No, there's generally no specific length restriction on the primary side feeder to a transformer as long as you meet all other applicable code requirements. The 25-foot tap rule specifically applies to secondary conductors.Here's a breakdown of the considerations for the primary side feeder: Overcurrent Protection

The primary side conductors must be protected against overcurrent. You need to size the primary conductors and overcurrent protection (breaker or fuses) according to the transformer's primary current rating. This calculation is based on the transformer's kVA and primary voltage. You're allowed to use the next higher standard breaker/fuse size above the calculated current if your calculated current doesn't match a standard breaker/fuse. However, specific rules govern how much higher you can go, so check your local electrical code.

Ampacity

The primary conductors must have sufficient ampacity to carry the calculated primary current, taking into account derating factors for ambient temperature, conductor bundling, and conduit fill.

Voltage Drop

This is perhaps the most critical consideration for longer runs. Excessive voltage drop on the primary side will negatively impact the transformer's output voltage and performance. If the voltage at the primary terminals of the transformer is too low, the output voltage will be proportionally lower, potentially causing equipment malfunction or damage on the secondary side. Voltage drop calculations must be performed to ensure the voltage at the transformer's primary terminals remains within acceptable limits under load. While the NEC doesn't mandate specific voltage drop limits, informational notes recommend a maximum of 3% voltage drop on feeders and 5% total voltage drop (feeder plus branch circuit).

Conductor Type and Installation

The conductors must be of an appropriate type (e.g., THHN, XHHW) for the environment and installed according to code requirements regarding conduit, raceways, supports, and clearances.

Grounding

Proper grounding of the transformer enclosure and the primary side conductors is essential for safety.

Short-Circuit Current Rating (SCCR)

The entire system, including the transformer, overcurrent protection devices, and conductors, must have a sufficient SCCR to withstand the available fault current at that point in the electrical system.*

Coordination

Overcurrent protection devices should be coordinated to ensure that the device closest to a fault trips first, minimizing disruption to the rest of the system.

Key Takeaway

While there's no hard length limit,

voltage drop is the primary factor that will limit how long you can run the primary conductors. You need to calculate the voltage drop based on the conductor size, length of run, load (kVA), and power factor to ensure it's within acceptable limits. Increasing the conductor size can mitigate voltage drop over longer distances.

Recommendation:

Consult with a qualified electrical engineer or licensed electrician to properly size the conductors, overcurrent protection, and perform the necessary calculations to ensure compliance with all applicable codes and regulations, especially regarding voltage drop, for your specific installation. They can also help with SCCR calculations and coordination studies. They will also be familiar with the specific local codes and interpretations. Flag for review