Most block heaters will do about 80% in 1-1.5 hrs. IMHO if it won't go at 80%, it ain't going anyway.

I don't usually plug in my N14 Cummins until it gets to single digits and it fires right up no arguments after about an hour. I have fired it unaided at -15 but it makes quite a fuss.

If I might suggest Gary, maybe check the current draw on the starter on that DT. At -5, in my experience it should have fired albeit with some difficulty unaided. Plugged in for an hour it should have fired like you just shut it off. It is only 466 cubes.

I think you might find that the starter is drawing a good bit north of 200 amps.

Kevin