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How hot is too hot for a breaker


Neal Lewis
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100 amp service panel from the 1960's. 3 or 4 window A/c units were running constantly today: no other considerable draw.

The main Square D breaker was 140 degrees and located in a basement that was probably 80 degrees today. No other outward signs of overheating. Is there some type of allowable limit for heat buildup? I did recommend a sparky take a look.

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100 amp service panel from the 1960's. 3 or 4 window A/c units were running constantly today: no other considerable draw.

The main Square D breaker was 140 degrees and located in a basement that was probably 80 degrees today. No other outward signs of overheating. Is there some type of allowable limit for heat buildup? I did recommend a sparky take a look.

That's a good question. I don't know the answer.

I'd suggest dropping a line to Square D. They're very good about answering technical questions about their products.

- Jim Katen, Oregon

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Found this:

Underwriter Laboratories Inc. (UL) standards require that molded-case

circuit breakers rated at 125 amperes or less be marked with the conductor insulation-temperature rating. Table 1 contains a listing of wire temperature ratings for Square D circuit breakers. The wire temperature rating is determined by testing the circuit breaker under full-load current with conductors sized for the appropriate temperature rating—60°C or 75°C. The temperature rise at the circuit breaker terminals must not exceed 50°C above ambient per UL Standard 489.

50C = 122F + 80F = 202F. That's seems awfully hot.

The complete PDF is here:

http://ecatalog.squared.com/pubs/Circui ... DB0103.pdf

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I don't follow you Marc. How did you end up with 50C = 90F? Me thinks you forgot to add 32°.

The 32 is added when you want the Farenheit equivalent of a temperature that is expressed on the Centigrade scale. In this case, it's not a temperature value that we are dealing with, it's a change in temperature.

Marc

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The 32 is added when you want the Farenheit equivalent of a temperature that is expressed on the Centigrade scale. In this case, it's not a temperature value that we are dealing with, it's a change in temperature.

Marc

Well I must be slow.

The temperature rise at the circuit breaker terminals must not exceed 50°C above ambient per UL Standard 489. The variable is ambient, not 50C per se. 50C=122F. With the above statement they could have easily said: The temperature rise at the circuit breaker terminals must not exceed 122F above ambient per UL Standard 489.

If your ambient is 80F and you shouldn't exceed 50°C above ambient doesn't that mean 50C ((°C x 1.8) + 32 = °F)= 122F + the ambient which is 80F?

I'm getting the feeling I'm wrong but I don't see where.

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Let's say that the ambient temperature is 80 degrees Farenheit. What's the equivalent temperature in Centigrade?

(80 -32) * (5/9) = 26.7 C

The temperature rise at the circuit breaker terminals must not exceed 50°C above ambient which in our case is:

26.7 + 50= 76.7 C

When we change back to Farenheit units, we get:

(76.7 * (9/5)) + 32 = 170 F

The difference between the two Farenheit temperatures is:

170 - 80 = 90 degrees.

So, a delta 50C is equivalent to a delta 90F.

Marc

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