TFFN / THHN Wire Test
Observations of TFFN / THHN wire installed in Asphalt or Concrete as a Saw-Cut Loop.

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Purpose:

Observe and record what happens when TFFN / THHN wire is installed in asphalt or concrete in direct sunlight.

Hypothesis:

If TFFN / THHN wire is installed in asphalt or concrete, then the wire will short to ground intermittently.

Materials:

- Pot half full of water*
- Burner
- Thermometer
- Electronic Insulation Tester (Megger)
- Camera for documentation purposes
* Water was chosen to simulate asphalt/concrete because its temperature is easily changed through heat.

Procedure:

Testing area was prepared by placing the pot with water on top of the burner with the heat off. TFFN wire was put inside water so the wire was completely submerged. The megger was attached to the wire and a reading was taken. The thermometer was put into the water and the burner was turned on to heat the water. As the temperature rose to 65º, 90º, 125º, 145º, and 160º readings were recorded, photographed, and documented. The study was completed once the temperature reached 160º.

TFFN wire being testedFigure 1
Image of TFFN wire used and testing area

Results:

TFFN Results

Results show a decrease in Meg-ohms as temperature rises. Detector manufacturers generally recommend a reading of over 100 mega-ohms for a detection to work properly. The reading returned to normal when the water cooled down.

Conclusion:

TFFN (or equivalent THHN) wire will short to ground and fail to work if installed in concrete or asphalt and exposed to high temperatures. However, the short will go away when the surface cools down creating an intermittent loop problem.

BD Loops - What these Results Mean for Installers

TFFN / THHN wire is a cheap home building wire bought from Home Depot, commonly used in saw-cut applications. TFFN / THHN wire is designed to be installed in walls run through conduit, it is not designed for direct burial installations. On a normal summer windless day asphalt can easily reach up to 160ºF in direct sunlight.

During the hot part of the day a loop wrapped with TFFN/THHN wire can short to ground, later when the sun has gone down and the surface cools the short will go away and the system will continue to work properly. Intermittent loop problems caused by heat can be hard to diagnose, resulting in unnecessary repeat service calls and functional loop detectors being returned. Therefore it is to your benefit to never use or recommend TFFN / THHN wire.

BD Loops Test Results - We run the Same Test on our Saw-Cut Loops

Conducting the same test with BD Loops 3/16” preformed saw-cut loops.

BD Loops Results

Results show that installation of BD Loop’s preformed loop is far superior than TFFN / THHN wire. Even at 180º BD Loops still perform at their best.

BD Loops Preformed Saw-Cut Inductance Loop Being Tested
Figure 2
Picture of a BD Loops saw-cut in 180º water with a reading over 2000 MΩ

Always recommend and install a BD Loops preformed loop for all saw-cut applications to avoid costly repeat service calls, failed loops, and returned detectors.