Help sizing circuit breaker feeding a motor via a VFD

[ARB247]

Hello everyone,

I have a 75hp, 3 phase, 480v, 85 FLA motor with 1.15 service factor fed via a 96 amp VFD connected to a 200 amp, 3 phase breaker on a 480/277v panel.
My question is what size breaker should be used on the panel to feed the VFD and motor? And also why?
I calculated breaker size is 300A assuming no VFD and using inverse time breaker. But what breaker size do we use with the VFD in between the motor and our circuit breaker?

Motor model: CEM2551T
VFD model: HMX09634NA

 

[David Castor]

The VFD will have a rated maximum current. You have to use that to size the upstream feeder and protection. This could be a bit higher than the normal motor FLA

 

[david luchini]

The circuit breaker sizing for a motor with a vfd is essentially the same as for the motor without it, except where the vfd specifies a maximum scgf protection rating.

Not sure about your 300A number though. Seems to me it would be 250A.

 

[infinity]

The VFD conductors are sized at 125% of the drive current. The OCPD is typically specified by the drive manufacturer. Without the VFD your inverse time CB would be up to 250% of the FLC for a 75 HP motor in Table 430.250 which is 96 amps.

 

[Jraef]

Since 2005, UL listing requires that VFDs provide short circuit protection for the programmed motor. It doesn’t however relieve you from the NEC 430.52 requirements. But the UL listing for the VFD often dictates the size AND TYPE of the OCPD. Be aware that a lot of VFDs are only listed with fuses. So if you have to use fuses, you will want them to be as large as possible so that they don’t clear needlessly.

 

[ARB247]

The circuit breaker sizing for a motor with a vfd is essentially the same as for the motor without it, except where the vfd specifies a maximum scgf protection rating.

Not sure about your 300A number though. Seems to me it would be 250A.

1) Full load amps on the 75 hp motor operating at 460v = 85 amps.

2) Service factor of the 75 hp motor = 1.15, therefore we must multiply our 85 FLA by 1.25 according to NEC 430.32. 1.25 multiplied by 85 FLA = 106.25 amps which equals the overload protection minimum size.

3) To size the branch circuit protection we now must refer to NEC 430.52. Multiply 106.25 amps by 250% to determine the correct inverse time breaker. We arrive at 266 amps, since 266 amps is not a standard circuit breaker we must round up to a 300 amp circuit breaker.

I am really confused on how the VFD between the motor and circuit breaker will affect my calculations. Besides that I believe the 300 amp breaker is appropriate assuming the VFD doesn’t change the breaker size.

 

[Jraef]

2) Service factor of the 75 hp motor = 1.15, therefore we must multiply our 85 FLA by 1.25 according to NEC 430.32. 1.25 multiplied by 85 FLA = 106.25 amps which equals the overload protection minimum size.

Careful with that…
Many manufacturers of overload relays build the 1.25 factor into the relay design, so you do NOT want to add it in again. The only proper method of OL protection selection or setting is to read the instructions for the specific relay you are using.

 

[david luchini]

3) To size the branch circuit protection we now must refer to NEC 430.52. Multiply 106.25 amps by 250% to determine the correct inverse time breaker. We arrive at 266 amps, since 266 amps is not a standard circuit breaker we must round up to a 300 amp circuit breaker.

430.52 says to multiply 96A (from Table 430.250) by 250%. That arrives at 240A. 250A is the next standard size up.

 

[ARB247]

Careful with that…
Many manufacturers of overload relays build the 1.25 factor into the relay design, so you do NOT want to add it in again. The only proper method of OL protection selection or setting is to read the instructions for the specific relay you are using.

@david luchini @David Castor @infinity
My VFD states it has a max output of 96 amps at 75hp @ 480VAC it is fed from a 200 amp breaker 480v, do you believe my vfd stopped working because the breaker was over sized? Seems I should have used a 100A breaker to feed the VFD as the VFD at 75hp 480v is 96 amps. Please advise, thanks gentlemen, much appreciated

 

[jim dungar]

@david luchini @David Castor @infinity
My VFD states it has a max output of 96 amps at 75hp @ 480VAC it is fed from a 200 amp breaker 480v, do you believe my vfd stopped working because the breaker was over sized? Seems I should have used a 100A breaker to feed the VFD as the VFD at 75hp 480v is 96 amps. Please advise, thanks gentlemen, much appreciated

The breaker should be based on the input amps of the VFD, nothing else matters when sizing the input protection device.

There is little in rush current on a VFD so you do not need to size the input breaker at much more than 125%, but it would be best to check the VFD manufacturer.

Oversizing the input protective device would have absolutely no impact on the performance of the VFD.

 

[Dennis Alwon]

Shouldn't the VFD itself be sized 125% of the motor current also?

 

[david luchini]

@david luchini @David Castor @infinity
My VFD states it has a max output of 96 amps at 75hp @ 480VAC it is fed from a 200 amp breaker 480v, do you believe my vfd stopped working because the breaker was over sized? Seems I should have used a 100A breaker to feed the VFD as the VFD at 75hp 480v is 96 amps. Please advise, thanks gentlemen, much appreciated

96×250% = 240A.

200A doesn't seem oversized as 250A is the max allowed.

 

[petersonra]

The VFD manual will tell you the maximum rating of the ocpd. It is not something you can calculate.

 

[Jraef]

The breaker should be based on the input amps of the VFD, nothing else matters when sizing the input protection device.

There is little in rush current on a VFD so you do not need to size the input breaker at much more than 125%, but it would be best to check the VFD manufacturer.

Oversizing the input protective device would have absolutely no impact on the performance of the VFD.

Technically, no. The rules for breaker sizing are NOT affected by the existence of the VFD in the circuit. The OCPD for a motor must be sized per 430.52, there is no modifier listed in that section for VFDs.

The only thing that changes WRT VFDs is that the feeder conductors must be sized at 125% of the input amps, which is usually LOWER than the output amp rating of the VFD.

But again, one of the "gotchas" to be aware of is that not all VFDs are UL listed to be used with a CB as the ONLY OCPD, many of them, especially lately, must have specific fuses on the input. RTFM.

Another gotcha is that since about 2005, in NO CASE are VFDs listed behind Mag-Only (MCP) breakers any longer. They used to be, so any legacy installs would still be, but UL changed the listing rules and no longer allow that. So if a VFD is listed with a circuit breaker as the only OCPD, they must be thermal-mag (inverse time) breakers.

 

[Jraef]

Shouldn't the VFD itself be sized 125% of the motor current also?

Nope, unless you really love your VFD supplier and want to spend extra money needlessly...

But you should understand the difference between sizing a VFD for "Variable Torque" / "Normal Duty" vs "Constant Torque" / "Heavy Duty". In a nutshell, VT/ND = centrifugal pumps and fans, because you basically cannot overload them, so the drives (transistors) are not rated for very much overload capability. Everything else that is not a centrifugal pump or fan is CT/HD, where the VFD has additional OL capability. It generally just boils down to the CT/HD rating being the next size up from the VT/ND rating.

 

[Jraef]

@david luchini @David Castor @infinity
My VFD states it has a max output of 96 amps at 75hp @ 480VAC it is fed from a 200 amp breaker 480v, do you believe my vfd stopped working because the breaker was over sized? Seems I should have used a 100A breaker to feed the VFD as the VFD at 75hp 480v is 96 amps. Please advise, thanks gentlemen, much appreciated

Nothing about the size of the breaker is going to affect the lifespan of the VFD, at least not directly. If the breaker is TOO SMALL, and for some reason continually nuisance trips, then that repeated powering / unpowering of the VFD can slightly shorten the life of the pre-charge circuit in some cases. But the breaker would likely wear out first, which would be a huge red flag...

I looked up the installation manual for that drive, as I suspected, it requires 250A Class T fuses as the OCPD, it does not list a circuit breaker of any size as an option. This also is a "packaged" drive, so it likely has those fuses in the package already, because this also has a Bypass built-in, so the fuses are going to need to be protecting the motor regardless of the VFD operation.

So if the fuses are in fact already inside of that packaged drive unit, then all you are needing is to size the FEEDER breaker for it (not the "branch"). The Feeder breaker would be sized to protect the conductors. The conductors would be sized at 125% of the INPUT current rating of the drive, which Eaton does not provide in their literature, but they DO show a minimum cable size of 1/0, so I would likely go with a 150A breaker to protect that. If you try to go with 250A it will require 250kCMIL cable, and the lugs on the drive only handle up to 2/0.