You're asking a classic question in electrical engineering, and your intuition is good! Here's a breakdown of the "best practice" approach to sizing a feeder to an MCC, considering the NEC (National Electrical Code in the US, adapt to your local code) and industry experience

Key Considerations & Steps:

1. NEC Requirements (and your Local Code)

This is the foundation. Always adhere to your local electrical code. The NEC (or equivalent) has specific rules about sizing feeders, overcurrent protection, and derating factors.2.

Connected Load



Calculate the total connected load

Sum the nameplate current of every motor, lighting panel, transformer, or other load that could be connected to the MCC at some point. This is a maximum theoretical value. Get accurate information on the nameplate data - don't guess.

Consider "after diversity" maximum demand

This is the peak current demand that the MCC actually experience after taking into account the diversity of the loads connected to the MCC.

Future Capacity

The bus rating needs to be considered to ensure the MCC can actually handle this future load.3.

Demand Factors & Load Diversity

This is where you start to bring reality into the equation. Not everything runs at its maximum simultaneously.

Demand Factor

This is the ratio of the maximum demand of a system (or part of a system) to the total connected load of the system (or the part of the system under consideration). It's always less than or equal to 1. Consult your local code (NEC Art. 220 and others) for allowed demand factors for various types of loads (lighting, HVAC, motors). Also, look at your experience with similar facilities.

Load Diversity

This is closely related to the demand factor. It represents the probability that all the loads will operate simultaneously at their maximum rating. Diversity factors are generally only applicable when sizing the service entrance or a feeder to the entire facility, not to a single MCC.

Apply Demand Factors Carefully

Don't arbitrarily apply high demand factors. Base them on real-world operating data or well-justified assumptions. Err on the side of caution, especially for critical loads.4.

Motor Calculations (NEC Art. 430)

Motors require special attention.

Largest Motor

The NEC requires specific calculations for the largest motor in the group, including its locked-rotor current (LRA) for overcurrent protection.

Group Motor Calculations

Determine the total current contributions from all motors, including the largest motor's extra current requirements for starting.5.

Non-Continuous vs. Continuous Loads



Continuous Load

A load is considered continuous if it operates at its maximum current for 3 hours or more. NEC requires that the overcurrent protection (OCPD) be rated for at least 125% of the continuous load.

Non-Continuous Load

Loads that don't meet the definition of continuous.

Apply the 125% Rule

This rule (related to continuous loads) is a minimum. You might choose a higher percentage for reliability or future expansion.6.

Voltage Drop

Crucial for motor performance and overall system efficiency.

NEC Informative Annex B

Provides guidelines for voltage drop. A common industry goal is to limit voltage drop to 3% for power feeders and 5% overall (feeder + branch circuit) under full load.

Calculate Voltage Drop

Use appropriate formulas (considering cable impedance, length, and current) to ensure voltage drop is within acceptable limits. This is especially important for long feeder runs. Software tools can simplify these calculations.

Consequences of Voltage Drop

Low voltage can damage motors, reduce lighting output, and cause equipment malfunction.7.

MCC Bus Ampacity



The Bus is the Bottleneck

As you mentioned, the MCC bus ampacity is the absolute maximum current the MCC can handle.

Feeder Ampacity >= Calculated Demand <= Bus Ampacity

The feeder's ampacity must be greater than or equal to your calculated demand (after applying demand factors, motor calculations, etc.) but cannot exceed the MCC bus ampacity.8.

Overcurrent Protection (OCPD)



Protect the Conductors

The OCPD (breaker or fuses) protects the feeder conductors from overloads and short circuits. Its rating should be selected based on the conductor ampacity and NEC guidelines.

Coordination

Coordinate the OCPD with downstream protective devices (motor starters, branch circuit breakers) to ensure selective tripping (only the faulted circuit trips, not the entire feeder). This is vital for system reliability.

Series Ratings

Understand and apply series ratings where applicable.9.

Conductor Type & Installation Method



Ampacity Tables

NEC provides ampacity tables for various conductor types (THHN, XHHW, etc.) and installation methods (in conduit, free air, etc.).

Derating Factors

Apply derating factors for ambient temperature, conduit fill (number of conductors in a conduit), and other conditions that can reduce the conductor's ampacity.10.

Future Expansion



Factor in Growth

Anticipate future load increases. Leaving some extra capacity in the feeder and MCC bus is a smart investment to avoid costly upgrades later. How much extra depends on the likelihood of future expansion and the cost of upsizing now versus later. 15-25% extra is a good starting point.

Modular MCCs

Consider modular MCCs that allow for easy addition of new buckets (motor starters, VFDs) as needed.

In Summary - The Decision Process


Start with the Code

Adhere to all applicable local electrical codes.2.

Calculate Connected Load

Determine the absolute maximum load possible.3.

Apply Demand Factors

Reduce the calculated load based on realistic operating conditions and code allowances.4.

Motor Calculations

Properly account for motor starting currents.5.

Check Voltage Drop

Ensure voltage drop is within acceptable limits. Adjust conductor size if needed.6.

Select Conductor Size

Based on the calculated demand, conductor ampacity (after derating), and voltage drop considerations.7.

Select OCPD

Based on the conductor ampacity and NEC requirements.8.

Verify Bus Ampacity

Ensure the MCC bus ampacity is greater than or equal to the calculated demand (after applying demand factors).9.

Consider Future Expansion

Add extra capacity to the feeder and MCC bus if future load increases are anticipated.

Example (Simplified)

Let's say you have an MCC with the following: Total connected motor load: 200 amps (nameplate) Lighting load: 50 amps Miscellaneous loads: 20 amps MCC Bus: 400A1.

Connected Load

200 + 50 + 20 = 270 amps2.

Demand Factors

Let's assume, after reviewing your local code, you can apply a demand factor of 80% to the motor load (200 0.8 = 160 amps). Assume all other loads are on constantly.
3.

Adjusted Load

160 + 50 + 20 = 230 amps4.

Motor Starting

Determine the LRA of the largest motor and add the LRA to running currents of the rest of the motors. Let's say this bumps the initial adjusted load up to 280A.5.

Future Capacity

Let's add 20% for future growth: 280 1.2 = 336 amps.
6.

Conductor Selection

You would need to select conductors with an ampacity of at least 336 amps, after considering derating factors.7.

OCPD Selection

Choose an OCPD (breaker or fuses) with a rating that protects the conductors.8.

Bus Ampacity

The MCC bus is rated for 400 amps, which is greater than the calculated demand of 336 amps, so it's acceptable.

Key Takeaways



Consult the Code

Always start and end with your local electrical code.

Be Conservative

Err on the side of caution, especially for critical loads.

Document Your Assumptions

Keep a record of all your assumptions, calculations, and justifications. This is important for future reference and troubleshooting.

Coordination is Key

Coordinate OCPDs to ensure selective tripping.

Voltage Drop Matters

Don't neglect voltage drop calculations.

Future Proofing

Plan for future expansion to avoid costly upgrades later.

Consider Harmonics: If there are significant non-linear loads, include calculations/considerations of harmonic loading.By following these guidelines, you can ensure that your MCC feeder is properly sized, safe, and reliable. Good luck! Flag for review