DANGERS OF THERMAL SHOCK
If conductors are energized rapidly, the cable’s insulation can be damaged due to thermal shock—like glass that cracks under sudden changes from hot to cold. THHN, for example, is prone to cracking or even shattering under sudden thermal loads because PVC expands quickly when exposed to heat. This issue can be compounded when THHN is stored or installed below the cable’s thermal threshold.
Avoid the risk altogether by using XHHW-2—it has a lower cold and a higher maximum temperature rating.
PROPER CABLE WARM-UP & RAMPING
Cables stored in cold conditions should never be brought to full current load immediately. A gradual current ramping strategy allows the material to expand slowly, minimizing the risk of insulation damage.
When current flows through a cold cable, the conductor heats up instantly, while the insulation remains cold and brittle. This creates internal mechanical shear, where the expanding conductor pushes against the rigid insulation that cannot yet flex, potentially causing:
Insulation Splitting
Longitudinal cracks that form as the conductor forces the insulation to stretch beyond its elastic limit.
Bond Failure
Separation of the insulation from the conductor that leads to mechanical failure.
Stress Fractures
Permanent fatigue in the polymer chain of the insulation, shortening the cable’s operational lifespan.
Before any current is applied, cables stored in freezing conditions should be allowed to reach the ambient room temperature of the installation site. For large reels, this can take 24 to 48 hours because the dense copper core retains cold longer than the outer jacket suggests.
Rather than switching a circuit to 100% capacity immediately, electrical engineers recommend an incremental load-up:
Apply a quarter of the rated current for 1 to 2 hours to "pre-warm" the conductor core.
Increase to half capacity, radiating heat through the insulation to the outer jacket.
Transition to full capacity once the cable surface shows uniform temperature.