Reflection tank heater wiring and fusing

**rwcran** · 09-16-2020, 03:51 PM

The following pertains to my 2020 Reflection 337RSL 5th wheel. I searched all my GD documentation regarding the tank heaters and found nothing helpful. What I have discovered is the wiring to the tank heaters originates in my front compartment where the factory installed battery is located. I have attached a picture pointing to the wire supplying power from the busbar to the relay shown in the picture. The tank heater on/off switch located at the main control panel in the coach energizes this relay and the output of the relay contacts delivers 12 VDC power to the four tank heaters, two gray, one black and one fresh water tank.

The datasheet for the Picker relay.
https://www.pickercomponents.com/pdf/Relays/PC792A.pdf

I confirmed with Grand Design there is no over current protection in the circuit and we can see from the relay data sheet it provides no over current protection. I have added an inline DC fuse in the wire between the busbar and the relay. Because it has not been cold enough where I am for the thermostats to activate the heating pads I have not been able to measure the total current of all four heating pads. I am assuming the total current is about 24 amps when they are all energized. I have fused it at 30 amps which matches the relay's rated 75C amperage. Why Grand Design did not fuse this circuit is unknown. Should the tank heater wiring short to the trailer frame there are two possible outcomes. First, the relay contacts would fuse together from the excessive heating of the short circuit current and the wire insulation would begin to melt, or second, the relay contacts could fail in an open condition. There is no way to know which would happen without testing a reasonable sample of relays under short circuit conditions and observing the results. I felt a fuse was appropriate and wanted to share this with others so they can add over current protection if they feel the necessity.