Wire Size and Circuit protection on Servo Drives on an Industrial Machine.

[RABrillhart]

I have a client that manufactures a machine using 13 Servo Drives.
All drives start at the same time.
We are selling him the Drives. He is asked for help sizing the wire and Circuit protection devices.
I used 430 part X to size the wire per servo input current.
The Servo is listed as having overload protection for the Servo motor.
I sized the circuit breaker based on Servo input current and table 430.52(C)(1) for a synchronous motor at 250% of the current.

The client is telling me they have never sized the CB's for similar Servo's as large as we recommended.
looking at the system it looks like someone specified a servo system larger than needed, When looking at a total of 13 drives the current really adds up.
They have typically sized the main control breaker for the machine at 30 amps. If I add up all of the rated input currents to size the main control panel breaker I am at closer to 200 amps.
So here are a few questions.
1, was I wrong using 430.52(C)(1) at 250%?
2. Is it acceptable to size the wire as stated above, but size the CB's based on empirical data?
3. The method I used to size the Main Control Breaker was sizing the main panel breaker still a sum of all loads (current) except the largest load, added to the largest breaker. Is this still acceptable or should I look at another method?

Thanks in advance for your assistance!

 

[zbang]

Not the expert here, but that doesn't stop me ....

Does this machine become a NRTL listed device? If so the premises wiring rules stop making sense and the NEC often flies out the window. For instance, if there are real servos the concept of inrush doesn't exist, or if they're conventional motors on VFDs they still don't have the same inrush as an across-the-line starter; you could size for the max draw of the VFD (or possibly smaller, say if you have a 1hp motor on a 2hp VFD).

You ought to be able to size the wire from drive to motor/servo to what the drive can deliver, not the branch circuit supplying it.

The main should be able to carry the full startup and running loads of the machine, but that very well may be a lot lower than the NEC would suggest if the startup is sequential.

Consider that you're building a system, not wiring a bunch of unrelated parts - if some parts are interlocked so they can't run at the same time, you wouldn't count them as if they can.

 

[RABrillhart]

Thank you for the information. The machine is not sequential. All drives are geared to a master. My big question is whether it is acceptable to size the main control panel with empirical data and if so should wire size still be accounted for from NEC?
It appears that the drives
May be oversized.

 

[jim dungar]

I would be using UL 508A and or NFPA 79 for designing the industrial control panel for the machine, then I would use the NEC to size the main and the feeder.

You do not have synchronous motors as used by the NEC. What you have is closer to VFD controlled motors. You based your selections on the drive input not the motor HP.

Ideally the machine will be Listed as a piece of equipment so your control panel would not be a stand alone item.