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Thread: heat vs. resistance

  1. #1
    Join Date
    Jan 2004
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    heat vs. resistance

    1. heat increases resistance of copper wire.
    2. we derate wire 125% for continuous use.
    3. If the resistance goes up the amperage goes down
    at a given voltage.

    Is the derating to keep the temperature down on the connections ie. 75 degree C.?

    Is the resistance so small that it does not come into play in this situation.

    I have amped out resistive heaters that were supposed to draw 90 amps at 240 volt, and found them drawing 100 amps when installed to #2 aluminum.When the proper wire was installed the amp draw went down and the excessive heat went away. At the time I thought that the heat was the culprit that increased the amp draw.
    I*E=P
    90a * 240v = 2160 watts.

    R= E/I = 2.25 OHMS

    If the resistance changes in the wire, that should cause a voltage drop ( wire in series with load), would not the amperage also go down? Would that also cause the watts/heat to go down also.

    Or is it just that over time the heat buildup will degrage the insulation value?

    I am prblbaly missing something very simple and elementary here, but its been buggin me for a while now, and this seems to be the place to get great answers. thanks for any help.

  2. #2
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    Re: heat vs. resistance

    With that aluminum wire adding resistance in series you should have read less amps.Was the voltage at service the same on both checks?Was this a digital reading ?
    Tampa Florida

  3. #3
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    Re: heat vs. resistance

    This was a time ago and it was analog. So maybe the my problem was wrong readings?
    SO in the end it is the heat buildup that is the factor not the amp draw particularly, current draw does produce some heat in any circuit.

  4. #4
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    Re: heat vs. resistance

    Everything has resistance to electric,the amount is what determines if it is an insulator or conductor.Any time current goes thru resistance we have a voltage drop.That drop (e) times the amps = the watts we lost in unwanted (usually)heat.The idea behind conductor size is to keep this safe and low.This voltage drop should stay under 5% but is not a rule.
    Tampa Florida

  5. #5
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    Re: heat vs. resistance

    jimwalker so waht you are saying is that the continuous use rating is for voltage drop?

  6. #6
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    Re: heat vs. resistance

    No we could live with the voltage drop.The heat from that voltage drop is the real problem.Long term heat damages insulation.
    Tampa Florida

  7. #7
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    Re: heat vs. resistance

    I'm not sure what kind of element your heater had, but here is my theory:

    1. The heater has a heating element like a toaster. It's resistance would be very low when it is cold, and the resistance would go up as it heats up.

    2. Supply the heater with wire that is too small, and there is a voltage drop across the wire. This allows less voltage across the heater.

    3. The smaller voltage across the heater delivers less power to the heater. This allows the heater to stay cooler and have less resistance.

    4. Less resistance at the heater allows more current to flow.

    Thus, it may be possible for a smaller supply wire to cause the current draw to go up. However, the increased current causes the opposite effect (more current delivers more power to the heater, increasing it's resistance). A little math could verify if the current would actually go up or down for supply wire that is too small.

    STeve

  8. #8
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    Re: heat vs. resistance

    Steve your on the right track.A 10 kw heater is rated as 10 kw @ 240 volts across the element (usually a coil)it has a lower ohms reading when cold than when it is hot.Should we supply this with a wire that say for a 20 foot run has a 2 volt drop ,then we actually only have 138 volts.It will not heat the coil as high as if we had 240.Without setting this up in a lab ,i believe we will have something slightly less than 10 kw.Take same heater and lets make the run 100 feet.Now if that was #6 aluminum we can expect a large voltage drop,maybe as high as 10 volts.With 230 volts we should be under 10 kw by a noticeable amount.If we plotted this on papper and as the wire got longer and longer at some point the voltage at the heater might only be say 10 volts (the coils will be cool)i don't believe as we add resistance that the kw will go up.At the point where we have 10 volts on the heater coils and 130 volts on the wire i think we would be far under 10 kw.This difference in coil readings from cold to hot is not that great.In electronics we do have something called a thermister.Its intended purpose is as it gets hot it goes from something like 3 or 4 ohms to maybe 200 ohms.This coil does not have these properties to that exstent.Using your theory if we supply the heater with 120 volts we would get more heat.In reality (assuming we had a 120 blower( not likely)
    we would not likely feel much heat.
    If anyone has any curve results from an actual test please post them.
    This is only my opinion based on theory.
    Without a sheet on that heater would be hard to be exact.
    Will assume at 0 volts across coil we agree there is no KW and @240 we agree it has 10 kw.
    This would make a strange chart if at some voltage between 0 and 240 that the KW goes up past the 10 KW.
    Tampa Florida

  9. #9
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    Re: heat vs. resistance

    caosesvida
    All of this can be computed using known formulas and constants.
    A good learning reference can be found at
    www.sea.siemens.com/step/
    steve

  10. #10
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    Re: heat vs. resistance

    thanks this is getting very interesting.

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