Running wire in parallel to reduce resistance.?

Question

I am trying to run a 20 amp circuit 300 feet to a place I need electricity. I already have 12 gauge exterior wire but at this distance, I don't know what kind of amperage it could handle. So can I run two lengths in parallel in order to operate a higher amperage circuit?

Maybe a parallel of about 200 feet with a single run for the remaining 100 feet. Will either of these methods work?

Thank you in advance.
Dave

Answer 1

Each of the above answers, apart from the first, gets to part of the question.

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Edit: ... with the exception of the one right above which came in while I was typing, and almost addresses your problem correctly (the NEC comment is incorrect). If you WERE trying to double the wire to get more current capacity, that would be a different story, but that's NOT what you're doing here.

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Edit again: ... Jesem47's example further down assumes a 20A load on your 20A circuit. I assumed you'd be a bit more "gentle" on average at 15A. If indeed you're running a full 20A load on this circuit, you'll need to adjust my figures accordingly, and you will indeed achieve a greater voltage drop. Your max effective load resistance changes to 6 ohms instead of 8 ohms.

BTW, I'm confused about that post's use of 6ga wire. The link below shows a resistance per 100' for 6ga that would give significantly lower voltage drops than his calculator.

http://www.radiolocman.com/electrical-engineering/circuit-cache.html?di=18899

Further, the calculator assumes a maximum allowed voltage drop as a percentage of the source voltage -- rather than looking at the net effect on voltage available to the load at a given current. Perhaps there's a rule of thumb that requires this, but I'm not familiar with it.

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Edit yet again, to Jesem47:

THE ORIGINAL POSTER'S SUGGESTION IS PERMITTED UNDER NEC 310.4 under one of the 310.4 exceptions mentioned in your post. This further strengthens my point that the purpose of the 2nd pair is NOT to increase ampacity, but to reduce voltage drop.

The information Jesem47 provided was a bit 'selective'.

From 310.4

"(b) The ampacity of each individual conductor is suffıcient to carry the entire load current shared by the parallel conductors"

The OP's use of 12 gauge wire is indeed "sufficient".

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First, let's establish what longer runs of wire cause. No wire is a perfect conductor, and all wire is spec'd with a given amount of resistance per 100' or 1000'. This impedes the flow of current, and causes a voltage drop across the wire. The first answer is mistaken in that while longer runs will cause a greater voltage drop, that drop will be along a longer length of wire -- so there's no danger of "overloading" a length of wire due to its length.

So this isn't a safety issue, and losing one side of your parallel system isn't going to cause the other side to "overload"... you'll just get a somewhat lower voltage to your loads at the end of the 300' run. The wire actually conducts its maximum current over the *shortest* possible run. Your 12 ga, being adequate for 20A, is adequate. End of that story.

However (and there's always a however), another poster mentioned 'voltage drop', and that's your only real consideration here, and the drop will vary with the current draw on the line.

SO LONG AS EACH cable is capable of handing your 20A load (and your 12ga certainly is), there is no risk of overload, and running parallel circuits is acceptable, if a bit peculiar.

Solid 12 gauge wire has a resistance of about 1.6 ohms per 1000 feet. If you were to run 300 feet of wire, you'd be talking about roughly 0.5 ohms of resistance for the run.

A resistive load (like a bunch of light bulbs) of 15A at 120V means that the load's resistance is about 8 ohms. (R = E / I). If the voltage is 120V, part of it drops across the wire, and part of it drops across your light bulbs, and it's proportional to the resistance. I think you'll be surprised to see the difference.

A)

So for a single 12 gauge wire, you get

Bulbs = (8 / (8 + 0.5) ) * 120 = 113V
Wire = (0.5 / (8 + 0.5)) * 120 = 7V

If your power at the box is 120V, this means you'd have 113V at the end of your 300' run with a fairly heavy (15A) load. If that works for you, then you don't need to run parallel wires, and you can ignore the 7V drop across your wire.

B)

What if you run a parallel wire the whole way?

If you run two 12ga wires in parallel for the whole run, they'd have a resistance 1/2 that of normal, or 0.25ohms per 300'. As a result, using the same 15A load, you'd have 116V at the end of your 300 foot run.

Bulbs = (8 / (8 + 0.25) ) * 120 = 116.4V
Wire = (0.25 / (8 + 0.25)) * 120 = 3.6V

C)

Your "hybrid" suggestion of single and and parallel:

Your "hybrid" example is a bit different as is the formula since you've got one section of wire 100' long at about 0.16 ohms and a 200' section of wire at about 0.16 ohms (half the resistance, but twice the length, so it comes out the same). We'll add them and call it 0.32 ohms for the two sections together.

Bulbs = (8 / (8 + 0.32) ) * 120 = 115.4V
Wire = (0.32 / (8 + 0.32)) * 120 = 4.6V

Answer 2

Parallel Wire

Answer 3

This Site Might Help You.

RE:
Running wire in parallel to reduce resistance.?
I am trying to run a 20 amp circuit 300 feet to a place I need electricity. I already have 12 gauge exterior wire but at this distance, I don't know what kind of amperage it could handle. So can I run two lengths in parallel in order to operate a higher amperage circuit?

Maybe a parallel of...

Answer 4

Paralleling small wires to increase their current carrying capability is definitely contrary to the National Electrical Code. Use a bigger wire instead. That solution is also likely to be cheaper.

The 12 gauge should be able to handle the current unless it is bundled with several other wires. However, voltage drop may be a problem, so you may need to go with a bigger wire than would be necessary to keep IR voltage drop under control.

If the run is very long, you might want to consider operating at a higher voltage at the line end stepping the voltage down at the load end with a transformer to reduce the effect of IR voltage drop. There is a good reason that the voltage is stepped up for transmission over long distances.

Answer 5

NEC Provision:
If the wire is smaller than 1/0 awg then parallel wires are not allowed except per 310.4 exceptions.

anyway i'll give you the data of my voltage Drop Calculator;
1. Voltage= 120
2. %VD= 5%
3. Phase=single
4. Wire size= #6 stranded Copper Coated
5. Wire lenght= 300 ft.
6. total load= 20 amps
7. # of parallel wires = 1
Results Window:

8. Max. VD allowed= 6 volts_______Actual= 6.12 (failed)
9. Min. voltage allowed= 114 V_____Actual= 113.88(failed)
10. Multiplier= 2_____________voltage difference=-0.12(failed)
11. Wire size= AWG6_____Recommended= AWG4
12. Distance=300 ft. ____Max. distance w/this load=294ft.
13. Max amps=20 _____Max. amp of wire= 55 amps@60°F
14. Min. no. of parallel wires=1____amps above or below load= 35 amps(ok)
now, its up to you if you consider your voltage drop.

Answer 6

The best procedure would be to test the circuit under load and make sure the wire does not heat up. It would depend also on the temperature and location of the circuit.

Have checked this out with a certified/registered electrician and he said he uses a 12 gauge wire at a distance of 300 feet and has no problem. He also said that a good test is to look at the voltage drop and check to see if the wire heats up under max load. Slightly warm is okay, but if there is more than just a slight warmth you should go to a 10 guage wire. If you already have the wire, the two parallel wires would solve the problem.

Answer 7

What was said just before is quite true, the shorter of your wires will have to handle a lot of the total current.
But I actually think you don't need to worry about separating your wires : no matter how long it is, a wire will withstand the same current.
The power loss will be proportional to the length of wire, though.

Answer 8

Yes both will work.I have done this before with 3 phase wiring. Used the 4 wires to make 2 larger for 120v. If you want to be real safe. Run two wires the 300 then put two fuses on the wires both rated for 12amps or so. So if one wire pops both lines will drop.

Answer 9

What you suggest will help but as stated before the remaining 100 foot section of single wire will be the weak link and may overheat. What you have to consider is the voltage drop that is acceptable for your application and plan for that. It is best to run a consistent size conductor all the way.

Answer 10

It's not recommended and the short run will still be handling full current. If one wire fails, you may have a fire.