Three Phase wiring & Appliances

Old 01-23-01, 02:01 PM
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I have a house built in 1920's that is wired three phase. An electrician friend has told me that I need to use the stinger leg (208 volts) to set up a seperate load center for the 240 appliances. When wired with one of the 120 legs the 208 will give 240.
My question is that a water heater does not care which wire carries the 208 but a stove or dryer it matters which wire carries the 120 since some of the controls are 120. How can I tell which wire carries the 120 on the plug for a stove or dryer??? Do I need to contact the manufacturer?
Old 01-24-01, 10:44 AM
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You have a three phase delta or 'hi leg' system. This means that you should have four total wires coming from your electric utility: phase A, phase B, phase C, and a grounded nuetral wire. To summarize, you should get the following readings with a voltmeter:

120 volts between phase A and nuetral.
208 volts between phase B and nuetral.
120 volts between phase C and nuetral.
240 volts between phase A and phase B.
240 volts between phase B and phase C.
240 volts between phase A and phase C.

For practical purposes, you should not need a new panel. I can find no NEC requirement that says all residences shall be fed with single phase power. This is a highly uncommon situation, and I would believe that your local utility may want to convert you to typical single phase power. I would recommend that you explore this option, as it will only make any future or present work on the home easier and less expensive.

That said, you should have the hi leg or phase B installed in your panel to the middle bus. This means that from left to right (or from top to bottom) on the power lugs/main breaker lugs you should have A,B,C depending on your panel.
The B phase or hi leg should be marked orange, or have orange phase tape on it at every termination, junction box, or lug in the building. If it does not, then make it so. This will serve as a reminder to anyone that this hot wire is the hi leg of a delta system, and should only be used under specific situations.

As you alluded to in your post, this hi leg has a higher voltage, and it will literally smoke any equipment attached to it that is not rated for 208 volts. This includes standard residential receptacles, light fixtures, furnaces, and equipment as well as residential ranges and dryers, as the internal circuitry uses 120 volts.


I would highly recommend that you set up your breaker panel as follows: on the left side of the panel, you should arrange the circuits so that position 1(top left, from top to bottom) has a single pole breaker in it, position 2 should be empty, position 3 and 4 can be two single pole breakers or one 2 pole breaker (240 V). Continue this pattern down the panel (two breakers, one emty, two breakers, one empty), until the final spot, where you will be limited to a single breaker again. Mirror this set up on the right side of the panel. This will only work if the phase B wire is in the center of the bus.

This will create a situation where the phase B hi leg is not continuing into the residence for any circuits, this will minimize the chance of some future handyman, or inexperienced electrician from inadvertently connecting to a 208 volt line.

If you don't feel comfortable with doing this yourself, then hire a journeyman electrician, and ask to see his license. A rookie, or a residential electrician, in my experience, will not know an adequate way to safely solve your problem. You also may consider waiting for one of the other experienced posters on this site to chime in, a lot of them have quality ideas that may vary slightly from mine.

Enjoy your day!
Old 01-24-01, 01:05 PM
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Whew! Thanks for the details. What I am trying to do is make use of the hi leg. I have a 3 phase AC condenser that is to new to get rid of so I will need to keep the three phase for a while. What I am trying to do is set up 2 different load centers: the first has both 120 legs and is set up like a standard single phase breaker panel. the second uses the stinger leg and one of the 120 legs to run all of the 240 appliances and both waterheaters.
Right now in the winter I do not use the hi leg at all and have been told that I can average my elecrical costs and bring them down if I use this leg more.
All of the homes in my neighborhood were built in the 20's and 30's and the power was run from a nearby industrial area so we all have 3 phase. Most are not using the hi leg at all, just ignoring it.
Old 01-24-01, 06:28 PM
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After reading your post and s1nuber's reply, it is my opinion that what he said as about as good as you are going to get considering what you have existing as a service as you describe it.

However warning flags come up with me the same as s1nuber, you need a qualified electrician that has been proven by testing that he knows what he is doing. This is a major accident invited to happen.

Now I feel you should look at your meter and see if you have a sweep meter in the glass with the spinning wheel. If you do then you are being charged what is called a DEMAND CHARGE. All three phase meters that I have experience with supplied by the utility company charges both a commercial rate from 30 to 100% more per kw usage than a normal residential use.

IF you are being charged with a commercial use meter costing you more money, you have an alternative that I might suggest to save you money, still be able to use that 3 phase a/c unit, and make the house much more manageable and safe.

I suggest that you install a single phase panel large enough to carry you house only add 100 amp more capacity.
Then buy what they call a "roto-phase". You could put a three phase sub panel off of the new single phase panel designed just for two circuits and with an amp load that will carry your a/c unit only. Then run both of your factory legs from a breaker in the single phase panel to provide power for line 1 and 3 of the three phase panel and put you stinger leg generated by the roto-phase to the middle leg of that three phase sub panel. This roto-phase will produce your stinger leg creating your three phase for you a/c unit but get you back into a more safe service design. This roto-phase stinger under load is only 240 volts as limited in a dwelling.


Check your maximum voltage in the NEC allowed to serve a dwelling 210-6, 410-75 these are the only two places that I know of that limits voltage in a dwelling. I see no NEC violation having three phase high leg in a dwelling either. Maybe there should be.

Be Careful

Old 01-24-01, 08:14 PM
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Thanks for the warnings. I have a licensed electrician I am working with & he has industrial experience. He is the one who recognized the 3 phase to begin with. I will check meter & see what I have here. All of the houses in neighborhood are originally 3 phase but most either ignore old hi leg or have been converted.
My house has 7 different breaker/screw-in fuse boxes that have been added over the 80 years the home is in age. Some of the old knob & tube wiring is still in place though it has all been disconnected and other wiring put in place. What I will probably do is what you have suggested. I will put a 200 amp panel in place with the two 120 legs and there is already a sub-panel that runs the 3 phase for the AC in place.
I just feel like I could utilize that hi leg. But if it will not save me $$$ then I don't want to waste time & effort and/or create the potential for problems in my home.
I really appreciate the candor everyone is giving me here and on another forum that I posed the same question.
Old 01-25-01, 06:13 AM
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Glad to help Fontana.

WG - Good point on the voltage limits, I don't like his situation personally, but as long as that hi leg does not get used for anything other than hard wired equipment of the proper voltage rating, I don't think he has violated any national codes. My local power authority and code enforcement agencies specifically forbid that type of service to a residence, so I don't have the problem here.

Enjoy the day!

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