Electrical Service Size using Variable Frequency Drives

[Boiler79]

When suppling information for electrical service size when using VFD's, do you use the max drive rating or FLA from the motors. There is a debate in our office on what to use.

 

[Cow]

Drives consume energy as well, so why wouldn't you use the drive input amps as the baseline?

Now, starters are a different story....

 

[texie]

See 430.122. This has been debated here before and I don't think there is consensus on this. For example, there was a recent post here about something like 20 each 3 HP motors with 5 HP drives and how to size the feeder. You can see that in a case like this 430.122 is a problem. I've said before that this is something I don't understand the reasoning on behind 430.122. Is this where we get to use sound engineering judgment?

 

[eric9822]

Like texie says 430.122 is where you want to look. I vaguely remember some debates going on but the section seems pretty clear to me. Feeders are sized at 125% of the drives rated input power.

 

[Jraef]

Like texie says 430.122 is where you want to look. I vaguely remember some debates going on but the section seems pretty clear to me. Feeders are sized at 125% of the drives rated input power.

Yes, notice it says the DRIVE'S rated power, not the motor. That's because of two things:

1. VFDs, like NEMA starters, only come in certain sizes. People often put in VFDs larger than the load it is attached to, then later because the VFD is rated for a higher current, they CHANGE the motor to one that takes advantage of that, leaving the circuit incapable of handling the new current if it was sized for the original motor FLA.
2. As mentioned, the VFD has losses and harmonic heating effects that do not factor into the circuit size if only looking at the motor FLA. The total amount of current represented in those losses and effects can themselves be affected by the design and appurtenances of the VFD, such as filters, reactors etc.

So to avoid having to engineer each installation, the NEC just made it easy by saying 125% of the max Amp rating of the VFD, then there is no question of whether or not it is adequate. Overkill? Maybe a little, but better safe than sorry.

 

[texie]

Yes, notice it says the DRIVE'S rated power, not the motor. That's because of two things:

1. VFDs, like NEMA starters, only come in certain sizes. People often put in VFDs larger than the load it is attached to, then later because the VFD is rated for a higher current, they CHANGE the motor to one that takes advantage of that, leaving the circuit incapable of handling the new current if it was sized for the original motor FLA.
2. As mentioned, the VFD has losses and harmonic heating effects that do not factor into the circuit size if only looking at the motor FLA. The total amount of current represented in those losses and effects can themselves be affected by the design and appurtenances of the VFD, such as filters, reactors etc.

So to avoid having to engineer each installation, the NEC just made it easy by saying 125% of the max Amp rating of the VFD, then there is no question of whether or not it is adequate. Overkill? Maybe a little, but better safe than sorry.

Item 1: I disagree, just because you have a size 1 starter but the motor only needs a size 0, you size the wire to the motor, not the size 1 starter. Of course, not so for the VFD.
Item 2: I'll stipulate to that. It just seems like there must be a better way to account for this instead of a blanket 125% adder to better accomodate situations like I mentioned in my previous post.

 

[cadpoint]

I can except the noise of 125% in respects to a VFD...

 

[don_resqcapt19]

The rule in 430.122 only applies to the size of the conductor that feeds the VFD. It does not have anything to do with load calculations.

 

[texie]

The rule in 430.122 only applies to the size of the conductor that feeds the VFD. It does not have anything to do with load calculations.

Well, if that is true, you could end up with smaller service conductors than feeders in some situations.

 

[Boiler79]

I have based my opinion on 430.122. So a system with multiple drives and not oversized has a total Max Drive Rating of 186 amps, total FLA of motors 145. I say the electrical service has to be able of handling the 186 amps.

 

[kwired]

Item 1: I disagree, just because you have a size 1 starter but the motor only needs a size 0, you size the wire to the motor, not the size 1 starter. Of course, not so for the VFD.
Item 2: I'll stipulate to that. It just seems like there must be a better way to account for this instead of a blanket 125% adder to better accomodate situations like I mentioned in my previous post.

I am with you. I have mentioned this before when this kind of topic comes up. I have a place I do work for that had a 250Hp drive that is connected to equipment they do not use. This equipment came with the plant - and was used by previous owner - current owner has no use for that equipment - they need to sell it but that is another complication with company HQ. We had an existing machine with 50 Hp motor and a need to vary speed, and and existing 250 hp drive that at the time was like new. I did help install it for previous owner and would guess it only was used for just a few hundered hours if even that before they sold the plant.

So we have a 250 Hp drive supplying a 50 Hp motor, we have overcurrent device originally supplying the 50 hp motor that was properly sized, and the overload protection paramaters do go low enough to include what is required for this motor. If it had been a 40 Hp motor other overload protection would have been needed as paramaters do not go low enough. Never had a problem with this installation that was because the drive was oversized - it has been that way for 8-10 years now. Why purchase a 50 hp drive when you have a 250 that you have no need for. If we have a need for the 250 I guarantee we will be purchasing a 50 and moving the 250 when that time comes.

There is no chance of installing a larger motor on this machine - there almost is not enough room for the 50. If we were to install a larger motor for some reason - we would change conductor sizes, overcurrent devices, as needed. We would have to do the same anyway even without a VFD, if we changed the motor size.

 

[Jraef]

Item 1: I disagree, just because you have a size 1 starter but the motor only needs a size 0, you size the wire to the motor, not the size 1 starter. Of course, not so for the VFD.
Item 2: I'll stipulate to that. It just seems like there must be a better way to account for this instead of a blanket 125% adder to better accomodate situations like I mentioned in my previous post.

I didn't mean to imply that you need to size a circuit for maximum with a NEMA size starter, it was only an example of devices that only come in certain sizes. The difference with starters and drives is that because drives represent a singificant investment, people were tending not to swap them out, but rather to use what they have. For every 250HP drive on a 50HP motor, there are likely 100 250HP drives on 150-200HP motors. I'm doing a project right now with 7 150HP pumps, the Consulting Engineer has specified that the drives be rated 300A minimum, that's essentially a 250HP drive the way they are sized with most mfrs. I'm not to question the reasoning, I'm just putting in the drives. But I know, some day, some plant foreman will look at upgrading that system and in his budget assessment, he is going to take advantage of the fact that those drives are over sized. If the line conductors were sized for 150HP motors, he might be tempted to look the other way. That sort of stuff happens all the time and especially on retrofits where there is no inspection. That's the scenario that 430.122 is trying to address in my opinion.

Is it a perfect solution? No, but like many of the code provisions it is a compromise to try to have people err on the side of safety.

 

[don_resqcapt19]

Well, if that is true, you could end up with smaller service conductors than feeders in some situations.

It is not referenced in Article 220 and there is nothing in 430 that tells us to us use that value for load calcualtions.

 

[petersonra]

See 430.122. This has been debated here before and I don't think there is consensus on this. For example, there was a recent post here about something like 20 each 3 HP motors with 5 HP drives and how to size the feeder. You can see that in a case like this 430.122 is a problem. I've said before that this is something I don't understand the reasoning on behind 430.122. Is this where we get to use sound engineering judgment?

Are you a sound engineer?

 

[petersonra]

Well, if that is true, you could end up with smaller service conductors than feeders in some situations.

that is actually pretty common.

 

[texie]

Are you a sound engineer?

Took me a minute to get what you mean.... guess I'm a little slow.

 

[texie]

I didn't mean to imply that you need to size a circuit for maximum with a NEMA size starter, it was only an example of devices that only come in certain sizes. The difference with starters and drives is that because drives represent a singificant investment, people were tending not to swap them out, but rather to use what they have. For every 250HP drive on a 50HP motor, there are likely 100 250HP drives on 150-200HP motors. I'm doing a project right now with 7 150HP pumps, the Consulting Engineer has specified that the drives be rated 300A minimum, that's essentially a 250HP drive the way they are sized with most mfrs. I'm not to question the reasoning, I'm just putting in the drives. But I know, some day, some plant foreman will look at upgrading that system and in his budget assessment, he is going to take advantage of the fact that those drives are over sized. If the line conductors were sized for 150HP motors, he might be tempted to look the other way. That sort of stuff happens all the time and especially on retrofits where there is no inspection. That's the scenario that 430.122 is trying to address in my opinion.

Is it a perfect solution? No, but like many of the code provisions it is a compromise to try to have people err on the side of safety.

I know you didn't mean to imply starters are the same, and I respect your opinions. I follow your thinking here in your example, but if we apply the same practice to, say, a service with 6- 200 amp disconnects and a calculated load of 800 amps, you would need 1200 amp of service conductor because somebody might come along and add load.
It just seems to me that there ought to be some better way than 430.122. Look at Kwired case. I don't see a problem, but it is not code compliant.

 

[texie]

that is actually pretty common.

Well, take Jaerf's case. 7, 150 HP motors with 250 HP drives. Assume that is all of the load on the service. If we use Iwire's argument and size the service conductors to the motors and size the feeders to the drive size, this is going to be a bit odd, don't you think? To me, it doesn't pass the common sense test. I'm of the opinion that maybe 430.120 needs a little fine tuning.

 

[texie]

I didn't mean to imply that you need to size a circuit for maximum with a NEMA size starter, it was only an example of devices that only come in certain sizes. The difference with starters and drives is that because drives represent a singificant investment, people were tending not to swap them out, but rather to use what they have. For every 250HP drive on a 50HP motor, there are likely 100 250HP drives on 150-200HP motors. I'm doing a project right now with 7 150HP pumps, the Consulting Engineer has specified that the drives be rated 300A minimum, that's essentially a 250HP drive the way they are sized with most mfrs. I'm not to question the reasoning, I'm just putting in the drives. But I know, some day, some plant foreman will look at upgrading that system and in his budget assessment, he is going to take advantage of the fact that those drives are over sized. If the line conductors were sized for 150HP motors, he might be tempted to look the other way. That sort of stuff happens all the time and especially on retrofits where there is no inspection. That's the scenario that 430.122 is trying to address in my opinion.

Is it a perfect solution? No, but like many of the code provisions it is a compromise to try to have people err on the side of safety.

Just a comment to say I don't mean to infer that in your case, if you think it is a possibility that the motors might be upsized at a later date, that it is not a good design practice. I'm just suggesting that maybe it should not be a code violation.

 

[Besoeker]

My two pence worth.....
The input and output power factors of a variable frequency drive usually differ by quite some margin with the output PF usually being lower than the input power factor.
This results in input current being lower than output current.

Just an observation.
I don't know whether code would permit smaller conductors on the input.