I replaced an electric furnace that had two feeders, one 240 volt 20 amp for the motor and control and one 240 volt 60 amp for the resistance heaters. This is a heat pump system but the compressor is on a third circuit.
The replacement unit installed takes a single feeder and only has 5 kw of resistance heat. I used the 20 amp feeder to feed the electronic air cleaner only and used the 60 amp feeder to feed the furnace (including the motor, control and 5 kw resistance grid). I replaced the 60 amp breaker with a 40 amp breaker. The resistance grid takes approximately 23 amps and the motor around 3 amps (running load). The manual says that the ampacity of the feed must be at least 29 amps and to fuse it at 30 amps.
If I had installed 7.5 kw heaters, it would have to be fused at 45 amps and if 10 kw, it says to fuse at 60 amps. Any higher than 10 kw requires multiple feeders (branches).
Question: Is is OK, legal and safe to leave the unit fused at 40 amps or do I have to go back in and replace the breaker with a 30 amp unit. The branch circuit wire is sized for 60 amps.