Overhead Insulated Wires for Underground Use

Hello tlagtarap

600v-35kv Power >> 600v Power Type THHN TYPE THHN POWER CABLE - 600V, PVC Insulation, Nylon Thermoplastic Jacket, Single Conductor, 90°C Dry Location 75°C Wet Location Type THHN cable is suitable for most current wiring solutions for residential, commercial and industrial environments. It is used in service entrance, feeders and branch circuits for permanent installations. Because of its protective covering, it is ideal for installing in petrochemical plants or service stations. The 1/0 AWG and larger cables can be installed in cable trays. Due to their high thermal resistance, THHN conductors are used for appliances wiring. For installation in conduits and raceways Sizes 1/0 AWG and larger can be installed in cable trays Nylon jacket offers mechanical protection and resistance to petroleum by-products, chemical agents and oils THHN insulation is flaming retardant PVC Maximum continuous conductor temperature: 90°C DRY Damp 75°C Wet The above is from: http://www.calvert-wire.com/show_product.php?id=1 Trust that assists you. Kind Regards....

Thanks for the speciification of THHN cables.

Since the application is in the Philippines with only two seasons, wet and dry- dry during summer 6 months and wet during rainy season 6 months .If I install these THHN cables in the conduits laid underground as part of our underground power cable system , we cannot dismiss the possibility of water intrusion into the conduits and submerging the THHN cables for a long time. But with its property "nylon jacket offers mechanical protection and resistance to petroleum by-products, chemical agents and oils" , thus this property of THHN cable then gives me the leverage to use it as underground cable?

Hello to you

I would like to add to your statement. What you said is true, THHN cables are design for hot condition but not on wet condtion especially on water submerged conditioned because its insulation is a Heat resistant thermoplastics only unlike THHW which is Moisture & heat resistant thermoplastic, they can withstand moisture for a certain period of time. The best type of cables use for underground service entrance and power lines are UF and USE type cables.

tlaqarap, I have a hard time thinking that you will be installing 230/460 volts in a house, or would it be 240/416 volts using the 240 volt phase?

The THHN will break down in the underground conduit before the wet period is over. It is not designed for standing water. We have problems with our underground cable under the same conditions in Florida. Basic science, water and electricity do not mix. Get your cable elevated if at all possible in the climate that you are operating in. The Monsoon is a killer on cables.

We use THWN or THHW in underground installations in Panama with no evidence of abnormal or significant degradation over 5 to 7 years (as determined by periodic meggger tests). We have the same climate conditions as the Phillipines. I would hesitate to use THHN in the same installations, although I have no direct experience with THHN in underground installations (other than the fact that most of the cable we purchase is rated THHN/THWN). The systems I work with are generally 480 V 3 Phase transmission between transformers separated by less than 1500 feet, generally on the order of 250 kW capacity...

Thank you for the info specially the use of THWN and THHW in underground installations. This helps in finally deciding not to use our existing THHN cables for our underground cabling system.

Autozam is correct with respect to the W in THHW, I would reccommend THWN, to maintain the nylon protection, and typically, almost any conductor, installed underground will be exposed to moisture. I.e. bad glue joints in the PVC schedule 40 or DB120 underground conduit. As far as running THHN overhead and exposed the Nylon jacket is not rated for UV protection and will Breakdown over time and the insulation will become brittle. Just check out old roof mounted AC or heater units where the wires are exposed over time. They become an electrical hazard.

Good luck