VFD WIRE SIZING

[steve66]

430. 122:

I have a VFD that is going to be in an ambient temp. greater than the VFD standard rating of 104 degrees.

So per the manufacturers spec sheet, I de-rate the VFD output. Just as an example, say the motor is 25 HP. So using with the required de-rating, I need a 30 HP VFD.

Article 430 says the VFD input wiring needs sized based on the VFD rating. So, can I use my "de-rated" 25HP for sizing the wiring, or do I have to use the nameplate rating of 30 HP?

The informational note tends to indicate I can use the de-rated 25 HP rating.

 

[Besoeker]

430. 122:

I have a VFD that is going to be in an ambient temp. greater than the VFD standard rating of 104 degrees.

So per the manufacturers spec sheet, I de-rate the VFD output. Just as an example, say the motor is 25 HP. So using with the required de-rating, I need a 30 HP VFD.

Article 430 says the VFD input wiring needs sized based on the VFD rating. So, can I use my "de-rated" 25HP for sizing the wiring, or do I have to use the nameplate rating of 30 HP?

The informational note tends to indicate I can use the de-rated 25 HP rating.

I don't know your rules but, as the difference in conductor size if any, is minimal I'd go with the 30HP rating.

 

[infinity]

The VFD conductors are based on 125% of the input current to the drive. The fact that you're using a larger drive does not matter.

 

[steve66]

I don't know your rules but, as the difference in conductor size if any, is minimal I'd go with the 30HP rating.

I would agree if it were a new install. However, all the motors and branch circuits are existing, so its a matter of having to replace existing wiring or not.

The VFD conductors are based on 125% of the input current to the drive. The fact that you're using a larger drive does not matter.

But there is the informational note that says equipment may have multiple power and input current ratings.

And the handbook even adds that:

"Equipment might have only one rating on the nameplate.  However, additional ratings can be provided in the manufacturers instructions for use."

To me, that directly addresses my situation where I have a 30HP drive, but due to temp. de-ratings, I'm using it as a 25 HP drive. Therefore, the input wiring should basically be sized for a 25HP drive.

 

[Jraef]

I would agree if it were a new install. However, all the motors and branch circuits are existing, so its a matter of having to replace existing wiring or not.



But there is the informational note that says equipment may have multiple power and input current ratings.

And the handbook even adds that:

"Equipment might have only one rating on the nameplate.  However, additional ratings can be provided in the manufacturers instructions for use."

To me, that directly addresses my situation where I have a 30HP drive, but due to temp. de-ratings, I'm using it as a 25 HP drive. Therefore, the input wiring should basically be sized for a 25HP drive.

If you can sell that argument to an AHJ, you are golden. I wish you luck...

 

[just the cowboy]

30 hp drive size feed for that

30 hp drive size feed for that

I was always told the feed had to be sized to the drive, just in case someone later says " thats a 30 hp drive lets put a 30 hp motor on it" .

 

[Besoeker]

I would agree if it were a new install. However, all the motors and branch circuits are existing, so its a matter of having to replace existing wiring or not.

What is the existing wiring rated for?

 

[steve66]

I was always told the feed had to be sized to the drive, just in case someone later says " thats a 30 hp drive lets put a 30 hp motor on it" .

That's what I've always thought too.

But the informational note is there in black and white, and if de-rating a VFD isn't exactly what they are talking about, then what is the note all about?

What is the existing wiring rated for?

There are actually various sizes motors and wires. I assume all the existing wire sizes are per the NEC with #12 being the minimum. (The original drawings don't have any wire sizes listed).

With that said, there are really only 7 motors that would be an issue. That would be:
1. Some 10 HP motors wired with #12, that would be connected to 15HP VFDs rated 21 amps FLA. (Gee, I'm only one amp over here anyway.)
2. Some 20 HP motors wired with #10, that would be connected to 25HP VFD rated at 34 amps FLA.

Actually, I'm not sure why the SQ D table lists #8 for a 25 HP motor. #10 should be good for 35 amps, which means the 10 HP motors would be the only ones that are an issue.

But looking up conductor ampacities made me think of something I hadn't thought of before: The conductor ampacities are listed for 86 deg F ambient, so I should really be de-rating the conductors more than the VFD's.

 

[kwired]

I was always told the feed had to be sized to the drive, just in case someone later says " thats a 30 hp drive lets put a 30 hp motor on it" .

Once had a customer that already had a 250 HP drive on machinery in an area of the plant they don't use - was equipment former owner of plant used, current owner has no active operations in that area.

Came up with an need for a VFD on a 50 HP motor, why buy a 50 HP drive when you already have a 250 hp drive not being used?

I may have been in violation of NEC, but that thing worked just fine on a circuit sized for 50 hp motor.

When the day came we needed a VFD for a 100 HP motor - why buy 100 hp drive when you have a 250? We bought a 50 hp drive to put on the 50 currently driven by the 250 HP drive and moved it to the 100 HP motor.

"30 hp drive lets put a 30 hp motor on it" Sort of no different then having size 3 contactor and electronic overload - hey that thing says it works up to 50 HP, and the overload can be turned up that far as well. That don't mean the Branch circuit device or the conductors were sized for 50 HP and all one has to do is turn up the overload:happysad:

I have seen many irrigation wells that were originally set up for 60 or 75 hp and at some time someone upgraded the motor to 75 or 100 but never changed anything but the overload setting, if it wasn't cranked all the way up anyway:happysad::happysad:

My thoughts is NEC is not entirely right on requiring us to size the input of the VFD to the rating of the drive, if the drive is not going to be used to it's full capacity. Change the motor to a different size -then you need to change whatever else is now wrong just like you do with any other control scheme. And what does it hurt to run 80 amp conductors protected with 80 amp OCPD even though the drive is rated for up to say 250 amps when it will never be loaded more then ~ 64 amps? If it were overloaded for some reason then OCPD should trip:slaphead:

 

[Jraef]

I first saw this in the 2002 NEC as Article 430.2, but it was not shown with the margin bars indicating that it was "new" at that time, so it must have been added earlier. I just don't have any older versions available to me now.

430.2 Adjustable-Speed Drive Systems. The incoming
branch circuit or feeder to power conversion equipment
included as a part of an adjustable-speed drive system shall
be based on the rated input to the power conversion equipment.
Where the power conversion equipment is marked to
indicate that overload protection is included, additional
overload protection shall not be required.
The disconnecting means shall be permitted to be in the
incoming line to the conversion equipment and shall have a
rating not less than 115 percent of the rated input current of
the conversion unit.

In 2011 it got it's own section (Section X) but this issue regarding conductor sizing remained the same, albeit in 430.122 now. The code doesn't really give any leeway so if you decide to ignore a portion of it, you take your chances.

The Informational Note they added at that time is because the 'HP rating" of a VFD can be different based on the type of load that connects to it, i.e. Variable Torque or Constant Torque. That means a single drive can have two different HP and current ratings based on the nature of the load that the motor is used on, because part of how a drive is rated has to do with its ability to keep working under overload conditions. With a VT load such as a centrifugal pump or fan, you literally CANNOT overload it under normal operating circumstances, the nature of the machine doesn't allow it. You cannot make a centrifugal pump draw more current unless you somehow increase the flow, which if properly designed in the first place, means the only time that can happen is if a pipe breaks (open channel flow) in which case you WANT the drive to trip it off line. So a VT rated drive can handle a larger motor under those circumstances and the drive current rating will be based on 110% overload for 30 seconds, knowing that this should never happen. If the load is anything OTHER than a centrifugal pump or fan, then you would expect the drive to see brief overload conditions as it works to accelerate, or RE-accelerate the load as things change. For example dropping a large box on a conveyor, the conveyor initially slows, so the motor draws more current for a few seconds as it re-accelerates it. So a CT rated drive is going to be expected to handle 150% current for 60 seconds. So a VFD rated for 100HP VT will only be rated for 75HP CT. Same transistors, different load profiles, different expectations, different current ratings.

Initially UL did not recognize this difference, but the VFD mfrs promoted it anyway and it caused some conflict. So UL started allowing this to be shown on the nameplates in about 2009, hence the change in the NEC to add that Informational note, because the same drive can handle a larger motor if it is VT, ergo the Max Amp rating is different on the same drive depending on how you use it, and that affects the conductor ampacity.

 

[kwired]

I first saw this in the 2002 NEC as Article 430.2, but it was not shown with the margin bars indicating that it was "new" at that time, so it must have been added earlier. I just don't have any older versions available to me now.


In 2011 it got it's own section (Section X) but this issue regarding conductor sizing remained the same, albeit in 430.122 now. The code doesn't really give any leeway so if you decide to ignore a portion of it, you take your chances.

The Informational Note they added at that time is because the 'HP rating" of a VFD can be different based on the type of load that connects to it, i.e. Variable Torque or Constant Torque. That means a single drive can have two different HP and current ratings based on the nature of the load that the motor is used on, because part of how a drive is rated has to do with its ability to keep working under overload conditions. With a VT load such as a centrifugal pump or fan, you literally CANNOT overload it under normal operating circumstances, the nature of the machine doesn't allow it. You cannot make a centrifugal pump draw more current unless you somehow increase the flow, which if properly designed in the first place, means the only time that can happen is if a pipe breaks (open channel flow) in which case you WANT the drive to trip it off line. So a VT rated drive can handle a larger motor under those circumstances and the drive current rating will be based on 110% overload for 30 seconds, knowing that this should never happen. If the load is anything OTHER than a centrifugal pump or fan, then you would expect the drive to see brief overload conditions as it works to accelerate, or RE-accelerate the load as things change. For example dropping a large box on a conveyor, the conveyor initially slows, so the motor draws more current for a few seconds as it re-accelerates it. So a CT rated drive is going to be expected to handle 150% current for 60 seconds. So a VFD rated for 100HP VT will only be rated for 75HP CT. Same transistors, different load profiles, different expectations, different current ratings.

Initially UL did not recognize this difference, but the VFD mfrs promoted it anyway and it caused some conflict. So UL started allowing this to be shown on the nameplates in about 2009, hence the change in the NEC to add that Informational note, because the same drive can handle a larger motor if it is VT, ergo the Max Amp rating is different on the same drive depending on how you use it, and that affects the conductor ampacity.

I expect the drive loaded to 150% or even 300% for too long to trip on overload protection before it would ever trip the branch circuit overcurrent protection and don't see the point in being so particular on supply rating of the drive. You have to set your overload protection correctly even on across the line motor starters. Drives in general are pretty good at protecting themselves, only time a branch circuit device supplying a drive should trip is if the drive front end components have failed, whether the motor supplied is full drive rated, 1/3 of the drive rated, or even 3 times the drive rating.

 

[Jraef]

I've heard it was this issue of drives often being over sized that prompted the change, but it would be interesting to see notes on what actually led up to it back in 2001, or whenever it took place.

 

[kwired]

I've heard it was this issue of drives often being over sized that prompted the change, but it would be interesting to see notes on what actually led up to it back in 2001, or whenever it took place.

Still just as stupid to me as requiring me to run conductors and overcurrent protection sized for a 100 HP motor when I install a size 4 pump panel but am only connecting a 60 HP motor. And I have seen many cases where someone upgraded that 60 HP motor to 75 but not change conductor size, maybe even keep same overcurrent protection if it allows the motor to start.

 

[steve66]

Still just as stupid to me as requiring me to run conductors and overcurrent protection sized for a 100 HP motor when I install a size 4 pump panel but am only connecting a 60 HP motor. And I have seen many cases where someone upgraded that 60 HP motor to 75 but not change conductor size, maybe even keep same overcurrent protection if it allows the motor to start.

Now that I think about it, since the motors will be on a VFD, their speed is set. So the mechanical load is basically fixed, and changing a motor shouldn't really change how much current or power it draws, even if they change it with a larger motor.

 

[kwired]

Now that I think about it, since the motors will be on a VFD, their speed is set. So the mechanical load is basically fixed, and changing a motor shouldn't really change how much current or power it draws, even if they change it with a larger motor.

If the mechanical load is truly fixed. There are plenty of situations where one could install a larger motor, then open a gate or valve and get more mechanical load out of the machine, that don't necessarily mean the machine can handle the added load either.

Pretty common on grain elevator leg to put ammeter on leg motor, user adjusts gate on inlet to set loading rate so they don't overload the leg, may need to adjust gate frequently if different products/commodities are frequently run through same leg.

Had an owner of a newly installed leg one time tell me "we need to be able to unload a truck faster then that", to which my reply was you are at the motors maximum capacity where you are at, any more will lead to earlier failure of the motor. Presuming motor was sized to maximum abilities of your equipment, then even increasing the motor size so you can run faster just means the other equipment is now overloaded and will have earlier failures of components.

 

[steve66]

In my case, these are all just heating and cooling circ. pumps. I think all the piping and everything is sized for a certain flow, and I doubt if there are any gates or valves that are normally pinched down to limit the flow.

The more I think about this, the more I'm with you: sizing the wiring for the VFD just in case someone replaces the motor seems kind of silly.

The mechanical guys can overwork and undersize a motor all they want, but we have to consider a 'what if" case that may never happen, and that by itself, doesn't really change the current drawn anyway.