Can we use each hot wire from 220v line separately

ramremNovember 17, 2010

We live in California. Years ago, we replaced an electric cook top with a gas cook top. The installer used one of the hot wires from the cook top 220 line to put in a 110v receptacle box in our lower cabinet, and we plug the gas cook top into one of the two outlets. The other hot wire from the 220v line is capped inside the receptacle box. We are now replacing our old and anemic vent hood with a more powerful hood that is rated at 10 amps. We decided to also replace the very old hood wire and discovered that it is tied into the circuit that includes several kitchen plugs. I read that the hood should be on a dedicated line, or, if need be, on the kitchen overhead light circuit. To use either of those approaches would require a great deal of extra work and cost. The hood wire is only about 5 feet above the 110v receptacle that we use for the gas cook top. We can easily get the new hood wire down to the receptacle. Can we safely connect the new vent hood hot wire to the now capped hot wire in the 110v receptacle, and then connect the neutral wire from the hood to the 220v line's neutral wire that is in the receptacle and that is currently connected to the receptacle? If not, can we bring the hood wire down to the receptacle area, have it exit the wall, attach a plug and then stick the plug into the unoccupied outlet? If not, are there any other simple solutions. Thanks.

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fa_f3_20

What you want to do is called a multiwire branch circuit. It's permitted if you do two things: (1) you must breaker the two hots using a double-pole breaker, and (2) the two hots must be on opposite phases of the service (which, with modern breaker boxes, will happen automatically when you use a double-pole breaker).

    Bookmark   November 17, 2010 at 11:10PM
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DavidR

One thing could throw a wrench into your plan - if the range circuit has only 3 wires (2 hots and a neutral), with no separate ground. It was once permitted to wire ranges that way, but no more.

If you don't have a proper, separate ground in the box, you'll have to pull a new homerun back to the panel. This can be a multiwire circuit as Fa describes above, and that way the old gas range receptacle will also be properly grounded (if it wasn't before).

If the old box IS properly grounded with a 3-wires-plus-ground feeder, then hurrah! But don't forget that you'll still need to install a 20 amp double pole breaker in the panel to replace the old 40 or 50 amp range breaker, if that hasn't already been done.

When you do this, you may find that the large wire to the range won't fit into the lugs on the 20a breaker. In that case, you'll have to use smaller pigtails from the breaker, and splice them to the range circuit wires.

    Bookmark   November 18, 2010 at 2:44AM
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brickeyee

"the two hots must be on opposite phases of the service"

How do you get another phase from a one phase service?

    Bookmark   November 18, 2010 at 8:25AM
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kurto

Brickeyee,

Even though the phase nomenclature is incorrect, and typical US residential service is referred to as "single phase", the advice was sound. From an EE perspective, the two 120V legs (RMS measured with respect to ground) are 180 degrees out of phase with each other. (But you already knew that, probably better than anyone else on this board).

Kurt

    Bookmark   November 18, 2010 at 2:08PM
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ramrem

Thanks for helping us out. Our line does have the ground, and I am looking for a double pole breaker.

    Bookmark   November 19, 2010 at 11:52AM
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brickeyee

"Even though the phase nomenclature is incorrect, and typical US residential service is referred to as "single phase", the advice was sound. From an EE perspective, the two 120V legs (RMS measured with respect to ground) are 180 degrees out of phase with each other. (But you already knew that, probably better than anyone else on this board). "

The 120/240 V system used for residential power in the US is an Edison circuit, just like a multi-wire branch circuit.

It is NOT 2-phase.

2-phase was experimented with long ago to allow for starting torque in induction motors (wit single phase power an induction motor has zero starting torque).

2-phase was discarded when 3-phase power proved easier to distribute.
2-phase uses a 90 degree phase shift and minimum of four conductors.
3-phase power requires only three conductors for a delta configuration.

The more common (and far more correct) description of a 120/240 V service is as legs, to make sure they are clearly differentiated from a 3-phase system that uses 120 degrees of shift.

    Bookmark   November 19, 2010 at 12:17PM
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DavidR

The 240/120v home mains is nothing more than a center tapped transformer. The tap is the neutral.

Those of us who spent our early years keeping company with tube amplifiers might be more accustomed to the idea of center-tapped transformers, as used in push-pull output circuits and full-wave 2-rectifier power supplies. (Old timers: remember 5Y3s and 5U4s? Selenium rectifiers were supposed to be an improvement! :)

Another model could be made with flashlight cells. Take your basic 2-cell battery holder and stick a paperclip or something between the cells. Now you have a center tap, and can derive two 1.5v circuits or one 3v circuit. This is great for experimenting to help understand how the mains work. For example, with an ammeter, you can convince youself that when the 1.5v loads are perfectly balanced, the center tap (the "neutral") carries no current, and that in all other cases it carries only the difference current.

    Bookmark   November 20, 2010 at 12:16AM
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