Conductor Temperature Ratings

[Dave Loucks]

Conductors and bus generate heat when current flows through them.  How much heat?  That depends on the conductor size, the application and many other factors.

For electrical equipment, such as switchgear, switchboards, motor control centers and panel boards, the manufacturer has received certification from UL that at rated current the bus, terminals, connection points and lugs are below the maximum temperature rating permitted by UL.

But what if you want to operate the equipment at an even cooler temperature than this maximum allowable temperature?  How much would you need to reduce the current to reduce the temperature?

That's a tricky question because the temperature of equipment likely isn't proportional to the power dissipated.  Cooling plays a factor and that cooling too is non-linear with temperature.

Can we at least estimate changes in temperature based on changes in current flow?

One place to start as a first pass approximation is IEEE 399-1997 (Brown Book) where in section 13.4.1, equation 13-5, an "adjustment factor" is calculated for when you want to change either the conductor temperature rating or the ambient temperature:

Where:

Tc		Conductor rated temperature in oC at which the base ampacity is specified
Tc'		Maximum allowed conductor temperature in oC
Ta		Ambient temperature in oC at which the base ampacity is specified
Ta'		Other ambient temperature oC at which the conductor will be applied

I'm assuming copper conductors here.  You'd use equation 13-6 if you wanted aluminum conductors (just change 234.5 to 228.1 both times).

In our example:
Tc            = 105^oC (40 + 65)
Tc'          = 70^oC (40 + 30)
Ta           = 40
Ta'          = 40 (no change)


So, this would imply that if you ran the conductor at no higher than 72% of its rated capacity (or better, 71%) the conductor temperature would not rise more than 30^oC above a 40^oC ambient.

Consider this only back of the envelope calculations since it could be better (there is might be better air circulation) or worse (you might have harmonics which will induce eddy current heating)

Here's a calculator that automates this math:


You can click on the image above to download, or select "conductor_temp_rerating.xlsx" from below.