Bob, you're right to be concerned. This setup presents several issues and potential code violations. Let's break down the problems

Understanding the System:
Delta-Delta Transformer

You have a 3-phase transformer converting 480V delta to 240V delta.

Corner Grounded Delta vs. High-Leg Delta

You mentioned a "center tap" which, when grounded on a delta secondary, typically indicates a high-leg delta configuration. It is very important to confirm if it is a high leg and how the secondary is phased to the primary.

"Neutral" Misnomer

In a high-leg delta system, the grounded center tap isn't a true neutral in the same way it is in a wye system. It's more of a grounded reference point that allows for 120V phase-to-ground loads.

Problem

You're using one leg to supply BOTH 240V line-to-line loads AND all the 120V line-to-neutral loads. This creates significant imbalance and can lead to overloading.

The Problems and Why Your Concern is Valid


Overloading Concerns on the "Neutral" (Grounded Conductor)



The Issue

While the circuit breaker on the grounded conductor is rated at 30A, that doesn't mean it's safe. The grounded conductor is carrying the entire imbalance of your 120V loads. You stated 24A is being used.

Code Implications (NEC - National Electrical Code)



NEC 250.24(C)

This section generally addresses grounding of alternating-current systems. For high-leg delta, you'd likely be violating the spirit (and potentially the letter) of these requirements, as the grounded conductor is intended to serve as a reference and not a primary current-carrying conductor like a neutral in a balanced wye system.

NEC 310.15

This and subsequent sections cover ampacity of conductors. You need to calculate the current carrying capacity of the wire being used as the grounded conductor and ensure it is sufficient for the load.2.

Overloading of the One Transformer Leg



The Issue

The one transformer leg feeding both the 240V and 120V loads is likely carrying significantly more current than the other two legs. This can overheat the transformer winding, shorten its life, and potentially lead to failure.

Code Implications



NEC 450.3

This section covers transformer overcurrent protection. You need to ensure the transformer is properly protected against overloads based on its KVA rating and voltage. The uneven loading will make this difficult to achieve.3.

Voltage Imbalance and Distortion



The Issue

Uneven loading on the transformer phases leads to voltage imbalance. This can negatively affect the performance and lifespan of motors and other sensitive equipment connected to the system.

Harmonic Content

Significant imbalance can also introduce harmonic currents, further stressing the transformer and other equipment.4.

"High-Leg" Voltage Concerns (If Applicable)

If this truly is a high-leg delta system, one phase (the "high leg," often marked orange) will have a voltage to ground that is approximately 208V (1.732 x 120V). This voltage MUST be clearly identified. Any equipment connected to this high-leg expecting 240V or 120V will be damaged.

Code Implications



NEC 110.15

This requires the high leg to be durably and permanently marked (typically with orange) wherever it is accessible.

The 5% Rule and the Grounded Conductor

You mention the 5% rule. This is a general guideline, often used in power quality discussions, related to acceptable voltage drop. While the 5% rule isn't directly applicable to the current on the grounded conductor in this specific scenario, the principle of minimizing voltage drop due to excessive current does apply. The grounded conductor isn't designed to carry such a high load current on a constant basis. The voltage drop across the conductor due to the 24A will impact the voltage at the load, especially under load.

What to Do


Verify the Transformer Configuration



CRITICAL. Confirm if it's a true high-leg delta or a different configuration entirely. Trace the wiring, measure the phase-to-ground voltages, and check the transformer nameplate. Understanding the configuration is the first step.2. Measure Current on All Legs

Use a clamp meter to measure the current on all three secondary legs and on the grounded conductor under normal operating conditions. This will give you a clear picture of the load imbalance.3.

Consult with a Qualified Electrical Engineer or Licensed Electrician

This is not a DIY situation. This situation requires careful analysis and correction by someone who understands three-phase power systems and the NEC.4.

Corrective Actions (Likely Required)



Redistribute Loads

The primary goal is to balance the load across all three phases as much as possible.

Separate 120V Loads

Consider using a separate 1-phase transformer to supply the 120V loads. This would isolate those loads from the 240V delta system.

Wye-Connected Transformer

The best long-term solution is likely to replace the transformer with a delta-wye transformer. This provides a true neutral and balanced 120/208V three-phase power distribution. This would require rewiring the entire load.

Upgrade Conductor Sizes

If redistribution or a separate transformer isn't immediately feasible, you may need to upgrade the size of the conductor used for the grounded conductor to handle the current safely, but this is not a long-term solution.5.

Proper Grounding and Bonding

Ensure the grounding and bonding system is installed correctly per NEC requirements.

In summary, your concerns are valid and the system as described is likely not safe or compliant with code. Do NOT duplicate the system without correcting these issues. Get professional help to properly analyze and correct the situation. Flag for review