Question for the Electrical Panel Builders

edited September 2016 in General

I have built a few heating element control panels for myself over the years using ssr, and ohm control pots and have had no troubles with them. I recently decided to build a new panel with a auberin dspr210 as the head of the panel. All my other builds were set up to just run one element. I decided i wanted to try and run 2 elements off of one panel. So i installed 2 auberin 25 amp ssr's i fed the box with 8 awg cable in and ran 10 awg cable out to each heating element. i have din mount breakers inside the box and built everything to be as safe as i possibly could. I got two runs out of it with no issues. The last time i used it i had a over heating issue and melted down a wire. I repaired it and started monitoring things very closely. Just a little bit more information before i get into my questions.
I am running 2 4500 watt low watt density elements off of the control panel. After i started watching things more closely i noticed the temp of the wires inside the panel are running around 115 degrees F, That is higher then i would like to see

is that to high to be safe though? What should the temp be?

When i was building the panel my thoughts were as follows 4500 watts/ 240 volts =s 18.75 amps if my understanding is correct so that should be low enough for 10 awg wire out to the elements and should also be under and 37.5 amps combined for the 8 awg main power line. (and yes i understand it is just barely under)

I did some more checking with a volt meter and my main power line to the panel runs right around 230 volts so by doing the math that comes out to 19.56 amps 4500/230 =19.56 amps would that be correct? should still be just under the 40 amp max rating that i see by google searching cable ratings.

So then i thought well if i back the power down with the auberin 210 to 60 percent that should help. But under use the wires are still around 115 degrees. So i started to wonder if simply just backing the power out put down while effectively cutting the power to the element if that might be just creating that much more heat and energy for the ssr to try and dissipate and why it is still running hot? What is my best bet for fixing this issue? backing the heating elements down to 3500 watt each? I obviously do not wish to have a fire starter prone unit on my hands. thanks in advance for any suggestions


  • 10AWG = 5 mm2 in our way which should be good for 30 amps but our norm for that would be 6 mm 2.

    8AWG is 8 mm2 which should handle 40 amps again more normal for us would probably be 10 mm2. It all depends on the run and the layout , how many cores etc.

    It appears close but do you have a fan in the enclosure to dissipate all the heat. You want it to work at any power level and not be limited by fool wire size - which is readily changeable. I will wait to see other responses who are more familiar with the application. My gut feeling on the incoming is that it is a bit close.

  • edited September 2016

    Somewhat related, but my 2c. If your SSR is in your box, with no external heat sink, it would be relatively easy to hit 115F in the enclosure.

    That said, heat dissipation of a typical SSR device is pretty low.

    I think the voltage drop is something like 1.5v, as typical.

    So for a typical 5-5.5kw heating element (about 21 amps at 240v), we're only talking about 31.5w of heat being generated in a period of time.

    SSR does not function like a resistive control, which needs to dissipate more heat at lower output power, they actually operate in the opposite way, only when high current is flowing do they generate highest heat output.

    So, for example, at 25% output power in the example above, we're really only talking about 8 watts of heat generation.

    These things need to be heat sinked because they can't dissipate the heat on their own, and we know that operating them at high temperatures is going to reduce their life.

    Ideally, a simple external mount passive heatsink is all you really need. Auber sells a couple different versions and they are pretty cheap. They are also much simpler - from a circuit design perspective (no fans, no low voltage), and they keep your enclosure sealed - which is nice if you are using something like an IP66 box, or something generally sealed to splashes, water, etc. Easier to install and build the box too, who cares about mounting fans and the like, just cut a hole and bolt it in.

    Plenty quiet too.

    Just make sure your heat sink fins are up/down - vertical - to maximize passive air flow.

  • edited September 2016

    Back to topic.

    To me, melt down of wire means a short, especially at those gauges. Why didn't your breakers trip long before this? The difference between 115F and meltdown are very large, not even in the same universe.

    What are the breakers you are using? Do you have model numbers/brands? Can you post?

    But, remember, at these amperages, even the minuscule resistance of wire will generate heat. Go run your vacuum cleaner for an hour, and feel the cord. It's going to be hot. Now, run it through a skinny extension cord for a couple minutes, even hotter.

    What kind of wire are you using anyway? Something like THW - which would be typical here, would have absolutely no problem running at 75C/167F all day long, anything below that would be well within the guidelines of THW (PVC, not rubber).

    Even with a 25% derating, your wire gauges are fine. At an ambient temp of say 30C/86F, it wouldn't be to of the realm of normal for a conductor carrying that amperage to be around 115F. BUT THIS IS VERY DIFFERENT FROM MELTING A CONDUCTOR.

  • edited September 2016

    If you want to play around, here is a good calculator.

    This takes into account ambient temperature and wire type.

  • edited September 2016

    To answer your questions grim i am using the auberins box with the largest heat sink 60 amp, the auberins breakers and for the wire i am using coleman cable seoprene 105c seo p-241-3 msha and for the wire inside the box i just stripped the out side jacket off the coleman cable and used the individual wires threw out the inside of the box.
    There is no fan inside the box, i do run a 4 inch fan outside the box across the external heatsink.

    I do not want to sit here and tell you there absolutely was not a short, But i feel as if there was not a short but how can i say for sure, but yes i would of thought if there was a actual short that a breaker would of tripped. so that leads me to think a wire was not shorted.

    Also just to point out. I did not say the wire melted at 115 degrees. I said that 115 degrees is the temp it is running at now after being repaired and ran again. I DID NOT get a temp reading at the time of the melting, I was in the room doing other things and could smell something was not right like melting or burning wire, so i ran over and turned everything off and then was to busy trying to figure out what was going wrong to think about taking any temps.

    the coleman cable has a rating of 105c on the outside jacket but i do not know if that applies once the jacket is removed and your simply just using the wires with their standard coating on them.

    GD50- you are correct i do wish to be able to use this at 100% when i so chose to that is why i am trying sort out my issues, I only did that to see if it would help lower the temps, I do not recall right now but i believe it was running about the same temp at 60% as it was at 100% which if i am understanding what grim is saying that may not make sense so i may have to run it again and watch it to be sure.

  • Grim I keep getting a error when trying to use the link you posted to the calculator. keeps going to a 404 message saying page not found. If you feel 115 degrees is not unreasonable maybe i will just keep running things keeping a very close watch on it until i feel comfortable.

  • You are using the 50 amp breaker?

    I'm saying a conductor temp of 115f in an enclosure with an ambient of 80f wouldn't be out of the ordinary at these amperages.

    Also saying that a conductor temp of 115f has nothing to do with melting off insulation, and that the conductor temp is not evidence of a problem.

    With the heat sink external, this further leads me to believe the issue is elsewhere, as the ambient temp in the enclosure can't possibly be getting too high.

    Where I am having real problems is why the breaker did not trip. You would have had to pull some serious amperage to melt the sheath.

  • And two 25a? One on each heater circuit?

  • Yes that is correct, 50 amp plus 25 amp for each heating element, Maybe the 50 amp is to large? i had the issue between the 50 amp and the 25 amps so basically the issue was on the incoming side which made me wonder if 2 4500 watt elements was just slightly to much for the system. Maybe a defect in the coating of the wire making it unable to handle the temp it is supposed to be able to? I really do not know just grasping at straws there.

  • I will add, I was not running the 4 inch fan across the external heat sink at the time of the melt down, I started doing that after the problem happened. I had thought the external heat sink would be ok on its own as its rated amperage was well over what i thought i would be drawing. But there is a very large difference in the temp of the box between using the fan across the heat sink and not using the fan. I do not have any recorded temps before using the fan

  • The high ambient still seems like a stretch to me.

    There is simply not that narrow of a gap between conductor rated temperatures and failure of the jacket.

    I believe the jacket rated temps are based on 500 hours at stated temp, and even then we are talking about embrittlement and not melt.

    The interior of that box would have been absolutely blazing, and your enclosure would have melted before the wire jacket.

  • Or if metal, you would have been burned.

  • edited September 2016

    Go to Home Depot and pickup a couple feet of XHHW in the right colors. It's cheap, they sell by the foot. Rewire with that.

    Check your circuits to make sure those breakers will trip. Or, safely make them trip/manually trip them and check for voltage.

  • I believe everything you are saying it makes sense, I am no means very experienced with this sort of thing, Just willing to do the work involved in the building. I believe i did everything right but again not a expert. I have never ran temp checks on the the wires of any other box i have built. Maybe they run as high as 115 degrees as well. My old boxes were just plastic box with internal heatsink and vent holes drilled in the side with a fan. I only started monitoring things after having this melt happen. Maybe i am just nervous and panicked now at the thought of it happening again or worse it catching fire. I just wish i knew if it was a short or not and if it was why the breaker did not trip off?
    I am going to be watching things very closely for a while

  • I did not see your last two posts before i posted my last one, How do i check the circuits to make sure they trip?

  • edited September 2016

    Flip the breakers off and use a voltmeter to be sure you have no voltage with them off, one circuit at a time.

    Or, dead short your heater wires, the breaker should trip immediately upon turning on. This is dangerous though. If you have a problem, you'll melt down.

  • Yeah the dead short is what i was afraid you might of meant, I have accidentally caused a short a couple of times in the past. Not a big fan of recreating that on purpose its scary to have the arc and breaker blow right in your face. I will test with the volt meter and check and see that the power indeed does shut off. Thanks

  • edited September 2016

    @chevelle496 said: Grim I keep getting a error when trying to use the link you posted to the calculator. keeps going to a 404 message saying page not found.

    Fixed (the last character was missing).

    NEC Table 310.15(B)(16) Conductor Size, OCPD, Voltage Drop, and Equipment Grounding Conductor Size Calculator @

    Your Place to be >>> <<< Home of the StillDragon® Community Forum

  • also, use THNN or similar wire rated for 90C temp(dry), do not use 'primary wire' from the local auto store, that stuff is absolute, 100% crap....

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