VFD Branch/Feeder Circuit Conductor Protection

[mull982]

I wanted to confirm my understandings of branch/feeder conductor protection for VFD's. When sizing a VFD feeder you must adhere to 430.122 and size the feeder conducotr at 125% of the VFD rated input current. 430.122 only specified required conductor size, and does not specifically adress any requirements to the upstream OCPD protecing this conductor. My understanding is that the upstream OCPD falls under the requirements of section 240 and must be sized to adequately protect the VFD feeder conductors.

I am curretnly working on a coordination study for several facilities where I notice that the OCPD feeding VFD feeder circuits are rated much higher than the conducotr rating of these feeders. For example in once case there is a 900A breaker which feeds (2) 4/0 of a VFD feeder circuit. These (2) 4/0 cables are only rated for 460A and are obviously not protected by the upstream 900A OCPD. I do not have the VFD nameplate info so I am assuming that the conductors are sized at 125% of VFD input per 430.122, and am just focusing on their protection by the upstream OCPD. I am looking at several cases where the feeder calbes do not appear protected by the upstream breaker.

Am I missing something in article 430 or elsewhere that allows for the breaker to be signifigantly larger than the conductors (similar to rule applied in 440.22 for compressors)? The only other thing I can think of is the design engineer is using the tap rule and basing the conductor size off of some main breaker or fuse located in the VFD's. I do not have access to the VFD's to verify these main fuses or breakers.

I'd appreciate any input on understanding this.

Thanks

 

[jusme123]

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[mull982]

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Wouldn't the overload protection as you referenced only apply to the VFD load side conductors? I wouldn't thing that the overlaods would apply the protection of the conductors on the line side of the VFD?

 

[Cow]

Have you looked up the manufacturers specs on the recommended breaker/fuse size for that particular drive? If the conductors are sized to 125% of the drive input current and the breaker size is correct, it sounds like you're in good shape.

 

[petersonra]

IMO it is a motor circuit. Just like any other motor circuit.

You just have a special rule that applies to a motor circuit that contains a VFD for sizing the input conductors.

 

[mull982]

Have you looked up the manufacturers specs on the recommended breaker/fuse size for that particular drive? If the conductors are sized to 125% of the drive input current and the breaker size is correct, it sounds like you're in good shape.

So are you suggesting that as long as the conductors are sized at 125% of drive input and upstream OCPD is sized per the VFD manufacturers specs then that is all that is required. The VFD manufactuers specification on OCPD is in lieu of any code requirement or takes priority over any code requirment?

IMO it is a motor circuit. Just like any other motor circuit.

You just have a special rule that applies to a motor circuit that contains a VFD for sizing the input conductors.

Are you saying that since this is considered a motor branch circuit then the maximum OCPD values in Table 430.52 appy for the OCPD upstream of the VFD? Basically the VFD is considered as any other type of branch circuit motor controller and thus 430.52 can apply? If that is the case than my upstream Inverse Time OCPD can be sized at 250% per this table. But is this 250% of the motor FLA, or the Drive input current? Typically its the motor FLA but in this case I'm not quite sure.

 

[petersonra]

So are you suggesting that as long as the conductors are sized at 125% of drive input and upstream OCPD is sized per the VFD manufacturers specs then that is all that is required. The VFD manufactuers specification on OCPD is in lieu of any code requirement or takes priority over any code requirment?




Are you saying that since this is considered a motor branch circuit then the maximum OCPD values in Table 430.52 appy for the OCPD upstream of the VFD? Basically the VFD is considered as any other type of branch circuit motor controller and thus 430.52 can apply? If that is the case than my upstream Inverse Time OCPD can be sized at 250% per this table. But is this 250% of the motor FLA, or the Drive input current? Typically its the motor FLA but in this case I'm not quite sure.

X. Adjustable-Speed Drive Systems
430.120 General. The installation provisions of Part I
through Part IX are applicable unless modified or supplemented
by Part X.

How does part X modify 430.52 in any way?

 

[Cow]

So are you suggesting that as long as the conductors are sized at 125% of drive input and upstream OCPD is sized per the VFD manufacturers specs then that is all that is required. The VFD manufactuers specification on OCPD is in lieu of any code requirement or takes priority over any code requirment?

I would look at both. The recommended breaker size by the drive manufacturer as well as the NEC tables breaker size for just that motor. You should find some overlap on the breaker sizes and that's where I'd pick my size. The only instance I can think of that might get you in trouble is if someone installed a WAY OVERSIZE drive on a motor, then you could get into breaker sizing problems.

You need to get the VFD nameplate info and their recommended breaker sizes.

 

[kwired]

I would look at both. The recommended breaker size by the drive manufacturer as well as the NEC tables breaker size for just that motor. You should find some overlap on the breaker sizes and that's where I'd pick my size. The only instance I can think of that might get you in trouble is if someone installed a WAY OVERSIZE drive on a motor, then you could get into breaker sizing problems.

You need to get the VFD nameplate info and their recommended breaker sizes.

Something else to consider is if the drive were to be bypassed if it fails. But most of the time if the drive is sized to the motor your conductor size will be same either way. It can be an issue like you said if the drive is oversized in comparison to the motor. I do have an install where a 250 hp drive is supplying a 50 hp motor. Circuit conductors and overcurrent device are sized to the 50 hp motor as if it were not connected to a VFD. Maybe a code violation, but what sense does it make to size the conductors and breaker in this case for a 250 HP motor? Thing has run fine for around 10 years now. We had an existing 50 hp driven machine and a need to adjust speed. We also had 250 hp drive available on machine no longer being used, why not use it?