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  1. #1
    Join Date
    Aug 2006
    Location
    Colorado
    Posts
    288

    Confused Strange amp draw - heat lamps

    So I was called because a bank of heat lamps melted down at the junction box above the lamps. This is a set of three hatco lamps, 120 volt, two with double elements and 1 single shorty one. From the looks of it, the connection at one of the neutrals melted down and took down the rest of what was in the box. So I fixed the wiring, fixed the breakers, and doubled up the neutral because the total amp draw of the 3 lamps is about 34 amps. So when all was said and done and I went to test it with the amp clamp on the double neutral. As I turned on the 5 elements one by one, the amp draw went up accordingly......except for the last one. With 4 elements running, I was at 29 amps, when I turned on the last one, instead of going up to 34 amps, it went DOWN to 25 amps! But all the elements were running! I tried a different meter, and the same reading happened. I talked to several other techs about it and they had never seen such a thing. Anyone?? Is one of them leaking to ground because electricity takes the shortest path??

    Never saw anything like it.

  2. #2
    Join Date
    Jan 2001
    Location
    Nebraska
    Posts
    319
    Do the elements feed from multiple breakers.?

    Is the last element you turned on by chance fed from another breaker that is on the other side of the 240 so that the potential difference between the first 4 elements line voltage and the line voltage fed to the 5th element is 240 volts?

  3. #3
    Join Date
    Aug 2006
    Location
    Colorado
    Posts
    288

    Question Hmmmm

    Thanks for responding - I was losing hope. That sort of makes sense, but the weird thing about it is: There are 6 heat lamps total - all 120 volt. There were six circuits coming from 2 double pole breakers that feed into 3 double pole mercury contactors. There were problems before where the double pole breakers were not tripping because they are feeding 120v heat lamps. So I took out the double breakers and put in 6 single pole breakers so that each lamp has its' own breaker now. But if that last element was getting 240, wouldn't it have to be 'backfeeding' from the neutral? The restaurant was getting ready to open, so I didn't have time to get so detailed. I may go back and check today.

  4. #4
    Join Date
    Jan 2001
    Location
    Nebraska
    Posts
    319
    Quote Originally Posted by tuba View Post
    Thanks for responding - I was losing hope. That sort of makes sense, but the weird thing about it is: There are 6 heat lamps total - all 120 volt. There were six circuits coming from 2 double pole breakers that feed into 3 double pole mercury contactors. There were problems before where the double pole breakers were not tripping because they are feeding 120v heat lamps. So I took out the double breakers and put in 6 single pole breakers so that each lamp has its' own breaker now. But if that last element was getting 240, wouldn't it have to be 'backfeeding' from the neutral? The restaurant was getting ready to open, so I didn't have time to get so detailed. I may go back and check today.
    The 5th circuit is only getting 120 volts across it: but if I understand what you are saying is that you have created is a 3 wire 240v circuit so the neutral will carry only the unbalance between the two power legs.

    If you are going to feed from 6 single pole breakers the correct way to do that would be to separate the neutrals and pull the proper size conductors including a neutral for each separate circuit.

    Normally, if you have the space in the panel to be selective, you would put 3 circuits on one leg and 3 on the other leg to balance the panel load.

  5. #5
    Join Date
    Aug 2006
    Location
    Colorado
    Posts
    288

    Confused Hmmmmm

    This is mind boggling. So the 3 lamps with individual breakers, tied into a doubled neutral wire to handle the 34 amp load if all were on full blast, is turning into a 240 volt circuit? Strange.......

  6. #6
    Join Date
    Jan 2001
    Location
    Nebraska
    Posts
    319
    Quote Originally Posted by tuba View Post
    This is mind boggling. So the 3 lamps with individual breakers, tied into a doubled neutral wire to handle the 34 amp load if all were on full blast, is turning into a 240 volt circuit? Strange.......
    Yep could be, depending on which side of the panel your breakers are connected to.

    If you take a 250watt lamp rated at 120volts it will pull 2.08333 amps (I=P/E) 250/120.

    Which would equate to 57.6 Ohms (R= E/I) 120/2.0833

    If you took 2 of these light bulbs and connect one side of each one to a circuit breaker and the other side of each one to a shared neutral; if the breakers were attached to the same pole in the distribution panel you would have two 120 volt circuits with a shared neutral. In that case each of the circuit breakers would see 2.0833 amps of current flow and the shared neutral would have 2.0833 amps + 2.0833 amps or 4.166 amps of current flow in the shared neutral or return path.

    If the second breaker is connected to the opposite pole of the distribution panel there will now be 240 volts between the two power leads feeding the lamps because of the shared neutral you have created a 240v 3 wire circuit. Because the lamps are balanced each will pull 2.0833 amps and the shared neutral will handle the current difference between the two lamps. Since the lamps current is balanced the difference is zero

    In this case you could unhook the neutral and would not notice any difference in the lamps brightness because basically you have created a 240v circuit with two equal loads in series so each will drop 1/2 of the applied voltage across them.

    If the loads in the above case (no neutral) were not balanced each load in the series circuit would have different voltage drops across them. If one of the lights was 100 watts and the other was a 250 watt we would see one bright light and one dim light and the bright one would probably burn out before long (that is the typical indication of an open neutral condition).

    Which light would be the brightest the 100 watt or the 250 watt?

    A shared neutral or return path when used in a 240volt shared neutral situation will only carry the unbalance of the 2 currents.

    Now all of the above assumes that you are in the USA where the typical power distribution for 240 volts comes from a center taped transformer where each of the power legs measure 120v to neutral and measures 240V between the two power legs because they are 180 phase displaced from one another.

    Draw out the circuit using 2 lights, first two 120 volt circuits with a shared nuetral and then draw it connected to as a 240 volt 3 wire circuit and I think you will see what is going on.

  7. #7
    Join Date
    Apr 2010
    Location
    Red Deer, Alberta
    Posts
    653
    Quote Originally Posted by Tom R View Post
    Do the elements feed from multiple breakers.?

    Is the last element you turned on by chance fed from another breaker that is on the other side of the 240 so that the potential difference between the first 4 elements line voltage and the line voltage fed to the 5th element is 240 volts?
    This sounds good to me...

    Check across the 120V (hot) leads of each feed when the breakers are turned on one by one, there should be 0 VAC across the hot leads if they all have the same source. If you see a voltage of about 120 VAC across any hot lead and the one you just turned on, select another breaker for that (the one you just turned on) feed...

    Assuming as Tom says you are in the USA or Canada...
    Last edited by enb54; 03-09-2012 at 10:11 PM. Reason: Assuming

  8. #8
    Join Date
    Sep 2011
    Location
    Maryland
    Posts
    35
    Is it possible that the last element you turn on is in series with one of the other 4? Given the info you provided I figure that the first four elements are about 17 ohms creating a current of about 7 amps each. The last element is about 29 ohms drawing 5 amps. All together the total draw is 33-34 amps. If you series the last element with one of the others it will be about 46 ohms and draw only 2.6 amps. So if you add the first three and then the last one (in series) it adds up to 24 amps.
    This senario would mean one of the first four elements would be dimmer then the other three.

    When I run into this sort of problem I draw it out on paper and the problem will usually be evident.

  9. #9
    Join Date
    Aug 2006
    Location
    Colorado
    Posts
    288

    That would make sense but..

    That would make sense, but given that it is 3 hatco lamps, 2 of them are really long, 2 elements each, and have a control at each end, and the short one is just that. The amps were fine as I turned on each of the long ones, then the amps dropped when I turned on the short one. I'm going to draw out the possibilities like Tom R. said to do.

  10. #10
    Join Date
    Sep 2011
    Location
    Maryland
    Posts
    35
    Yeah, I was just thinking that unless you have been the only one with their hands in it you never know what you might have. And since you mentioned multiple breakers and contractors almost anything is possible.

    The Lang MM36 cheese melter has a series-parallel arrangement that will make your head spin but it works. Just not very conventional.

    Good luck. Let us know what you find.

  11. #11
    Join Date
    Aug 2006
    Location
    Colorado
    Posts
    288

    So I found the book explanation, but

    I was looking through the 'Wiring Simplified' book and I found what I needed.

    A 120 volt 3-wire circuit will use a common neutral, and one of each leg off of the 240. If the amp draws are equal, then the effective amp draw on the common neutral will be zero. So that makes sense, And somehow, one of those lamps is getting power from the other side of the 240, even though all the breakers are on the same side of the box.......

  12. #12
    Join Date
    Apr 2010
    Location
    Red Deer, Alberta
    Posts
    653
    Quote Originally Posted by tuba View Post
    I was looking through the 'Wiring Simplified' book and I found what I needed.

    A 120 volt 3-wire circuit will use a common neutral, and one of each leg off of the 240. If the amp draws are equal, then the effective amp draw on the common neutral will be zero. So that makes sense, And somehow, one of those lamps is getting power from the other side of the 240, even though all the breakers are on the same side of the box.......
    Generally, every alternating breaker (vertically) is a different phase, while horizontally, they are in phase. Therefore, your breakers to be all the same phase should be fed from every 2nd one vertically. That's generally why the 240V feeds use a double thickness breaker. Use the method of checking with your voltmeter across adjacent vertical breakers, you'll see the difference right away.

    Of course, that's assuming that the wiring is normal...

  13. #13
    Join Date
    Jul 2002
    Location
    SE Texas
    Posts
    435
    Quote Originally Posted by tuba View Post
    And somehow, one of those lamps is getting power from the other side of the 240, even though all the breakers are on the same side of the box.......
    As has been mentioned more than once, that doesn't necessarily mean the breakers feeding the elements are on the same phase and that doesn't necessarily mean the breakers feeding the elements are on opposite phases. That would depend on which bus bar they're installed. If they're "on the same side of the box" and they're stacked vertically, one on top of the other, each breaker would be on a different phase. If they're stacked vertically with each breaker in every other slot, each breaker would be on the same phase. I hope that makes sense to you.

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