You are on the right track with your concerns. Let's break down the sizing requirements for the SBJ and GEC.

Understanding the Code Requirements Separately Derived System (SDS)

A transformer creating a new neutral point qualifies as an SDS.

Grounding Electrode Conductor (GEC)

Connects the grounding electrode system to the SDS.

System Bonding Jumper (SBJ)

Bonds the neutral point of the SDS to the transformer enclosure.

Key Code Sections



NEC 250.30(A)(1)

Sizing the Grounding Electrode Conductor (GEC) for a separately derived system. It states, "The size of the grounding electrode conductor for a separately derived system shall be based on the derived phase conductors and shall be in accordance with Table 250.66."

NEC 250.28(D)(1)

Sizing the System Bonding Jumper (SBJ) on the secondary side. It states, "For a grounded system, an unspliced system bonding jumper shall be used to connect the equipment grounding conductors and the service-equipment enclosure to the grounded conductor of the system... The size of the system bonding jumper for a separately derived system shall not be smaller than specified in 250.28(D)(1) through (D)(3) and shall be sized for the derived phase conductors."

Your Scenario

You have a 150kVA transformer with two sets of conductors coming from the secondary:1. 250 kcmil to a 250A MCB2. 600 kcmil to a 400A MCB

Analysis

The SBJ and GEC are sized based on the derived phase conductors. The code language specifically mentions that the sizing needs to be per conductor.

NEC 250.66 Table| Size of Largest Ungrounded Service-Entrance Conductor or Equivalent Area for Parallel Conductors | Size of Grounding Electrode Conductor

:--------------------------------------------------------------------------------------------- | :-------------------------------------

Copper | Copper

Not over 1100 kcmil | 1/0 AWG

Over 1100 kcmil | 3/0 AWG |NEC 250.28(D)(1)

The bonding jumper shall not be smaller than specified in 250.28(D)(1) through (D)(3) and shall be sized for the derived phase conductors. Where the derived phase conductors are larger than 1100 kcmil copper or 1750 kcmil aluminum, the bonding jumper shall have an area not less than 121⁄2 percent of the area of the largest derived phase conductor or equivalent area for parallel conductors. The system bonding jumper shall not be required to be larger than the largest ungrounded derived phase conductor or equivalent area for parallel conductors.

SBJ and GEC Sizing

The language "equivalent area for parallel conductors" applies, BUT this is for if they paralleled conductors to feed the single main breaker of the panel. In your situation, you are sizing the SBJ and GEC based on the total secondary conductor size based on the largest ungrounded conductor. (600kcmil)This means that you will use table 250.66 and 250.28(D)(1) to size the SBJ and GEC.
1. The largest secondary conductor is 600 kcmil2. The GEC and SBJ should be sized per 250.66 and 250.28(D)(1), respectively.
3. This would result in a minimum 1/0 AWG.

Important Considerations



Parallel Conductors

The "equivalent area for parallel conductors" wording in Table 250.66 would apply if, for example, they used two 300 kcmil conductors per phase instead of a single 600 kcmil conductor to feed the 400A panel. In that case, you would add the areas of the parallel conductors to determine the "equivalent area."

Minimum Size

Always ensure the GEC meets the minimum size requirements in 250.66 and is not smaller than what is required for the grounding electrode to which it is connected (typically concrete-encased electrode, ground ring, etc.).

Engineering Judgement

In some specific cases, an engineer might choose to upsize the GEC or SBJ for improved performance or future capacity, but this is not a code requirement.

In summary: The 1/0 AWG is likely sufficient based on the largest ungrounded conductor, and the table that applies. Ensure there is also a properly sized equipment grounding conductor running with each set of phase conductors. Flag for review