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I know it's not NEC and far from the right thing to do. I need 30 amps in the garage.

I already have two unused 12/2 cables (w/ground) running to the garage. I was thinking to use one 12/2 cable, combine the black and white from one, putting it on one phase, and use another 12/2 wire, combine the black and white from that one, and put it on the other phase. The grounds of both would be tied to neutral.

In the garage, I'd make a breakout box and attach each 12/2 to a wire of a 6/3. Each 12/2 would be a hot, 120VAC of a leg of the 2 double circuit breakers.

I'd be pulling about 30 amps for an electric car charger.

Please give me all the reasons why NOT to do this, and that it's a bad idea.

  • 12/2 - Black & White - Phase 1, 120VAC > Black wire on 6/3
  • 12/2 - Black & White - Phase 2, 120VAC > Red wire on 6/3
  • Ground > White on 6/3
  • Grounds of each would be tied to neutral/ground.
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  • Is this an attached or a detached garage? Commented Jul 11, 2017 at 3:18
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    Redundant "me too" comments are normally discouraged but what you are proposing is so dangerous that I'm going to add my voice to the clamoring throng. Please please do not do this. Commented Jul 11, 2017 at 14:08
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    You know so little about this that you're adding extra wrong beyond the normal amount of wrong, by putting each pole on a separate cable, maximizing the current imbalance between cables. Those are called cables by the way. Commented Jul 11, 2017 at 15:00
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    Possible duplicate of How can I get 30 Amps @120V using several 12AWG and/or 14AWG cables?
    – isherwood
    Commented Jul 11, 2017 at 15:26
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    @ThreePhaseEel Was your comment to evaluate the risk of burning just the garage down (detached) vs. burning the whole house down (attached)? ;-) Commented Jul 11, 2017 at 20:03

7 Answers 7

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  1. With only a couple exceptions (of which yours is not one of them) the National Electrical Code does not allow the paralleling of conductors less than 1/0 AWG in size. The reasons for just about everything in the Code is safety.

  2. In your scenario you will have neutral current flowing on a bare equipment ground wire. Another code violation because it is very unsafe.

  3. Current flowing in one direction in a cable and not in the other direction can cause electromagnetic inductive heating if passing through ferrous metal components like cable clamps, locknuts, panel knockouts, etc. (The principle used by induction cooktops) This leads to melting insulation, FIRE 🔥 and everybody dies. Not a pretty scenario. Again this is prohibited by the code for the, hopefully, obvious reasons above.

So, short answer to your unsafe proposal is NO don't do it

Run a properly sized cable. It is not worth your life or your family's lives to save a few bucks.

Good luck!

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  • " In your scenario you will have neutral current flowing on a bare equipment ground wire. Another code violation because it is very unsafe. " It is being used for EV charging and 240 volt electric car chargers don't use the neutral conductor, at least not the ones from Tesla. I've seen adapters for plugging in a Tesla to charge and they have a plastic neutral peg for NEMA 14-50 outlets, that peg is not conducting anything. They also use NEMA 6-30 outlets for charging, no neutral conductor on that. Why use 14-50 plugs? My guess, few places stock 6-50 parts.
    – MacGuffin
    Commented Nov 30, 2021 at 7:27
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Tell the 'charger' to run at 20A

You do not need 30A of charging. You can git-r-dun on a 20A/240V circuit. Really.

Your "car charger" is actually an EVSE and not a charger. Here is how those really work.

The EVSE tells the car how much power it is allowed to draw. Simply tell the EVSE it's on a 20A breaker instead of a 30A breaker. Since EVSEs by law run at 80% of breaker, that will derate it to 16A (3840W) instead of 24A (5760W). If for some bizarre reason this EVSE does not allow you to configure that, get a better one. They all use the same connector (J1772).

"That was easy"

No?

You have several options. Paralleling is not one.

Also, I hope you're not married to that charger, because it sucks. It requires a neutral wire, and there's no earthly reason for an EV charger to need neutral. Get another one, or push back on the builder and ask "How do I wire this 240V-only?" They're crazy not to - all the market growth is in Europe, Australia, Asia or the middle east. Tell them you're taking the car to Europe.

While you're talking to them, ask them if they support 380V (middle-east 2-leg) or 480V (industrial) because that's a backup plan.

Also ask them if the charger can be programmed to only draw 20A, because that solves it right there.

Just run the darn cable

No electrician would install your ideas, so you're clearly doing this yourself, armed with "a little knowledge". And you're no pauper if you're getting an electric car. The proper 10/3 cable is just not that expensive when you DIY, so stop being cheap and go do it.

In fact, with modern pricing, you can get a 2-2-2-4 Aluminum feeder cable for the same price as 10/3, and that is legal for 90A. That means you are ready for "next-generation" multi-EV charging at 80A shared.

Now you're playing with power

Your power draw is 7200 VA (240V x 30A). Instead of carrying it that way, use two common 10KVA transformers to step it up to 480V x 15A for transmission. That's within the range of your 12/2 cable. If the charger can intake 480V directly, you'll only need one transformer. Ground remains ground. You will need a grounding rod out at the vehicle.

If the charger really needs neutral, then your second 10KVA transformer out at the charger can be jumpered for 240+120V output, giving neutral.

Now you're playing with funny fractions of power

If the transformer can take a Euro voltage of 360-417V, you do something vaguely similar by using transformers in a buck-boost fashion to step up a 240V/20A to higher voltage (still at 20A). The direct supply needs to be breakered at 20A, and since the connection is series, they will suffice to protect the wires. The boost transformer(s) will need their own appropriate breakers.


Everything from here will probably be nixed by your electrical inspector, and/or your insurance company will decline to pay for the fire, leaving your wages garnished for a long time (can't escape an illegal act with bankruptcy).

Oh, you're going to do it anyway?

That's foolish, but at least you can shave some excessive and gratuitous foolishness by complying with the rules for paralleling (only violating the rule that says paralleling can only be done on #0 and larger cables, and with equipment approved for paralleling). First, you spread conductors, not gang them: each cable gets one of each. Phase L1 goes on black of both cables. Phase L2 goes on re-marked white of both cables. This balances currents in each cable, and reduces vibration and eddy current heating (which will be serious at this amperage). Grounds are grounds. Neutral would not be used, which is why to push back on the charger manufacturer and tell them you want a no-neutral charger.

Further, every conductor must be fused individually including neutral, no matter how hard that is (except ground). Your breakers provide hot protection, if you use neutrals you can kit-box something to protect them, or you can add a small three-phase subpanel and use one phase for neutral. Remember, this is still super stupid.

Intentionally overload one 12-2 cable

This is also bad, but not as bad as paralleling, but there may be a way to "make it legal". If you look in the code book, 12 AWG wire is legal to 30 amps if you can run it at 90 degrees C, but this is forbidden in 240.4(D). So we can make this legal-safe except for the 240.4 violation. The first impediment is terminations (breakers and plugs). You could solve at least that problem by pigtailing with 10AWG wire, and use a 90C rated splice to transition to 10 AWG pigtails.

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You do not do this. You already state that you know it is not the right thing to do so STOP.

The worst part of your crazy plan is trying to wire up without a safety ground being routed to the destination from the power panel.

Secondly the current carrying conductors of any circuit are supposed to be run in the same cable or conduit so that there is cancellation of the fields around the current sourcing wires and the return neutral wires.

You really need to run new suitably sized wiring to the new car charging receptacle. For a 30A service at 240V you are looking at adding a proper two pole 30A breaker into the panel with locked handles. Then from there running #10-3/wGND to the 30A rated receptacle. How the wire is run will depend upon if you are all within one building structure or running between structures. Your location may also determine if conduit must be used in the garage if the wires run in or on walls in an exposed manner.

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While basic electric theory says that two parallel resistances have half the resistance of either, there are other factors in play. These other factors make the short answer, no, don't do this.

For starters, common home wire (like Romex) isn't built to exacting-enough specs. Assuming you had exactly the same wire length (not more than a quarter inch difference or so) the figures that are used for things like NEC are averages. There may be hot spots. Depending on how they ended up, the excess resistance could cause heating to the order of the square of the single-conductor DC result. In short, this could burn though a wire and cause an arc very quickly. The longer the run, the worse it gets.

Second, standard home wiring doesn't carry DC. The phase and loading calculations are very complicated. They're already done for you by some smart folks, and they're incorporated into the NEC, with margin for error. For instance, a loop or even a bend could cause inductance between your conductors, which, once again, could cause heating, melted insulation and an arc. The longer the run, the worse it gets.

There are devices that gang power sources, watching for differences and actively balancing draw. They're common in solar power installations. It's possible to have two separate circuits that run into such a thing, each with the requisite 20A breaker; this could give you close to 40A (minus system losses).

While you may install whatever you like in your own home, you probably have home insurance. They'll likely never come to inspect but if there's a problem, they will flag anything not to NEC (and UL for active equipment) and they may not pay a claim. It's probably in your insurance contract somewhere if you read the whole thing.

Grounding neutral is a bad idea. Everything in a garage must be grounded and on a GFCI outlet (or breaker). New installations must also have arc fault circuitry. Using 12/2 to a garage is a very poor idea. If you must run a separate ground, there are ways to do it with a local ground rod. Ultimately, you'll probably end up spending more to have this work correctly.

Designing on the cheap is a good idea but this is not a place where you can cut corners. Your best bet is to run a separate fat wire to a 50A subpanel that's close to the source. Then you can run shorter appropriate wiring to a dedicated 30A outlet.

For what it's worth, if your device draws 30A, it doesn't have a regular plug. At least, it shouldn't. That should be your sign that you can't just run a normal circuit with a normal outlet.

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NEC Article 310.10 (H)(1) forbids any conductors run in parallel smaller than a 1/0.

-2

You can combine 12/2 but run exact lengths parallel and solder the two black together, the two white together and the two ground together. That is the only way you can do this. The math says you will then have a 9 gauge wire. It will fail inspection but it will be safe.

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    No, it wont be safe, if one cable fails for instance, or develops a break, increasing its resistance. Commented Jan 23 at 11:07
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    It will fail inspection because it is inherently unsafe. (Known) unsafe things are not allowed by code.
    – FreeMan
    Commented Jan 23 at 13:58
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Go for it. I've been wiring everything from houses to schools and hospitals for over 10 years. I've seen 12/2 eomex carry 50 amps on a temp pole at a school before. It was hot to the touch, but it wS working, and remained working throughout the job. Don't listen to these naysayers, they haven't done enough experimenting with electricity, they just know what they've been TOLD is safe.

It'll work. Probably forever. Nuff said

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    How many people died in building fires as a result of your work? Commented Nov 30, 2021 at 7:10
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    Not a pro here, but I'd say there's a difference between cable strung in open air and a cable buried in the wall. The former will get cooling from the air around it while the latter won't.
    – DoxyLover
    Commented Nov 30, 2021 at 7:33
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    While this might "work", it is a code violation and any damage caused by such wiring wouldn't be covered by insurance if the company were to find out. It would probably also cause an electrician to lose his license if he were found doing this, and would probably expose one to civil or criminal lawsuits for damages. All-in-all, it's REALLY bad advice.
    – FreeMan
    Commented Nov 30, 2021 at 12:24

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