Okay, let's break down the NEC requirements for sizing secondary conductors from a transformer to switchgear. You're right, there's often confusion, and various interpretations exist. Your calculations are a good starting point, but we need to refine them to align with the NEC.

The NEC Perspective: Key Sections NEC 450.3(B)

Transformer Overcurrent Protection Secondary Side (This is THE key section!) This section dictates how the secondary overcurrent protection device (the main breaker in your switchgear) relates to the transformer's ampacity. It will drive the conductor sizing.

NEC 240.4

Protection of Conductors. This section establishes the general principle that conductors must be protected against overcurrent in accordance with their ampacities.

NEC 240.21(C)

Location of Overcurrent Protection, Secondary Conductors. This section allows for a secondary conductor run without overcurrent protection at its supply end, but it sets specific requirements. This is commonly referred to as the "Transformer Tap Rule."

NEC 310.15

Ampacity of Conductors. This section provides the tables and information for determining the ampacity of conductors based on insulation type, temperature rating, and other factors.

NEC 110.14(C)

Termination Temperature Ratings. Important for ensuring connections are made with the correct temperature rating connectors.

Understanding the NEC Approach

The NEC focuses on protecting the conductors and the transformer. It's not just about carrying the load; it's about preventing overloads and faults from damaging equipment.

Let's Analyze Your Example and Apply the NEC

You have a 750 kVA, 4160V-480V Delta-Wye transformer.1. Transformer Secondary FLA (Your Calculation is Correct)

`Secondary FLA = 750 kVA / (1.732 0.480 kV) = 902.13 A`2.

NEC 450.3(B) - Secondary Overcurrent Protection Requirements

This is crucial. NEC 450.3(B) dictates the maximum size of the secondary overcurrent protective device (your main breaker in the switchgear). You have two main options, depending on what type of transformer you have.

If the overcurrent device is rated 125% or less, this becomes the determining factor. if the Overcurrent device is rated over 125% then it needs to be sized to the max allowance per NEC 450.3(B).3. Applying NEC 240.21(C) - Transformer Secondary Conductor "Tap Rule"

This is where the 20-foot distance comes into play. If you meet ALL the conditions of 240.21(C), you can size the secondary conductors smaller than the transformer's secondary FLA. Here are the main conditions you need to meet: Condition (1)

The secondary conductors' ampacity must not be less than the calculated load AND not less than the rating of the overcurrent device at the load end (your switchgear main breaker).

Condition (2)

The secondary conductors' ampacity must be at least 1/3 of the overcurrent protection device (fuse or breaker) protecting the primary side of the transformer.

Condition (3)

The total length of the secondary conductors cannot exceed 25 feet. (You meet this requirement.)

Your Calculations Revisited (and Corrected) Your Primary Side Calculation

Correct. You have a 150A fuse, and that is a standard Fuse size that is slightly higher than the primary FLA of 104.1A.

Your Secondary Side Calculation (Method 1 - Based on Primary Protection)

This method (150A 4.16kV / 0.48kV = 1300A) is incorrect and potentially unsafe and NOT per NEC. It doesn't consider the transformer's actual capacity or the need for secondary overcurrent protection. You cannot derive secondary conductor ampacity directly from the primary protection device.

Your Secondary Side Calculation (Method 2 - 1.25 x Secondary FLA)

This (1.25 x 902.13A = 1127.66A) is a closer approximation IF you are using a secondary overcurrent device that is greater than 125%. This is a common, safe way to size the conductors IF it complies with 450.3(B) and 240.21(C).

Recommended Approach1. Determine the Main Breaker Size

First, determine the actual load that the switchgear will serve. Calculate the required ampacity of the main breaker in the switchgear based on the connected load (NEC 220). Then determine the maximum Size for the Overcurrent protection based on NEC 450.3(B). This value is the maximum allowed. You might choose a smaller breaker based on your load calculations.2.

Check NEC 240.21(C) (Transformer Tap Rule)



Condition (1)

Ensure the conductor ampacity is at least the rating of the main breaker.

Condition (2)

Ensure the conductor ampacity is at least 1/3 of the primary overcurrent protection (150A / 3 = 50A). This is likely already met.

Condition (3)

The 25-foot rule is met.3.

Check NEC 310.15 for Ampacity

Select the appropriate conductor size and insulation type (e.g., THHN, XHHW) to meet the ampacity requirements you've determined. Remember to adjust for ambient temperature (NEC 310.15(B)(2)) if necessary. Also, check termination temperature requirements (NEC 110.14(C)).
4.

Consider Voltage Drop

For a 20-foot run, voltage drop might not be a major concern, but it's good practice to check it. Use a voltage drop calculator to ensure the voltage drop is within acceptable limits (typically 3% for power circuits).

Example: Applying the Process

Let's say you determine your connected load requires a 1000A main breaker in the switchgear. We also know the maximum ampacity allowable from NEC 450.3(B) is 125% of 902.13 (FLA) or 1127.66A. Because the 1000A Overcurrent device is less than 1127.66A we will use the value for the secondary wire size.
1. Main Breaker

1000A2.

NEC 240.21(C) Checks



Condition (1)

Conductors must be rated for at least 1000A.

Condition (2)

Conductors must be rated for at least 50A (150A / 3). This is easily met.

Condition (3)

Length is less than 25 feet.3.

Conductor Selection

You would need to select conductors that have an ampacity of at least 1000A, considering the insulation type, temperature rating, and installation method (e.g., conduit, cable tray). You would likely need multiple conductors in parallel per phase. You might use (3) 500 kcmil conductors per phase (check ampacity tables).4.

Voltage Drop

Verify voltage drop is acceptable with the selected conductor size and length.

Important Considerations Parallel Conductors

If you use parallel conductors (more than one conductor per phase), make sure they are the same length, material, and insulation type (NEC 310.10(H)).

Grounding

Ensure proper grounding per NEC 250.

Coordination Study

A full coordination study is highly recommended. This analyzes the interaction between all overcurrent protection devices (primary fuse, secondary main breaker, downstream breakers) to ensure proper selective tripping and minimize outages. This is especially important for critical systems.

Local Codes and Regulations

Always check with your local authorities (AHJ - Authority Having Jurisdiction) for any specific requirements or interpretations of the NEC.

In Summary

Don't rely solely on a "formula" based on the primary protection. The most important thing to understand is that the NEC wants you to protect the transformer and the conductors on the secondary side. Your calculations need to be driven by the requirements in NEC 450.3(B) and 240.21(C), along with proper conductor ampacity selection from NEC 310.
15.Always consult with a qualified electrical engineer or licensed electrician for specific designs and installations. Electrical codes can be complex, and safety is paramount. Flag for review