Okay, Bob, let's break this down and get you some solid guidance. You're right to question the wire sizing, and using a distribution block in this scenario is perfectly viable and often a better approach.Here's a detailed explanation and calculation

Understanding the Situation
5kVA Transformer

5000 VA

Secondary Voltage

120 VAC

Secondary Current Capacity

5000 VA / 120 V = 41.67 Amps

Two Circuits

Each circuit presumably has a dedicated circuit breaker.

Short Distances

Wiring distances are very short within the control panel (1-2 feet).

Why 8 AWG Might Be Overkill (Initially)8 AWG is often chosen for higher currents, especially over longer distances. While it's certainly safe, it might be unnecessarily large and difficult to work with inside a control panel for the individual circuits.

The Distribution Block Approach: A Better Solution

Using a distribution block allows you to:1. Run the Correct Size Wire for the Load

Calculate the actual load for each of the two circuits. You only need to size the wire to carry the maximum current expected on each individual circuit, not the full 41.67 amps of the transformer.
2.

Simplify Wiring

A distribution block provides a convenient and organized way to split the single larger secondary wire into multiple smaller wires.3.

Maintain Overcurrent Protection

The individual circuit breakers will protect their respective circuits from overloads.

Detailed Steps & Calculations1. Calculate Circuit Loads



Most Important Step

Find out the maximum current draw of each circuit connected to the transformer. Let's say Circuit 1 has a maximum load of 15 amps and Circuit 2 has a maximum load of 20 amps.

Don't Oversize Unnecessarily

Just because the transformer can supply 41.67 amps doesn't mean it will. Size the wires based on the actual load.2.

Wire Sizing (Secondary to Distribution Block)

The wire from the transformer secondary to the distribution block

must be sized to handle the full transformer secondary current (41.67 amps). Therefore, you DO need to ensure this wire is sized appropriately for the 41.67 amps. 8 AWG Copper (THHN) is acceptable for this section. You could potentially go to 6 AWG, depending on the temperature rating of the terminals and wire used, and your local code requirements. Always check the ampacity tables (NEC Table 310.16 or similar) and apply any necessary correction factors for ambient temperature or bundling. Remember to use the 75 degree C column for termination temperature unless you know that your breakers and transformer terminals are rated for 90 degrees.
3. Wire Sizing (Distribution Block to Circuit Breakers)



Circuit 1 (15 amps)

You can use 14 AWG copper THHN wire. (15 amp breaker).

Circuit 2 (20 amps)

You can use 12 AWG copper THHN wire. (20 amp breaker).

Important

Always refer to the NEC (National Electrical Code) or your local electrical code for the minimum allowed wire sizes. The values above are based on common practices and NEC Table 310.16.
4.

Distribution Block Selection

Choose a distribution block with an ampacity rating at least equal to the transformer's secondary current (41.67 amps). It's good practice to choose one slightly higher. Ensure the distribution block is rated for the voltage (120V) and has the appropriate number of terminals for your application. Verify that the distribution block's terminals are compatible with the wire sizes you'll be using (both the large secondary wire and the smaller circuit wires).5.

Neutral

Yes, you can absolutely use a distribution block for the neutral conductor in the same manner as the hot (ungrounded) conductor. The neutral conductor from the transformer secondary to the neutral distribution block must be sized to handle the maximum unbalanced load. In many cases, this can be the full transformer secondary current (41.67 amps), especially if you don't know the load balance between the two circuits. However, if you know the maximum potential imbalance, you can size it accordingly. If you do not know, then you need to plan for 41.67 amps. From the neutral distribution block to each circuit, size the neutral conductors according to the load on each circuit (same as the hot conductors).

Important Considerations Overcurrent Protection is Key

Make sure each circuit is protected by a properly sized circuit breaker. The breaker should be rated to protect the smallest wire in the circuit.

NEC and Local Codes

Always consult the latest edition of the NEC (or your local electrical code) and follow its requirements. This response is for guidance only and should not be considered a substitute for professional electrical advice.

Terminal Temperature Ratings

Ensure that the terminals on your transformer, distribution block, and circuit breakers are rated for the temperature rating of the wire you are using. (Typically 75°C).

Conductor Type

THHN wire is a common choice for control panel wiring. Make sure it's rated for the voltage and temperature requirements of your application.

Professional Inspection

It's always a good idea to have a qualified electrician review your design and installation to ensure compliance with all applicable codes and best practices.

In Summary:

Using a distribution block in your control panel is a safe and efficient way to distribute power from your transformer to multiple circuits. Size the wire from the transformer to the distribution block for the full secondary current capacity. Then, size the wire from the distribution block to each circuit breaker based on the actual load of each circuit.By following these steps and adhering to the NEC and local codes, you can ensure a safe and reliable electrical installation. Flag for review