120/240v 3 wire circuits

Discussion in 'General Electronics Chat' started by Buck Stinger, Feb 15, 2012.

  1. Buck Stinger

    Thread Starter New Member

    Feb 15, 2012
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    Ive been reading blogs regarding exisitng 240v 3 wire circuits being converted into 4 wire 120/240v circuits. We all know that using a bare ground conductor as a combination neutral conductor is a bad idea and will not comply with code. I am curious why there isnt much mention regarding using a buck/boost transformer and creating a neutral conductor in particular situations. For Example: in a residential setting where there is currently an oven installed and connected with 8/2 romex. The oven is upgraded and required to have a 120/240v 34.8 amp single phase circuit. Obviously there is not an insulated neutral conductor present and installing a new 8/3 romex with ground is prohibitive. Installing a 10 kva buck/boost transformers, 240v primary / 120/240v secondary are very expensive but could save money and a headache vs. installing a new circuit from the main service. These transformers will create a neutral.

    Theoretically, can anyone identify an issue with this application? (other than cost), or should this theory work? I have not attempted this installation yet, but found this to be a good alternitive if it will work.

    Buck,
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    This was neither feedback nor a suggestion, so I have moved it to a more appropriate forum.

    A buck boost design is DC in nature (as far as I know) while you are wanting to discuss AC option.

    I assume you are talking American standards (but this is an assumption). The USA uses a split phase system. This consists of 2 separate 120AC circuits and a 240VAC circuit. Far as I know there are not 4 120VAC legs for residential houses, though industrial applications regularly use 3 phase at 270VAC and more.

    Generally we recommend a licensed electrician at this point, since there is real danger doing something like this without the proper training.
     
  3. BSomer

    Member

    Dec 28, 2011
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    Do you have any information on the transformer you have in mind for this job? Most buck/boost transformers are designed to decrease/increase the input voltage. Not necessarily create a neutral and the same voltage. You could get away with just connecting the neutral of your new stove to the ground connection point. The neutral is bonded to the ground at the electrical panel anyway, at least it should be per NEC. The only issue that I can think of when connecting it this way is if your oven is powered by a GFCI circuit breaker you may get some nuisance tripping of the breaker.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    A transformer could give a neutral but you still need a safety ground.

    8KVA transformers rather big and expensive. I found a listing for some 7.5KVA isolation transformers and they were over $2000.
     
  5. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    and I'm not sure where you think code is going to allow you to hang one. If your willing to give up those 120v conviences within your stove, than a much smaller control xtrans is all you require.
     
  6. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Last edited: Feb 16, 2012
  7. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    sub (3) would seem to denounce the suggestion?
     
  8. gerty

    AAC Fanatic!

    Aug 30, 2007
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    How so? The way I read it, the conductor can be insulated or not, part of the SE cable, and originate at service panel.

    And before we go too much further, I just realized this is from the California code, which may or may not be different. What made me look is the fact that it specifies SE cable instead of just NM.
     
  9. gerty

    AAC Fanatic!

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  10. BSomer

    Member

    Dec 28, 2011
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    So... Like I had stated above. You can get away with connecting the neutral to the ground point on the new stove. I suppose I could have stated it differently to be a little more clear.

    As to the current cable that is "most likely" to be feeding power to the stove, it will be a 8 AWG / 2 conductor with a 10 AWG bare copper ground. This fits the requirements in the code for the wire type. It should be coming from the main electrical panel of the home and not a "sub panel" that would be fed from the main panel.

    To be honest, I have not done too much residential wiring. Most of the stuff I did while in the electrical construction over the last 17 years has been industrial and commercial. I have recently got out of construction because it is so volatile right now and hard to get any work.
     
  11. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    I was actually going from the angle of a trans installed. Interesting leeway in code.
     
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