I am in the early stages of reading about converting 120V AC to 240V AC to power an electric dryer.

One way to create 240V AC is to "find" two out of phase 120V outlets.

My question is HOW does an electrician establish which outlets will have what phase relationship? Can I measure these phase relationships at the level of the circuit breaker box, rather than searching the house for pairs that create 0v versus 240v?

  • 2
    why do you need to convert 110 VAC to 220 VAC? ... the breaker panel should have the required voltage already ... it is scary that you are working with something that maybe you should not work with – jsotola Sep 6 '19 at 16:31
  • The way modern electrical panels are implemented, two adjacent breaker spaces will always be on opposite phases, and you can use a two-pole breaker to get 240V. However, be careful with "double-stuff" breakers, since they will be on the same phase. See Harper's answer here for more details + pictures: diy.stackexchange.com/questions/110151/… – Nate S. Sep 6 '19 at 16:45
  • 10
    Also, you should definitely not be "searching the house" for two outlets on separate 120V circuits to create 240V -- you need a proper dedicated 240V circuit on a double pole breaker to safely and effectively power your electric dryer. – Nate S. Sep 6 '19 at 16:48
  • 7
    @user391339 we are telling you to take any idea of tapping two outlets right off the table, because it's a dead-end investigation. I get you're new to this. We're not new to this. If there was any viable way to do this, half of us would own suicide cords. There's not :) And yes, that idea is attempted often enough that it has a name :) – Harper - Reinstate Monica Sep 6 '19 at 16:57
  • 1
    Does your apartment have the correct fittings (mostly, a vent path to the outside) to host a dryer to begin with? (Some apartments simply don't.) – ThreePhaseEel Sep 7 '19 at 0:45

How it's done by electricians is that they are familiar with the busing inside the service panel in question, and they know what they're punching down into. Also, anytime phase matters, they are using a multipole breaker (or at least handle-ties) which enforce phase choice for them, via keying.

Generally, there are only four different regimes: the standard alternating stabs (CH, BR, QO, HOM, Siemens), standard with space-straddling (GE and FPE), side-by-side busing (Pushmatic), or both-at-once busing (Zinsco). All the named panels except GE are obsolete.

Don't even think about it

What you're trying to do there is extremely bad at a variety of levels.

And it isn't going to work anyway, because the outlets you're trying to tap can't possibly deliver enough power. Not by a long stretch.

The best you can hope for is to rearrange the dryer's wiring so its heating unit runs on 120V instead of 240V (whilst the tumble motor and controls continue to get 120V). This will give 1/4 the heat, so drying will take much longer, but will draw within the delivery range of a 120V outlet.

You could also do something outlandish with AC-DC-AC double conversion using fully isolated DC supplies, but we're over the moon at this point, and it would be cheaper just to pay the landlord to fit a 240V circuit. (And no, you can't run the heating element on DC because the switchgear can't handle that because DC is a very nasty customer above about 40V.)

Buying an electric dryer was just a bad idea. Sell it on Craigslist and get a gas dryer and hook it up the normal way. If you weren't paying attention when you bought the dryer, welcome to the world of people who bought 3-phase machine tools without thinking that mattered.

Why suicide cords don't work

The first problem is one they share with any cord that has 2 male connections (like a generator cord built by someone who has apparently not heard of inlets): When one is plugged in, the other is lethal. Even if they are on opposite poles, you can't rely on unplugging the load first, because someone might knock out a cord by mistake.

Then you have the various sides of the circuit not running together in the same cable - making a big loop. (This blindsides DC electronics people everytime because it doesn't matter in DC). AC circuits throw considerable EMF. That's why we run conductors together, so the EMF cancels out. And this is a lot of current - a reed switch can operate on 10 ampere-turns, and you'll have 23 ampere-turns anywhere along the wire! Everything in the middle becomes the core of a transformer. This causes vibration, wire chafing and eddy current heating on anything nearby that's metallic. Like the receptacle itself! This vibration and heating takes energy, which reduces the effective capacity of the circuit.

Length is also an issue. Unequal lengths means phase and echo differences, that can themselves cause wire heating. It's complicated, but another energy loss.

There's just not nearly enough power on the circuits. A typical 120V outlet is good for 1800W intermittent, and your dryer is pulling 5600W or so continuous, which calls for a 125% derate, so 5600W becomes 7000W.

Even paralleling two 1800W circuits to make 240V, you're still only at 3600W - not even close.

I gather this isn't for a one-time use, so now you have the problem of routing all these cables. I gather the core issue is that a landlord won't give permission for a 240V connection, so I gather they won't give permission to fish or run Legrand Wiremold all over the walls, so no competent wiring method will be possible. So this becomes an ad-hoc connection with extension cords draping across doorways, which is a wretched fire hazard. This problem will also haunt the AC-DC-AC conversion method. The only way that can work is if a huge storage battery is involved, but 5600W for an hour is a whole lot of storage.

| improve this answer | |
  • "Sell it on Craigslist and get a gas dryer and hook it up the normal way." ...but obviously only if you have a gas line in your home and you're sure you know how to hook it up safely. Or, better yet, have a professional take care of the installation for you. A gas fire is just as bad as an electrical fire, if not worse — at least electrical fires rarely if ever explode… – Ilmari Karonen Sep 7 '19 at 15:39

A typical electric clothes dryer in the US runs on 120V/240V. 120V for controls, motor (could be 120V or 240V) & light, 240V for the heat. The key is the 240V for the heat - that is the most practical way to get enough heat with an electric dryer. In almost all cases, the incoming power will provide 120V/240V without much effort.

However, you need to have the proper equipment installed - a double-pole breaker (not two separate breakers, and most definitely not "use a couple of existing circuits") to provide power in a safe manner (double-pole means it will trip if there is an overload in either 1/2 of the circuit) and a proper 4-wire receptacle installed (there are older 3-wire receptacles but they are not for new installations, and any dryer built in the last 25 years (probably longer) can be easily switched from 3-wire to 4-wire connection). The wire between the breaker and the receptacle needs to be sized correctly as well - typical 10 AWG for a 30 Amp circuit.

Don't give up on the idea just yet. Post pictures or additional information about the breaker panel and it is likely (but not guaranteed) that there is a reasonable solution.

| improve this answer | |

You wrote in the comments that the unit was sold to you as being usable with 120V. That's not possible, and the person who sold it was being misleading if they told you such. There's just no way to get enough heat from 120V to dry a load of clothes with resistive heating. The current you'd need just doesn't admit appropriate residential wiring.

In more civilized countries you can get electric clothes dryers that operate with a compressor and refrigerant, like a dehumidifier, to warm the clothes with heat from the condenser and condense the moisture for removal on the evaporator (naming is opposite becaue there's a reciprocal relationship between phase transitions of the refrigerant and water/water vapor). But I've never seen such a thing in the US. Here, "energy efficient" just means the auto-sensing cycles stop drying your clothes when they're still 25% wet.

| improve this answer | |
  • 2
    Speaking from such civilized country: a typical heat pump dryer takes a 1000 W connection, so that is no problem at all for a single phase outlet. 3-phase outlets (3x 230V to neutral, 400V phase-to-phase) are only ever used for kitchen stoves, and for heavy equipment (welding, woodworking, EV charging). – Jeroen van Duyn Sep 7 '19 at 18:48
  • "There's just no way to get enough heat from 120V to dry a load of clothes with resistive heating." -- for Americans who use so hot driers it destroys their clothes but that's no problem for them because clothing is so very cheap there. A low heat drier which might take like 1.5-2 hours to dry a load wouldn't be a problem but that's simply not available in North America. – chx Sep 7 '19 at 20:03
  • @chx: Dryers here (US) take 1.5-2 hours to actually dry a medium to large load of clothes at medium heat. – R.. GitHub STOP HELPING ICE Sep 7 '19 at 21:29
  • I am in Canada and drying usually take 25 min... – chx Sep 7 '19 at 22:31
  • 1
    @chx Depends on ambient humidity, also, and there's no way you've got dry clothes after 25 minutes unless you're using the highest heat setting and have a light load. R. here is talking about medium heat - making the point that NA dryers are capable of longer, lower heat drying just the same as EU appliances. Also keep in mind that high humidity areas will extend drying times (without internal aircon). – J... Sep 8 '19 at 10:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.