Sizing Circuit Breaker for Drive Panel Feeder

[remlapw]

What is the proper procedure to pick the main CB size for a drive panel with servo drives and VFDs?

Each drive will also have individual fusing as per manufacture recommendations

All of the examples I find to reference are related to using starters, and not VFDs.


I have heard many differing opinions, such as 250% of largest FLA motor + the sum of the FLA of all other motors.

I would like to get a better grasp on the correct methodology and reference material.

 

[jraef]

Minimum size must be 125% of the largest load plus the sum of the remaining loads.


Maximum size is up to you, but it must match the conductors used.

 

[remlapw]

Minimum size must be 125% of the largest load plus the sum of the remaining loads.


Maximum size is up to you, but it must match the conductors used.

So in this case, that's 125% of the input current rating for the drive + sum of remaining drive input current ratings?

Do you have a reference to a standard as to where this is stated?

TIA!

 

[Rson]

So in this case, that's 125% of the input current rating for the drive + sum of remaining drive input current ratings?

Do you have a reference to a standard as to where this is stated?

TIA!

Not sure where you are from, but in the US it is called out in NFPA 70 Article 670.4(C).

The rating for the protective device supplying a machine cannot be greater than:

[Largest protective device or setting] + [sum of all motor FLA ]+ [ sum of all resistance loads * 1.25]

 

[jraef]

UL508A (in my version).

32.3 Sizing of overcurrent protection
32.3.1 The size of the overcurrent protection shall not exceed:
a) The rating of the largest branch circuit protective device in the circuit plus the full-load
currents of all other motors or other loads in the group; or
b) The ampacity of the conductors or bus bars on the load side of the overcurrent device.

If they are all VFDs or servos, then the miminum size of the OCPD for any VFD, including the largest, will be 125% of its input current rating. The maximum can be higher, so then if you use that value, you must ensure that your conductors are protected.

 

[g.mccormick]

UL508A (in my version).

If they are all VFDs or servos, then the miminum size of the OCPD for any VFD, including the largest, will be 125% of its input current rating. The maximum can be higher, so then if you use that value, you must ensure that your conductors are protected.

Don't you treat VFDs as motors when it comes to conductors and ocpd?

I believe NEC says minimum conductors sizing is 115 (or 125%) of drive input. Like on a motor circuit, you can end up with branch OCPD sized higher than the conductors behind it correct?