Submittal Drawings’ Core Pipe Sizing
Care should be taken in selecting a self-regulating heating cable to be used in a channel on a pre- insulated pipe. Because of the air surrounding the heating cable in the channel, the cable's operating temperature is higher because of the reduced heat transfer; this will reduce the effective power output of the self-regulating heating cable. The channel's position on the circumference of the pipe also has an influence on the operating temperature. Derating factors for such applications are available from the cable manufacturers.
Although self-regulating heating cables are ideally suited for the electric tracing of metal pipes, care must be taken when using self-regulating heating cables for freeze protection on plastic pipes. Certain self-regulating heating cables are capable of exceeding the Vicat melting point of plastic pipe. More importantly, self-regulating heating cables never turn "off" regardless of the operating or the surrounding temperature. Unless a thermostat is used to control these cables, they will consume enormous amounts of power trying to raise the temperature of the pipe to the limit of the system, when only freeze protection is required.
Since plastic pipes are pressure rated at 23°C (73.4°F), dual sensing electronic thermostats must be used not only to control the heating cable temperature, but also to protect the plastic pipes from overheating at the heating cable location because of the upper temperature limit of such cables. If dual sensing thermostats are not used, the plastic pipes may heat above their rated temperature (softening the plastic) and rupture because of the internal pressure and reduced strength of the pipe wall.
THERMOCABLE® maximum circuit lengths are longer than those of most standard self- regulating cables because # 12 AWG bus wires (except C8-120-COJ) are used as opposed to #16 AWG used in most self-regulating cables. The heavier gauge of bus wires in THERMOCABLE® results in reduced voltage drop and consequently longer circuit lengths. This feature permits fewer power feed points and temperature controllers, thus reducing costs.
With THERMOCABLE®, the circuit breaker protection can be rated normally. When trace cables are used to prevent freezing, the thermostats are set just above the freezing point, usually 3°C (37.4°F). Self-regulating cables have an inrush current at these low temperatures and the circuit breakers must be oversized according to the self-regulating cable manufacturers published recommendations. This requires that all the wiring in such tracing circuits be sized to accommodate the inrush load, even though it lasts only a number of minutes.