480 volt transformer tripping breaker - Page 2
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  1. #21
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    Cb's are designed so they trip even if the handle is held "on".

    Even zinsco breakers can pass that test.

    Anyhow dont use an ntc because you still need a time delay relay to short them out. Large enough ntc that doesnt need relay to short out will be spendy. Time delay relay is how much?

    Smaller ntc and the relay fails now the ntc burns up.

    3 lightbulbs and an extra manual disconnect? How much is that compared to the correct breaker in thr first place?

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    Quote Originally Posted by johansen View Post
    .

    3 lightbulbs and an extra manual disconnect? How much is that compared to the correct breaker in thr first place?
    Problem is the correct breaker is not available, generally. Show me a 100 amp adj mag trip breaker that goes in a Square D QO box.

    Those type breakers are usually higher current bolt in designed to be in their own enclosure and fed from a power bus. Standard fare in a huge factory but a real pain to implement in a small to medium shop.


    Sent from my iPhone using Tapatalk

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    This has been beat to death on many threads and I've dealt with the fun undersized circuits.

    Basically, if you size the breaker as if you were using the full transformer capacity you don't need to do anything special.

    If you are using a smaller primary breaker, you need to upsize the trip curve to accommodate the inrush.

    If you want a smaller breaker for circuit protection, use a full sized breaker and full sized wires, going to a fused disconnect with slow blow fuses.

    Do not switch breakers on and off every day. They will fail. Use a disconnect for that.

    Hokey solutions such as heater coils and light bulbs work but are a hassle. If you cannot feed the inrush of the transformer you need to sell the transformer and buy a smaller one. You will not come out ahead with resistors and contactors or switches.

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    Now if you want a really hokey, F*'d up solution, wire in the old 30A as well as the 50A, just make sure the phases are all on the same legs. Flip the 30A on for the inrush tolerance, then the 50A and then turn the 30A off. Of course if you leave both of them on you could potentially draw 80A before tripping on a true overload, which is certainly not good.

  6. #25
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    Lots of replies here so I will just answer in a general sense.
    Also sorry I did not do a search on this problem not realizing it has been beat to death.

    All I really need to do is find a 50 amp three phase breaker that is the same vintage as my 30 amp which worked fine. Everything else is sized properly and works fine. I only used the 30 amp originally as it was in the panel when I bought it used along with a lot of other expensive breakers.
    I thought why not give it a try and see if it works as the load on the other side was pretty small, maybe 10horse motor, might be 7.5 I will have to check.

    The wire to the transformer is sized for a 50amp breaker. I added a plasma cutter and a heat treat furnace both of which draw more power so I needed to upgrade to the 50 amp breaker not realizing that this would become a problem. I will be careful and try to remember not to run all three at once.

    The transformer is larger than required but not common here so difficult to replace and it is already installed and works well other than the breaker problem. So how was I (or my electrician) to know that the new breaker would not perform like the old smaller breaker? As Spock would say “It is simply not logical captain”.

    Now after all these replies If I cannot find a suitable breaker I will have to change the supply to a fused disconnect and all should be well.

    Thank you to the PM for another interesting bit of education.

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    Quote Originally Posted by M. Moore View Post
    So how was I (or my electrician) to know that the new breaker would not perform like the old smaller breaker? As Spock would say “It is simply not logical captain”.
    You shop at a gasoline station that maybe has three different Octane of gasoline, a Diesel nozzle, same pump, and heating kerosene at a pump across the carpark?

    Why would a motor-Mechanic give flying f**k which liquid, so long as he got 50 gallons of it instead of 30 gallons?

    That's "simply not logical Captain!"

    He'd have to ask a "soccer mom?"

    Hockey fans don't give a s**t ...so long as it don't give them worse hangover than usual.


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    Quote Originally Posted by Strostkovy View Post
    ..............Hokey solutions such as heater coils and light bulbs work but are a hassle. If you cannot feed the inrush of the transformer you need to sell the transformer and buy a smaller one. You will not come out ahead with resistors and contactors or switches.
    A "soft start" that really does limit current from turn-on will work. As for "coming out ahead"? That depends on whether you have any other option.

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    The OP has two paths forward. He can eliminate the severe inrush current as a path or he can tolerate the severe inrush. His call. He has both solutions now.

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    It sounds like you might have upsized the cable from the panel to the transformer? Impedance reduces inrush current and longer, thinner cable can be the cheapest way to do that.

    B, C, and D trip curves are the European names; you're unlikely to find them on US equipment unless it's dual-rated DIN-rail breakers; the funky ones Eaton & AB (amongst others) sell with the extra-large terminal shrouds.

    I'm not sure what the US equivalents are; they all seem to be more-or-less the same trip curve in the light-duty space.

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    Quote Originally Posted by SomeoneSomewhere View Post
    It sounds like you might have upsized the cable from the panel to the transformer? Impedance reduces inrush current and longer, thinner cable can be the cheapest way to do that.

    B, C, and D trip curves are the European names; you're unlikely to find them on US equipment unless it's dual-rated DIN-rail breakers; the funky ones Eaton & AB (amongst others) sell with the extra-large terminal shrouds.

    I'm not sure what the US equivalents are; they all seem to be more-or-less the same trip curve in the light-duty space.
    All he needs is to use the ones already mentioned for the general type of high-inductance, high-inrush loads.

    Square-D house here, so I'd buy ONE QO-HID. Covers the whole QO range. See Schneider's ordinary brochures.

    Extract, boldface added:

    QO-HID and QOB-HID circuit breakers are manufactured with larger contacts than standard QO and QOB circuit breakers to allow switching of high inductive loads.

    They also have magnetic characteristics similar to QO-HM and QOB-HM high-magnetic circuit breakers to allow the circuit breaker to hold in against the high starting inrush currents which are typical in HID lighting systems.
    The QO-HM would do, but... is not standard in the Ampacity he needs. 15 A & 20 A, only.

    Any OTHER major-maker will have comparable goods.
    It is not a rare need, nor anything close to a new need.
    His sparks should have KNOWN that. It is no Big Deal.

    "Soft start?" Well.. I have a demand-component to my billing here, even on vanilla 200 A @ 24(6) VAC split-phase residential service.

    Could was it might still be a good idea for his environment, too?

    YMMV

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    I have already pulled out a 60amp disconnect from the overcrowded storage shed and 3 50 amp slow blow breakers.
    This will be installed soon as it is the cheapest option to solve the problem given that I had the parts.
    The very nice brand new 50 amp breaker will retain its spot in the panel and forever be a reminder of this saga.
    How can these new breakers handle a motor start inrush when they can’t handle a bare transformer start? Obviously I don’t understand the difference.

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    Quote Originally Posted by M. Moore View Post
    Obviously I don’t understand the difference.
    A one-time go at experiencing a severe pain in the ass out of ignorance doesn't make you a "bad person".

    You "understand" the hole in the wallet by now well enough?

    That'll do to find someone ELSE who does "understand", NEXT go, yah?

    BFD

    I heard of a guy who had the same dam' problem with DIY circumcision.

    Yah didn' wanna be next to him at a urinal.
    Pissed on the bias.

    Your ONE shoe got wet. BOTH, one might blame on a puddle in the carpark. The imbalance was harder.

    Electricity is just that way, some weeks, too.

    Half pissed-on, half pissed-off.

    Might as well be politics, yah? And even GOD can't understand THAT!

  14. #33
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    Quote Originally Posted by M. Moore View Post
    I have already pulled out a 60amp disconnect from the overcrowded storage shed and 3 50 amp slow blow breakers.
    This will be installed soon as it is the cheapest option to solve the problem given that I had the parts.
    The very nice brand new 50 amp breaker will retain its spot in the panel and forever be a reminder of this saga.
    How can these new breakers handle a motor start inrush when they can’t handle a bare transformer start? Obviously I don’t understand the difference.
    motors typically have a 7 fold FLA peak starting amps. the inrush for the first 2 line cycles is negligible.

    transformers have an asymetrical current draw.. only sometimes. sometimes none at all.

    a good quality 1kva toroidal transformer can draw 50-100 times its nominal full load current for the first couple line cycles. but this spike in current isn't for the full cycle, its thermal rms value isn't particularly high.
    when i plug my 2kw variac in, i can hear it take 3 seconds for the dc bias to work its way out of the winding.

    basically the peak current in the first line cycle trips the magnetic circuit breaker. older breakers with no magnetic component won't trip.

  15. #34
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    Quote Originally Posted by M. Moore View Post
    ............
    How can these new breakers handle a motor start inrush when they can’t handle a bare transformer start? Obviously I don’t understand the difference.
    It is all about the "response curve" of the breaker.

    This is a curve of the trip time vs current, at a particular temperature. Generally it is similar to a fuse, although it is not exactly the same. As current increases, the trip time is reduced. At a light overload, it may never trip. At a very high current, it may trip "instantly" (really a very fast trip with a delay that you cannot perceive).

    If the current is an overload, but is not present for the trip time on the response curve at that current, it is within the "hold"region, and the breaker does not trip.

    If the breaker trips, either it is weakened for some reason (heat, many trips, use as a switch, etc), OR the load passed outside the "hold region" of the response curve.

    A different higher rated breaker may trip at a lower current-time point than a lower rated breaker, due to constructional differences. This is often true of fuses as well. A 3A fuse from a particular maker may stand up indefinitely against repeated overloads of a particular current and time. But the same maker's 4A fuse may fail after only a few of the same short overloads.

  16. #35
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    Thank you all again for the info and suggestions.
    The disconnect was installed today with 50 amp slo-blo fuses and all is well.
    Anybody need a 50 amp Siemens 3phase breaker? Hardly used!

  17. #36
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    Quote Originally Posted by M. Moore View Post
    Thank you all again for the info and suggestions.
    The disconnect was installed today with 50 amp slo-blo fuses and all is well.
    Anybody need a 50 amp Siemens 3phase breaker? Hardly used!
    That's good. I'm wondering though how did you feed the disconnect? Not from the breaker panel I assume otherwise that breaker would trip.

  18. #37
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    ADH,
    We connected the new box into the gutter that feeds all the other disconnects. Not much room and we had to add a connection point, (3 actually) all done live.

    Used the roller and then the plasma today and not a whisper of annoyance when I turned on the transformer, just the sweet humming sound.

    I have already forgotten how much the breaker cost and the cost to install the disconnect was actually very reasonable considering that I had all the equipment.


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