This is a good question with a few layers to it. Let's break down the considerations

1. Current per Phase & Transformer
Switchboard Load

4000A

Number of Transformers

3

Current per Transformer (Ideal)

4000A / 3 = 1333.33A per phase (assuming balanced loading).

Number of Cables per Phase

The 9 runs of (4) 750 kCM cables leaving the switchboard means each phase needs to carry 4000A. Since each transformer carries 1/3 of this load, we can calculate the number of cables required per phase on each transformer, assuming the load is balanced.

Current per Cable

The ampacity of 750 kCM copper conductors is usually between 545A - 690A depending on temperature rating and installation method. Lets assume its at least 545A.

Number of Cables Required per Transformer Phase

1333.33A/545A = 2.44 cables. Since you cannot install fractions of cables, you will need to install at least 3 cables per transformer phase (2 legs) to carry the load, as you have done (4 cables on X2 and 5 cables on X4).

2. Paralleling Conductors and Ensuring Equal Current Sharing

This is where it gets critical. The NEC (National Electrical Code) provides guidance on paralleling conductors. Specifically, refer to NEC Article 310.10(H) and 310.10(H)(2) for 1/0 and larger conductors. The key requirements include: Same Length

All paralleled conductors MUST be the same length. This minimizes impedance differences.

Same Conductor Material

(Copper or Aluminum)

Same Size in Circular Mil Area

(750 kCM in your case)

Same Insulation Type

(e.g., THHN, XHHW)

Terminated in the Same Manner

(Same type of lug, same torque, etc.)

3. The Purpose of the Jumpers (X1-X3 and X2-X4)

The jumpers are absolutely essential to ensuring current sharing between the transformer windings and the paralleled cables. Without them, you're relying solely on the internal construction of the transformer to evenly distribute current, which is unlikely.The purpose of the jumpers is to create an equipotential connection that forces the current to split as evenly as possible between the transformer legs. They are a vital component in ensuring the installation meets the requirements of NEC 310.10(H)(2).4. Number and Size of Jumpers One Jumper vs. Multiple Jumpers

While a single jumper might work in theory, it is generally considered poor practice and not recommended for the following reasons:

Single Point of Failure

If that one jumper fails or becomes loose, you lose paralleling entirely.

Increased Impedance

A single jumper may have higher impedance than multiple smaller jumpers, leading to uneven current sharing.

Reduced Reliability

Redundancy is always a good idea in high-current applications.

Industry Best Practice



Multiple jumpers (2 or more) of appropriate size are highly recommended. Jumper Sizing

The jumpers should be sized to carry at least the maximum expected current in each leg of the transformer winding. In your case, each transformer carries 1333.33A. Therefore, each leg in the transformer will see 666.66A. If there is a fault on one leg of the transformer, the single leg would likely carry over 1333.33A. It is best to size to this number or larger.

Therefore, you need at least (2) jumpers per transformer phase Using (4) jumpers is ideal as it will allow the load to be distributed between each cable. If using 750kCM for the cable terminations, use 750kCM cables as your jumpers5. Connecting Cables to the Transformer Cable Lengths

The cable lengths coming from the switchboard to the transformer lugs X1, X2, X3, and X4 MUST be the same length. This is essential for equal current sharing. Any difference in length will cause unequal current distribution.

6. Critical Considerations Torque Connections

Properly torque ALL connections (lugs, jumpers, etc.) to the manufacturer's specifications. Use a calibrated torque wrench.

Thermographic Survey

After commissioning, a thermographic survey (infrared scan) is highly recommended to verify balanced loading on all conductors and connections. Hot spots indicate poor connections or unequal current sharing.

Calculations

Complete load calculations for the entire system to ensure the transformers are adequately sized and that the conductors have sufficient ampacity.*

Coordination Study

Conduct a full coordination study to ensure proper protection of the transformers, cables, and switchboard.

In Summary - Recommendations1. Install multiple (4) jumpers of substantial size (ideally 750 kCM) between X1-X3 and X2-X4 on each transformer.2. Ensure all paralleled conductors (including jumpers) are the same length, material, size, insulation, and termination method.3. Torque all connections to the manufacturer's specifications.4. Conduct a thermographic survey after commissioning.5. Complete load calculations and a coordination study.Important Disclaimer: I am an AI chatbot and cannot provide professional engineering advice. Consult with a qualified electrical engineer to ensure your installation complies with all applicable codes and standards and is safe and reliable. Flag for review