On my Freightliner with a CAT C7 there're 4 wires coming to the outlet plug on the driver's side of the truck, 2 go to the coolant heater element and 2 go to the oil pan heater element. I rewired my plug and when doing so I measured the resistance of both heater elements. Coolant is 17 ohms and oil is 97 ohms, at 120 volts this implies about 850 Watts for the coolant heater and 150 Watts for the oil heater for a total of 1,000 Watts which requires an 8.3 Amp total current.

Since I'm a fulltime RVer I always have a "free" 20A plug handy, and I plug in for an hour or so in the morning when the temp falls below 50F overnight and several hours below 40F and head farther south if it starts going below freezing! This saves some wear and tear from starting a cold engine, and I can't find the number just now but 1 cold start causes the same amount of wear as X number of miles of normal driving.

For my 300 HP CAT C7 at full power the combustion process causes the release of 812 HP worth of heat ENERGY, and the rejected heat is 287 HP, and about half of this rejected heat goes out the exhaust which gives about 144 HP worth of heat ENERGY going into the coolant, then add to this about 30 HP worth of heat ENERGY due to friction, and that gives about 174 HP worth of heat ENERGY going into the coolant, which is almost 130,000 Watts compared to only 850 Watts from the coolant heater!!!

Of course when operating at full power there's a lot of airflow through the radiator to carry away the rejected heat ENERGY in the coolant but even considering that it's hard to see how 850 Watts from the coolant heater is going to damage an engine even if you leave it plugged in 24/7!

On C7 powered motorhomes the coolant/oil heaters are controlled by a switch in the bedroom so you don't have to go outside on a cold morning like I do. I've read several reports where drivers forgot to turn the switch off and only discovered at a rest stop after several hours of driving that the coolant/oil heaters were still receiving power from their generators which they had on to run their air conditioners, and apparently they suffered no ill effects which doesn't surprise me since the coolant heater is only putting out 0.7% of the heat ENERGY that's being rejected by the engine when under a full load!

 

With out knowing the exact details of what caused it I am taking an educated guess that the fire under the hood of the truck that burnt was caused by a short circuit. The truck was probably not connected to a GFI outlet whick would have tripped withen milliseconds of sensing the short. I have seen circuits that have not tripped on a short due to FPE breakers, pennies under fuses, to long of a run from the panel and yes an extension cord that was undersized and way to long for the purpose