Derating Cables in Water

[marko333]

Hello.

Do cables that sit in water have to be derated?

The situation I am refering too, is where i have a submains cable that is installed underground in heavy duty conduit. The conduit is filled up with water completely and the water is not draining anywhere.
Should the cable current rating be derated?

Thank you.

 

[alehman]

It depends. For low voltage, dual rated insulation the answer is yes. THHN/THWN is rated for 90C in air and 75C in water. For medium voltage cable, I don't think the ratings change, but you need to calculate the temperature rise based on the thermal conductivity of the surrounding material which is effected by water content.

 

[jghrist]

As alehman stated, if the cable has dual rated insulation, you need to limit the temperature to 75°C in wet locations. The easy way is to derate according to NEC ampacity tables from 90°C to 75°C. In reality, the fact that the conduit is filled with water will reduce the temperature for a given current because the thermal resistance of water is much lower than that of air. To take advantage of this, however, you would have to use ampacities calculated under engineering supervision using Neher-McGrath's methods and the general formula shown in NEC 315.15(C). This is not an easy task. For medium voltage cable which isn't dual rated, you would normally ignore the cooling effect of the water in the conduit. You can't depend on it being there at all times anyway.