Message Board Thread - "Temperature Calculations"

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Temperature Calculations Dave 2/9/2004
I seem to recall a formula to estimate the projected temp. increase of a connection running at a fraction of it's rated capacity if the capacity is increased "X". . Eg. ... fuse clips running at 42 deg.C at 24% of rated current. What would be the projected temp. increase if the current draw was doubled, trippled... Any guidence appreciated. Dave
Re:Temperature Calculations Gary Orlove 2/10/2004
This can be a complicated subject because it depends on many variables, notably the physical characteristics of the part and the environment. We know that load increases the temperature, but calculating exactly how much is the problem.

Please refer to this paper for more details and formulas for the parts we tested.
"The relationship between current load and temperature for quasi-steady state and transient conditions."


Gary Orlove
Infrared Training Center
Re:Temperature Calculations Bernie Lyon 2/10/2004
George Baird published a paper in Thermosense, 1987: "The effect of circuit loading on electrical problem temperature." For the following loads, he measured these temperature rises:
10% load 4 Celsius
50% load 20.6 Celsius
100% load 50 Celsius

Thomas Perch-Nielsen and J.C. Sorensen published a Thermosense paper in 1994: "Guidelines to thermographic inspection of electrical installations." Here are some of their results, with respect to current and temperature rise:
Current 100A 12 Celsius
Current 150A 25 Celsius
Current 200A 40 Celsius
Current 250A 65 Celsius

Norman Shackman wrote an article for the April 2002 edition of Maintenance Technology Magazine: "Using IR Data to Estimate Circuit Upgrades." This method uses logarithmic graph paper to predict the temperature rise in Celsius degrees, by drawing a line at an angle of 28 degrees through a known current and temperature rise coordinate. I tried to find this on line, but it was not available at their website. (

I published a paper with Gary Orlove and Donna Peters, in Thermosense, 2000: "The relationship between current load and temperature
for quasi-steady state and transient conditions."
Based upon my experimental results, I came up with the equations below to predict temperature rises. To predict the delta temperature rise for an increased load you would do the following:
Measure the current and temperature rise for a component that has been operating at that current for an hour. Divide the full load current by the measured current. Raise that value to the X power, where X is a value that ranges from 1.5 to 1.8. (NOTE: In the two extremes below the exponents do not appear as superscripts!) This will give you an estimation of the new temperature rise.

Min. temp. rise at full load (C°) =
(full load current / measured current)1.5
x measured temp rise in C°

Max. temp. rise at full load (C°) =
(full load current / measured current)1.8
x measured temp rise in C°

Dr. Bob Madding has produced a program that allows you to enter in load and temperature rise values to predict new temperature rises with increasing loads. This is a Beta copy and it is available on the ITC Utility CD

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