2

I've got a situation where I have a 240V 30A dryer line, and I want to power something else from the same circuit, but NEVER at the same time as the dryer. I'm looking for a switch to use, like a DPDT ON-ON break-before-make switch. There are tons of DPDT switches I've found, and most seem to be ON-OFF. Hardly any show their wiring diagrams, so if they don't give more details, I don't know if they'll work. Note: the second circuit will not use more than 15A, maybe just 12A.

I'm hoping someone can recommend specific switches that will do what I want. Manufacturer and exact part# is what I'm looking for.

Personally, I don't care if it's a knife switch, something that looks like a standard light switch, two buttons, has relays, I don't care. The breaker box is full, it's a long and difficult wire run, and I do not need a separate circuit because I'll NEVER need to run both devices at once.

I could resort to just using one plug and unplugging one then plugging the other one in, but I don't see the point of constantly exposing live wires (the plugs) where something could inadvertently fall on them or water could be sprayed, instead of keeping things all sealed in the wall with a safe, simple switch between two plugs.

  • 1
    of course this is just a power switch : amazon.com/s/… Your best bet will probably be a contactor , operated by LV on the control side via a light switch and have the 220 V pass though the different contacts depending on if the contactor is active or not. This would need to be enclosed in a case - probably not acceptable as a permanent install - but perhaps in a cheater. – Ken Mar 28 '18 at 9:25
  • 1
    Here is what you need for a Dbl Pole Dbl Throw Switch amazon.com/Leviton-1288-30-Amp-120-Volt-Double-Pole/dp/… – Ken Mar 28 '18 at 9:28
  • I don't see the difference between the 3032-2x, the 1288, 1286, and the MS302-DS or MS303-DS. This is what's so infuriating about this search ... they all seem to be the pretty much same based on the specs given, but a with a huge price difference. – DPS Mar 28 '18 at 9:59
  • 2
    Is there a reason (code or otherwise) why you just can't put two outlets on the same circuit, and trust the circuit breaker to do its job? We do this all the time with 120V/15A circuits; it's trivial to plug more than one hairdryer into the same circuit. – Daniel Griscom Mar 28 '18 at 10:57
  • 1
    The breaker protects the wire, not the load. Do you think it would be a violation to put a 1 amp light bulb on a circuit protected by a 15 amp breaker? – Philip Ngai Mar 28 '18 at 16:34
2

From the 2017 National Electrical Code:

210.19(A)(2) Branch Circuits with More than One Receptacle. Conductors of branch circuits supplying more than one receptacle for cord-and-plug-connected portable loads shall have an ampacity of not less than the rating of the branch circuit.

210.21(B)(3) Receptacle Ratings. Where connected to a branch circuit supplying two or more receptacles or outlets, receptacle ratings shall conform to the values listed in Table 210.21(B)(3), or, where rated higher than 50 amperes, the receptacle rating shall not be less than the branch-circuit rating.

The table referenced shows 30 amp circuits require 30 amp receptacles.

No switch is required, you just need to make sure all the conductors and receptacles are rated for 30 amps.

Good luck!

  • I appreciate all of the comments and terrific information. After looking at the various costs and benefits, I've decided this is the approach I'm going to take. The switches I'd use are just too expensive, and I'm never going to be running both (electrical) loads at once anyway, so the switch is not really providing any benefit in this case. However, if circumstances were a bit different, then it would probably be more useful. So while this is my preferred answer in this case, I wish I could select the answer above as well, mainly for its thorough and detailed explanation. – DPS Mar 31 '18 at 5:08
3

You need the right switch.

Poles are the number of independent channels the switch has - single-pole, double-pole, triple-pole, or n-pole. You need double-pole but triple would be nice as I consider it elegant to also switch neutral, it being a conductor.

Throw is the number of output options. "Single-throw" means it can only connect Common to output 1, or not. "Double" means it connects Common to outputs 1 or 2. You need double-throw.

  • The 3032-2x and MS302-DS are double-pole single-throw (DPST) switches.
  • The 1286 is SPDT, i.e. a common 3-way switch.
  • The MS303 is 3PST since it's for 3-phase power.
  • The 1288 is DPDT. Bingo!

The different prices reflect different ratings (MS=Motor Start, tungesten, ballast etc.) or the varying difficulty of cramming all that into the small 1-gang form-factor. The 1288 works pretty hard, hence the price.

If you don't like paying $60 for a switch that crams this functionality into a 1-gang yoke, then visit your local electrical supply house and ask them for other options which may trade form-factor for price. A big-box store is not an electrical supply house.

Better, a subpanel with a "generator interlock"

If you spend time around here, you'll hear us preach a lot about getting panels with lots of extra spaces. That's because a full panel is intolerable. It cripples you and forces you into bad or expensive compromises. So with an eye toward that and future expansion...

... fit a subpanel somewhere appropriate. Feed it normally, from the current 30A dryer circuit. Then "mis"use a generator interlock kit (such as this sensibly priced Siemens one for Siemens panels) to interlock your dryer's and other load's breaker so they can't be on at once. Voila. (Normally with these interlocks, you backfeed the panel with one of two power sources. In this case you're normal-feeding two loads, and interlocking the loads.)

At some later happy day, you retrofit larger cable and turn this panel into a more useful subpanel.

Automatic knock-off

Ironically this may break out as the cheapest option. The idea is to power both receptacles all the time unless the dryer is drawing power, then shut off the other one with a normally-closed relay. You'll need a large steel box to put this in (a 5" square deep box is not excessive), and then a listed relay+transformer combo device, in which a transformer makes just enough 24V to operate the integral relay. That way you will be using low voltage with oddball parts, and Code is greatly relaxed.

The relay needs to be DPST-NC (normally closed) (meaning 2-pole, normally on). Typically these mount in a 1/2" knockout hole. It will provide 2 low-voltage wires, and when they are shunted (shorted), the relay operates. This means it'll have at least 8 wires: 4 for the DPST contacts, 2 to supply lineside power to the transformer, and 2 for the aforementioned 24V wires.

The two 24V wires go to both sides of a magnetic reed switch. These typically throw at 10 ampere-turns. Since a dryer draws 23 amperes on full, that's plenty. It's simply a matter of finding the right geometry that the reed switch likes. Generally, in wiring, it is mandatory to lay all hots and neutrals tightly parallel to each other so their EMF fields cancel each other out. In this case we do the exact opposite on purpose. Experiment with laying the reed switch crossing at 90 degrees, or parallel to the wire. All of this must happen inside the steel box.

enter image description here

On a dryer, 80% of current flow will be on the two "hots". The neutral can be kept out of it. Also feel free to calibrate it so the dryer doesn't trip on "fluff/no-heat".

Retrofit grounds

The only thing that scares me about this advice is a lot of dryer circuits have 3-wire (ungrounded) service. This is a Very Bad Idea, because the switching/interlocking scheme creates so many new and unimagined ways for "the NEMA 10 problem" to emerge -- in which all grounded surfaces of your appliances are energized at 120V.

The safe answer here is to retrofit a ground. I know, I know, long and difficult wire run -- fortunately the ground can be retrofitted as a separate #10 wire, and it can follow a separate route, and it only needs to reach any grounded location that has #10 wire back to the same panel, or anywhere on the grounding electrode system.

  • I like the first part of your answer best. The other parts seem to be what the op is trying to avoid the large expense of changing the panel. While the auto-disable function is interesting; the reed switch - I really hate them, they can be finicky or get 'sticky' , do they work yes, a hall effect I like better - because it does not get 'sticky'. There are voltage / current sensing relays but probably too expensive and again more complicated than the DPDT Switch. Otherwise I like that you gave the op all those options and info! – Ken Mar 29 '18 at 8:44
-1

I don't think you need this kind of switch: main breaker is there to allow only the power flow the wiring can safely hold, so just daisy chain the new 220V outlet. If (by mistake) you'll leave both appliances on, the breaker will do it's job.

  • 1
    With the breaker being well over 200% of the load this would be a code violation in most cases. – Ed Beal Mar 28 '18 at 14:06
  • 1
    Breaker should be rated as big as CABLE can HOLD, the load connected is non-influent. You can (legally) wire a circuit with 6mm^2 cable, 30A breaker and connect only a single 3W led bulb to it and all is safe. Breaker should rated according to the wiring, not to the loads connected. So connecting the same bulb to a 1mm^2 5A circuit is still totally safe. The same is wiring a 3kW oven to a 5A circuit, in this case the worst case is the breaker will trip because it can't witstand the load. And it's how it's supposed to work. Breaker is there to protect the cables not what's connected to those. – DDS Mar 28 '18 at 14:54
  • 1
    From your answer you don't use the NEC the maximum ocpd for the NEC other than motor loads is 150% since the OP is in the U.S. this is a wrong answer. – Ed Beal Mar 28 '18 at 23:38
  • Can you post theexactly norm of the code. I wire some industrial appliance sent to US and for 'general' usage we must rate the protection just as what cables can safely bring (ignoring the load). NEC states for outlets 'breaker should be AT LEAST as capable as the outlet. AT LEAST, not AT MOST – DDS Mar 29 '18 at 8:42
  • I am currently in the hospital and don't have my code books. There is a big difference between residential and commercial, industrial controlls in the NEC. For example within a control enclosure # 16 wire is allowed to handle 40 amps but residential I think it is 7 amps ,the NEC limits over current protective devices to #14 wire to 15 amps, #12 to 20 and #10 to 30 a residential, things like motor loads that have a high in rush current are allowed to have larger ocpd and there are also tables for these but a listed manufacture that has UL approval can use higher values. I hope this is helpful. – Ed Beal Mar 29 '18 at 9:49

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.