conductor ampacities

[msf]

Table 310.13 lists THHW and XHHW as 90C for dry locations and 75C for wet locations.

When de-rating conductor ampacity for conduit fill and/or temperature,is it permissible to use the 90C rating in a wet application and apply the de-rating factors as long as the 75C rating is not exceeded? OR must you use the 75C rating and apply the de-rating factors for selecting the proper conductor.

 

[sparky_magoo]

Re: conductor ampacities

It is my understanding that use must use the 75 degree rating and then apply any additional deratings( i.e. conduit fill) on top of that.

 

[charlie b]

Re: conductor ampacities

It's a good question, but I have to agree with sparky. The entire issue revolves around protection of the conductor's insulation system. Any reasons for derating would be based on limiting the degradation of the insulation. If the insulation is only good for 75C in a wet environment, then that must be your starting point for applying other derating factors.

 

[bphgravity]

Re: conductor ampacities

When an insulation is good for 90? in dry location, what happens to it that makes it only good for 75? when in a wet location?

 

[charlie b]

Re: conductor ampacities

I do not know. But I'm not one to fear guessing.

 

[steve66]

Re: conductor ampacities

Brian;

You stole my question I was wondering the exact same thing. It seems like the water would conduct the heat away from the wire faster, allowing a higher ampacity.

Steve

 

[jim dungar]

Re: conductor ampacities

Water would only improve the heat transfer if it was flowing. Once all the water reached the same temperature it would actually make it harder for the conductors to cool down.

 

[sparky_magoo]

Re: conductor ampacities

I always assumed that contact with water somehow breaks down the insulation, hence the need to derate the conductors.