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Ambinet Air Temperature for Conductor Derating

Ambinet Air Temperature for Conductor Derating

Ambinet Air Temperature for Conductor Derating

(OP)
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

RE: Ambinet Air Temperature for Conductor Derating

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.

RE: Ambinet Air Temperature for Conductor Derating

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

RE: Ambinet Air Temperature for Conductor Derating

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.

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