You're on the right track, but let's break this down and clarify some key points regarding transformer secondary protection, conductor sizing, and applicable code sections (NEC, generally). I'll assume you're referencing the US NEC.1. Transformer Secondary Conductor Protection



NEC 240.21(C) - Secondary Conductors

This is the main section you need to focus on. It outlines the requirements for protecting transformer secondary conductors. You are correct that the general rule requires overcurrent protection (fuses or a circuit breaker) on the secondary side.

NEC 240.21(C)(1) - 10-Foot Tap Rule

This is the most common exception to the secondary protection rule. However, based on your description, this exception

does NOT apply to your situation for several reasons: Length

The conductors are more than 10 feet long (your run is 50 feet).

Ampacity

The secondary conductors must have an ampacity not less than: (1) the calculated load of the secondary conductors or (2) the rating of the device supplied by the secondary conductors. You are using a 150 amp main breaker, so the conductors feeding it need to be rated for AT LEAST 150amps.

NEC 240.21(C)(2) - 25-Foot Tap Rule

This is another exception, but it's likely not applicable here either.

Length

While the conductors might be less than 25 feet (though the total distance from the transformer to the MBP is greater than 25'), this still requires additional criteria to be met

Ampacity

The conductors must have an ampacity of at least 1/3 of the overcurrent device protecting the primary of the transformer.

Protected from Physical Damage

The conductors must be suitably protected from physical damage and installed in a raceway.

Terminated in a Single Overcurrent Protective Device

The secondary conductors are terminated in a single circuit breaker or set of fuses that will limit the load to no more than the ampacity of the conductors

2. Your Situation & Required Protection

Based on your description: You have a 75 kVA transformer. You have a non-fused disconnect 6 feet from the transformer. You have a 50-foot run of conductors from the disconnect to a 150A MBP. The conductors, therefore, are far exceeding the 10 foot tap exception and are NOT meeting the criteria for the 25 foot tap.

Therefore, YES, you are correct. The disconnect MUST be fused (or replaced with a fused disconnect or circuit breaker) to provide overcurrent protection for the secondary conductors.3. Sizing the Fuses/Circuit Breaker



Transformer Secondary Current

Calculate the full-load secondary current of the transformer using this formula: For a 3-phase transformer: I = (kVA x 1000) / (Voltage x √3) For a single-phase transformer: I = (kVA x 1000) / VoltageAssume your transformer is wired for 480V primary and 208Y/120V secondary (a common voltage for construction trailers). And let's also assume it's a 3-phase transformer:I = (75 kVA x 1000) / (208V x √3) = Approximately 208 Amps

NEC 450.3(B) - Transformer Overcurrent Protection (Secondary Side)

This section dictates the maximum overcurrent protection allowed on the secondary side of the transformer. The maximum overcurrent protection cannot exceed 125% of the rated secondary current. In our example, 208 Amps x 1.25 = 260 Amps.

Select a Standard Fuse or Circuit Breaker Size

You must use a standard fuse or circuit breaker size (see NEC 240.6). Choose the next lower standard size that is less than or equal to your calculated value. In this case, a 250 amp fuse is a standard size that is less than 260 amps.

Conductor Ampacity

The conductors from the transformer to the 150A MBP

must be sized to carry at least the 150A load of the breaker and must have a rating of at least 125% of the continuous load.4. Distance from Transformer

You are also correct that there's no specific maximum distance requirement for placing the overcurrent protection from the transformer,

as long as you meet the secondary conductor protection requirements of 240.21(C). The code focuses on protecting the conductors based on their ampacity and the overcurrent device's rating. In your case, the 50-foot run is what makes the tap rules inapplicable.5. Summary & Recommendations


Replace the non-fused disconnect with a fused disconnect or a circuit breaker.2. Calculate the full-load secondary current of your transformer.3. Size the fuses/breaker to no more than 125% of the transformer's secondary current. Use the next lower standard size.4. Ensure the conductors from the transformer to the MBP are sized appropriately for at least the 150A of the breaker plus any additional load. Use the correct ampacity tables in the NEC (Article 310) and account for temperature correction and adjustment factors. The conductors would need to have an ampacity of at least 150A and also meet 240.4(B), allowing next higher standard overcurrent protection where the ampacity is not exactly matched by an overcurrent device.
5. Bonding and Grounding

Make sure the transformer, disconnect/breaker, and MBP are properly bonded and grounded according to NEC Article 250.6.

Labeling

Label the equipment clearly (e.g., "Transformer Secondary Disconnect").

Important Considerations



Voltage Drop

With a 50-foot run, voltage drop is a very important factor. Ensure your conductor size is adequate to keep voltage drop within acceptable limits (typically 3% for power circuits). Calculate voltage drop!

Aluminum vs. Copper

Consider using aluminum conductors for cost savings, but ensure they are properly sized and terminated. Aluminum conductors require special considerations for termination to prevent corrosion and loosening.

Local Codes and AHJ

Always check with your local electrical inspector (Authority Having Jurisdiction - AHJ) to ensure you are meeting all local codes and requirements. They may have specific interpretations or amendments to the NEC.*

Competent Electrician

This is not a DIY project. It is critical to have a qualified and licensed electrician perform this work. Electrical work can be dangerous, and incorrect wiring can lead to fire, shock, or equipment damage.

Disclaimer: I am an AI chatbot and cannot provide professional electrical advice. This information is for general guidance only. Consult with a qualified and licensed electrician for any electrical work. Always refer to the latest edition of the National Electrical Code (NEC) and local codes for specific requirements. Flag for review