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Thread: 120/240 Three Phase Transformer

  1. #1
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    120/240 Three Phase Transformer

    I am reviewing a design package that has a couple of these. I have not encountered them before. My guess is that the secondary is a delta with the center point of one coil (A-C, perhaps) being grounded. In that case, you cannot use a single-pole breaker to supply a 120 volt load, unless you limit yourself to using phases A or C. Is that right? Am I also right in understanding that if you used a single pole breaker on the B phase, the voltage to neutral would be 208V?

    TIA.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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    Correct.
    In addition, in the event you have any 240 single phase loads that are connected to "B" phase,a slash rated breaker(240/120) is not allowed.
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    Thanks, Gus. The design does show some 240V loads connected from A-B and from B-C. I had not heard of a slash rated breaker. Can you explain what that means? I need to know whether to add a comment on my review for this issue.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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    Quote Originally Posted by charlie b View Post
    Thanks, Gus. The design does show some 240V loads connected from A-B and from B-C. I had not heard of a slash rated breaker. Can you explain what that means? I need to know whether to add a comment on my review for this issue.
    Two pole breakers commonly used in 120/240 single phase or 208Y/120 systems are commonly rated as 120/240V (or is it 240/120?)
    What this indicates is that the voltage from pole to pole may be up to 240V while the voltage from either pole to ground cannot be more than 120V nominal.
    Single pole breakers in the same panels may also be slash rated, indicating for all practical purposes that they cannot be used for more than 120V line to ground.

    Three pole breakers are not as often slash rated, since corner grounded or ungrounded delta would always have to be rated for full delta voltage from line to ground. In some cases it may be cheaper to use a three pole breaker rather than a full-rated two pole.

    You are not likely to find tandem or half width breakers which are rated for full voltage to ground.

    What it amounts to technically is that the slash rated breakers are not tested for full voltage to ground and may have smaller insulation gaps and clearances as a result. More compact designs are possible.

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    Quote Originally Posted by charlie b View Post
    Thanks, Gus. The design does show some 240V loads connected from A-B and from B-C. I had not heard of a slash rated breaker. Can you explain what that means? I need to know whether to add a comment on my review for this issue.
    NEC is 240.85 Because the voltage to ground on phase B exceeds 120v the "standard" two pole breaker listed as being rated 240/120 isn't allowed. "Straight rated" 240v 2 pole breakers are available but often not off the shelf and at a higher price.
    Someone with more knowledge of the listing will have to explain the technical difference.
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    I understand it now. Many thanks to you both.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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    Quote Originally Posted by charlie b View Post
    I am reviewing a design package that has a couple of these. I have not encountered them before. My guess is that the secondary is a delta with the center point of one coil (A-C, perhaps) being grounded. In that case, you cannot use a single-pole breaker to supply a 120 volt load, unless you limit yourself to using phases A or C. Is that right? Am I also right in understanding that if you used a single pole breaker on the B phase, the voltage to neutral would be 208V?

    TIA.
    You are correct on all accounts. This would likely not be one transformer, but two (maybe 3), one being center tapped. Usually an open delta. Using the B phase for 208V loads is possible, tho there are load limitations; iirc, newer transformers would allow ~35%, whereas older designs were limited to ~5%. As previously mentioned, if the voltage exceeds 120V to ground, as the B phase would (208V), you must use full 240V rated breakers.

    In my experience, nearly all of these installs skip the B leg or bus except for 3ph loads, which are quite light. iow, when I see a panel with every third spot skipped (blank), I know it's most likely a high-leg delta.
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    Quote Originally Posted by GoldDigger View Post
    Two pole breakers commonly used in 120/240 single phase or 208Y/120 systems are commonly rated as 120/240V (or is it 240/120?)
    ...
    Just FYI for the naming convention (per IEEE):

    • If discussing THREE phase systems, you always put the Line to Line voltage first, as in 208Y120 or 208/120; 480Y277 or 480/277; or in this case, 240/120V 3ph 4 wire.
    • If discussing a SINGLE phase service, you always use the Line to Neutral voltage first, as in 120/240V.


    Where it gets a little fuzzy is if discussing a single phase feed off of a three phase Y service, you use the single phase naming convention, so 208Y120V source, you could call it 120/208V if referring to a single phase feed from it. Likewise, and more slippery, is a 120/240V single phase feed from a 240/120V three phase service. So, to the point you made GD, the BREAKERS, if 2 pole for a 208/120V system, would be "slash" rated as 120/240V because you are speaking of a single phase device.

    But that said, when you buy "slash" rated breakers for using in a 480Y277V system, you will usually see them listed as 480/277V. Why is it that way? Because, up until the Europeans started invading the North American electrical market, there was no such thing as a "slash rated" 480V breaker, they were all 600V phase to phase rated, no need to distinguish. But the Europeans had these little cheap 415/240V rated breakers that they submitted for UL listing that would not pass the 480V L-L fault duty, so UL allowed them to be "slash rated" as 480/277V in the same way as the lower voltage versions already sold here. The reason it's different however is that in this case, the slash rating applies to THREE PHASE breaker ratings as well, not just single phase.

    The purpose of the numerical order naming convention was (ostensibly) so that if someone describes a voltage to you, you should know immediately by the order of numbers whether they were referring to single phase or three phase. That unfortunately has been somewhat lost over time because hardly anyone ever teaches it so most people mess it up unknowingly.

    Case in point, per the IEEE rules the title of this thread is technically incorrect... In discussing the service transformer, it would be described as a 240/120 3 phase 4 wire delta transformer.
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    Quote Originally Posted by Jraef View Post
    That unfortunately has been somewhat lost over time because hardly anyone ever teaches it so most people mess it up unknowingly.
    And then there are those of us who mess it up intentionally. Or more to the point, we are willing to not notice or be concerned over which way we write it, and will randomly change from one way to the other.

    But seriously, thanks for the explanation. I had not encountered that before. Now I have a reason to remember the formal "right" way. Not sure I will adopt the convention, even though I am a Senior IEEE Member with 29 years of dues-paying experience. I like the feeling of freedom that comes with resistance to convention.
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    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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    Quote Originally Posted by charlie b View Post
    And then there are those of us who mess it up intentionally. Or more to the point, we are willing to not notice or be concerned over which way we write it, and will randomly change from one way to the other.

    But seriously, thanks for the explanation. I had not encountered that before. Now I have a reason to remember the formal "right" way. Not sure I will adopt the convention, even though I am a Senior IEEE Member with 29 years of dues-paying experience. I like the feeling of freedom that comes with resistance to convention.


    I didn't know (or maybe remember) any of this until late in my career when I went to work for Siemens in 2006. I attended a training module on Switchgear and Transformers and thought "I can teach this, I'll just sleep through it and ace the exam." I got that wrong on the exam, which forced me to go back through some of the stuff I slept through and there it was; something "new to me".

    Like I said, the convention makes sense, but few people adhere to it, so it's mostly a convention in name only. Kind of like Switchgear and Switchboard, which have specific official definitions, but most people use the terms interchangeably. So if you are a stickler and try to correct everyone that uses the terms wrong, nobody wants to talk to you twice...
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