Okay, let's break down your questions and provide guidance on how to approach this situation. Sizing circuit protection for servo drives can be tricky, especially with multiple drives running simultaneously.

Understanding the Situation

You're dealing with a machine that uses 13 servo drives, all starting simultaneously. You're providing the drives and responsible for helping your client determine proper wire and circuit breaker sizing. The core issue is that the calculated circuit breaker size based on NEC 430 seems significantly larger than what your client has historically used. There's a suspicion that the servos might be oversized for the application, which is contributing to the high calculated current.

Addressing Your Questions

Here's a breakdown of your questions and recommendations

1. Was I wrong using 430.52(C)(1) at 250%?
Maybe not wrong, but potentially overly conservative and potentially incorrect. NEC 430.52(C)(1) is a maximum value and depends on if the servo is even considered a synchronous motor. The code also states, if the listed CB or fuse is smaller, you may use that value instead. The key question is: What type of motor is controlled by the servo drive? A servo drive typically controls either a permanent magnet synchronous motor (PMSM) or an induction motor. If it controls a PMSM, then using Table 430.52(C)(1) for "Synchronous Motor" with a maximum of 250% for an inverse time breaker could be applicable, if there is no other data to specify a lower overcurrent protection value. If it controls an induction motor, the allowable percentage is different (typically higher). Important Considerations



Manufacturer's Recommendations

Crucially, you need to check the servo drive manufacturer's documentation. They will often specify recommended circuit breaker types and sizes. These recommendations often consider the drive's internal protection and inrush characteristics. The manufacturer's data takes precedence.



Short-Circuit Current Rating (SCCR)

The circuit breaker must have an adequate SCCR to protect the drive from fault currents at the installation location.

Type of Circuit Breaker

Consider using time-delay fuses or specific motor-rated circuit breakers. These are designed to handle motor inrush currents without nuisance tripping.

2. Is it acceptable to size the wire as stated above, but size the CB's based on empirical data?

No, not generally acceptable. You cannot arbitrarily ignore code requirements and rely solely on empirical data. That said, code is not always a black and white issue.

Wire Sizing

Sizing the wire based on the motor nameplate current is appropriate as long as you account for adjustments due to ambient temperature, conductor bundling, and voltage drop. If you are using the servo input current as specified by the servo manufacturer and the value is higher than the motor current, you are following the code requirement.

Circuit Breaker Sizing

You can't simply reduce the breaker size based on past experience if it violates the NEC. However, you CAN do the following As stated above, use the manufacturer's recommendations. Determine if the synchronous motor is truly a synchronous motor. The code has an exception to use less than 250% if the breaker trips during startup.

However, using empirical data can be a starting point for investigation

If your client's past experience is drastically different, it's a flag to:

Verify the Drive Specifications

Double-check the servo drive model numbers, input current ratings, and motor horsepower.

Review the Application

Are the drives truly loaded to their full capacity? If the machine is under-utilized, the drives may be oversized.

Analyze Inrush Current

The high calculated breaker size likely stems from the inrush current at startup. Consider using inrush current limiting devices or soft starters if appropriate for the application. (Note: Servo drives often have built-in soft start functionality.)

Consider Power Factor Correction

If the servo drives have a low power factor, this can increase the current draw.

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?

This is a common and generally acceptable method, BUT with a few caveats



Standard Calculation

Sum the continuous load currents, add 125% of the largest motor's full-load current, and add 100% of the remaining motor's full-load currents.

Consider Simultaneous Starting

The method you described assumes that not all motors start simultaneously. However, you have stated that all motors start simultaneously. This can be a problem for a couple reasons:

Voltage Sag

Starting all 13 drives at once will cause a significant voltage sag on the system. This can cause problems with other equipment.

High Inrush Current

Starting all 13 drives at once will create a very high inrush current. This can trip the main breaker.

Demand Factors

Carefully consider the actual operating characteristics of the machine. Are all 13 drives always operating at their maximum current simultaneously? If not, you might be able to apply a demand factor to reduce the calculated current. Be very cautious when applying demand factors. Get your calculations approved by an engineer or inspector.

Feeder Conductor Size

Don't forget to ensure the feeder conductors (wires feeding the entire control panel) are sized appropriately for the calculated load.

Recommendations1.

Obtain Servo Drive Manufacturer Documentation

This is your most important step. Get the installation manuals for the specific servo drive models you are supplying. The manufacturer's data sheet and installation guide should provide clear guidance on recommended circuit protection, including type and size.2.

Verify Motor Type and Ratings

Confirm the type of motor the servo drive controls (PMSM, induction, etc.) and obtain the motor's nameplate data (horsepower, voltage, FLA).3.

Review the Application with the Client

Understand the actual load requirements of the machine. Are the drives truly running at full capacity simultaneously? Could the machine operation be modified to stagger the starting of some drives?4.

Document Your Calculations

Clearly document all your calculations, including the NEC code sections you used, the servo drive manufacturer's recommendations, and any demand factors applied.5.

Consider a Professional Engineer (PE)

Given the complexity of the system and the potential safety implications, it may be wise to engage a licensed electrical engineer to review your design and calculations. They can provide a stamped engineering drawing and ensure compliance with all applicable codes and standards.6.

Communicate with the Authority Having Jurisdiction (AHJ)

Discuss your approach with the local electrical inspector (AHJ) to ensure they are comfortable with your proposed solution.

In summary, do not solely rely on empirical data, and prioritize the manufacturer's recommendations. Use 430 as a guide, but also, if needed, as a tool, but follow the data and documentation provided to you to ensure a safe install. Flag for review