Understanding the 25-Foot Secondary Conductor Rule in the 2023 NEC: Section 240.21(C)(6)

The 25-foot secondary conductor rule, outlined in 240.21(C)(6) of the 2023 National Electrical Code (NEC), allows transformer secondary conductors to extend up to 25 feet without overcurrent protection at the transformer under specific conditions. This rule offers flexibility in design while maintaining safety and compliance.

This article explains the rule, its requirements, and provides a practical example with detailed insights into conductor ampacity, overcurrent protection device (OCPD) selection, and the transformer’s capacity.

What is the 25-Foot Secondary Conductor Rule?

The NEC permits transformer secondary conductors to extend up to 25 feet without overcurrent protection at the source, provided all of the following conditions are satisfied:

Conditions for Compliance

• Conductor Length: Secondary conductors must not exceed 25 feet in length.
• Ampacity:

  The ampacity of the secondary conductors must be at least:

  Ampacity ≥ (Primary OCPD Rating ÷ Primary-to-Secondary Voltage Ratio) × 1/3

  This ensures the secondary conductors can handle potential faults or overloads safely, accounting for the voltage transformation between the primary and secondary sides of the transformer.

• Termination into Overcurrent Protection: Secondary conductors must terminate into a single circuit breaker or a set of fuses that limit the load current to no more than the conductor’s ampacity as permitted by 310.14.
• Physical Protection: Secondary conductors must be installed in an approved raceway or by other approved means to protect them from physical damage.
• Termination Location: The secondary conductors must terminate in an overcurrent device located as close as practicable to the point where the conductors emerge from the raceway or enclosure.

Example: Applying the 25-Foot Secondary Conductor Rule

Scenario:

• Transformer Details: A 75 kVA, 480V to 208/120V three-phase step-down transformer.
• Primary OCPD Rating: 300 amps.
• Primary-to-Secondary Voltage Ratio: 480 ÷ 208 = 2.31.
• Secondary Conductors: 3/0 AWG copper, rated for 200 amps at 75°C.
• Conductor Length: 20 feet (within the 25-foot limit).
• Secondary OCPD: 150-amp breaker.

Application:

1. Conductor Length: The conductors are 20 feet long, satisfying the 25-foot length limit.
2. Ampacity Check (Voltage Ratio Formula):

   Ampacity = (Primary OCPD Rating ÷ Voltage Ratio) × 1/3

   Ampacity = (300 ÷ 2.31) × 1/3 ≈ 43.29 amps

   The conductor ampacity (200 amps) far exceeds the minimum required ampacity (43.29 amps).

3. Termination into Overcurrent Protection: The secondary conductors terminate in a 150-amp circuit breaker that limits the load current to no more than the conductor’s ampacity, as required by 310.14.
4. Physical Protection: The conductors are installed in a raceway, satisfying the NEC requirement for physical protection.
5. Termination Location: The overcurrent device is located adjacent to the termination point of the conductors, meeting the "as close as practicable" requirement.

Why a 150-Amp OCPD is Used with 200-Amp Conductors

While the secondary conductors are rated for 200 amps, the overcurrent device is sized based on the connected load and NEC requirements:

• Load-Driven Design: The connected load only requires 150 amps, so the OCPD is chosen to protect this load appropriately. Using a 150-amp breaker ensures that the current does not exceed the load requirements, maintaining safe operation.
• Conductor Ampacity: The conductors can handle up to 200 amps, but NEC Section 310.14 requires the OCPD to limit the current to the conductor’s allowable ampacity. A 150-amp breaker meets this requirement while providing headroom for future load increases.
• Future Expansion Consideration: The 200-amp conductors allow for potential future load increases without needing to upgrade the wiring. However, the breaker size reflects the current system requirements.

Comparison to the Transformer’s Maximum Output

The 75 kVA transformer has a maximum secondary output of approximately 208 amps, calculated as follows:

Transformer Current = Transformer Rating (VA) ÷ (Voltage (L-L) × √3)

Transformer Current = 75,000 ÷ (208 × 1.732) ≈ 208.2 amps

Key Observations:

• OCPD Limits Current Below Full Output: The 150-amp breaker limits the load current to less than the transformer’s maximum capacity, ensuring the transformer operates within safe limits.
• Load-Driven Design: The system is designed for a connected load of 150 amps, so the full transformer capacity is not utilized, providing flexibility for future growth.
• Safety Margin: The conductor ampacity (200 amps) and the transformer capacity (208 amps) both exceed the OCPD rating, providing a safety buffer for the installation.

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