noob question about wire/fuse size on a transformer

Discussion in 'General Electronics Chat' started by ynot, Dec 9, 2008.

  1. ynot

    Thread Starter New Member

    Dec 9, 2008
    3
    0
    Hello , I am new here and a noob when it comes to transformers, I could really use some advice. I'm not familiar with large transformers, I mostly do electric motor control, for motors under 10 hp.

    Where I work , they ask me to install a combination panel as well as a 50 kva single phase, 600v/240v transformer , to replace a 17kva of the same. ( because they cooked the 17 )
    Basically I need to know what size fuses do I need on the 600v disconnect to the transformer, and what wire size from the secondary side to the 100 amp combination panel do I need.
    Just doing very very basic calculations from my old college text books, which i haven't opened in many years, I figured 80 amp fuses would be safe on the primary, and that the secondary is capable of 210 amps?
    Is this correct?
    Since the new combination panel has a 100 amp main will #3 /600v, copper wire be ok to supply the comb. panel, as well at the primary on the transformer?

    Another question if I can.... if they every decided that a 100 amp combination panel was not large enough to meet their needs , and wanted to go with a 200 amp combiation panel, would #2 aluminum be safe to supply the panel?

    thanks from you time
    Tony
    just a note, there was no combination panel with the 17 kva transformer, just a load centre ( no main or fused disconnect ), also they had 40 amp fuses on the primary... hence the cooked transformer
     
    Last edited: Dec 9, 2008
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Well, you really should have a licensed electrician inspect your current system and wire it in.

    80A breakers on the primary side and 200A worth of breakers on the secondary side sounds reasonable to me, but I'm not a licensed electrician.

    There may be other concerns with the power distribution panel that you may be overlooking, such as load distribution or possibly exceeding the ratings of the service panel, when combined with other existing loads. If the panel gets burned up, you'll wind up having a good bit of down-time while the damage gets repaired.

    By comparison, a licensed electrican's time is quite a bargain - and they will perform the work to comply with the electrical code.
     
  3. ynot

    Thread Starter New Member

    Dec 9, 2008
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    The thing is no work at this place has ever had a licensed electrician performing the work, expect for the back up generators, and the main hydro.. because the owner didn't have a choice ....

    I recently became completely deaf, both ears ... I don't lip read well at all . I haven't been successful finding another job ... period. and I have looked very hard for another job
    I'm good at what I do, I'm just not very familiar with large transformers.There are many things here that were done ( not by me ) just plain wrong, and I am trying to correct them.

    Though I value your advice, and realize that's what I should do...if I don't do it, Ill get laid off and not called back...( that's how thing work at this place ) how then am I suppose to support my family?
    If I don't do it ,someone else with much less knowledge about electrical work will do it... and Ill be out of a job
    So can a electrician please answer my questions...please
     
  4. floomdoggle

    Senior Member

    Sep 1, 2008
    217
    2
    Fiirst of all, what is the input voltage? And why are fuses being used? A breaker should be used. Single phase over 220 is unusual, as would be the motors. 600v is not only dangerous, but, also unusual. I would imagine the original electrician wasn't very good. You probably have 440v three phase wiring, wired up to simulate 600v single phase.
    A diagram of the input wiring would be helpful. What country are you in? Would be helpful.
    Dan
     
  5. floomdoggle

    Senior Member

    Sep 1, 2008
    217
    2
    Also, what type of wire is coming in to, or out of the box, copper or aluminium? dissimilar metal conductors need special attention. "#" can mean many things. Pounds, or AWG, wire size?
    Dan
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    I do not endorse any work performed by unlicensed/unqualified persons. Is a man's pride really worth more than the lives of other people? The plant in question should be reported to the government and shut down until it can be made safe.

    I will share the following, from the NEC, for information purposes only:

    #2 aluminum is way too small for 200 Amperes. At least 4/0 is needed, and then ONLY IF the correct insulation type is used and the wiring is used in low enough ambient temperature. 350kcmil aluminum can handle 200 Amperes regardless of insulation type IF used in low enough ambient temperatures.

    #3 copper is okay for 100 Amps, but ONLY if used in low enough ambient temperatures, and NOT if the insulation is type TW or type UF.

    #3 aluminum is too small for 100 Amps.

    All of the above assumes not more than 3 current carrying conductors in a raceway or cable. If there are four or more conductors in the same raceway or cable, one must apply de-rating factors. So if you have a 4-wire hree phase system with non-linear loads & harmonics on the neutral, the above listed wiring is too small per NEC 310.15(B)(4)(c).

    Antioxidant compound must be used when aluminum wires are connected to copper busses or vice versa.

    There are also codes for maximum conductors of a given size in a conduit of given size. Ditto for enclosures.

    This ain't even 10% of the tip of the iceburg.

    There are some darn good reasons why American and Canadian laws specify this kind of work being done only by qualified persons. Bad wires start fires!
     
    Last edited: Dec 10, 2008
  7. Pich

    Active Member

    Mar 11, 2008
    119
    4
    Ynot
    I think you should pass this one on to a licensed electrical contractor, too many code rules involved from grounding to the size of the feeder conductors and most of it boils down to safety and a safe installation.
     
  8. ynot

    Thread Starter New Member

    Dec 9, 2008
    3
    0
    1st off I want to thank you all for replying to my questions, I guess I will go back and talk to my employer once again ( quoting some of your replies), and see what he says, maybe, if I explain the liability aspect of this endeavor he will see it from a different perspective.

    to floomdoggle ... in answer to your questions, the the main hydro feed from the hydro ploe to the main diconnect in the pump house is 3 phase 600v. from there it goes to a splitter box. the main disconect has a #3 green to 2 ground plates buried outside, the splitter box is bonded to the main disconnect as well as to the transformer
    the transformer uses two legs of the 3 phases, and is stamped on the side of the transformer 600v/240v single phase transformer. # = number,
    I live in Ontario Canada
    Thank you all once again
    regards
    ynot

     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    An excellent approach. Please, please, do so. I most sincerely hope that you will retain your position afterwards.

    I ran all of the wiring in my folks' "new house" in the early 1960's. I was 9 years old. I've worked on the radar systems of quite a few fighter jets, missiles, and spacecraft.

    None of that qualifies me to be an electrican.

    Connecting motors to an existing properly designed and wired electrical system infrastructure is one thing. Making changes to the infrastructure is something entirely different.

    You and your company need someone physically there to inspect the system to see what needs to be done to get things within code. There is no way that even a Master Electrican could tell you how to do that without being there.

    I spent a number of summers in Sarnia, ONT (SYC) in the 60's and early 70's. The Bridge Tavern had good food.
     
  10. floomdoggle

    Senior Member

    Sep 1, 2008
    217
    2
    Do not touch this wiring, unless you want to disconnect the main input, wear gloves so thick you cannot move your fingers, and have your boss as a ground. Unless you want really curly, burnt hair.
    Come down to Texas, good workers are always welcome. And tell that asshole of a boss if he wants a reason to save money, while endangering his workers to make a 10 cent profit, he can meet my foot. Excuse my vehemence. A licensed electrician, is the only way this is going to be done properly. And without more damage than would cover the original cost.
    As you are in Canada, call, anonymously, your local authorities to let them know what this slob is trying. An electrical inspection of your workplace seems to be in order.
    Dan
     
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