Feeder Protection on tankless water heater

[User2]

I was working on a 74 amp tankless water heater in which they sized the branch circuit at 125%. This was 92.5A so they put in #3 awg/8 ground @75C terminations, and a 100A disconnect with dual element time delay fuses at 100A. This was fed from a motor control center. What i found was odd was that the fusing at the motor control center was 200A dual element time delay fuses. I was told the previous electrician never wanted anything to trip out on the MCC side. Is it ok to have the #3 branch circuit wiring protected by these 200A fuses because the heater fusing downline is at 100A and cannot let through anymore than that, which the wiring is rated to handle.

MCC >>>>>>>>>>>>>>>>>>>>Disconnect>>>>>>>>>>>>>>>>>>>>>>>Heater
200A Dual-element fuse 100A Dual Element Fuse
#3AWG #3 AWG

 

[david luchini]

I was working on a 74 amp tankless water heater in which they sized the branch circuit at 125%. This was 92.5A so they put in #3 awg/8 ground @75C terminations, and a 100A disconnect with dual element time delay fuses at 100A. This was fed from a motor control center. What i found was odd was that the fusing at the motor control center was 200A dual element time delay fuses. I was told the previous electrician never wanted anything to trip out on the MCC side. Is it ok to have the #3 branch circuit wiring protected by these 200A fuses because the heater fusing downline is at 100A and cannot let through anymore than that, which the wiring is rated to handle.

MCC >>>>>>>>>>>>>>>>>>>>Disconnect>>>>>>>>>>>>>>>>>>>>>>>Heater
200A Dual-element fuse 100A Dual Element Fuse
#3AWG #3 AWG

The #3 protected by the 200A fuse would be allowable if it met one of the tap rules in 240.21(B). If it doesn't meet the requirements of 240.21(B), then it would be a violation.