How To Properly Install Thhn Cable During Cold Weather

Cold weather can wreak havoc on your skin, your heating bill and the air pressure in your tires, but did you also know that it can destroy your cabling if its not handled properly? One such cable, easily susceptible to cold weather deterioration, is THHN cable.

What is THHN cable?

While the terminology of some cables is convoluted and difficult to understand, most of the names of cables are acronyms or abbreviations. The name “THHN” simply comes from “Thermoplastic, High-Heat Resistance, Nylon-Coated.” THHN cable is classified under Hook-up Wire and is a very popular cable, typically used as general building wire. “General” simply means that it has several different applications, all falling under the building industry. Like all general use cables, there is no one “correct” application for THHN cable. However, it is very important to understand any cable’s strengths and weaknesses, what environments it can and cannot be used.

THHN Cable Strengths

Knowing where the name THHN comes from helps identify the cable’s strengths. Because it is a thermoplastic material it can be heated and cooled several times to reshape it, but like any material it has high and low temperature limits that it can withstand before becoming unstable. These specific temperatures can differ between individual manufacturers.

With the “High Heat” rating in its name, that infers it can be used in high temperature applications. What specific applications depend on the actual specifications of the cable, and the operating temperature ranges must be known to determine this. “Nylon-coated” identifies a specific thermoplastic material being used. THHN also has thermoplastic PVC material as insulation, but the Nylon is used as the cable’s jacket and is exposed to the elements. Nylon is a strong material, and is so widely used that THHN cable is more cost effective than other building cables made to withstand more demanding environments.

THHN Cable Weaknesses

The very same materials that give THHN cable it’s strengths also lend weaknesses as well in the right (or should I say wrong?) environments.

Because of the Nylon coating, THHN cable can become stiff at lower temperatures. Temperatures lower than 14F can make it susceptible to cracking. Thermoplastic materials need to undergo a process called plasticizing to function at constant low temperatures, but for cables that will be installed and then never move this isn’t necessary. Proper handling will allow for the cable to withstand installation and then cold temperatures up to the lowest end of the operating temperature range.

The term “thermoplastic” is defined as a material that melts into a liquid at high temperatures and freezes at lower temperatures. The colder it gets, the more it freezes, until the slightest agitation will cause fracture. Bending the cable causes one side of the jacket to compress and the other to expand, which is just the type of agitation needed to cause cracking.

Proper Installation of THHN Cable

To make sure that your THHN cable doesnt crack during installation, there are a few simple steps to follow:
Store cable indoors when the weather turns cold; a stable temperature of 65F is best.
If this is not possible, bring the cable inside at this temperature for 24 hours before installation
Never install THHN when the temperature outside is below 14F
While exposed to cold temperatures, do not drop it to the floor and protect the cable from impacts from other objects as this can crack the stiffened material.

After proper installation, THHN cable can safely operate in outside temperatures below 14F given that it remains stationary.