Operating temperature of PVC conduit for X90 and X110 rated XLPE cables

[tofulover]

Hi all,

Where we use X90 and X110 rated XLPE cables and we "push" the cables to run at a higher ampacity with a higher conductor operating temperature, say we take 80deg in this instance.

If we run these cables in PVC conduits (typically buried in-ground), would these PVC conduits not be an issue considering that most PVC conduits are indicated on their data sheets to have a service temperature of around 60degC or so.

Curious to know what others think?

Cheers
tofu

 

[7anoter4]

In my opinion, it is not important if the cable outside surface in contact with the duct is 90oC but the total inner temperature of the duct which is not more then 60 oC.
For instance, a 4/0 awg copper conductor single core cable of 12.07 mm conductor diameter and 15.7 mm over insulation , according to NEC Table 310.60(C)(77) outstands 290 A at 90oC. The outside surface of a cable is 87.4oC.
The heat flux of 3 cables of 290 A each one, passing through the duct will be 52.9 W -at 90oC.
The thermal resistance between cables and duct will be about 1 K.m/W- calculated according IEC 60287-2-1-then the ttemperature
drop through the duct will be 53 oC[or K].Then the duct wall will be at 90-53=37oC [if the surrounding earth will be at 20oC].

 

[FacEngrPE]

Ignore code for a moment.
Engineering Toolbox states:

Maximum Short Time Operating Temperature
- for pipes without pressure.
PVC : 95 [sup]o[/sup]C

Vicat Softening Temperature
- is the temperature where a needle shaped penetrator sinks into a test piece a specified depth when a specified vertical load (1 kg) is applied.
PVC: 92 [sup]o[/sup]C

If the cables get hot enough, the PVC pipe may shrink onto the cables. This will make pulling the cables out of the conduit impossible. My opinion is that you are operating too close to the softening point.

 

[tofulover]

In my opinion, it is not important if the cable outside surface in contact with the duct is 90oC but the total inner temperature of the duct which is not more then 60 oC.
For instance, a 4/0 awg copper conductor single core cable of 12.07 mm conductor diameter and 15.7 mm over insulation , according to NEC Table 310.60(C)(77) outstands 290 A at 90oC. The outside surface of a cable is 87.4oC.
The heat flux of 3 cables of 290 A each one, passing through the duct will be 52.9 W -at 90oC.
The thermal resistance between cables and duct will be about 1 K.m/W- calculated according IEC 60287-2-1-then the ttemperature
drop through the duct will be 53 oC[or K].Then the duct wall will be at 90-53=37oC [if the surrounding earth will be at 20oC].

Thanks for your comments. Sounded very logical to me!