Underground duct banks

[frankelectric]

Hello:

I want to know if the THHN or THWN wire can be installed in underground duct banks. Also if the outer cover of this wire is damaged or broken; the wet proof application is loss?


Your comments will be appreciated.

Frank Rodriguez

 

[bphgravity]

Re: Underground duct banks

Underground installations of conductors or cables in raceways must be listed for use in wet locations. (300.5(D)(5)) The insulation types with a "W" have this listing.

I believe damaged or broken coverings on conductors violates Article 110.3 and should be removed and not used.

 

[roger]

Re: Underground duct banks

The outer covering of THHN and THWN has nothing to do with the water proofing or the dielectric constant of the insulation.

This information is available from most manufacturers. My information comes from the Southwire catalog, I'm sure if you go to their website you can probably find this same info.


If it is a nylon covering it is more apt to absorb moisture.

Roger

 

[Guest]

Re: Underground duct banks

THWN:
Insulation : Flame-retardant, heat, abrasion, and moisture-resistant PVC with polyamide nylon

STANDARDS:
UL 83 : Thermoplastic insulated wires and cables
TYPE THWN : 75C wet or dry

APPLICATION:
General purpose building wiring insulated with polyvinyl chloride(PVC) and covered with a tough protective sheath of nylon intended for lighting and power circuits wiring as specified in the National Electrical Code. The wire may be operated at 75C in wet locations.

 

[bphgravity]

Re: Underground duct banks

Originally posted by roger:
The outer covering of THHN and THWN has nothing to do with the water proofing or the dielectric constant of the insulation.

Do you know what its purpose is then?

 

[roger]

Re: Underground duct banks

Bryan,

Do you know what its purpose is then?

not really, except possibly to help protect the thin insulation itself, 310.13.

I remember an argument on this forum once where one poster said he was told from a manufacturers rep it was only to assist in pulling it thru a conduit.

I have a hard time believing this is the case, but who knows.

Roger

 

[bphgravity]

Re: Underground duct banks

Note 1 to Table 310.13 states: "Some insulations do not require an outer covering."

Does this mean that if an outer covering is listed, it is then required?

 

[don_resqcapt19]

Re: Underground duct banks

Look at the report of a Goerge Washington Chapter IAEI meeting where the speaker was the Technical Director of Southwire's electrical division. (you will have to scrol down about 10 pages to get to the report of the Nov 18, 2003 meeting) He said in part:

Unless conductors are exposed to gasoline or oil, or submerged for an extended period we're talking many months--the nylon covering offers strictly a mechanical protection, Dave explained, with the underlying PVC supplying the dielectric properties as Gil had told us. The nylon actually is stripped away for the UL acceptance test. Therefore, if the nylon is scraped off, but the plastic underneath is intact, an appropriate response usually is "Good; it's done its job," and a green tag.

Don