Branch circuit protection for single-motor application with VFD

[tomb53]

Looking for interpretation of Article 430.130(A)(1) and (2) with respect to Branch Circuit Short Circuit and Ground Fault Protective device rating for single motor applications when a VFD is involved. The article refers to 430.52 which really addresses only manual motor controller and overload relay. If a higher BCP value is specified in the VFD manufacturer's instructions, can this override Article 430.52 and Table 430-52 since overload and short circuit protection is provided by the VFD? The VFD is UL Listed with higher BCP value than 400% of full-load motor current. This is especially true for single-phase input VFD with 3-phase output but applies to others as well.

 

[tom baker]

Did you look at part X of Article 430?

 

[Jraef]

Looking for interpretation of Article 430.130(A)(1) and (2) with respect to Branch Circuit Short Circuit and Ground Fault Protective device rating for single motor applications when a VFD is involved. The article refers to 430.52 which really addresses only manual motor controller and overload relay. If a higher BCP value is specified in the VFD manufacturer's instructions, can this override Article 430.52 and Table 430-52 since overload and short circuit protection is provided by the VFD? The VFD is UL Listed with higher BCP value than 400% of full-load motor current. This is especially true for single-phase input VFD with 3-phase output but applies to others as well.

Yes, this is confusing, and there is some code history that makes it even more muddy in my opinion. So hold on, it's a bumpy ride.

This issue was raised after the 2002 NEC came into effect when the rules on VFD circuit sizing were added (at that time, 430.2):

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.

This resulted in the OCPD ahead of the VFD possibly ending up too large for the motor down stream of it. So it was addressed in the UL listing requirements for VFDs starting in around 2005 to where in order to be UL listed, VFDs must now provide the SC protection of the motor it is programmed to operate. So for example if you use a 10HP VFD to run a 5HP motor, the OCPD required to protect the VFD had to be sized for the VFD, so your OCPD sometimes ended up well outside of the range allowable in the NEC 430.52. After the change in the UL listing procedure, when you program the VFD for protecting the 5HP motor, you are ALSO (behind the scenes, transparent to you) programming the SC protection of that motor.

HOWEVER, this is NOT what it says in the NEC. In 2005 they CHANGED the original wording in 2002 430.2 and replaced it with 430.122, which exists today, stating that only the CONDUCTORS must be sized for the VFD rated input current. They REMOVED the wording regarding the branch circuit because people (including UL) were interpretting that to include the OCPD. But they left that issue unaddressed, meaning it was unclear again.

So they AGAIN fixed in in the 2014 code by adding 430.130, which SPECIFICALLY addresses "power conversion equipment", which means VFDs and Branch Over Current Protection.

430.130 Branch-Circuit Short-Circuit and Ground Fault Protection for Single Motor Circuits Containing Power Conversion Equipment. (too long to copy, read it yourself, but it essentially refers BACK to 430.52)

When that code section came out, UL, in the process of changing the VFD section of UL508C to the new IEC harmonized standard for VFDs (UL 61800-5-1) in 2015, changed again to harmonize with what the NEC says in 430.130.

Bottom line. What this means is that (as of the 2014 NEC) what you do is to:
Size the OCPD feeding the VFD for the motor using 430.52 rules, but
Size the CONDUCTORS feeding the VFD per 430.122.

The problem with that however is that some VFDs allow as much as a 10:1 size ratio, meaning you can operate a motor that is 1/10th the size of the VFD (example a 100HP VFD operating a 10HP motor). In that case you might be trying to make connections to a small breaker with conductors that will not fit in the lugs. IMHO the NEC and UL need to address this again with that in mind, since UL had ALREADY required that VFDs act as the SC protection for the motor.

*Side note: 430.52 is NOT just about OL relays, Section 1 is all about the Branch OCPD. Also you are applying the EXCEPTION that allows you to go to 400%, but that only applies if the STARTING CURRENT is so high that the normal size in the table is inadequate. With a VFD, that exception would never apply.

 

[swptln]

Looking for interpretation of Article 430.130(A)(1) and (2) with respect to Branch Circuit Short Circuit and Ground Fault Protective device rating for single motor applications when a VFD is involved. The article refers to 430.52 which really addresses only manual motor controller and overload relay. If a higher BCP value is specified in the VFD manufacturer's instructions, can this override Article 430.52 and Table 430-52 since overload and short circuit protection is provided by the VFD? The VFD is UL Listed with higher BCP value than 400% of full-load motor current. This is especially true for single-phase input VFD with 3-phase output but applies to others as well.

If this part of a field installed VFD and field installed motor, then you size your BCP just as you would any other motor controller, based on motor nameplate ratings. If this a VFD and motor system, which is what it sounds like......the motor and VFD are a listed and tested system and you install your BCP per the manufacturers specifications/rated input.

 

[kwired]

Yes, this is confusing, and there is some code history that makes it even more muddy in my opinion. So hold on, it's a bumpy ride.

This issue was raised after the 2002 NEC came into effect when the rules on VFD circuit sizing were added (at that time, 430.2):

This resulted in the OCPD ahead of the VFD possibly ending up too large for the motor down stream of it. So it was addressed in the UL listing requirements for VFDs starting in around 2005 to where in order to be UL listed, VFDs must now provide the SC protection of the motor it is programmed to operate. So for example if you use a 10HP VFD to run a 5HP motor, the OCPD required to protect the VFD had to be sized for the VFD, so your OCPD sometimes ended up well outside of the range allowable in the NEC 430.52. After the change in the UL listing procedure, when you program the VFD for protecting the 5HP motor, you are ALSO (behind the scenes, transparent to you) programming the SC protection of that motor.

HOWEVER, this is NOT what it says in the NEC. In 2005 they CHANGED the original wording in 2002 430.2 and replaced it with 430.122, which exists today, stating that only the CONDUCTORS must be sized for the VFD rated input current. They REMOVED the wording regarding the branch circuit because people (including UL) were interpretting that to include the OCPD. But they left that issue unaddressed, meaning it was unclear again.

So they AGAIN fixed in in the 2014 code by adding 430.130, which SPECIFICALLY addresses "power conversion equipment", which means VFDs and Branch Over Current Protection.


When that code section came out, UL, in the process of changing the VFD section of UL508C to the new IEC harmonized standard for VFDs (UL 61800-5-1) in 2015, changed again to harmonize with what the NEC says in 430.130.

Bottom line. What this means is that (as of the 2014 NEC) what you do is to:
Size the OCPD feeding the VFD for the motor using 430.52 rules, but
Size the CONDUCTORS feeding the VFD per 430.122.

The problem with that however is that some VFDs allow as much as a 10:1 size ratio, meaning you can operate a motor that is 1/10th the size of the VFD (example a 100HP VFD operating a 10HP motor). In that case you might be trying to make connections to a small breaker with conductors that will not fit in the lugs. IMHO the NEC and UL need to address this again with that in mind, since UL had ALREADY required that VFDs act as the SC protection for the motor.

*Side note: 430.52 is NOT just about OL relays, Section 1 is all about the Branch OCPD. Also you are applying the EXCEPTION that allows you to go to 400%, but that only applies if the STARTING CURRENT is so high that the normal size in the table is inadequate. With a VFD, that exception would never apply.

Yes they need to address it again, I haven't run into a drive yet that won't respond to a short circutit or ground fault faster than conventional overcurrent devices, those drives do this if for no other reason to protect the drive itself from the fault current.

IMO conductor ampacity needs to be sized to the connected motor and branch circuit SC/GF protection really doesn't matter can be per conductor ampacity or per allowances in art 430. Overload protection is already going to be done by the drive. If you have a oversized drive and don't set the overload protection in accordance with the motor rating - then that in itself is a failure on your part. Sort of no different than needing a 35 amp fuse but needing to use a 60 amp fuseholder to accommodate it and then installing a 60 amp fuse anyway.