john2016
Member
- Location
- California
- Occupation
- Electrical Engineer
I'm working on the design for an AHU replacement project that has four 30HP, 460V, 3 phase fans with each motor having its own VFD (soft start). The mechanical engineer is telling me that the fans will run at a low speed normally (around 18 or 19HP) but if one of the 4 fans breaks then the other fans would automatically speed up to accommodate the load.
I was planning on providing a breaker in the switchboard and run a feeder up to the roof where the AHU is located. Then I would provide a wireway and four separate 3P100A disconnect switches with 80 amp fuses for each 30HP motor.
My question is how to properly size the breaker in the switchboard that will feed the four 30HP fans with VFD's. Do I take 125% of the largest motor plus the sum of the remaining motors to size the breaker?
Here are the specs for the AHU. 4 supply fans, 460/3/60, 35.85FLA, 44.81MCA, 80.0A FUSE SIZE.
Would I size the breaker in the switchboard this way? 35.85 x 125% = 44.81 + 35.85 + 35.85 + 35.85 = 152.35A. Provide a 3P160A breaker?
Also, the new AHU will be located outside of the existing AHU which is located in a roof top mechanical room. The new AHU will be put in place before the existing AHU is removed. The mechanical engineer is placing the new VFD's inside the mechanical room. Since the new AHU will be located outside the mechanical room would I place the 4 disconnect switches outside next to the new AHU even though the VFD's will be located inside the mechanical room? I'm not clear on where the correct placement for the disconnect switches should be since they are supposed to be in line of sight. The VFD's will not be in line of sight since they will be located inside the mechanical room and the new AHU will be located outside of the mechanical room.
I was planning on providing a breaker in the switchboard and run a feeder up to the roof where the AHU is located. Then I would provide a wireway and four separate 3P100A disconnect switches with 80 amp fuses for each 30HP motor.
My question is how to properly size the breaker in the switchboard that will feed the four 30HP fans with VFD's. Do I take 125% of the largest motor plus the sum of the remaining motors to size the breaker?
Here are the specs for the AHU. 4 supply fans, 460/3/60, 35.85FLA, 44.81MCA, 80.0A FUSE SIZE.
Would I size the breaker in the switchboard this way? 35.85 x 125% = 44.81 + 35.85 + 35.85 + 35.85 = 152.35A. Provide a 3P160A breaker?
Also, the new AHU will be located outside of the existing AHU which is located in a roof top mechanical room. The new AHU will be put in place before the existing AHU is removed. The mechanical engineer is placing the new VFD's inside the mechanical room. Since the new AHU will be located outside the mechanical room would I place the 4 disconnect switches outside next to the new AHU even though the VFD's will be located inside the mechanical room? I'm not clear on where the correct placement for the disconnect switches should be since they are supposed to be in line of sight. The VFD's will not be in line of sight since they will be located inside the mechanical room and the new AHU will be located outside of the mechanical room.