Labeling Full Load Current for panels with VFDs.

[crispy_copper]

How is the Full Load Current determined for an industrial control panel when motors are controlled by VFDs? What sections in NFPA70 or other texts can I reference?

Specifically in reference to the labeling requirements of section 409.110(2). I have a panel with a combination of VFDs, ATL starters, soft starters, and control transformers.

409.110 Marking. An industrial control panel shall have permanent markings that are visible...
(1) Manufacturer's name, trademark...
(2) Supply voltage, number of phases, frequency, and full-load current for each incoming supply circuit.
(3) ......

 

[mayanees]

How is the Full Load Current determined for an industrial control panel when motors are controlled by VFDs? What sections in NFPA70 or other texts can I reference?

Specifically in reference to the labeling requirements of section 409.110(2). I have a panel with a combination of VFDs, ATL starters, soft starters, and control transformers.

409.110 Marking. An industrial control panel shall have permanent markings that are visible...
(1) Manufacturer's name, trademark...
(2) Supply voltage, number of phases, frequency, and full-load current for each incoming supply circuit.
(3) ......

I think we're working on the same thing.
At work we have a paper copy of UL508A and it says that for markings: 52.1 pretty much says what your first two say - b) Complete electrical rating of each source of supply as spec'd in 49.1.
I'll look in the paper copy at work next week for what 49.1 says, but I would expect that your marking would be the fla of the motor, or for a fault, the max output of the drive which is fixed at some % of the drive's output.

 

[Strathead]

I think we're working on the same thing.
At work we have a paper copy of UL508A and it says that for markings: 52.1 pretty much says what your first two say - b) Complete electrical rating of each source of supply as spec'd in 49.1.
I'll look in the paper copy at work next week for what 49.1 says, but I would expect that your marking would be the fla of the motor, or for a fault, the max output of the drive which is fixed at some % of the drive's output.

Just because a motor has a VFD doesn't change the tables that size minimum circuit ampacity or Maximum overcurrent protection for a motor. So I would think 430.247-430.250 would provide you with most of the information you need with 430.52 providing the means to calculate. I am not experienced at this, but it doesn't seem that we need to complicate it.

 

[Jraef]

Just because a motor has a VFD doesn't change the tables that size minimum circuit ampacity or Maximum overcurrent protection for a motor. So I would think 430.247-430.250 would provide you with most of the information you need with 430.52 providing the means to calculate. I am not experienced at this, but it doesn't seem that we need to complicate it.

That's correct. The VFD changes nothing in terms of labeling and circuit calculations, you still go by the motors that they drive.

There ARE differences in conductor size selection and OCPDs. Conductors FEEDING a VFD must be sized per 430.122 at 125% of the VFD rated INPUT current, which is usually LESS than the rated output current (because the VFD input power factor is higher), whereas the motor leads (VFD to motor) must be sized per the motor rules. That sometimes creates a situation wherein the input conductors are smaller than the output conductors, (although there is nothing wrong with just making them the same and thus OVER sizing the input...) The OCPD for a VFD must be sized as per the listing of the VFD (110.3). People often want to take umbrage with this, but the fact remains that if a VFD was listed with a specific fuse, you cannot deviate. This sort of issue really gets squirrely when you use a VFD that is larger than the motor it is controlling. But rules are rules...

 

[iLeKtRiKoS]

That's correct. The VFD changes nothing in terms of labeling and circuit calculations, you still go by the motors that they drive.

There ARE differences in conductor size selection and OCPDs. Conductors FEEDING a VFD must be sized per 430.122 at 125% of the VFD rated INPUT current, which is usually LESS than the rated output current (because the VFD input power factor is higher), whereas the motor leads (VFD to motor) must be sized per the motor rules. That sometimes creates a situation wherein the input conductors are smaller than the output conductors, (although there is nothing wrong with just making them the same and thus OVER sizing the input...) The OCPD for a VFD must be sized as per the listing of the VFD (110.3). People often want to take umbrage with this, but the fact remains that if a VFD was listed with a specific fuse, you cannot deviate. This sort of issue really gets squirrely when you use a VFD that is larger than the motor it is controlling. But rules are rules...

I have this squirrely issue for sizing the OCPD. Here is the story, I have 3- motors rated (2x4.8A, 3.4A). I put individual fuse on each of them with respect to the VFD manufactures recommendation. I also add an AC-Line reactor rated 30Amps max, that will support all the 3 motors. My question is that, can I used a 20Amps(fuse) OCPD for all 3-motors? Using these; (4.8*300%+4.8+3.4=22.6Amps). Is right to consider the AC line reactor max Current to properly size the branch OCPD for all 3motors?

 

[petersonra]

I don't think the code has any special requirements for oc protection of reactors so you are left with what the manufacturer tells you to do.

The reactor uses no current so it is not part of the equation as far as determining the upstream feeder ocpd rating.

 

[iLeKtRiKoS]

That's correct. The VFD changes nothing in terms of labeling and circuit calculations, you still go by the motors that they drive.

There ARE differences in conductor size selection and OCPDs. Conductors FEEDING a VFD must be sized per 430.122 at 125% of the VFD rated INPUT current, which is usually LESS than the rated output current (because the VFD input power factor is higher), whereas the motor leads (VFD to motor) must be sized per the motor rules. That sometimes creates a situation wherein the input conductors are smaller than the output conductors, (although there is nothing wrong with just making them the same and thus OVER sizing the input...) The OCPD for a VFD must be sized as per the listing of the VFD (110.3). People often want to take umbrage with this, but the fact remains that if a VFD was listed with a specific fuse, you cannot deviate. This sort of issue really gets squirrely when you use a VFD that is larger than the motor it is controlling. But rules are rules..

I don't think the code has any special requirements for oc protection of reactors so you are left with what the manufacturer tells you to do.

The reactor uses no current so it is not part of the equation as far as determining the upstream feeder ocpd rating.

I don't think the code has any special requirements for oc protection of reactors so you are left with what the manufacturer tells you to do.

The reactor uses no current so it is not part of the equation as far as determining the upstream feeder ocpd rating.

Is my equation correct? Given that I used the FLA of the motors. I also upsize the VFD and used the recommended fuse for the VFD. Question for the branch OCPD that will protect the VFD to the motor. Should I use only the motor FLA?

 

[petersonra]

Is my equation correct? Given that I used the FLA of the motors. I also upsize the VFD and used the recommended fuse for the VFD. Question for the branch OCPD that will protect the VFD to the motor. Should I use only the motor FLA?

There is no ocpd between the VFD and the motor. The VFD protects the wiring to the motor and the motor.

 

[iLeKtRiKoS]

There is no ocpd between the VFD and the motor. The VFD protects the wiring to the motor and the motor.

How should I compute the Branch OCPD for the 3-motors then?

 

[iLeKtRiKoS]

How should I compute the Branch OCPD for the 3-motors then?

Heres is the block diagram

BrOCPD>Line Reactor > F1 > VFD1 > Motor1
> F2 > VFD2 > Motor2
> F3 > VFD3 > Motor3

How to size the BrOCPD?

 

[petersonra]

Heres is the block diagram

BrOCPD>Line Reactor > F1 > VFD1 > Motor1
> F2 > VFD2 > Motor2
> F3 > VFD3 > Motor3

How to size the BrOCPD?

I would probably use a 30 Amp OCPD and #10 wires, and use #14 downstream of the fuses.

My question is that, can I used a 20Amps(fuse) OCPD for all 3-motors? Using these; (4.8*300%+4.8+3.4=22.6Amps).

I don't know where the 300% came from. If I am correctly remembering, the largest rating of the feeder OCPD is the largest rating allowed for the largest motor plus the full load currents of the rest of the motors.

You will need to look in the manual for the largest drive and figure out what the maximum OCPD rating allowed is because you are required to use the numbers in the manufacturers instructions. But you will end up with the minimum allowed. It is just easier to use a 30 A OCPD and not worry about trying to make it smaller. But conceivably, you might be able to get it down to 20 Amps since most drives max OCPD is about 250%. of the VFD rated input current. Is 4.8 Amps the rated VFD input current?

Why waste time with this kind of thing? The difference in cost between a 20 A fuse and a 30 A fuse is likely nil, and the difference between 5 feet of #12 and 5 feet of #10 wire is a few pennies. The 30 A fuse might even be cheaper than a 20 Amp fuse.

 

[iLeKtRiKoS]

I would probably use a 30 Amp OCPD and #10 wires, and use #14 downstream of the fuses.


I don't know where the 300% came from. If I am correctly remembering, the largest rating of the feeder OCPD is the largest rating allowed for the largest motor plus the full load currents of the rest of the motors.

You will need to look in the manual for the largest drive and figure out what the maximum OCPD rating allowed is because you are required to use the numbers in the manufacturers instructions. But you will end up with the minimum allowed. It is just easier to use a 30 A OCPD and not worry about trying to make it smaller. But conceivably, you might be able to get it down to 20 Amps since most drives max OCPD is about 250%. of the VFD rated input current. Is 4.8 Amps the rated VFD input current?

Why waste time with this kind of thing? The difference in cost between a 20 A fuse and a 30 A fuse is likely nil, and the difference between 5 feet of #12 and 5 feet of #10 wire is a few pennies. The 30 A fuse might even be cheaper than a 20 Amp fuse.

-as far as I know for group of motors. Largest FLA x (300% for fuse or 250%for breaker) + FLA of the remaining motor in the group.
-4.8Amps and 3.4amps are the FLA of the motor.
-looks like 30amps is acceptable (w/in 400% of the largest FLA) if ever the 20amps cannot hold the load.

-this is what makes me confused. Given that the motor used VFD which has a recommended OCPD rating from manufacturers. Which one should I used for the selection of Branch OCPD. Input current of the VFD or the motors FLA

 

[petersonra]

The branch circuit rating of the ocpd device protecting the input to the VFD MUST be selected in accordance with the manufacturers instructions. With some VFDs this is a cb or fuse and the manual will tell you the maximum rating. For some vfds, the manual may only list fuses and in some cases only fast blow fuses (e.g. type t fast blow). You MUST look it up in the VFD manual. There is no formula involved. It does not matter what the VFD rated input current is or the motor fla. The VFD instructions will tell you the maximum rating of the ocpd feeding the VFD

 

[iLeKtRiKoS]

What about for feeder overcurrent protection for the 3-motors

 

[petersonra]

What about for feeder overcurrent protection for the 3-motors

If you did not have to protect the reactor you could make the feeder breaker just about anything, although there are some limits both code and practical. I think 150 amps would be the max rating without the reactor because of the #14 taps.

Protecting the reactor would seem to limit it to 30 amps max.

 

[iLeKtRiKoS]

If you did not have to protect the reactor you could make the feeder breaker just about anything, although there are some limits both code and practical. I think 150 amps would be the max rating without the reactor because of the #14 taps.

Protecting the reactor would seem to limit it to 30 amps max.

Isn’t that #14 are rated 15amps @60C? How did you come up to 150Amps? Why not 50 amps?

 

[ptonsparky]

You are not required to use the maximum fuse/CB. I've used the same style but smaller size. I don't believe the fuse protects against anything other than rapid disassembly.

 

[petersonra]

Isn’t that #14 are rated 15amps @60C? How did you come up to 150Amps? Why not 50 amps?

Because the #14 branch circuit wires are a tap and the minimum ampacity rating of the wires going to the branch circuit fuse blocks is 10% of the rating of the upstream ocpd.

If you made them #10s you could use an upstream ocpd with a rating of 300 amps.

 

[iLeKtRiKoS]

Seems like to much for the circuit.

 

[iLeKtRiKoS]

That's correct. The VFD changes nothing in terms of labeling and circuit calculations, you still go by the motors that they drive.

There ARE differences in conductor size selection and OCPDs. Conductors FEEDING a VFD must be sized per 430.122 at 125% of the VFD rated INPUT current, which is usually LESS than the rated output current (because the VFD input power factor is higher), whereas the motor leads (VFD to motor) must be sized per the motor rules. That sometimes creates a situation wherein the input conductors are smaller than the output conductors, (although there is nothing wrong with just making them the same and thus OVER sizing the input...) The OCPD for a VFD must be sized as per the listing of the VFD (110.3). People often want to take umbrage with this, but the fact remains that if a VFD was listed with a specific fuse, you cannot deviate. This sort of issue really gets squirrely when you use a VFD that is larger than the motor it is controlling. But rules are rules...

Following on this thread. Do we still need to consider the inrush current of a motor? Given that your using VFD controller