Table 310.104(A)

[CodeCheck]

Sorry if this is too basic a question. I'm having difficulty finding an explanation for why some conductors are rated at lower temperatures in wet locations than they are in dry locations. Is the reasoning that water trapped around the conductor could reach a higher temperature more readily than air, and then reduce the rate of heat dissipation from the conductor?

Thank you

Douglas Hansen

 

[GoldDigger]

... some conductors are rated at lower temperatures in wet locations than they are in dry locations.

Example please?

 

[augie47]

Example please?

Don't know the answer to his question, but an example would be THHW or XHHW

 

[GoldDigger]

Don't know the answer to his question, but an example would be THHW or XHHW

But in that case, they are not the same conductor and insulation unless the wire is dual-rated. The limits on the wire are set by the overall standard for that type, and may be more relaxed for a particular environment with the idea that finding an insulation that is both water resistant and high temperature may be difficult. The fact that some manufacturers are able to find an insulation formulation that meets both type requirements does not automatically change the rating requirements imposed by the standard for that type.


CodeCheck should be aware, as has been frequently mentioned here, that it is the insulation that is being temperature rated, and not the conductor part of the wire.

 

[Dennis Alwon]

Ex. Xhhw is rated 90C in dry and damp locations but 75C in wet locations

I do not know why either

 

[CodeCheck]

Ex. Xhhw is rated 90C in dry and damp locations but 75C in wet locations

I do not know why either

A change in 2008 to 300.9 categorizes the interior of above-ground raceways as a wet location whenever the raceway is in a wet location, regardless of conductor arrangement or types of fittings. That reduces the allowable ampacity of some installations.

My question isn't so much what the code is saying, but why. What is the property of water as compared to air that causes the same conductor to have a lower rating in a wet location than in a dry one?

Douglas Hansen

 

[GoldDigger]

My question isn't so much what the code is saying, but why. What is the property of water as compared to air that causes the same conductor to have a lower rating in a wet location than in a dry one?

One possibility is that the thermoplastic insulation becomes more permeable to and absorbs moisture as the temperature increases and therefore cannot maintain its resistance value and voltage limit if exposed to moisture while hot.
Another less likely possibility is that exposure to moisture while hot could cause chemical changes in the insulation.

And finally, number three is that for some obscure reason UL did not want to add a test to the higher temperature when wet to the specification.

 

[MarineTech]

A change in 2008 to 300.9 categorizes the interior of above-ground raceways as a wet location whenever the raceway is in a wet location, regardless of conductor arrangement or types of fittings. That reduces the allowable ampacity of some installations.

My question isn't so much what the code is saying, but why. What is the property of water as compared to air that causes the same conductor to have a lower rating in a wet location than in a dry one?

Douglas Hansen

In my experience, the limit on the exposure to water effecting derate, is a function of the insulation in constant contact with water over time. However, I cannot find my report source at this time.

What I could find is:

In contrast to electrical treeing, water treeing is the deterioration of a solid
dielectric material which is simultaneously exposed to moisture and an electric
field. It is a significant factor in determining the useful life of buried power
cables. Water trees initiate from sites of high electrical stress such as rough
interfaces, protruding conductive points, voids, or imbedded contaminants but at
a lower field than that required for electrical trees. In contrast to electrical
trees, water trees are characterized by: (a) the presence of water is essential
for their growth; (b) they can grow for years before reaching a size where they
may contribute to a breakdown; and (c) although slow growing they are initiated
and grow in much lower electrical fields than those required for the development
of electrical trees.

Whenever I have a question about insulation properties I contact the wire manufacturer, which could refer the question to the plastic manufacturer for chemical makeup and process mix properties.

 

[delectric123]

The 14 or 12 AWG wire i have is THHN/THWN-2 so i get the 90degC rating plus it can be used for wet location.

 

[jumper]

The 14 or 12 AWG wire i have is THHN/THWN-2 so i get the 90degC rating plus it can be used for wet location.

Note that 90C rating is for derating and correction factors and your final ampacity for OCPD selection cannot exceed 75C, or 60C in some cases. 110.14(C)

For example: A #8 THHN/THWN-2 has an ampacity of 55A yet you would be limited to a 50A in most cases. You can use 55A for derating if needed.

 

[delectric123]

thats right, all OCPDs i know are rated for only 75degC. for like 12 AWG with a 90degC rating of 30amps you can derate considerably and still get a good amperage out of it.

 

[Dennis Alwon]

A change in 2008 to 300.9 categorizes the interior of above-ground raceways as a wet location whenever the raceway is in a wet location, regardless of conductor arrangement or types of fittings. That reduces the allowable ampacity of some installations.

My question isn't so much what the code is saying, but why. What is the property of water as compared to air that causes the same conductor to have a lower rating in a wet location than in a dry one?

Douglas Hansen

I realized what you were asking but I was responding to another post--- I stated I did not know why wet location was rated 75C and dry 90C.

 

[George Stolz]

One possibility is that the thermoplastic insulation becomes more permeable to and absorbs moisture as the temperature increases and therefore cannot maintain its resistance value and voltage limit if exposed to moisture while hot.
Another less likely possibility is that exposure to moisture while hot could cause chemical changes in the insulation.

And finally, number three is that for some obscure reason UL did not want to add a test to the higher temperature when wet to the specification.

So you're saying you don't know either? Same here.

 

[GoldDigger]

So you're saying you don't know either? Same here.

Yup. But happy to speculate.
If I can convince myself that there might be a reason, I don't worry so much about it as when it just seems contradictory.