Ciruit Breaker Heat Rejection Calc.

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Ssups

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I have a question about circuit breaker heat load. We are planning on putting a Square D I-line power center in an equipment shelter that will be located in the desert. Our HVAC tech asked what the heat rejection is for the breakers so that he can properly size the air conditioning system (new one on me). The breakers are 800 amp type MG main, (1) 600 amp type MGA, (1) 400 amp type MGA, and (2) 125 amp type HG. Any ideas?

Thanks!
 
Ha Ha!!

That is basically what I was thinking, but the question caught me off guard and made me stop and think about it. I guess from the HVAC tech point of view he would want to knonw the information.

If there is 200 amps passing through an 400 amp thermal magnetic breaker, it will have to heat up the trip mechanism some what.

I was just wondering if anyone had any faced this type of question before.
 
That is unusual - especially for me. I'm thinking you need to suck enough heat out to get the inside ambient down to 100F. I'd say you have a smart HVAC tech.

There is always the check with the MFG statement - and I tend to that if it matters.

However, for a first approxomation, you might consioder half load - say 400A, and a few milliohms end to end.

So I^2R = (400)^2 x 10 X 10^-3 = 1600W

Now if you are at 80% load, even for an hour or so, then
I^2R = (800 X .8)^2 X 10X10^-3 = ~4000W

carl
 
If you had an 800A breaker with 10 milliohms of resistance, then the voltage drop across the breaker at full load would be 8V. Given that the a voltage drop in millivolts is normally expected, I expect the contact resistance to be in the microohm range.

Try to get the manufacturer specifications on expected millivolt drop across the breakers. Power = voltage * current.

-Jon
 
A 800A MCCB should have about a 50 microhm contact resistance. A 100A MCCb should be about 500 microhms. Of course these v alues vary and there is no spec for contact resistance but these are typical values I see in the field on new equipment.
 
jon, zog -
You are both correct.

Now add the incomming line connection resistance, 10 bolted bus connections, breaker stabs in and out (X2)., outgoing line connection That's why I called the number, "a few milliohms end to end", and a "first order approximation".

I won't feel bad if you want to pick another number - you could even decide there were only 9 bolted bus connections instead of 10:rolleyes:

carl

edited to add "end to end"
 
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I've been selling and supporting molded case circuit breakers for over 18 years and don't have the slightest clue what heat rejection rating is.
However, regarding HVAC I did provide heating watt values in order for HVAC systems to be sized.
 
I have used heat dissipation as the term and have seen it as a watts/pole value either cataloged or provided by the manufacture's tech support. All the values I have seen are based on full load conditions. IMO the HVAC tech is going off his lingo because he isn't sizing his air unit to provide cooling, but to reject heat from the enclosure.

tony
 
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