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Commercial mains wiring: 12ga wire for 30 amp load?
On Thu, 10 Nov 2005 22:40:32 -0800, Esther & Fester Bestertester
wrote: Thus spake : You have 2 issues. Foiust, is the motor supposed to be running on 230 or 208? That couild be the extra current. If the motor is really exceeding the nameplate amps I would investigate futher. Nameplate: 230V, 16.5A. I presume that if an autotransformer boosts the voltage 10%, it must draw 10%(?) more current in order to do so. Is this correct? (Nothing is free.) FBt Correct, except that there will also be an additional bit of current draw due to losses in the autotransformer, which will probably be over 95% efficient: For 208V in, 230V out at 16.5A out to motor and 95% autotransformer efficiency: Iin = (16.5*(230/208))/.95 = 19.2A Which is slightly under your measured 20A, which indicates that you are running slightly over 100% load on your motor if my voltage guesses are right. Hopefully the nameplate service factor is at least 1.15 continuous (if the motor runs continuously). BTW congratulations on responding to one of only 2 posters who understand your situation (the other being Tom Horne of the Tacoma Park VFD). But consider that anyone responding to your question here cannot be aware of all pertinent circumstances such as possible derating requirements for high ambient temperature or more than 3 current carrying conductors in a conduit, which a competent local electrical designer would take into consideration, as well as considering the length of the wiring run and resulting voltage drop. Since you did not seem to understand the very basic fact that the 90 C wire ampacity rating is provided only to allow for various derating requirements, and the usable ampacity cannot exceed the rating for the conductor at the max temp ratings of the connections at either end (probably 75 C max for your circuit breaker, which will be labeled with temp rating), I suggest you find a competent local electrical designer or inspector to examine your installation. The $100 or so could save a lot more in the long run. |
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Commercial mains wiring: 12ga wire for 30 amp load?
Glen Walpert wrote: On Thu, 10 Nov 2005 22:40:32 -0800, Esther & Fester Bestertester wrote: Thus spake : You have 2 issues. Foiust, is the motor supposed to be running on 230 or 208? That couild be the extra current. If the motor is really exceeding the nameplate amps I would investigate futher. Nameplate: 230V, 16.5A. I presume that if an autotransformer boosts the voltage 10%, it must draw 10%(?) more current in order to do so. Is this correct? (Nothing is free.) FBt Correct, except that there will also be an additional bit of current draw due to losses in the autotransformer, which will probably be over 95% efficient: For 208V in, 230V out at 16.5A out to motor and 95% autotransformer efficiency: Iin = (16.5*(230/208))/.95 = 19.2A Which is slightly under your measured 20A, which indicates that you are running slightly over 100% load on your motor if my voltage guesses are right. Hopefully the nameplate service factor is at least 1.15 continuous (if the motor runs continuously). BTW congratulations on responding to one of only 2 posters who understand your situation (the other being Tom Horne of the Tacoma Park VFD). But consider that anyone responding to your question here cannot be aware of all pertinent circumstances such as possible derating requirements for high ambient temperature or more than 3 current carrying conductors in a conduit, which a competent local electrical designer would take into consideration, as well as considering the length of the wiring run and resulting voltage drop. Since you did not seem to understand the very basic fact that the 90 C wire ampacity rating is provided only to allow for various derating requirements, and the usable ampacity cannot exceed the rating for the conductor at the max temp ratings of the connections at either end (probably 75 C max for your circuit breaker, which will be labeled with temp rating), I suggest you find a competent local electrical designer or inspector to examine your installation. The $100 or so could save a lot more in the long run. Since he went with an autotransformer, he should derate the wiring for NEC "general use" and that puts 12ga THHN at 20A. He needs to reinstall at 10ga- maybe- depending on circuit run length. 10ga would be good for up to 100', 8ga would be recommended for over 150', 6ga for up to 300' etc at 2.5% voltage drop..This may explain why he went with an autotransformer- burned the old motor out with overload at low voltage and upped the breaker from 20A motor time delay to 30A to eliminate LRA trips. He should therefore reinstall the 20A breaker with the 10ga or larger and get rid of the autotransformer unless the motor specifically calls out 230V which is rare these days. |
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