?Is a Kinetix 5500 circuit considered a Motor Branch Circuit?

Good afternoon,
Does anyone know if a circuit containing 3 Kinetix 5500 drives (& their motors) is considered a Motor Branch Circuit according to NEC 430?

My design steps for Motor Branch Circuits would be:

• Determine FLC of motor from NEC[430.150]
• Determine conductor size/ampacity using [430.22]
• Size motor overload relays
• Size motor branch circuit short circuit and ground fault protective device using [430.52]

Alternatively, for a Non-Motor Branch Circuit:

• Add up the load
• Determine conductor size/ampacity using [310.15,16] and [210.19]
• Size branch circuit short circuit and ground fault protective device using [210.20(A)] and [240.4(D)]

But a branch circuit with 3 servo drives (in series) and their motors seems to be a hybrid of the two types of circuits. I heard somewhere that you just disregard the motor FLC and use the Amp rating of the drives for determining the load. But I haven't gotten any good information on which of the two code sections I should be using for conductor ampacity or circuit protection. Should I be in the NEC[430.xx] section or the NEC[2XX.X]-[3XX.X]?

Thanks for any clarification!

Does anyone here have any insight into this?

Is this for academic purposes or an actual project? The corresponding AB manuals have pretty good sections regarding the power circuitry.

What do you mean by (in series)? The load side of Your branch circuit protectors could be in series in some applications but I would probably strangle someone if I ever saw drives wired in series with a single BCP.

To answer your other question. VFD’s have a max rated output current, the branch circuit and the branch circuit protective device should be sized for that rating. In theory you could hook a 10 hp vfd to a 15 hp motor, not really a scenario that you would normally come across. In that scenario you could size the branch circuit to the vfd max output current not the motor.

However, a more likely scenario is for a VFD to be oversized above the motor hp. Say, 15 hp vfd to a 10 hp motor. In that situation you should size the branch circuit the same as above, the max output current of the vfd. In this scenario the vfd output current would be above the fla of the motor. Since the vfd has the potential to output current above the fla of the motor then the branch circuit should be sized to the vfd.

JonHopper said:

Is this for academic purposes or an actual project? The corresponding AB manuals have pretty good sections regarding the power circuitry.

Click to expand...

I guess it's a mix of both. I work for an OEM, and my current project has already been designed and built but I just want to try to learn how to design the next one.

_Dock_ said:

What do you mean by (in series)? The load side of Your branch circuit protectors could be in series in some applications but I would probably strangle someone if I ever saw drives wired in series with a single BCP.

Click to expand...

I believe the Kinetix 5500s are designed to be used together in sets of multiple axes on one branch circuit. AB calls the feature "Multi-Axis, 3-phase bus sharing". The power is bussed together across the top of the drives.

After looking deeper into the NFPA 79 chapter 7, it actually has a lot of guidance for circuit protection as it relates to motor controller circuits (I'm assuming this is referring to circuits with VFD + Motor). (See 7.2.10.1 through 7.2.10.5).