IEC 60364-5-52 - Ampacity vs Derating factors

[NickParker]

IEC 60364-5-52 , tables b. 52.2 to b.52.13 indicates the current carrying capacities of the cable ( in air & ground for different insulations).

My question is, why the ampacities are different for air & ground in those tables, although there are separate tables for derating factors (correction factors) which would be considered.

Isn't the ampacity should be same on which the derating factors to be applied?

 

[cuky2000]

The tables of current-carrying capacities of cable are developed for specific conditions.
For instance, for underground cable installation, cable ampacity is developed for soil thermal resistivities of 2,5 K·m/W. The correction factors for this value is 1 but, different adjustment is required otherwise

 

[Mbrooke]

Heat dissipation. Wire in open air will give up far more heat then wire in the ground.

 

[che12345]

Dear Mr. NickParker

" ... IEC 60364-5-52 , tables b. 52.2 to b.52.13 indicates the current carrying capacities of the cable ...My question is, why the ampacities are different for air & ground in those tables, although there are separate tables for derating factors ...."
As per IEC B.52.2.1 :...the current carrying capacities...the following reference temperatures:
- ... cable in air 30deg C
- ...buried cables in ground 20deg C ...with soil thermal resistivity of 2.5 K.m/W...
It should be noted that they are based on:
a) different ambient temperatures,
b) thermal resistivity of air at ambient temperature of 30deg C deffer from that of the soil with ambient temperature of 20deg C with thermal resistivity of 2.5 K.m/W.

Che Kuan Yau (Singapore)

 

[waross]

The ampacity tables are based on the temperature rise of the cable due to I[sup]2[/sup]R losses. This in turn depends on the ability of the cable to reject the heat. Hence different ampacities for different installations. ec: Free air, in raceway, direct buried, buried in ducts, higher than normal ambient temperatures etc..

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

First, the underground cables are in conduit but cables in air are not.
Second, the heat is evacuated in three way: conduction, convection and radiation.
On underground run the heat is evacuated from the conductor towards the ambient by conduction mainly-except from outer surface of the cable to the conduit. The thermal resistances in thermal units are: 1 from conductor to insulation,0.09 from insulated core to jacket 0.56 from cable to conduit and 1.3 from conduit to earth at 20oC ambient by conduction.
On the cable exposed run the thermal resistances are : 1 from conductor to insulation,0.09 from insulated core to jacket, no conduit and 0.5 from cable to air [but the air temperature is 30oC]. From the outer surface of the cable to air the heat is evacuated by convection and radiation.