Yes, you are correct. There is a special provision for sizing the equipment grounding conductor (EGC) in an impedance grounded system, and it is NOT solely based on Table 250.122.Here's the breakdown and why: Table 250.122 is generally for overcurrent protective device sizing

Table 250.122 is used for sizing EGCs based on the rating of the overcurrent protective device (OCPD) protecting the circuit. In an impedance grounded system, the OCPD might not immediately trip on a ground fault. The purpose of impedance grounding is to limit the ground-fault current, often allowing for continued operation while the fault is located and corrected.

Article 250.36 for Impedance Grounding Systems

This section of the NEC (National Electrical Code) addresses impedance grounding systems. Crucially, 250.36(A) lays out specific requirements.

Key Requirements from 250.36(A)(2)

This part of the NEC is most relevant. It states that the equipment grounding conductors shall be sized according to the largest of the following:1.

250.122

The EGC must meet the sizing requirements of Table 250.122, based on the rating of the circuit's overcurrent protective device, like you mentioned.
2.

Calculated Ground-Fault Current

The EGC must be sized large enough to carry the available ground-fault current that could flow through it. This is the current that the impedance grounding resistor limits (your 400A in your example).

This is the most common way to size the EGC3. Minimum Size

The equipment grounding conductors shall not be smaller than the sizes given in Table 250.122 for the rating of the largest circuit overcurrent device.

Why 250.36(A)(2) is Important

It acknowledges that the OCPD might not immediately clear the fault. Therefore, the EGC must be able to handle the limited ground-fault current for the duration it might flow without exceeding its temperature rating and potentially damaging the conductor or creating a fire hazard.

Engineering Judgment Required

Determining the adequate size for the EGC when using the "calculated ground-fault current" method requires careful consideration. You must factor in:

The Resistor's Current-Limiting Value

In your case, 400A.

The Duration of the Fault Current

1-2 seconds in your example, although you should verify this assumption. The longer the fault duration, the larger the EGC needs to be.

The Conductor Material

Copper or aluminum.

The Conductor Insulation Type

Affects the temperature rating.

The Installation Method

Determines the allowable ampacity.

Example

Let's say your 400A ground fault current lasts for 2 seconds. You would need to select an EGC size that can withstand 400A for 2 seconds without exceeding its temperature rating. Using conductor ampacity charts and engineering calculations based on the conductor's thermal capacity (I²t), you would determine the appropriate gauge. A larger gauge conductor would have more mass and better heat dissapation which makes it more resilient to the current for the given amount of time.

Caution: Always consult a qualified electrical engineer to perform these calculations and ensure the EGC is adequately sized for your specific impedance grounded system. There are software tools available to help with these calculations. Do not rely solely on Table 250.122 in this situation.In summary, while Table 250.122 provides a baseline, 250.36(A)(2) governs the sizing of the EGC in an impedance grounded system. You must size the EGC based on the largest of the requirements, and usually, the calculated ground-fault current and the duration of the current flow will dictate the minimum size of the EGC. Therefore, the 400A current you mentioned must be used in your calculations. Flag for review