Your colleague's concern about inrush current is valid, but not necessarily a guaranteed problem. Here's a breakdown of why he might be concerned, why he might be wrong, and how to approach the situation

Why Your Colleague Might Be Concerned:
Voltage Dips

Large inrush currents from the 600A and 400A loads can cause a temporary voltage dip on the panelboard bus. This dip can affect the performance of the smaller 30A loads, potentially causing them to malfunction, drop out, or experience premature failure.

Sensitive Equipment

Some mechanical equipment, especially those with sensitive electronic controls (even if installed in the 80s), are very susceptible to voltage fluctuations. These might include equipment with PLC's, VFD's or other solid state controls.

Harmonic Distortion

Large nonlinear loads (like motor drives) can inject harmonics into the electrical system. The impedance of the smaller branch circuits to harmonics would be different, resulting in a more complex and unbalanced voltage distortion.

Why He Might Be Wrong (or Why It Might Not Be a Significant Problem)



Adequate System Impedance

If the overall impedance of the system (upstream transformer, feeders) is low enough, the voltage dip caused by the inrush current might be minimal and not significantly affect the 30A loads. A short circuit study will provide the impedance value and available fault current which can be used to determine the voltage drop.

Equipment Robustness

Older equipment, particularly those installed in the 80s, was often designed with a greater tolerance for voltage fluctuations than more modern, highly optimized electronics. They might be more robust than your colleague assumes.

Load Characteristics

The nature of the 600A and 400A loads is crucial. Are they across the line motors with a very high inrush? Or are they electronic motor starters with soft start characteristics? If the starting current is well controlled, there may be little or no voltage sag.

CB Coordination

As long as the system is properly coordinated, the 30A breakers will not trip due to the high inrush currents of the larger loads.

How to Approach the Situation (Best Practices)


Gather Detailed Load Information



Type of Loads

Exactly what are the 600A, 400A, and 30A loads? (e.g., motors, pumps, chillers, fans, lighting, etc.)

Motor Starting Method

If motors, how are they started? (Across-the-line, reduced voltage starter, VFD, etc.). Get inrush current multipliers or NEMA code letters for the motors. VFDs will drastically reduce inrush.

Load Profiles

How often do the large loads start/stop? How long do they run? Is there any overlap in starting times?

Sensitivity of 30A Loads

Find out the voltage tolerance of the 30A loads. Consult their manuals or manufacturer.2.

Perform a Voltage Drop Calculation and Short Circuit Analysis

This is critical! Calculate the expected voltage drop at the panelboard bus during the inrush of the largest loads. This requires knowing the system impedance (transformer impedance, feeder impedance, etc.) and the inrush current of the loads. Compare the calculated voltage drop to the voltage tolerance of the 30A loads. If the drop is within the tolerance, you're likely fine.

Short Circuit Analysis

A short circuit analysis can help determine the available fault current at the panelboard, which is needed for proper breaker selection and coordination.3.

Consider Mitigation Strategies (If Necessary)

If the voltage drop calculations show a potential problem, consider these solutions:

Subpanel

Your colleague's suggestion of a subpanel for the 30A loads is a viable option. This isolates the smaller loads from the voltage dips caused by the larger loads.

Buck-Boost Transformer or Constant Voltage Transformer (CVT)

Install a buck-boost or CVT on the 30A loads. This helps to mitigate the impacts of voltage sag by boosting the voltage.

Soft Starters/VFDs

If the large loads are motors started across-the-line, consider replacing them with soft starters or VFDs. These significantly reduce inrush current. This may be outside the scope of the panel replacement, but can be evaluated on a cost basis.

Oversizing Feeders

Increasing the size of the feeders to the panelboard will lower impedance and reduce voltage drop.4.

Breaker Coordination Study

Ensure that the circuit breakers are properly coordinated. The larger breakers should trip before the smaller breakers for downstream faults. This will minimize the impact of faults on the entire system.5.

Field Testing

After installation, consider monitoring the voltage at the panelboard during startup of the large loads to verify the voltage drop calculations and ensure the 30A loads are not adversely affected.

In summary: Don't dismiss your colleague's concern outright. It's a good engineering practice to consider potential voltage dips. However, quantify the problem with calculations. If the calculations show a significant voltage drop that exceeds the tolerance of the 30A loads, then take steps to mitigate the issue. If the voltage drop is minimal or within tolerance, then you may not need to make any changes.Because of the nature of this project, I highly recommend you perform a short circuit and coordination study. This would require an engineer with electrical power system experience and the use of electrical power system modeling software.Without more detailed information about the loads and the system impedance, it's impossible to say definitively whether a subpanel is necessary. But, a thorough analysis will provide a data-driven answer. Flag for review