Okay, let's break down your questions regarding your 600A tap to a new service. This is a common scenario, and understanding the code requirements is critical.1. Downsizing Grounded Service Entrance Tap Conductor (Neutral)



Yes, you can likely downsize your grounded service entrance conductor (neutral) based on 250.24(D)(1) and Table 250.102(C)(1), BUT with careful consideration.

Here's the breakdown: 250.24(D)(1) General

This section generally permits the grounded conductor (neutral) to be sized based on the calculated load it is expected to carry, rather than simply being the same size as the ungrounded conductors. It directs you to Table 250.102(C)(1).

Table 250.102(C)(1)

This table provides the minimum size of the grounded conductor based on the size of the largest ungrounded service-entrance conductor or equivalent area for parallel conductors. Critically, it states: "See 220.61 for reduction in size of grounded conductor for dwellings." 220.61 does not apply in your situation, which is a commercial service.

Crucial Considerations



Load Calculation

You must perform a thorough load calculation for the 600A service you're adding. This is not just estimating; it's a formal calculation based on Article 220. This calculation will determine the maximum unbalanced load that the neutral is likely to carry. The neutral conductor must be sized to carry that calculated unbalanced load at a minimum.

Harmonic Currents

Pay very close attention to the potential for harmonic currents in the load you're feeding. If the load includes significant non-linear loads (e.g., variable frequency drives (VFDs), LED lighting with poor power factor, UPS systems, computers), harmonic currents will flow primarily on the neutral conductor. This can significantly increase the neutral current beyond what a simple unbalanced load calculation might suggest. Oversizing the neutral is often necessary in these situations.

Voltage Drop

Even if the calculated neutral current is relatively small, consider voltage drop. A smaller neutral conductor will contribute to greater voltage drop, which can affect the performance of sensitive equipment.

Minimum Size

Remember, Table 250.102(C)(1) provides the minimum allowable size. You may need to upsize based on your load calculation, harmonic considerations, or voltage drop concerns.

Examples

If you have (3) 500kcmil (per phase) the minimum neutral conductor would be a (1) 2/0 Cu. If you have (3) 350kcmil (per phase) the minimum neutral conductor would be a (1) #2 Cu.

Therefore, you can likely reduce the neutral size to the minimum listed in Table 250.102(C)(1), but the deciding factor will always be the calculated load2. Grounding Electrode Conductor (GEC) Connection



Yes, you can connect the GEC from your new 600A service to the existing service's ground bus, provided it complies with 250.64(D)(1) as a "single system."

Here's the breakdown: 250.64(D) Common Grounding Electrode

This section covers situations where you have multiple services or separately derived systems in the same building.

250.64(D)(1) Common Grounding Electrode Conductor

This section outlines an alternative method to comply with 250.64(C). As long as the main service (the one you're tapping) meets the requirements of 250.50, it allows you to run a GEC from each service to a common point.

Key Points Single Building/Structure

This only applies if both services are supplying the same building or structure. This is likely the case since you said its next to the gear.

Adequate Size of the Existing GEC

Verify that the existing GEC for the main service is sized adequately to handle the combined fault current from both services. This is crucial! Consider the worst-case fault current scenario and ensure the existing GEC is large enough per 250.66. If it's not, you'll need to upgrade the existing GEC.

Adequate Size of the Common Grounding Electrode Bus

The existing grounding bus in the main service gear must also be large enough to handle the combined fault current from both services.

Proper Connection

Ensure a proper, low-impedance connection to the existing ground bus. Use listed connectors and follow manufacturer's instructions.

GEC Size for the New Service

Size the GEC for the new 600A service according to Table 250.66, based on the size of the service-entrance conductors for that service (the 600A service). It doesn't matter that it will share a grounding electrode conductor with the existing service.

In summary, connecting the GEC from your new 600A service to the existing service's ground bus is generally permitted under 250.64(D)(1), but only if you ensure:1. Both services are supplying the same building.
2. The existing GEC and ground bus are adequately sized to handle the combined fault current.3. The GEC for the new service is properly sized according to Table 250.
66.4. Proper connections are made.

Important Considerations and Recommendations



Engineering Review

This is a significant modification to an existing service. Consult with a qualified electrical engineer. They can perform the necessary fault current calculations, load calculations, and ensure code compliance.

Permitting and Inspection

Obtain the necessary permits and inspections from your local authority having jurisdiction (AHJ). They will have the final say on the acceptability of the installation.

Documentation

Keep detailed records of all calculations, conductor sizes, grounding connections, and inspection reports.

Coordination Study

Consider a coordination study to ensure proper selective coordination of overcurrent protective devices (OCPDs) throughout the system. This will help to minimize outages and improve system reliability.

Short Circuit Study

Its good practice to run a short circuit study to ensure that the bus and equipment is rated for the available fault current.By carefully following these guidelines and consulting with qualified professionals, you can ensure a safe and code-compliant installation. Failure to comply with these requirements can result in serious safety hazards and code violations.

Disclaimer: I am an AI chatbot and cannot provide professional engineering advice. This information is for general educational purposes only. Consult with a qualified electrical engineer for specific design and installation guidance.* Flag for review