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Thread: What is the difference between 208 and 240?

  1. #11
    Join Date
    Feb 2007
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    3

    Explainations and maybe alternatives

    Ok, I guess I openned a can of worms. Residential 120/240 is derived from a single leg of three phase power. 120/208 is derived from two legs of three phase power. From the pics (Nova's), it seems that the delta could essentially carry 120/208/240, with the 208 from L2 to Grn. The Wye system looks like it won't achieve anything past 208V. So in a residential setting, would the transformer be a Delta? Then if this is the case, you have L1, L3 and Grn coming into the house, where's L2?

    Back on page 21 this guy asks this, "single phase to three phase converters", they talk about rotary vs. static phase converters. Are these creating three phases out of the one leg of three phase? and if they are, would it make a different again if it were a wye or a Delta Transformer?

    Following the follow-up of 422.60A, I also referred to 422.61 for the listing of the elements, the unit has been modified it seems (30 Amp cord and Plug for what I'm coming up with at 38.46 Amp). I found a wiring diagram on the junction box cover plate. After clean up I can barely make out that it shows Single and Three Phase wiring. All my numbers are lost so I will refer to the manufacturer. Why I was checking is that the newer units and replacement elements come as 208 and 240. Making it work isn't the issue, now it's about phase conversion and why would one system (Wye vs. Delta, 208V vs. 240V) be better than the other and for what structure application. Is it about money and the ease of power distribution? Or is there a valid reason for the difference? Then another explaination please, some of the guys doing the multiplexes were wiring with three phase and there were others wiring with the Single phase. I didn't have the chance to install the cans so I'm wondering now what woild be the difference? If in the three phase units wouldn't be hard to get the 240V for the household appliances? Thanks, if you can.

  2. #12
    Join Date
    Feb 2007
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    3

    I just typed a book and lost it!

    Ok I think I have the difference on how the voltages are derived. But in a residential application on the Delta transformer you have L1, L3 and Grn, where would L2 be? Back on page 21 "single phase to three phase converters
    ", this guy is creating phases. The differences between static and rotary? And does this mean that in again a residential application you could create three phases from one leg of one phase to derive at your 208V? Or would it be defeating the purpose depending on the transformer?

    I guess the other question is what are the advantages to 208V vs. 240V? The ease of power distribution, cost, both? As for installing a 208V single phase appliance on a 240V single phase circuit, wouldn't the unit draw only what was needed? Thanks.

  3. #13
    Join Date
    Dec 2004
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    Fort Collins, CO NEC: 2014
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    15,431
    Watabull, welcome to the forum. As a new member, your posts won't be immediately visible to you, so don't worry that you don't see them - they're not lost, just in need of approval behind the scenes first.
    Quote Originally Posted by watabull
    From the pics (Nova's), it seems that the delta could essentially carry 120/208/240, with the 208 from L2 to Grn.
    That's a little deceiving. A 120/240V, three phase, midpoint-grounded transformer has what is commonly called a "high-leg" or "stinger", which does have a technical voltage of 208V from the B phase to neutral. But it can't support 208V loads, because the voltage between those two points isn't stable.

    So in a residential setting, would the transformer be a Delta?
    No, it is a different transformer. Take a look over at the Technical Reference Area over at ECN forum.

    Single Phase Transformer has a phase and neutral on the line side:


    Delta Transformer uses three phases to create three phases (and a neutral) of a different voltage:


    As for the rest of what you wrote, I'm not sure what you're getting at.

    What are you trying to do? Are you trying to install a commercial range in a house? :confused:

  4. #14

    208 vs 240

    Quote Originally Posted by watabull
    I've been working in the field for nine years, residential new construction and have never dealt with a 208 system. I've looked through the NEC for a description/explaination and did not find one. If there is article on the matter, which one, I can take it from there. If not could someone please explain this for me, thanks. The motivation for this is for two reasons 1) Common knowledge so I will know in the future and don't have to ask silly questions. 2)Will a 208v 8kva(1 or 3 phase) griddle cook(be ruined, sorry for the pun) on a 120/240V 40 Amp circuit? This question I do not know the answer to. Thanks.
    I have the same problem, but with a commercial building, i.e. church. I need an article to present to the church so that the new construction is not changed from 208V 3P, as recommended by my PE EE, to 240V 3P as requested by the church. This assumes that my assumptions are correct. They have an existing 35,000 SF building that is 240V 3P. The new sanctuary is 22,000 SF. They want to consolidate the service and wire for 240V 3P. I do not need an opinion. I need a published article on the subject. Any help with a title and address would be appreciated.

  5. #15
    Join Date
    Dec 2004
    Location
    Fort Collins, CO NEC: 2014
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    Quote Originally Posted by Reynolds White
    I need an article to present to the church so that the new construction is not changed from 208V 3P, as recommended by my PE EE, to 240V 3P as requested by the church.
    I am confused.

    230.2 requires a single service unless special conditions apply. The church's request seems not only reasonable, but is arguably required by the NEC.

    I do not need an opinion. I need a published article on the subject.
    I am confused again. You want an article saying that 240V 3 phase power is undesirable? :-?

    I deleted your post in the other thread, so that your answers would be grouped in this thread, to minimize confusion for yourself and the other forum members. In the future, you might want to start your own thread, as your question is not directly related to the original post of this thread.

  6. #16
    Join Date
    May 2003
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    This are no such thing as a silly question. However, we can provide silly answers to reasonable questions.
    There was never a greater truth spoken on this site, than that statement, wish I had thought of it.
    Brian John
    Leesburg, VA

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