220 circuit

Do both legs have to be switched in order to break a 220 circuit. I was wondering because I have an electronic themostat that is wired for 220 but only has one switched 110 leg.


FWIW, NEC code probably requires that all ungrounded* conductors on a circuit be ganged together to trip the breaker together at the panel and to open both legs at a disconnect, if one exists. That said, I seem to remember my brother, a mechanical (air conditioning) contractor, saying that an AC unit may require a separate 110V circuit, along with the 220V circuit.

Strange, I thought most thermostats run off 24V, fed by the air conditioner.

Be warned: this is a caffeine-free post. And, as always, check voltages before electrocuting yourself. :eek:

*The neutral is a grounded conductor; grounds are considered grounding conductors.

I’m not real sure what you’re asking. Usually your condensing coil or heat pump are 220. Within that curcuit, there should be a 110 transformer. Off of that there’s either 3,5, or 7 wires and that serve a 24 volt thermostat. That’s my very basic understanding of how it works based on my personal experience as a homeowner.

Oops! Sorry cornflakes, I missed your post. I’m slow today.

Depends how the system’s set up. Some hot water based heating systems have the circulator connected directly through the wall thermostat, although most systems I’ve encountered have a relay somewhere in the circuit (certainly makes wiring a lot simpler).

The electronic thermostat that I mentioned is for an egg hatcher. It can be wired for 110 or 220. The thermostat is supposed to power a 220 strip heater and a 220 immersion heater for a humidity pan. I wired the thermostat according to the wiring schematic (I think) and it seems as if there is only one switched leg at 110 volts. The hatcher is supposed to run at 100 ° F, but its more like an autoclave, unfortunately. I’d appreciate any input you may send my direction.

The OP is asking a different question. The unit is getting 220V. The SWITCH [thermostat, on/off, etc] only breaks one 110V leg, always leaving the other 110V leg hot at the unit. The OP wants to know if that is OK.

In general, whenever you switch 220 VAC, it should be done to both legs. This is for safety; on a 220 VAC system, both legs are hot with respect to earth ground. If only one leg is switched, and you turn the unit “off,” one of the legs will remain hot.

Now having said that, I have seen many instances of the following:

A control device plugs into (or is hardwired into) 220 VAC. The main power switch switches both legs, which is good. It is internally fused on both legs, which is good. A 220 VAC heater (or some kind of load) plugs into the device. The heater is controlled, via a relay, on one leg only.

So is this kosher? Are you allowed to put a control switch or relay on only one leg of a 220 VAC load? I don’t know. But I know that, even if it is legal, I don’t like it.

Has anyone been hit with 220? Let me tell you, you wont need coffee for a week. I was wiring up a dryer for a customer, she thought it wasnt working yet because I had left the cord I was hooking up un-plugged. I just about threw the whole dryer at her.

So moral of the story: if you are working with electrical devices, keep the power cord out of the reach of the mental incompatents.

Chances are you weren’t jolted with 220 VAC.
Consider the following:

  1. When someone is electrocuted, 99% of the time it is between “hot” and earth ground.
  2. On a 220 VAC circuit, both legs are hot with respect to ground. The potential between either leg and earth ground is 110 VAC.

So even though you were working on a 220 VAC circuit, you were probably hit with 110 VAC (which can still pack a wallop).

FYI, here’s what you would have to do to receive a real 220 VAC jolt:

  1. Have unobstructed access to both legs of a live 220 VAC circuit.
  2. Ensure you are very isolated from earth ground.
  3. Touch one of the legs. You will probably not feel a shock because you’d be completely insulated from earth ground (see #2). I say “probably” because you might feel a little bit of tingle due to your body’s capacitance.
  4. Touch the other leg. Bzzzzzzt.

Oh it was 220, I was threading on the nuts for the pig tail, I do the ground first, then the other two at the same time, it went through both arms.


Have you ever heard of the OSHA lock out tag out system. I suggest you get one of the plug lock outs, and use it. It keeps the untrained people from electricuting you. You’re the only person that can enable the energy source.

A search under (OSHA lock out tag out) will get a large number of site hits.

If you’re certain you touched both legs at the same time then yep, you were jolted with 220 VAC. Ouch. The point of my previous post was that most people don’t get zonked that way when working on a 220 VAC circuit; they’re grounded, they touch one leg, and they get zapped with 110 VAC. (But they walk away thinking they were hit with 220 VAC.)

Also consider that you were subjected to “repeated hits” of an instantaneous voltage of around 330 V. Youch.