Can I combine two 12-2 cables to serve a NEMA 10-30 socket for dryer?

Question

By remodeling my bedroom, I discovered my current tub is using 240v 20A, which are powered by two 12-2 cable. As an update, I don't want a this fancy tub, and also want to add a dryer in my bedroom. However, considering that my bedroom is on second floor and the panel is in the basement, it needs to cut so many thing to run a regular 10-3 wire upstairs. I though of a short cut to use those two 12-2 wire to create a 240V 30A socket which doesn't need a neutral.

So please correct me if I'm wrong. For a dryer which doesn't need a neutral wire in most case (NEMA 10-30 doesn't have neutral), and by the code to run 30A need 10gauge wire. (However most dryer only need 22.5A, as 5400w), I can combine two 12 gauge wire equivalent to 9 gauge in size. I can use each 12-2 cable as one 10 gauge wire to each pole of the NEMA10-30 socket to form a 240V.

Please tell me any danger might happen, or why not to do thing such like this.

Otherwise I can only get a NEMA 6-15 socket which only supports 240V 20A to some smaller ventless dryer which I don't want.

Also, I know code won't allow something like this. But to run a new wire would cost $5000+, which I just want to know are there any risk other than code.

Answer 1

Are you trying to break a record for "most code violations"? Code exist for a reason, the reason is safety. Tech is supposed to serve us, not kill us.

Your entire logic is that "Code is a bunch of nonsense legalese" simply because you don't understand it. No, every rule has a safety reason.

Further, you have a bunch of beliefs that are wrong. And all your beliefs happen to support your plan. Coincidence? Not. You are "making these facts up" to justify what you want you do. That's the road to perdition.

NEMA 10-30 receptacles haven't been legal for 30 years.

Old ones that were installed properly prior to 1990 are grandfathered (you don't need to rip them out) but you're certainly not allowed to install any new ones. All dryers which might plug into a NEMA 10-30 are required to use a 14-30 today, and all are able to do that.

NEMA 10-30 has a neutral. Not a ground.

Neutral is not ground.

The whole problem with NEMA 10-30 is it doesn't have a ground. That pin you think is ground is actually neutral.

All the dryers you think don't need neutral, actually do need neutral. They are using an archaic exception in Code that allows plugging them into legacy NEMA 10-30 sockets if the socket was installed over 30 years ago.

The reason usual/cheap dryers need neutral is they have 120V tumble motor and controls. I don't know why they don't just make those 240V, SMH, but that would cost $10 more, and we can't have that!! They also use electric resistive heat strips to dry the clothes, which is the most expensive and inefficient way to do that thing. Cheap to buy, expensive to run.

If you want hot-hot-ground, use NEMA 6. But most dryers can't use that.

For instance your 12/2 cable could support a NEMA 6-20 recep, which indeed may be able to feed certain high-tech dryers such as the condensing types you don't like for some reason. (perhaps you do not pay the electric bill).

It doesn't cost $5000 to run a branch circuit. 5000 pesos, maybe.

Paralleling is right out.

It requires special equipment on the supply side that is specifically UL-listed for paralleling. That means it needs to meet the safety regs in the UL White Book also. Also paralleling is not allowed below #1/0 wire size (125A), and realistically nobody does it below 4/0 (200A). So you simply won't find appropriate equipment, nor would you want to pay for it if you could.

Answer 2

I'd like to expand on longneck's (correct) answer a bit and explain why code forbids this:

There's a saying, code is written in blood and ash. What that means is that in developing the code, experts looked at incident reports from thousands of fires, electrocutions, and other deaths, and figured out things that would have prevented those deaths. And of course, they balanced this risk assessment with what's practical and affordable to most people, because there's no point in writing regulations most people won't follow.

As a result, there's some things in the electrical code that look silly in isolation, but are very important when you consider that things could go wrong. Parallel conductors is a great example of that: as you note, two #12 wires would be capable of supporting the current your dryer requires even better than the #10 wire code requires.

But consider what happens when a wire breaks, or comes out of its screw or wire nut (which can be fairly common). If you're using a single #10 wire, then the circuit is broken, your dryer doesn't work, and you notice that and get it fixed. If you're using two #12 wires and one of them breaks, your dryer will still work, but it'll overheat it's one remaining #12 wire, causing a fire within the walls of your house. Paralleling #12 wire here creates a situation where a single fault could kill you before you even notice, and that's why it's not allowed.

Additionally, I think there's a bit of confusion in your question, as most dryers in North America do indeed use neutral -- NEMA 10 actually lacks a ground, not a neutral, and there's a special (and bad; it's killed people) exception in the code that allows dryers and ranges on old NEMA 10 circuits to "bootleg" ground by connecting it to neutral. Often, 240V is used for the heating elements, and 120V is used for the motor that spins the clothes. Also note that installing a new NEMA 10 today is not allowed; you must instead use NEMA 14 which supplies both ground and neutral.

Answer 3

What you are asking about is called "paralleling". It is not allowed on circuits of this size.

Answer 4

As already answered, code prohibits paralleling at this level and similar (15A, 20A, 30A, 40A, 50A - not sure but I think it only becomes an option somewhere > 100A). But the question you may be wondering is why?

A couple of reasons. One of them is uneven wire use and, by extension, heating. In a perfect split circuit of 30A on 2 12 AWG wires, each wire would get 1/2 the current = 15A on 12 AWG, which is totally safe. However, if there is a difference in resistance between the wires, which can happen for a bunch of reasons, including poor quality connections, then one wire will get more and the other wire will get less. Up to 20A + 10A and you're still OK. Once it gets past that, one of the wires is carrying more than it is rated to handle and no breaker will trip because the total current is only 30A.

Another reason is a broken connection. You connect all 4 wires at both ends (2 for one hot, 2 for the other hot). One wire comes loose. Now you have all 30A on one wire, with no breaker trip.

The bottom line is that there are enough "gotchas" that code prohibits doing this out of an abundance of caution. In this particular case though, if you violated code and everything worked you would not know something had gone wrong (e.g., a wire came loose after a few years) until a fire started.

Answer 5

Can I combine two 12-2 cables to serve a NEMA 10-30 socket for dryer?

TL;DR: Never.

Other answers cover the reasons pretty well.

I fail to understand how running a second cable in parallel should be any more expensive than just running a properly sized 10/3 cable with L1, L2, N and PE conductors, to accommodate any dryer on the market, and disconnecting the original #12 circuit. The only price difference would be the cost of the cable, and that's small compared to the labor involved anyway.

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The discussion below is from an abstract electrical engineering angle and completely ignores codes and similar requirements. Let's just think of what it would take to parallel the wires safely, in general, assuming a typical US residential panel as the source of power.

You'd need to protect each wire individually from overcurrents. For each paralleled wire, you'd need a circuit breaker rated for the ampacity of the wire (or less), to protect that wire from overheating.

Let's say we want to parallel #12 wires to use in a 30A circuit. You have 2 L1 and 2 L2 #12 wires (i.e. both wires live). In the electrical panel, you'd need a 20A (or less) circuit breaker for each wire - four circuit breakers total.

But I'd say that this is cutting things too close - by the time one of those 20A breakers would trip, there'd be a fairly large current imbalance. You'd want smaller breakers so that they'd trip when the imbalance is developing but far from overheating the wires. Thus a rating a bit above 15A, but less then 20A. But no such breakers are made for branch circuits in the form that would fit into typical proprietary (e.g. GE, Leviton, etc) panels.

So, we'd have to use DIN-rail mounted breakers, UL-listed for branch circuit use, and select e.g. 16A breakers. So this becomes fairly complicated now. Here's what it'd take:

1. 2 x 30A branch circuit breakers on the L1/L2 feeds in the main panel, going over #10 or #8 wire to a subpanel (I'd prefer #8 since it's more robust when feeding common DIN parallel feed terminals).

2. 4 x 16A branch circuit breakers on a DIN rail in a DIN-rail subpanel. The circuit breakers feeding each pair of wires would need to have a common trip mechanism, so that if one of the wires in the pair trips the breaker, the other wire would be automatically disconnected as well, since there's no way it could do its job safely then.

3. In the receptacle box fed from these paralleled #12 wires, each pair of wires would need to be connected to a #10 pigtail, and only that pigtail would connect to the receptacle. 30A receptacles only allow connecting a single feed wire, so the pigtail is necessary.

And when putting this into service, you'd have to use additional wire lengths somewhere to do equalization loops so that the "cold" current flow in both paralleled circuits would be equal. Obviously the wire lengths would have to be quite similar, and ideally both cables would run in the same thermal environment, e.g. next to each other (at which point you can just put a proper cable there!).

The above is not legal in US jurisdictions, but from an engineering standpoint it is a reasonable approach towards a solution assuming that nobody has an urge to tweak it later. And this assumption is laughably wrong. So, it's the sort of a "reasonable" solution that is only reasonable in a lab setting, and thoroughly impractical anywhere else, because humans are not perfect.

Let's recap: instead of just running the properly sized cable, we'd have to install a whole new subpanel with its own breakers, and then ideally run another cable right next to the original one. That doesn't make any sense anymore.

Now one could argue loudly that it's "unfair" that the NEC doesn't allow such kludges: there's a good reason for that. A kludge of that sort would have a very low probability of being installed right, and also would likely be installed in circumstances where the original cable run may not be up to par (e.g. there may be hidden splices or who knows what in the walls). And there would be DIY types who would be installing such kludges left right and center. And then whenever the 16A breakers would trip due to the inevitable imbalances as the old connections oxidize and so on, there'd be a tendency to use larger breakers instead or to bypass them altogether since they'd be "a nuisance".

Sometimes, the codes prevent theoretically reasonable engineering measures because there's no way to trust the humans executing the kludge to get it right, and then the humans using the kludge not to make things unsafe. This is also borne out of death and ashes. The codes have it totally right.

This sort of a kludge could probably be put in place on, say, the International Space Station, since there'd be enough oversight and understanding to mitigate any developing problem. But that's entirely hypothetical anyway, since none of the usual parts used terrestrially have any place on ISS due to offgassing, toxic fumes when overheated or when on fire, inability to operate in vacuum (suppose there's a decompression - last thing you want is for a power distribution unit to catch on fire because there isn't enough convection cooling), and so on. I'm just talking of the "class" of kludges, not about literally doing it using the same parts or even when solving the same problem.