Vfd cable breaker sizing

[hhsting]

Attached sketch i have panelboad breaker #1 feeding VFD 20HP rated. The 20HP vfd is feeding 50A fused disconnect at the 20HP motor 480V three phase.


My questions are as follows:

1. Which code section in NEC 2017 Article 430 would one use for sizing of Breaker #1, Cable #1, Cable #2, Cable #3? I understand their is part for power conversion 430 but I am confused as to what applies where

 

[ptonsparky]

#1. What is the MOC protection for the VFD in its installation manual?

The rest is protected by the VFD if you have sized it properly.
Why would you put a fused disconnect on the load side of a VFD?

 

[ptonsparky]

The size of the wire for 1 is determined by the input requirement of the VFD.
Use a 30 HP drive and you size the wire by its label. A 40? the same.
IDK if a 20 will vary from one mfg or another but again, whatever that label calls for.

 

[hhsting]

#1. What is the MOC protection for the VFD in its installation manual?

The rest is protected by the VFD if you have sized it properly.
Why would you put a fused disconnect on the load side of a VFD?

VFDs are located remote from the motors so disconnect is provided at the motors. The motor is actually cooling tower rooftop.

Their is a plaque next to disco to shut vfd disco first

I dont have MOC of vfd

 

[hhsting]

The size of the wire for 1 is determined by the input requirement of the VFD.
Use a 30 HP drive and you size the wire by its label. A 40? the same.
IDK if a 20 will vary from one mfg or another but again, whatever that label calls for.

What about cable #2,#3?

 

[david luchini]

Attached sketch i have panelboad breaker #1 feeding VFD 20HP rated. The 20HP vfd is feeding 50A fused disconnect at the 20HP motor 480V three phase.


My questions are as follows:

1. Which code section in NEC 2017 Article 430 would one use for sizing of Breaker #1, Cable #1, Cable #2, Cable #3? I understand their is part for power conversion 430 but I am confused as to what applies where

Breaker #1: 430.130
Cable #1: 430.122
Cable #3: 430.22
Cable #4: 430.22

VFDs are located remote from the motors so disconnect is provided at the motors. The motor is actually cooling tower rooftop.

Use a non-fused disconnect at the motor.

 

[hhsting]

Breaker #1: 430.130
Cable #1: 430.122
Cable #3: 430.22
Cable #4: 430.22

Use a non-fused disconnect at the motor.

Designer has selected fused disconnect??? Is their anything in code to use non fuse disco or any reason?

 

[ptonsparky]

Reason would be just another point of failure you don't need.
I would also suggest an overlapping control interlock on the disconnect.

 

[Jraef]

Fuses on the output side of a VFD are pointless from a protection standpoint and asking for trouble. If a fuse blows, it’s likely going to damage the VFD transistors. But luckily, it’s highly unlikely that the fuse will blow because the VFD will act faster to stop a fault than a fuse anyway.

As others have said, use a non-fused disconnect with an aux contact wired back to disable the drive if someone ignores the sign about disconnecting the drive first. People are inherently lazy. If the HVAC technician is on the roof to work on this and sees the sign telling him to go back to the panel to turn off the breaker first, then go back up to the roof again, he is going say “F-it” and open that disconnect under load, possibly damaging the VFD.

Breaker size is dictated by the VFD instruction manual, not to exceed NEC article 430.52. Conductor sizes are dictated by the NEC; input conductors per 430.122, output conductors per 430.22 motor circuit rules based on HP tables.