OCPD for VFD Circuit

[philly]

I am looking at an application where there is a VFD fed from a panelboard with a 60A breaker and #10 wire. The motor fed from the VFD is a 15hp motor with a 21A FLA rating.

In this applicaiton would the #10 cable be considered a motor feeder circuit or branch circuit?

If it is a branch circuit I'm thinking the #10 wire is adequatly protected by the 60A breaker since the NEC allows branch circuit breaker to be 250% of motor FLA. However if this is considered a feeder circuit I dont think the #10 wire is adequatly protected by the 60A breaker. Im not sure of the VFD input requirments for feeder ampacity of max OCPD size but regardless of what the VFD required the amapcity of the feeder circuit would still have to be adequate for the feeder breaker correct?

The only other thing that comes to mind is if the VFD has a main breaker rated at 30A which could then utilize the tap rule and protect the #10 cable however I dont see why the VFD would have a 30A breaker and the feeder breaker be a 60A breaker.

Any thoughts?

 

[Smart $]

I am looking at an application where there is a VFD fed from a panelboard with a 60A breaker and #10 wire. The motor fed from the VFD is a 15hp motor with a 21A FLA rating.

In this applicaiton would the #10 cable be considered a motor feeder circuit or branch circuit?

If it is a branch circuit I'm thinking the #10 wire is adequatly protected by the 60A breaker since the NEC allows branch circuit breaker to be 250% of motor FLA. However if this is considered a feeder circuit I dont think the #10 wire is adequatly protected by the 60A breaker. Im not sure of the VFD input requirments for feeder ampacity of max OCPD size but regardless of what the VFD required the amapcity of the feeder circuit would still have to be adequate for the feeder breaker correct?

The only other thing that comes to mind is if the VFD has a main breaker rated at 30A which could then utilize the tap rule and protect the #10 cable however I dont see why the VFD would have a 30A breaker and the feeder breaker be a 60A breaker.

Any thoughts?

VFD requirements are covered under 430 Part X. 430.120 says Parts I through IX applies to anything not modified in Part X. 430.122(A) says the supply conductors ampacity must be not less than 125% of the VFD's rated input current. However, its title is "Branch/Feeder Circuit Conductors." So it is not clear whether the supply conductors are one or the other. Is it possible to be both given their definitions?

There is nothing in Part X regarding OCPD rating, so it falls back on Parts I through IX. Never really considered the scenario you bring up, and am leaning in the same direction as your mode of thinking. The FLA at 250% would permit a 50A breaker, right?

 

[Jraef]

VFD requirements are covered under 430 Part X. 430.120 says Parts I through IX applies to anything not modified in Part X. 430.122(A) says the supply conductors ampacity must be not less than 125% of the VFD's rated input current. However, its title is "Branch/Feeder Circuit Conductors." So it is not clear whether the supply conductors are one or the other. Is it possible to be both given their definitions?

There is nothing in Part X regarding OCPD rating, so it falls back on Parts I through IX. Never really considered the scenario you bring up, and am leaning in the same direction as your mode of thinking. The FLA at 250% would permit a 50A breaker, right?

The conductors feeding the drive are the FEEDER circuit for the drive. 430.122 says that those conductors must now be sized at 125% of the VFD maximum nameplate amps, NOT the motor. You (Philly) did not say what that is. The motor FLA is no longer used if there is a VFD.

The conductors from the VFD to the motor are the BRANCH circuit, and per recent (2005) changes to the UL rules on VFDs, now the VFD must provide the BRANCH SC and GF protection for the circuit below it. This was because after the FEEDER conductor sizing rules established in 430.122, it became possible for the up stream device to become too large for the motor if the motor was smaller than the VFD size, which happens quite a lot.

So bottom line, you do not need to worry about the SP/GF PD on the motor side of the VFD (assuming it is UL listed as a "controller"* and less than 9 years old) and once you know the VFD max amp rating, you size the FEEDER conductors based on that. But the NEC is not going to dictate the size of the feeder OCPD in any manner than normal, i.e. size for the conductors you run, AND per the equipment mfr's instructions. If the VFD mfr states a maximum OCPD, you must abide by that, plus it's likely that OCPD was involved in the UL listing of it.

And by the way pay close attention of the wording, some low-end players only UL list their units behind fuses, because it is easier to get a favorable SCCR listing on it that way.

*UL listing as a "controller" allows the exception in the NEC that says (paraphrased), "unless the controller is listed to provide this". The tricky thing is, some of the bottom feeder VFDs are either NOT listed at all, or are UL listed as "Power Conversion Equipment" instead, which means they do NOT need to abide by that SC/GFPD requirement, and you are on your own.

 

[don_resqcapt19]

The conductors feeding the drive are the FEEDER circuit for the drive. ...

Are you saying that the drive itself is the branch circuit overcurrent protective device?

 

[Smart $]

Jraef,

Having already said what I had to say, let me state it in another way.

Where the supply conductors are the motor feeder circuit, the SC/GFP requirement defaults to 430.62, which says the motor feeder SC/GFP cannot be greater than the largest rating of the branch circuit SC/GFP for any motor supplied by the feeder. So the branch circuit SC/GFP in the OP case is 30A... which means the feeder SC/GFP rating cannot be greater than 30A, regardless of the size of conductor.


Where the supply conductors are the motor branch circuit, the SC/GFP is sized per the motor FLA, allowing in this case up to (21A x 250%=) 52.5A, rounded down to 50A for standard-rating breaker. In this scenario the 30A breaker in the VFD is considered supplementary OCPD.

 

[don_resqcapt19]

The typical application of the VFD supply conductor rules is that the standard overcurrent protection rules apply. The conductor has to be sized at least 125% of the rated input amps and those conductors must be protected at or below their ampacity. Maybe that is not what the code actually says, but that is the standard application of the rules.

 

[GoldDigger]

A perhaps critical difference is that the VFD is an active device that can have internal faults and during a fault can potentially (via braking resistors, for example) consume power that is not going to the motor.
That means that the pass through overload protection is not adequate for code purposes.

Tapatalk!

 

[petersonra]

The typical application of the VFD supply conductor rules is that the standard overcurrent protection rules apply. The conductor has to be sized at least 125% of the rated input amps and those conductors must be protected at or below their ampacity. Maybe that is not what the code actually says, but that is the standard application of the rules.

the std OC rules for a motor circuit?

 

[Jraef]

Are you saying that the drive itself is the branch circuit overcurrent protective device?

Yes, but only for the LOAD side of the VFD, not the line side. When the "125% of the VFD rating" rule was added in the 2002 NEC, it created an issue with motors significantly smaller being used on a VFD, because like an adjustable OL relay, the VFD could be programmed to protect that small motor from OL, but the new rule essentially resulted in people over sizing the conductors, and therefore the OCPD, with relation to these smaller motors. So UL stepped in around 2005 and made it so that the procedure for getting a drive UL listed as a controller had to include the SC/GF protection for the MOTOR side, just in case the motor was significantly smaller. It's basically listed just like one of those "self protected motor starters" that provide both the OL and SCPD in one. Most VFD mfrs deal with it simply by adjusting the SC/GF trip values behind the scenes in accordance with what you tell it the motor FLA is when you set it up, some deal with it by limiting the minimum motor size that they allow to be connected down stream without requiring separate OCPD devices (which becomes problematic because bi-metal current sensors don't like the high harmonics and older magnetic trips can over heat).

 

[don_resqcapt19]

Yes, but only for the LOAD side of the VFD, not the line side. When the "125% of the VFD rating" rule was added in the 2002 NEC, it created an issue with motors significantly smaller being used on a VFD, because like an adjustable OL relay, the VFD could be programmed to protect that small motor from OL, but the new rule essentially resulted in people over sizing the conductors, and therefore the OCPD, with relation to these smaller motors. So UL stepped in around 2005 and made it so that the procedure for getting a drive UL listed as a controller had to include the SC/GF protection for the MOTOR side, just in case the motor was significantly smaller. It's basically listed just like one of those "self protected motor starters" that provide both the OL and SCPD in one. Most VFD mfrs deal with it simply by adjusting the SC/GF trip values behind the scenes in accordance with what you tell it the motor FLA is when you set it up, some deal with it by limiting the minimum motor size that they allow to be connected down stream without requiring separate OCPD devices (which becomes problematic because bi-metal current sensors don't like the high harmonics and older magnetic trips can over heat).

But my point is that the supply conductors to a VFD are not feeder conductors...they are branch circuit conductors per the Article 100 definitions.

 

[don_resqcapt19]

the std OC rules for a motor circuit?

No. My next sentence in that post said that the supply conductors are protected at or below their rated ampacity. The applications that I see and most of what I have read do not use any of the motor circuit protection rules for the supply conductors to the VFD.

 

[Smart $]

... The applications that I see and most of what I have read do not use any of the motor circuit protection rules for the supply conductors to the VFD.

That may be.. but then the question arises: Where does the Code say that?

Part X does not cover SC/GFP, and Part X starts...

430.120 General. The installation provisions of Part I
through Part IX are applicable unless modified or supplemented
by Part X.

 

[petersonra]

the only thing I see in the code for VFDs that is different than for regular motor starters is the ampacity rules. and that only applies to the wire size.

you still have to follow the instructions and I have run across some VFDs that limit OCPD to 125% or thereabouts, but others come with instructions that specifically allow for much higher values of OCPD.

I am not sure why but the low end VFDs seem to be the ones with the 125% OCPD on them.

there are also some japanese drives that have some really strange input ratings.

 

[petersonra]

This is what UL508a says

31.3.2 The branch circuit protection for a single-motor circuit provided with a variable-speed drive shall
be of the type and size specified by the manufacturer?s instructions provided with the drive. When the
instructions do not specify the type and size, a branch-circuit fuse or inverse-time circuit breaker shall be
used and shall be sized in accordance with 31.3.1(a) based upon the full-load motor output current rating
of the drive.

31.3.1(a) is the normal OCPD sizing criteria for motors.

and this is the latest UL508a we just got a month or two ago.

 

[Jraef]

But my point is that the supply conductors to a VFD are not feeder conductors...they are branch circuit conductors per the Article 100 definitions.

I can't argue with you on that, I think it's too muddy and needs clarification. The VFD is kind of a specialized circuit in that what happens on the load side on terms of a SC or GF has almost no relation to what happens on the line side, because of the conversion/inversion nature of the VFD itself. To the VFD, the line side is only a raw material from which a NEW power source, dedicated to that motor circuit, is created. So really it's kind of like you have a branch, because feeding the VFD is like feeding a single load (but forget that it is a motor for a second), then a "sub-branch" behind the VFD, because it truly IS feeding the final load. But there is no definition for "sub-branch" of course, so the closest thing we have is Feeder and Branch.

 

[philly]

I can't argue with you on that, I think it's too muddy and needs clarification. The VFD is kind of a specialized circuit in that what happens on the load side on terms of a SC or GF has almost no relation to what happens on the line side, because of the conversion/inversion nature of the VFD itself. To the VFD, the line side is only a raw material from which a NEW power source, dedicated to that motor circuit, is created. So really it's kind of like you have a branch, because feeding the VFD is like feeding a single load (but forget that it is a motor for a second), then a "sub-branch" behind the VFD, because it truly IS feeding the final load. But there is no definition for "sub-branch" of course, so the closest thing we have is Feeder and Branch.

It sounds like it is just a matter of diiffering/confusing terminology as stated above however at the end of the day the NEC requires the supply circuit to the VFD to be rated at 125% of the VFD input circuit and have an OCPD that does not exceed the max OCPD rating listed by the VFD. Also the supply circuit to the vfd must be proteced by a OCPD that is rated at or less then the supply conductor.

With that being said in my particular case I did not note the input current to the VFD when I was in the field, however even if the #10 supply conductor meets 125% of the VFD input rating, and the 60A supply breaker is less than the max OCPD listed by VFD manufacturer, the fact remains that the #10 conductor with an ampacity of 30A is not adequatly protected by the 60A breaker? Do others agree that cable should be a #6 or feeder breaker be reduced to30A breaker?

Since it was discussed that the feeder breaker does not apply to the branch circuit sizing rules, then the breaker being able to be 250% of motor rating is thrown out the window as mentioned. Had their not be a VFD in the circuit and the 60A breaker fed a motor starter with overloads, then we could consider a breaker rated at 250% but since there is a VFD involved this is not the case.

So given the current circumstances the #10 cable would not be protected by the 60A feeder breaker. The only exception that I could think of would be if there was a 30A breaker located in the VFD (I did not check in the field) that would then allow the tap rule to be used with the cable terminating into a single OCPD, within 25ft, etc.....Could the provisions of 240.21 be used to protect the #10 cable in this case if there is a 30A breaker inside the VFD?

 

[petersonra]

Also the supply circuit to the vfd must be proteced by a OCPD that is rated at or less then the supply conductor.

where in article 430 does it say that?

 

[Smart $]

It sounds like it is just a matter of diiffering/confusing terminology as stated above however at the end of the day the NEC requires the supply circuit to the VFD to be rated at 125% of the VFD input circuit and have an OCPD that does not exceed the max OCPD rating listed by the VFD. Also the supply circuit to the vfd must be proteced by a OCPD that is rated at or less then the supply conductor.

With that being said in my particular case I did not note the input current to the VFD when I was in the field, however even if the #10 supply conductor meets 125% of the VFD input rating, and the 60A supply breaker is less than the max OCPD listed by VFD manufacturer, the fact remains that the #10 conductor with an ampacity of 30A is not adequatly protected by the 60A breaker? Do others agree that cable should be a #6 or feeder breaker be reduced to30A breaker?

Since it was discussed that the feeder breaker does not apply to the branch circuit sizing rules, then the breaker being able to be 250% of motor rating is thrown out the window as mentioned. Had their not be a VFD in the circuit and the 60A breaker fed a motor starter with overloads, then we could consider a breaker rated at 250% but since there is a VFD involved this is not the case.

So given the current circumstances the #10 cable would not be protected by the 60A feeder breaker. The only exception that I could think of would be if there was a 30A breaker located in the VFD (I did not check in the field) that would then allow the tap rule to be used with the cable terminating into a single OCPD, within 25ft, etc.....Could the provisions of 240.21 be used to protect the #10 cable in this case if there is a 30A breaker inside the VFD?

Charlie Trout, who answers the Code Question of the Day on the NECA Web site, seems to agree VFD supply conductor are motor branch circuit conductors and ratings are applied as such. Read first Q&A here...
http://www.ecmag.com/section/codes-standards/sizing-conductors-vfds-defining-terms-and-more

 

[don_resqcapt19]

Charlie Trout, who answers the Code Question of the Day on the NECA Web site, seems to agree VFD supply conductor are motor branch circuit conductors and ratings are applied as such. Read first Q&A here...
http://www.ecmag.com/section/codes-standards/sizing-conductors-vfds-defining-terms-and-more

He didn't really address the issue of the size or protection of the supply conductor to the VFD. There is nothing in the question or answer that even specifies the VFD input current.

 

[dkidd]

Look at the new 430.130 in the 2014.