed to splice conductors in the panel,,and I didn't even get into buying a jbox's to mount outside on the panels,,,,so you arn't really saving anything,,fact is you are costing yourself,,time and money

I am now working in Iraq in the support of Iraqi Freedom and our US Military,I am a Master Electrician Inspector for the Dept. of Defence,I have seen more electrical crap in the last 8 months than you can imagine,,every trick in the book used,,,open style disconnects just barly outside the water area of a shower that our soldiers are using nightly,,oh by the way,,everything is 230/400 50hz 3 phase over here,and they don't believe in grounding,there have been several soliders lost their lives due to poor wiring practices.

So do things right,,don't cut any corners,,heck,,hire it out if you have to,,,you and I will both sleep better at night,,you will only miss the extra money once,,but you won't have to worry about it for years to come either