Wire Size Calculations

[bjfitz]

I work for a company that manufactures industrial equipment with listed industrial control panels. Many of these have 1-3 VFDs and a mix of other loads, including a 24VDC power supply and smaller, fixed speed fans. We have gotten many questions from customers regarding our published supply wire sizing recommendations for variable speed control panels. We use the requirements of 409.20 to calculate required ampacity, taking the VFD input current rating as the full-load current rating of motor loads rather than the actual motor load per the requirements of 430.122. We use the following formula to calculate the required ampacity for a machine:

(1.25 * largest VFD input current rating) + remaining VFD input current rating + FLC off all fan motor loads + (1.25 * 24VDC control power supply input FLC rating)

We believe this method to be correct with NEC, but are there any comments or corrections that we should consider in our calculations?

 

[petersonra]

presumably by "listed" you mean UL508a.

UL508a does not have any requirement at all on how to size the feeder ampacity of wires coming into the panel.

In any case 409.20 says nothing about using the VFD input current to calculate the feeder size.

409.20 Conductor ? Minimum Size and Ampacity. The
size of the industrial control panel supply conductor shall
have an ampacity not less than 125 percent of the full-load
current rating of all resistance heating loads plus 125 percent
of the full-load current rating of the highest rated motor
plus the sum of the full-load current ratings of all other
connected motors and apparatus based on their duty cycle
that may be in operation at the same time.

As for the 24VDC load - it would be part of the

full-load current ratings of all other ... apparatus

In this case likely a transformer feeding a power supply, unless it has a 480V input. in any case, there is no need to add 25%.

 

[Smart $]

presumably by "listed" you mean UL508a.

UL508a does not have any requirement at all on how to size the feeder ampacity of wires coming into the panel.

In any case 409.20 says nothing about using the VFD input current to calculate the feeder size.

Bob,

The supply wire size is only covered under UL standard if it is part of the listing and furnished by the manufacturer of the listed equipment. Otherwise, supply wire size is determined per NEC.

@bjfitz

430.122 is the proper section... but as a manufacturer you cannot always determine what the actual motor load will be. Recommending the size for the largest motors you panel will handle is the safest approach. However, you could note something to this effect in your documentation.

 

[petersonra]

Bob,

The supply wire size is only covered under UL standard if it is part of the listing and furnished by the manufacturer of the listed equipment. Otherwise, supply wire size is determined per NEC.

@bjfitz

430.122 is the proper section... but as a manufacturer you cannot always determine what the actual motor load will be. Recommending the size for the largest motors you panel will handle is the safest approach. However, you could note something to this effect in your documentation.

409.20 deals with industrial control panel feeder sizing. It would seem to be more on point than anything in 430.

as for UL508a, UL508a does not provide any guidance for sizing incoming feeders therefore such a recommendation is not part of the listing, even if the manufacturer makes such a recommendation.

 

[Smart $]

409.20 deals with industrial control panel feeder sizing. It would seem to be more on point than anything in 430.

Agree... but the point is that 490.20 sizing is the same as 430 sizing, where VFD max current has no bearing on wire and OCPD ratings.

as for UL508a, UL508a does not provide any guidance for sizing incoming feeders therefore such a recommendation is not part of the listing, even if the manufacturer makes such a recommendation.

Agree... but also say that the recommendation should not be stated in a manner in which the consumer believes it to be part of the listing.

 

[bjfitz]

presumably by "listed" you mean UL508a.

Yes, that is correct.

Agree... but the point is that 490.20 sizing is the same as 430 sizing, where VFD max current has no bearing on wire and OCPD ratings.

Per 430.122 (A), I read the following as a requirement that conductors must be sized to the VFD rating rather than the motor load.

(A) Branch/Feeder Circuit Conductors. Circuit conductors
supplying power conversion equipment included as part of an
adjustable-speed drive system shall have an ampacity not less
than 125 percent of the rated input current to the power conversion
equipment.

Our understanding of the requirements of 430.122 is that the drive input rating is used rather than the output rating in order to account for the inefficiencies and power conversion occurring with this setup. Would this not apply when the VFD is in a control panel?

 

[Smart $]

...
Per 430.122 (A), I read the following as a requirement that conductors must be sized to the VFD rating rather than the motor load.

Our understanding of the requirements of 430.122 is that the drive input rating is used rather than the output rating in order to account for the inefficiencies and power conversion occurring with this setup. Would this not apply when the VFD is in a control panel?

Sorry, I didn't actually look at 430.122... because I'm semi-familiar with the requirements of Part X.

The thing is, you have to take the context of Part X in its entirety to see what I'm saying... but most notably...

430.130 Branch-Circuit Short-Circuit and Ground-Fault Protection for Single Motor Circuits Containing Power Conversion Equipment.

(A) Circuits Containing Power Conversion Equipment. Circuits containing power conversion equipment shall be protected by a branch-circuit short-circuit and ground-fault protective device in accordance with the following:
(1) The rating and type of protection shall be determined by 430.52(C)(1), (C)(3), (C)(5), or (C)(6), using the full-load current rating of the motor load as determined by 430.6.
(2) Where maximum branch-circuit short-circuit and ground-fault protective ratings are stipulated for specific device types in the manufacturer?s instructions for the power conversion equipment or are otherwise marked on the equipment, they shall not be exceeded even if higher values are permitted by 430.130(A)(1).
(3) A self-protected combination controller shall only be permitted where specifically identified in the manufacturer?s instructions for the power conversion equipment or if otherwise marked on the equipment.

Informational Note: The type of protective device, its rating, and its setting are often marked on or provided with the power conversion equipment.

(B) Bypass Circuit/Device. Branch-circuit short-circuit and ground-fault protection shall also be provided for a bypass circuit/device(s). Where a single branch-circuit short-circuit and ground-fault protective device is provided for circuits containing both power conversion equipment and a bypass circuit, the branch-circuit protective device type and its rating or setting shall be in accordance with those determined for the power conversion equipment and for the bypass circuit/device(s) equipment.

430.131 Several Motors or Loads on One Branch Circuit Including Power Conversion Equipment. For installations meeting all the requirements of 430.53 that include one or more power converters, the branch-circuit short- circuit and ground-fault protective fuses or inverse time circuit breakers shall be of a type and rating or setting permitted for use with the power conversion equipment using the full-load current rating of the connected motor load in accordance with 430.53. For the purposes of 430.53 and 430.131, power conversion equipment shall be considered to be a motor controller.

But otherwise, you are correct in that the wire is sized based on the rated input current of the VFD's.

 

[bjfitz]

Alright, thank you both for your help, I appreciate the input.