Protection of VFD feeder using tap rules

[mull982]

I was looking at a particular application wondering whether it is NEC compliant.

In a panel there is a 110A breaker protecting a #4 cable which runs for 50ft feeding the input of a VFD. I believe that VFD has an input breaker but am not exactly sure what size.

Now I know that typically the 110A breaker cannot protect the #4 cable (would need to be 100A breaker max) however I am wondering if using the tap rule that the breaker at the input of the vfd can protect the cable. For instance if the breaker at the input of the VFD is an 100A or 80A breaker would this protect the #4 cable by means of the tap rule.

I know also that the feeder breaker and cable must be sized at 125% of the VFD rated input current.

 

[kwired]

I was looking at a particular application wondering whether it is NEC compliant.

In a panel there is a 110A breaker protecting a #4 cable which runs for 50ft feeding the input of a VFD. I believe that VFD has an input breaker but am not exactly sure what size.

Now I know that typically the 110A breaker cannot protect the #4 cable (would need to be 100A breaker max) however I am wondering if using the tap rule that the breaker at the input of the vfd can protect the cable. For instance if the breaker at the input of the VFD is an 100A or 80A breaker would this protect the #4 cable by means of the tap rule.

I know also that the feeder breaker and cable must be sized at 125% of the VFD rated input current.

Unless you have 90 degree terminations (not likely) 4 AWG is rated 85 amps at 75 deg. next standard size up is 90 amps. If load would happen to be between 86 and 90 then 4 AWG is too small even with 90 amp protection.

Other than outside taps of unlimited length or in certain manufacturing applications with over 35 foot high ceilings there is no tap rule that works for 50 feet of tap conductor.

 

[petersonra]

Unless you have 90 degree terminations (not likely) 4 AWG is rated 85 amps at 75 deg. next standard size up is 90 amps. If load would happen to be between 86 and 90 then 4 AWG is too small even with 90 amp protection.

Other than outside taps of unlimited length or in certain manufacturing applications with over 35 foot high ceilings there is no tap rule that works for 50 feet of tap conductor.

we had this discussion a few weeks ago and the concensus seemed to be that it was ok. I remain unconvinced.

 

[kwired]

we had this discussion a few weeks ago and the concensus seemed to be that it was ok. I remain unconvinced.

What was ok about it?

Conventional motor circuit allows for higher overcurrent protection than for other loads on a conductor. VFD is not necessarily going to allow that though.

We need to know more about the drive for definite answers, but in general the overcurrent protection in OP is too high for the conductor he has. He has 85 amp conductor and wants to use 110 amp overcurrent device.

If he has overcurrent at load end of the feed then he could use tap rules, except his feeder tap will be longer than tap rules allow, unless outside where there is no limit on tap length.

 

[mull982]

Other than outside taps of unlimited length or in certain manufacturing applications with over 35 foot high ceilings there is no tap rule that works for 50 feet of tap conductor.

For the exception allowing the tap rule of "unlimited length" outside does it matter if the source termination at the panel in inside? For instance what if the panelboard is in an electrical room on a roof or elsewhere and then the cable travels outside of the electrical room where it terminates into the VFD which is located outside? Is this o.k. to use the tap rule enen though a small portion of the run is inside?

 

[kwired]

For the exception allowing the tap rule of "unlimited length" outside does it matter if the source termination at the panel in inside? For instance what if the panelboard is in an electrical room on a roof or elsewhere and then the cable travels outside of the electrical room where it terminates into the VFD which is located outside? Is this o.k. to use the tap rule enen though a small portion of the run is inside?

Most AHJ will probably allow similar amounts of conductors inside as they do for a service or a building supplied by a feeder where the length is limited and it terminates "nearest the point of entry". Some don't allow any service conductor to enter and also may not allow even short section of this tap to be inside just to be consistant, others may limit to 3 feet, 5 feet, or something like that to be inside the building - it would be a local rule and not a NEC rule.

 

[petersonra]

What was ok about it?

Conventional motor circuit allows for higher overcurrent protection than for other loads on a conductor. VFD is not necessarily going to allow that though.

We need to know more about the drive for definite answers, but in general the overcurrent protection in OP is too high for the conductor he has. He has 85 amp conductor and wants to use 110 amp overcurrent device.

If he has overcurrent at load end of the feed then he could use tap rules, except his feeder tap will be longer than tap rules allow, unless outside where there is no limit on tap length.

I am inclined to agree, however, others said otherwise.

 

[petersonra]

How is a VFD circuit not a motor circuit?

If you can run a wire sized for the motor load and not the OCPD to a motor starter, why can't you do the same thing for a VFD?

I am inclined to agree that it is a tap and subject to the tap rule length limits, but others have argued otherwise.

Here is an example from an actual project i am working on.

VFD being fed by a CB. Code allows the CB to be 250% of the motor load.

Are you advocating that this wire has to be sized for the motor load or for the CB size?

I have some 200HP VFDs fed by 400A CBs.

Are you saying I need 600KCM (400A CB) or 350KCM (sized for motor load)?

I am inclined to think that if the VFD is more than 25 feet away from the CB that it does not meet the tap rule requirements.

 

[mull982]

So it sounds like in my case since it is outside I may be able to use the tap rule, however the CB at the VFD must be a 90A breaker in order to adequately protect the #4 cable feeding it?

 

[petersonra]

So it sounds like in my case since it is outside I may be able to use the tap rule, however the CB at the VFD must be a 90A breaker in order to adequately protect the #4 cable feeding it?

I think we scared them with this question.

 

[petersonra]

Incidentally, I tried the motor circuit calculator at electrician.com.

http://www.electrician2.com/calculators/motor_ver_1.html

above 20Hp it wants bigger wires than my handy slide rule chart does.

for instance, it wants 500 KCM for 200HP @ 480V.

240A * 1.25% is 300A. Wouldn't 350 cover that adequately?

 

[kwired]

When using an adjustable speed drive 430.122 requires conductors to be rated a minimum of 125% of the rated input current of the drive. It does not matter what the rating of the driven motor is - you are supplying the drive. The drive is power conversion equipment and usually has its own internal protection for the output. If it does not have internal protection you are likely going to follow instructions provided with the drive for load side protection so it becomes 110.3(B) issue. Overcurrent protection for the drive itself (line side) would also be 110.3(B) issue and not from 430 general motor rules.

430.122 also says that if the drive has bypass equipment that the conductors will be larger of 125% of drive rating or 430.6 values (the usual size for the motor without the drive).

 

[kenaslan]

A VFD is not a motor, it is an electronic piece of equipment that converts 60hz to something else. It will have a rectifier inside, followed by a freq generator. On large motors you might have 3 seperate delta-tri delta 4160V transformers, each with 3ea 4160V outputs, these will feed three seperate DC rectifiers, which in turn feed a VFD

How is a VFD circuit not a motor circuit?

If you can run a wire sized for the motor load and not the OCPD to a motor starter, why can't you do the same thing for a VFD?/QUOTE]

 

[kwired]

How is a VFD circuit not a motor circuit?

If you can run a wire sized for the motor load and not the OCPD to a motor starter, why can't you do the same thing for a VFD?

I am inclined to agree that it is a tap and subject to the tap rule length limits, but others have argued otherwise.

Here is an example from an actual project i am working on.

VFD being fed by a CB. Code allows the CB to be 250% of the motor load.

Are you advocating that this wire has to be sized for the motor load or for the CB size?

I have some 200HP VFDs fed by 400A CBs.

Are you saying I need 600KCM (400A CB) or 350KCM (sized for motor load)?

I am inclined to think that if the VFD is more than 25 feet away from the CB that it does not meet the tap rule requirements.

A VFD is not a motor, it is an electronic piece of equipment that converts 60hz to something else. It will have a rectifier inside, followed by a freq generator. On large motors you might have 3 seperate delta-tri delta 4160V transformers, each with 3ea 4160V outputs, these will feed three seperate DC rectifiers, which in turn feed a VFD

How is a VFD circuit not a motor circuit?

If you can run a wire sized for the motor load and not the OCPD to a motor starter, why can't you do the same thing for a VFD?/QUOTE]

The drive is power conversion equipment. Input current does not have same characteristics as if motor were connected directly to the line. One major difference is there is not the same inrush at startup - the main reason we are allowed to put overcurrent devices rated higher than conductor ampacity on most motor circuits. NEC does not tell us what max size overcurrent device is allowed for these drives, but instructions for the drive do - so overcurrent protection is 110.3(B) driven normally. In absence of that I would say the general overcurrent protection rules of protecting conductors at their ampacity would be the next guide for overcurrent protection.

If you have a method of running motor across the line or through the drive then you will probably have overcurrent protection sized normally for motor with supplemental devices protecting the drive if protection is required to be different for the drive. (And a way to make sure both are not connected to the load at same time)

 

[petersonra]

I have often wondered why.

Say we have a 20A circuit. We need #12 conductors and #12 EGC. The #12 EGC is required to trip the CB when we get a short or a ground fault.

So why is it that magically in a motor circuit, we can change the 20A CB to maybe a 50A CB (250%), leave the #12 EGC and the CB will still trip on SC or GF? Why would a #12 EGC not work on all circuits with a 50A CB?

 

[kwired]

I have often wondered why.

Say we have a 20A circuit. We need #12 conductors and #12 EGC. The #12 EGC is required to trip the CB when we get a short or a ground fault.

So why is it that magically in a motor circuit, we can change the 20A CB to maybe a 50A CB (250%), leave the #12 EGC and the CB will still trip on SC or GF? Why would a #12 EGC not work on all circuits with a 50A CB?

Below 30 amps is different than above 30 amps. Start going above 30 amp overcurrent protection and your EGC is allowed to be smaller than the ungrounded conductors. EGC is never required to be larger than the ungrounded conductors though. I can understand a small EGC may fail when the larger ungrounded conductor may not during a fault. Why the 30 amp level was chosen as the point where this starts I don't know.

Keep in mind that other than service conductors and outside feeder taps there is limitations on conductor sizes and lengths when it comes to overcurrent protection higher than the conductor ampacity. The idea is that the overcurrent device is to open the circuit before the conductor can no longer take the fault current and burns open. Services and outdoor feeder taps is not as much risk to property if the conductor ends up burning open before an overcurrent device would open. JMO, but seems logical.

 

[mull982]

The drive is power conversion equipment. Input current does not have same characteristics as if motor were connected directly to the line. One major difference is there is not the same inrush at startup - the main reason we are allowed to put overcurrent devices rated higher than conductor ampacity on most motor circuits. NEC does not tell us what max size overcurrent device is allowed for these drives, but instructions for the drive do - so overcurrent protection is 110.3(B) driven normally. In absence of that I would say the general overcurrent protection rules of protecting conductors at their ampacity would be the next guide for overcurrent protection.

This is how I have always interpreted the code. The NEC only requires 125% rating for the cable and OCPD feeding the drive. There is no maximum on what the OCPD can be as mentioned above (except stated by manufacturer) however the cable size must adhere to the rules of 240.4. Taking it one step further if the cable is not protected by the OCPD then the tap rules may be used if there is a breaker located at the VFD itself.

This is my interpretation.

 

[kwired]

This is how I have always interpreted the code. The NEC only requires 125% rating for the cable and OCPD feeding the drive. There is no maximum on what the OCPD can be as mentioned above (except stated by manufacturer) however the cable size must adhere to the rules of 240.4. Taking it one step further if the cable is not protected by the OCPD then the tap rules may be used if there is a breaker located at the VFD itself.

This is my interpretation.

You are correct. If there is a OC device at the drive itself then the branch circuit starts at that device. Everything ahead of it is a feeder or a feeder tap. There is no limitations on length of feeder, there are limitations on length of some feeder taps.

 

[petersonra]

Below 30 amps is different than above 30 amps. Start going above 30 amp overcurrent protection and your EGC is allowed to be smaller than the ungrounded conductors. EGC is never required to be larger than the ungrounded conductors though. I can understand a small EGC may fail when the larger ungrounded conductor may not during a fault. Why the 30 amp level was chosen as the point where this starts I don't know.

Keep in mind that other than service conductors and outside feeder taps there is limitations on conductor sizes and lengths when it comes to overcurrent protection higher than the conductor ampacity. The idea is that the overcurrent device is to open the circuit before the conductor can no longer take the fault current and burns open. Services and outdoor feeder taps is not as much risk to property if the conductor ends up burning open before an overcurrent device would open. JMO, but seems logical.

I just picked that as an example because I can remember the required EGC sizes for those amperages.

Lets pick another case.

20 HP 480V 54FLA

#4 wire according to my chart

90 Amp TM breaker

But, I am allowed to go up to 250% which would 135A, the next size breaker is 150A.

Normally with a 150A CB one would need a #6 EGC, but with a 90A only a #8 EGC is required according to T250.122.

So how is a motor circuit with a 150A CB adequately protected against SC with #8 EGC, when any other circuit with a 150A CB requires a #6 EGC to be adequately protected?

 

[petersonra]

This is how I have always interpreted the code. The NEC only requires 125% rating for the cable and OCPD feeding the drive. There is no maximum on what the OCPD can be as mentioned above (except stated by manufacturer) however the cable size must adhere to the rules of 240.4. Taking it one step further if the cable is not protected by the OCPD then the tap rules may be used if there is a breaker located at the VFD itself.

This is my interpretation.

Where does the code say the OCPD has to be 125%? It only requires that for branch and feeder circuits. 430.122 (A)

It does not say anything about the feeder tap there though, so that would be covered in an earlier section.