VFD Protection Device Sizing

[D!NNy]

• 2-2

Size the cables per the VFD rule (which to be clear is 125% of the MAXIMUM RATED input current, not just what you plan to use with the connected motor), then size the breaker per the cable protection. Most VFDs protect their LOAD from short circuits, but not itself and cannot really be protected by any breaker from a short circuit within itself. All you can hope for is to protect the cables so there is no fire. Agree


Coming to PD sizing ???

I have this VFD used on a 400HP motor with 477A FLA Per 430.250.
VFD rating 411-519kVA with input amps max of 655A.

If I use 2-500MCMper phase i will have 427*2*0.88=751A
This is not going to work because instead of sizing conductors to motor FLA i am sizing for the VFD FLA which is 655*1.25=818A min
For this i need 3*350MCM =283*3= 849 really? i am assuming this cables to be sized like this all the way to the motor? Are we sizing this way just because VFD has the ability to connect higher motor sizes?? If so can i size conductors based on HP if it is changed the conductors to be changed as well for later??

PD sizing: if it has to be sized for the cables it has to be 800A. correct.
Now the overload settings in the VFD is 1.15 to motor or 1.15 to the VFD?

Option 1: If i do the overload 1.15*motor which is 1.15*477=548.5A. my protection device size has to be greater than 548.5 and below 849A so can i size 600A breaker. this way breaker is sized more than overload and protecting conductors. But if the motor is changed in the future breaker and overload to be changed as well.

Option 2: If i do the overload 1.15*VFDr which is 1.15*655=753. my protection device size has to be greater than 753 and below 849A so can i size 800A breaker. this way breaker is sized more than overload and protecting conductors. but there is no overload protection for the motors.

Option 3: Adjust the overload based on the motor and size thePD based on the condutors max of FLA of the VFD but not exceding it.


Please correct me which option is the best way to go by.

Thanks in advance.

• I have this 6-10, 09:22 AM

 

[D!NNy]

Conductors can be protected by the breakers only for the short circuit protection, unless the PD solid state device precise to trip for the overloads. most of the thermal magnetic breakers are rated for instantaneous tripping not for overloads... this is based on the TCC curves observed in the ETAP software.

If i have a motor of 30HP which draws 40FLA min conductor size is 40*1.25=50A. where as different breakers has different limits to be sized. recommended way is 150-300% of the FLA. if i take minimum that is 1.5*40=60A minimum PD size is 70A since 60A circuit breaker is not available. if i size the conductors based on the 1.25 how can CB will protect the cables for overloads.?

Thanks

 

[jim dungar]

When using VFD's, be careful not to intermingle NEC requirements.

Consider that you have two circuits which are inter connected by the VFD.
Size the incoming conductors and protective devices per the rated input to the VFD, ignore the motor, as you have done.

Now stop and take a deep breath.

Okay, you can now proceed with sizing the conductors and protection for the motor.

Confirm the VFD provides short circuit,ground fault and running overload protection to the motor (all UL Listed ones do).
Ignore the line side of the VFD.
Select your motor conductors as if they were being fed by any standard motor OLR (e.g. based on the VFD protection settings).

Only, if your equipment includes a 'bypass' circuit, would you pretend the VFD does not exist and follow traditional motor circuit sizing. Your required components will be the larger of either the VFD or traditional methods.

 

[Jraef]

It is only the INPUT cable sizing that must be sized for the maximum VFD input amps per 430.122, not the cables from the drive to the motor. Those can be sized normally, i.e. based on the HP charts in 430. But if your motor is smaller than the VFD, ie 350HP motor on a 400HP VFD, I would consider sizing conductors for the maximum HP size of the VFD just in case.

OCPD size feeding the VFD should be done per the recommendations in the VFD manual, because in all likelihood that is what the VFD was UL listed with. There is no need to over size the up stream OCPD as per normal motor rules, the VFD (assuming it is UL listed) is now the Overload and OCPD protection for the motor that is down stream of it. It became part of the UL listing process some time ago (wasn't always like that). When you program the motor FLA into the VFD, it is also accounting for the SC amps at the same time.

You ALWAYS set the motor thermal overload protection via the VFD programming instructions, assume nothing. The rules in the NEC are not the best when it comes to operating a motor from a VFD. For example, even if your motor says on the nameplate that it is rated 1.15SF and the NEC has rules to adjust OL protection for that, you will find that motor mfrs tell you that you cannot use a 1.15SF motor at any more than 1.0SF if operated from a VFD. The VFD mfrs know this and the OL protection scheme is designed around that fact.

So just RTFM and you will be fine.

 

[jim dungar]

Conductors can be protected by the breakers only for the short circuit protection, unless the PD solid state device precise to trip for the overloads. most of the thermal magnetic breakers are rated for instantaneous tripping not for overloads... this is based on the TCC curves observed in the ETAP software.

If i have a motor of 30HP which draws 40FLA min conductor size is 40*1.25=50A. where as different breakers has different limits to be sized. recommended way is 150-300% of the FLA. if i take minimum that is 1.5*40=60A minimum PD size is 70A since 60A circuit breaker is not available. if i size the conductors based on the 1.25 how can CB will protect the cables for overloads.?

You need to go back and look at your TCC. Thermal magnetic circuit breaker are rated for overloads (that is the thermal function). You are correct that their curve looks different than that of a motor running overload relay (OLR).

60A circuit breakers are available.

 

[hurk27]

Also your ampacity of the conductors are not close to what table 310.16 in the NEC says, 500kcmils copper is only rated at 380 amps under the 75? column, and copper 350kcmils is 310 amps, unless I'm missing something??

 

[D!NNy]

It is only the INPUT cable sizing that must be sized for the maximum VFD input amps per 430.122, not the cables from the drive to the motor. Those can be sized normally, i.e. based on the HP charts in 430. But if your motor is smaller than the VFD, ie 350HP motor on a 400HP VFD, I would consider sizing conductors for the maximum HP size of the VFD just in case.

VFD rating 411-519kVA with input amps max of 655A. Min conductor size =655*1.25=818.25

For this i need 3*350MCM =283*3= 849 using NEC 2008 table B.310.7

Minimum conductor size to the motor= 477*1.25=596.25A which will require =2*350MCM = 351*2*0.88=617.76 using NEC 2008 table B.310.7

We have three incoming cables and 2 outgoing cables, Is this recommended i rather go with same cables for both incoming and outgoing. But seems to over rated or 3rd 350MCM cable is sized for future just in case motor size is increased.

OCPD size feeding the VFD should be done per the recommendations in the VFD manual, because in all likelihood that is what the VFD was UL listed with. There is no need to over size the up stream OCPD as per normal motor rules, the VFD (assuming it is UL listed) is now the Overload and OCPD protection for the motor that is down stream of it. It became part of the UL listing process some time ago (wasn't always like that). When you program the motor FLA into the VFD, it is also accounting for the SC amps at the same time.

My concern is client is having a 600A existing CB feeding this VFD. Since the VFD has the max rated ampacity of 655A should i have to use a 800A breaker instead of 600A even though the load is only 477A at 1SF or 548.55A at 1.15SF. Either 600A or 800A CB can be protecting the cables.
But if i use the 800A CB in order to protect the cables i have to use 3*350MCM at least to protect the cables.

You ALWAYS set the motor thermal overload protection via the VFD programming instructions, assume nothing. The rules in the NEC are not the best when it comes to operating a motor from a VFD. For example, even if your motor says on the nameplate that it is rated 1.15SF and the NEC has rules to adjust OL protection for that, you will find that motor mfrs tell you that you cannot use a 1.15SF motor at any more than 1.0SF if operated from a VFD. The VFD mfrs know this and the OL protection scheme is designed around that fact.

So just RTFM and you will be fine.

A motor with the SF 1.15 DOL starter connected cannot be operated on a VFD with to SF 1.15 unless it is listed to do so.

 

[D!NNy]

You need to go back and look at your TCC. Thermal magnetic circuit breaker are rated for overloads (that is the thermal function). You are correct that their curve looks different than that of a motor running overload relay (OLR).

60A circuit breakers are available.

Please see the attached coordination curves for a Cutler Hammer LD600A Thermal Magnetic breaker: Showing the ampacity and clerrating time

3000 sec (50 Min approx) is overload condition and 350MCM is cable is not protected. Correct me if i am wrong.

 

[D!NNy]

Also your ampacity of the conductors are not close to what table 310.16 in the NEC says, 500kcmils copper is only rated at 380 amps under the 75? column, and copper 350kcmils is 310 amps, unless I'm missing something??

I was using ampacities from NEC 2008 table B.310.7 since they are underground buried for temperature de-rating i could use 90 Deg column in 310.16.

 

[D!NNy]

When using VFD's, be careful not to intermingle NEC requirements.

Consider that you have two circuits which are inter connected by the VFD.
Size the incoming conductors and protective devices per the rated input to the VFD, ignore the motor, as you have done.

I makes sense to size the conductors to the input rating of the VFD if you have multiple smaller motors connected on the same VFD so that you can use the full capacity of the motor. But in my application it is only one motor for ever HP is fixed ie., 400HP.

Now stop and take a deep breath.

Okay, you can now proceed with sizing the conductors and protection for the motor.

If i size the PD for the motor do my CB has to protecting the cables?
Confirm the VFD provides short circuit,ground fault and running overload protection to the motor (all UL Listed ones do). Yes
Ignore the line side of the VFD.
Select your motor conductors as if they were being fed by any standard motor OLR (e.g. based on the VFD protection settings).

Only, if your equipment includes a 'bypass' circuit, would you pretend the VFD does not exist and follow traditional motor circuit sizing. Your required components will be the larger of either the VFD or traditional methods.

 

[jim dungar]

3000 sec (50 Min approx) is overload condition and 350MCM is cable is not protected. Correct me if i am wrong.

Did you you account for the paralleled conductors?

UL489 listed breakers will almost always (maybe >99.99%) protect conductors sized using the 75?C ampacities. Circuit breaker testing requires about 4' of conductors in order to prove the breakers do actually protect the conductors.