VFD Breaker Sizing

Status
Not open for further replies.
K

kody916

Member
Location
Illinois
hi everyone,

For a motor when sizing the cable we typically use 1.5 x FLA current. For protecting the cable we then accordingly select the CB. I have a project where i am installing VFD on a 30HP motor. My calcs for normal starter are below.

30HP, FLA=40A per NEC. So, for cable sizing = 40*1.25=50A, #8AWG should work. Now the CB should protect the cable, hence 50A CB will be selected.

Now here is the concern. I now have to install a VFD for this motor and i need to select the proper breaker. SHOULD THE 50A BREAKER ABOVE WORK FOR THE VFD?

I was also looking at the NEC. Section 430.128 states "disconnecting Means. The disconnecting means shall be permitted to be in the incoming line to the conversion equipment and shall have a rating not less than 115 percent of the rated input current of the conversion unit."
I am using ABB VFD with input current of 52A. So as per above criteria, 52*1.15=60A. IS 60A BREAKER THE RIGHT SIZE?

I am caught up between 50A & 60A breaker. I would like to know which one should i select. Any suggestions?

Thank You
 
The code requires you to follow the manufacturers instructions for sizing the ocpd on the inlet to the VFD.

You are required to use conductors with an ampacity of at least 125% of the rated VFD inlet current.

The breaker rating really does not make much difference. Make it less than or equal to the maximum the manual tells you to use.

Most modern vfds seem to require an inlet breaker rating less than about 250% of the input current rating but this is not consistent and you MUST look in the manual to see what it says.

Incidentally, some vfds require input fuses. You will find this in the manual if so required.
 
Last edited:
The NEC should have two separate requirements for sizing drive breakers. One for a basic drive and another one for drives that have a bypass function. Where I retired from they removed starters out of buckets and used the existing bucket circuit breaker to supply power to well over a hundred motors that most had bypass function ( 3 contactors that on bypass dropped out power to drive and supplied power thru the M3 bypass contactors and an overload block. We always set ramp up speed to at least ten seconds to limit in rush current. All of our drives had input fuses inside the drive cabinets. Even when a motor shorted out or drive blow up ( Blown IGBT'S ) never tripped the old circuit breaker in the bucket.
 
kody916 said:
hi everyone,

For a motor when sizing the cable we typically use 1.5 x FLA current. For protecting the cable we then accordingly select the CB. I have a project where i am installing VFD on a 30HP motor. My calcs for normal starter are below.

30HP, FLA=40A per NEC. So, for cable sizing = 40*1.25=50A, #8AWG should work. Now the CB should protect the cable, hence 50A CB will be selected.

Now here is the concern. I now have to install a VFD for this motor and i need to select the proper breaker. SHOULD THE 50A BREAKER ABOVE WORK FOR THE VFD?

I was also looking at the NEC. Section 430.128 states "disconnecting Means. The disconnecting means shall be permitted to be in the incoming line to the conversion equipment and shall have a rating not less than 115 percent of the rated input current of the conversion unit."
I am using ABB VFD with input current of 52A. So as per above criteria, 52*1.15=60A. IS 60A BREAKER THE RIGHT SIZE?

I am caught up between 50A & 60A breaker. I would like to know which one should i select. Any suggestions?

Thank You
Click to expand...

Yes, 430.128 Disconnecting Means. " ......shall have a rating not less than 115% of the rated input current of the conversion unit."

(OCPD overcurrent protection device) So, T.240.6(A) list the standard sizes per NEC. There's nothing in between a 50A and a 60A.
You said to use 40 amps, (FLA) 40A x 115% = 46A. So next size up is a 50 A, and not less than 115 %, can't round down to a 45A.

This is not based on conductors but on the motor current. Conductors are based on 430.122 (A) at 125 % of the rated input current.

What kind of OCPD are you using as per T.430.52? You said 'breaker' so is it the IT trip breaker or Inverse Time breaker and type of motor eg., single phase or 3 phase. On the left side of that table you have 7 types of motors, is it one of the first 3 that are listed?

And the OCPD percentages range from 800 % for IT to 250 % for IT Breaker for the first 3 types of motors. Both are circuit breakers.

Less percentages for Nontime Delay Fuse and Duel Element (Time Delay) Fuse.

What is the voltage of the motor?
Thanks for reading.

TX+ MASTER #4544
 
Tex

You cannot normally use a motor circuit protector (mag trip only) on the input of a vfd. There used to be some drives listed for such combinations but it seems UL stopped listing them that way.

You cannot use the motor current for anything relating to the input of the drive.

Vfd input conductors are required to be not less than 125% of the VFD rated input current.

The ocpd feeding the input of the VFD has to be selected in accordance with the manufacturers instructions.
 
VFDs - 430.124 Overload protection to protect the motor.

430.128 Disconnecting Means is more meant for if you were to install a disconnect for the VFD. It doesn't have to be the feeder breaker. If you are trying to utilize that as both, then you are correct in assuming you have to met the minimum of 430.128.

430.130 will help in determining the ground fault protective device.

The conversion equipment protection and conductor selection might not be the same as the protection of the motor.
 
petersonra said:
Tex

You cannot normally use a motor circuit protector (mag trip only) on the input of a vfd. There used to be some drives listed for such combinations but it seems UL stopped listing them that way.

You cannot use the motor current for anything relating to the input of the drive.

Vfd input conductors are required to be not less than 125% of the VFD rated input current.

The ocpd feeding the input of the VFD has to be selected in accordance with the manufacturers instructions.
Click to expand...
Yes it's that simple. 125% for the conductors and the manufacturer recommended OCPD size. Nothing after the drive matters.
 
Status
Not open for further replies.
Top