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I'm trying to power a 30 Amp x 120 Volt RV plug near the perimeter of a five-acre property (200' from the breaker panel). I do not want to buy, dig and bury a new 10-gauge UF cable. However, the location is currently served by only 3 UF cables:

  • 12 gauge 12/3 to the RV site. At this point, the four wires of the 12/3 UF cable are unterminated at both ends (unused) but it is available and already buried from a 100 amp 240-volt subpanel, itself 120’ from the Main Breaker Panel for the property.
  • 12 gauge 12/2, which is currently providing power to a dozen (mostly unused) 20 amp outlets along the property perimeter. It terminates in the main breaker panel on the property.
  • 14 gauge 14/2, which is planned to power lights along the perimeter, but so far it too is unused. It terminates in the main breaker panel on the property

Let me describe my "Plan A" so far, and my Plan A-revised".

Plan A: With the 12/3, I want to parallel both the hots (the black and the red wire) to make a single hot. Then I want to parallel the neutral and ground wires to make a single neutral; able to carry the 30 amps (circuit 1 at 30 amps). I’d then connect both grounds from the nearby 12/2 UF cable (circuit 2 at 20 amps) and the 14/2 UF cable (circuit 3 at 15 amps) to provide a common ground for all cables on all three separate circuits.

Plan A-revised: Add a sub-panel at the RV site. The black wire from the 12/3 feeds into a 20A breaker in the sub-panel on pole A. The black wire from the 12/2 also feeds into a 20A breaker into the sub-panel, also on pole A. The neutrals from 12/3 and 12/2 go to the neutral bar in the subpanel, and for good measure, the neutral from the 14/2 also goes to the neutral bar, giving a total of 55A of neutral return. All grounds to the ground bar. Thus, all the hots are breakered at both ends of their run. Then, a single-pole 30A breaker goes to the TT30 receptacle.

My goal is to understand the best way to provide 30A @ 120V.

  • @ThreePhaseEel , Thank you! I'm guessing the code forbids using conductors in parallel because it greatly increases the risk of creating a poor connection at one end or the other and thereby increases the risk of creating excess heat on the circuit? Are there any other considerations that I should be aware of in the proposed configuration? – David Davido Feb 18 '17 at 14:23
  • I revised the question to make it less about the method and more about the end-goal. This invites answers which use other methods. – Harper Feb 20 '17 at 16:40
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    Why not use the 12/3 to provide 240V at 20 amps, and then use a step-down transformer at the sub-panel? Simple, code compliant, and anyone else that comes after you will understand it. – longneck Feb 20 '17 at 18:03
  • @longneck, Yes a transformer is the solution! The thought of using one had not occurred to me. Thank you. – David Davido Feb 21 '17 at 15:09
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Let's go over the ground rules and we'll build up to the solutions.

Code is your friend. Code can seem annoying when you're trying to do something, especially when you get fixated on one style of solution. And many Code requirements seem rather silly, especially to someone with an EE background. And newbies tend to think they can "outsmart" it. But the more you know Code, the more sense it makes. This isn't indoctrination, it's understanding the why's.

The Authority Having Jurisdiction (AHJ) is the decider. This is your local city inspector, the one who requires you pull a permit. If you don't know much about Code, you may not know about the need to pull permits. This means an electrical inspector will look it over, and this means you better be Code. Cheating around the permitting process will get you in big trouble later.

Redundant circuits disallowed. With outdoor work, you cannot have 2 circuits going to the same place unless they are different somehow: voltage, switching, etc. That means if both circuits from the main panel are 120V, you should put a switch on one of them. Nothing says you ever have to throw the switch.

Mixing disallowed. Each neutral must stay with its partner hot(s). Grounds can mix, but only with other grounds from the same panel. This is relevant since you have circuits served out of different panels in close proximity. You can't even mix their grounds.

The RV site needs a grounding system of its own

This is true no matter what you do. There are several ways to create a grounding system, but the usual method is to drive two 8' copper rods into the earth some distance apart. This is needed because under some conditions, earth under your feet is not at the same voltage potential as earth 200 feet away. This eliminates the possibility of being shocked while standing in a puddle holding a grounded thing.

This local earth ground does not replace the equipment safety ground wire brought over with the conductors. Ground needs to return enough current to assure a breaker trip in the case of a bolted hot-ground fault. Dirt doesn't conduct well enough to do that.

This is important because some people don't like installing a main service as now you need a grounding system. Makes no difference, you need it anyway.

Plan A (and revised) is hopeless

There are too many reasons to even discuss. Some of them sneak up on you because it's unexplored territory. (a bit like aluminum wire seemed like a keen idea until...) For instance, what about overcurrent protection for the neutral wires? Not an issue for a single neutral, so nobody thinks about it. You'll just keep running into more and more. What if the neutral between main and sub breaks? Dissimilar wire lengths. Dissimilar wire gauges. Eddy current heating. On and on. Given McCarthy evaluation there is no reason to deeply analyze this hopeless dead-end.

Plan B: Do as every RV owner does, everywhere.

Get a cheater cable with 30TT socket to 5-15 or 5-20 plug, and and be careful with use of RV accessories not to overload the circuit. Since a cheater is a removable cord and not part of fixed wiring, it's outside the permitting/inspection process. It's not Code, but doesn't seem to cause much trouble. RVs have lots of other trouble with 22% of all RVers report being shocked, but this won't do that.

Since you mentioned this doesn't work out for you, I assume you really need the full 30A at 120V - 3600 watts or 3600 VA (3.6 KVA).

Now things get fun.

Plan C: Take the red pill: 3.6 KVA

We use any one cable to send 240V @ 15A (3600VA) from the house. At the RV site, we use a transformer to turn that into 120V @ 30A. (also 3600VA). The transformer absolutely needs several properties.

  • Outdoor rated
  • 240V primary (240x480 is a common type, that's fine).
  • 120V primary (120-240 is fine if it can handle 30A).
  • 3.6 KVA or larger (5 KVA is common).

These are commonly seen on eBay or Craigslist for $100. Of course a new one is likely to be quieter. Shop around, prices vary a lot.

Since the long haul is 15A, you can use any cable. I'd use the 12/3 simply because it goes to the right place. In that case, neutral would be unused.

Now we go down the rabbit hole. Read my answer here, particularly plans 3-4, about how using a transformer makes it a main service. That's why you need a local grounding system, so you can have the one neutral-ground bond that every main service must have.

The post-transformer side is freakishly simple. The transformer secondary goes directly to hot and neutral on the TT30. It doesn't matter which one. However it does matter that the "neutral" side is bonded to your locally derived ground, and the ground pin on the TT30. This is the one place on this side of the transformer where a neutral-ground bond occurs. As ThreePhaseEel mentions, this is allowed under 240.21(C)(1). All of the overcurrent protection is provided by the 15A breaker back at the house (ah, the long walk).

Plan C-2: A panel, and a little more load: 4.8 KVA

Same as plan C, but use a 12AWG circuit, and breaker it for 20A. Since this is 4800VA, you would definitely need a 5K VA transformer. The transformer secondary goes to a main panel as discussed, and neutral and ground are locally derived, and bonded in the panel (the one place). It has a 30A breaker going to the TT30. If the RV trips the breaker, it'd be more likely to trip here.

You could also put 1 or 2 more circuits in there to other things, but the combined load could trip the breaker back at the house.

Plan C/D: Get ready

Same as plan C-2, except you use a 10KVA transformer, which definitely has 240x480 on the primary and 120/240 wiring on the secondary. This will allow you to later bump to plan D with only one more transformer.

Plan D: Now you're playing with power

This plan delivers 9.6 KVA over 12/2 cable, allowing you to power a larger RV which takes 240VAC/30A (7200VA). It uses particularly hazardous voltage, so you'll want professional help for this.

In this plan, you use two 10 KVA transformers: The one I mentioned in plan C-D, and a similar transformer at the house.

The transformer at the house will be back-fed (which is fine). Jumper both transformer primaries for 480V and connect them back to back. The power will be transmitted over the long wires at 480V and half the amps. That means you can deliver 240V/40A (9.6 KVA) on 12AWG wire - or if you prefer, you could jumper the transformer to serve 120V/80A. This would need to go into a "main" panel again with locally derived ground and neutral.

This should scare you a little, but the 480V will only be present in 2 places, and only inside transformer enclosures.

Alternately, you could bump it to 12 KVA by using 600V transformers instead of 480V.

The absolute limit of insanity here is 21 KVA over the 12/3 cable, by using a 3-phase converter and a transformer that sends the power as 20A/600V 3-phase "delta". That's enough for 6 of your RVs.

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    @DavidDavido OK. Give me a day. The fact that you're willing to spend a little coin really helps. I promise I won't make you trench any cable. Practical questions: other than the RV, is there anything else the 12/3 needs to power? Bigger RVs can also use 240V/30A, do you foresee wanting that? (it's possible). – Harper Feb 20 '17 at 16:59
  • Very well said Mr Harper! I've never installed a transformer's, so the thought of using one had not occurred to me. It is a wonderful solution. I enjoyed the way you started your answer with a review of the ground rules particularly appreciate that you pointed out creating a new May you be blessed! – David Davido Feb 21 '17 at 14:56
  • BTW -- 5kVA transformers show up brand new for < $500 from several suppliers, so the transformer approach is definitely less costly than the alternatives for a full 3.6kVA at the RV site. – ThreePhaseEel Feb 22 '17 at 3:33
  • Also -- 450.14 says he'll need a way to lock the primary side breaker off (as it's the disconnecting means for the transformer, and not at the transformer)...and if he wanted to take another red pill and toss another transformer or two into the mix -- he could get the 12/2 into play by using it as its own transformer feeder run and using a secondary-side tie to bond the two secondaries together -- that gives the potential for 24kVA without having to trench in feeders... – ThreePhaseEel Feb 22 '17 at 3:44
  • One other thing -- I think I may have been wrong in my interpretation of 240.21(C)(1) -- I suspect jumperable transformers may not count after all, so this is definitely something for the OP to speak to the AHJ about. (Thankfully, inserting the panel is cheap and also provides a convenient place to make the N-G bond on the separately derived service.) – ThreePhaseEel Feb 22 '17 at 4:51
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CODE SAYS NO (to your proposal and its revised version)!

Your proposal is a blatant violation of NEC 310.10(H) (there are a couple of exceptions to this Code section, but they do not apply to any situation that this Stack will ever encounter):

(H) Conductors in Parallel.

(1) General. Aluminum, copper-clad aluminum, or copper conductors, for each phase, polarity, neutral, or grounded circuit shall be permitted to be connected in parallel (electrically joined at both ends) only in sizes 1/0 AWG and larger where installed in accordance with 310.10(H)(2) through (H)(6).

You can get what you need, but not by playing with parallel wires

It is possible, as Harper points out, to get lots of power down thin wires -- that's precisely why AC won the war of the currents over DC, even, as we can step AC up to high voltages to distribute it efficiently then step it back down to convenient voltages for the end user to plug into. The devices that do this are called transformers, and are common fixtures not only on power poles and in utility cabinets and vaults, but inside larger buildings such as factories and high-rises as well. They aren't terribly expensive either -- appropriately rated ones cost under $500 brand new, and much less used.

However, they come with several wiring caveats, not the least that you are invoking the separately derived service rules in the NEC which basically mean the transformer has created a new main service out of what is in effect a feeder. This means you need service equipment (at the very least, a N-G bond, if you don't have a full panel present for the secondary conductors that is) and a grounding electrode system. See Harper's answer for the gory details.

DANGER: HIGH VOLTAGE

However, as the voltage goes up, the risks moronic behavior incurs go up quite sharply, which makes trying to mess with things like transformers a fool's errand unless you scrupulously follow Code and safe working practices. And right now, based on your half-baked attempts to kludge things together, we don't have any confidence in that.

  • Also, your plan calls for repurposing a fault ground wire as a current-bearing neutral wire. This by itself is a violation of both the NEC and of common sense. – A. I. Breveleri Feb 18 '17 at 4:01
  • @A.I.Breveleri , Thanks. Is the concern about using a fault ground wire as a current-bearing neutral, that it is not a properly insulated conductor? The ground wire in UF cable is protected from moisture in same sheath as the other 3 wires of the 12/3 cable, but may not be as well insulated from current crossover. Theoretically, it would be possible to instead use the 2 properly insulated conductors (the black & the white) from the unused 14/2 UF cable combined in parallel to create a 30 amp neutral conductor and thereby create a circuit with the black and red wires of the 12/3 as the hot. – David Davido Feb 18 '17 at 14:49
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    Is this a "how many rules can I break" contest? There are several practical reasons not to separate conductors even separate from the rules on paralleling. But trying to teach electrical theory to someone whose motivation is to do something... ends up being a battle of rationalizations. – Harper Feb 18 '17 at 20:25

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