Errors in C to F delta conversions

I got a headache figuring this out. I measured a breaker at 161 F. Ambient 77 F. OK, so I subtracted, getting 84 F. Convert 84 F to C and get 29 C delta. OK, within 40C delta, right? And I have seen here, in this forum where it has been said if you need a 40C delta that’s 104F. Plugging into a standard C to F converter, available on a google search with a click of the mouse, shows that to be true. Right? Wrong! I discovered this when cross checking. I looked at my previous conversions of F to C of the absolutes 161F and 77F which are 71C and 25C respectively. for a 46C delta. What the %*(^ ? Worked it all around geting the same results until I figured that the 32F (or 17.7C) offset in the conversion was getting in the way. (29C + 17 = the 46 which was right)
When dealing with deltas, only the scaling factor applies. (.555 F to C and 1.8 C to F) No offset is used. And the oft stated 40C delta being 104F is likewise wrong. (it is 72F)
A handy converter for deltas may be found at:

As you discovered, you can’t use temperature conversions to convert delta-T between C & F.

The quick way to convert a delta C to a delta F in your head is to multiple by 2 and reduce the result by 10%. The net result is the same as applying the 1.8 conversion multiplier. I don’t have a simple method to go the other way. For F to C delta conversion you divide by 1.8

You need a new converter 32 f = 0c

Why would you need a converter. Obviously anything heating up to 161 deg. is an issue. Or would you prefer to wait till it got a little warmer and watch it start to melt.

You are indeed correct Jim.

Mr. Graham is also correct.

The measured temperature of 32 degrees F is equivalent to 0 degrees C
However the calculated difference of 32 F degrees between two temperature reference points is equivalent to a difference of 17.8 C degrees between those same reference points.

This important fundamental knowledge and more is taught in most quality introductory courses provided by competent providers and instructors such as Infraspection Institute.

You made the same mistake. When dealing with deltas; a difference of 32F is obviously not a difference of 0C. Try using the delta calculator I provided;

Yes I know it is to hot, but not melt down hot (sheesh) in fact only 6C over Square D’s 40C over ambient maximum for this particular commercial breaker.


In the absence of a thermal image and load readings for the subject breaker, it is hard to infer any cause for the measured temperature. Assuming that your measured temperature is correct, the subject breaker is too hot for the following reasons.

The maximum operating temperature for a molded case breaker operating at 100% load is 60°C (140°F). With a measured temperature of 161°F (71.6°C) the breaker that you reference is out of spec and should be investigated by a qualified electrician as to cause. It is important to understand that the subject breaker’s temperature could increase dramatically were the cause of heat due to a loose connection and/or the breaker was lightly loaded at the time of your inspection.

When applying temperature limits, some will attempt to qualify the severity of exceptions by stating that they are ‘not that far out of spec’ or that they are ‘not that hot’. While it’s unlikely that your breaker is going to melt down at the temperatures you reported, you have made a significant find and it should not be overlooked or trivialized by the untrained or uninformed.

Temperature limits for electrical and mechanical components along with ways for assessing repair priorities are covered in depth in the Standard for Infrared Inspection of Electrical Systems and Rotating Equipment. This Standard and others like it are available through the Infraspection Online Store.

Hope this helps.

I was going to post up the same thing as far as the load goes.

What % is the load at? Even at 100% your breaker is over temp, period. If the load was only 50%, well now you have a really big issue.

Also don’t forget to add on the error factor of the camera itself. That is another 2% possibility.

Jason Kaylor
AC Tool Supply
Net Zero Tools
AZ Infrared Services

I just read the post wrong, I known better than to reply to this things in the feild on my tablet. :slight_smile:

Yes, I agree and have notified the customer that this is a hot breaker and should be addressed.