Ambinet Air Temperature for Conductor Derating

[buddy91082]

Looking to see if the 24 hour average ambient temperature can be used for conductor derating or do I need to use the maximum ambient temperature seen at ay given time (even tho it may only be for 5 minutes as an example.

I assume if I need to use the maximum ambient temperature the thermal time constant for 600v conductors is low.

Thanks.
b

 

[bacon4life]

Maximum. The thermal time constant for bare overhead conductors is on the order of minutes per IEEE 738. I assume insulation may make the time constant a bit longer, but certainly not 24 hours.

 

[aruskuat]

You need to use correction factor of temperature based on maximum operating temperature, typically standard ambient temp is 40C for above ground and 20 C for under ground installation. You can find it in IEEE 399

 

[7anoter4]

For cable heating at limited time see EPRI-Power Plant Electrical Reference Series vol.4 ch.4.4 cable sizing.Ampacity. Evaluation of Conductor Temperature Under Transient Conditions.
or:
IEEE 242/2001 CH.9.5.2.4 Development of intermediate characteristics:
“Cable, with the thermal inertia of its own and of its surroundings, takes from 1 h to 6 h to change from initial to final temperature as the result of a current change. Consequently, overloads substantially greater than its continuous rating may be placed on a cable for this range of times.”
If the 600 V cable lays on a free air-on racks, for instance-a cable of less than 2 awg the thermal constant K is only 20 min [0.33 h] but a cable with copper conductor more than 250 MCM directly buried in ground K will be 6 hours.
By-the-way, in my opinion IEEE 399 does not specifies the maximum ambient temperature but it indicates usual limits. The following chapter states the adjustment factors if the ambient could be different from “usual":
IEEE 399/1997 ch.13.4 Ampacity adjustment factors.