Understanding the 10-Foot Secondary Conductor Rule in the 2023 NEC: Section 240.21(C)(2)

The 10-foot secondary conductor rule, outlined in 240.21(C)(2) of the 2023 National Electrical Code (NEC), provides an exception to the general requirement for overcurrent protection at the source of transformer secondary conductors. This rule allows certain installations to omit overcurrent protection at the transformer, provided strict conditions are met.

This article explains the rule, provides a corrected example for a three-phase transformer, and clarifies why the connected load may differ from the transformer’s full-rated capacity.

What is the 10-Foot Secondary Conductor Rule?

The NEC permits transformer secondary conductors to extend up to 10 feet without overcurrent protection at the source, provided the following conditions are satisfied:

Conditions for Compliance

• Conductor Length: The secondary conductors must not exceed 10 feet in length.
• Ampacity: The ampacity of the secondary conductors must be at least:
  • Equal to the calculated load served by the conductors, or
  • Equal to the rating of the overcurrent protective device (OCPD) that terminates the secondary conductors, whichever is greater.

  For field installations, the ampacity of the secondary conductors must also meet the following formula:

  Ampacity ≥ (Primary OCPD Rating ÷ Primary-to-Secondary Voltage Ratio) × 0.1

• Overcurrent Protection: Overcurrent protection must be installed at the termination of the secondary conductors.
• Physical Protection: The secondary conductors must be installed in raceways for protection from physical damage.
• Proximity to the Transformer: The OCPD protecting the secondary conductors must be installed as close as practicable to where the conductors emerge from the raceway or enclosure.

Why the Secondary Load May Differ from the Transformer’s Full Capacity

For a three-phase transformer, the full-rated capacity is calculated using the following formula:

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

For a 75 kVA, 208V three-phase transformer:

Secondary Current = 75,000 ÷ (208 × 1.732) = 75,000 ÷ 360.25 ≈ 208.2 amps

While the transformer can supply up to 208 amps, the connected load often depends on the design of the electrical system and may not use the transformer’s full capacity. In this example, a 100-amp load is used, as the connected equipment and OCPD determine the actual current draw.

Example: Applying the 10-Foot Secondary Conductor Rule

Scenario:

• Transformer Details: A 75 kVA, 480V to 208V three-phase step-down transformer.
• Primary OCPD Rating: 125 amps.
• Secondary Conductors: 2 AWG copper conductors, rated for 115 amps at 75°C.
• Conductor Length: 8 feet (within the 10-foot limit).
• Load: 100 amps (connected equipment).
• Secondary OCPD: 100-amp breaker.

Application:

1. Conductor Length: The secondary conductors are 8 feet long, satisfying the 10-foot limit.
2. Ampacity Check (General Requirements):
   • The secondary conductor ampacity (115 amps) is:
   • Greater than the connected load (100 amps).
   • Equal to or greater than the OCPD rating at the termination (100 amps).
3. Ampacity Check (Field Installation Calculation):
   • Primary-to-Secondary Voltage Ratio: 480 ÷ 208 = 2.31.
   • Required Ampacity:

   Ampacity = (Primary OCPD Rating ÷ Voltage Ratio) × 0.1

   Ampacity = (125 ÷ 2.31) × 0.1 ≈ 5.41 amps

   • The conductor ampacity (115 amps) exceeds this minimum requirement.
4. Overcurrent Protection: A 100-amp breaker is installed at the termination point, providing the required protection.
5. Physical Protection: The conductors are installed in a raceway, meeting the NEC requirement for physical protection.
6. Proximity to Transformer: The termination point with the OCPD is adjacent to the transformer, meeting the "as close as practicable" requirement.

Conclusion: This installation complies with the 10-foot secondary conductor rule in 240.21(C)(2).

Key Takeaways

• Transformer Capacity vs. Connected Load: While a 75 kVA, 208V three-phase transformer can deliver 208 amps, the connected load and downstream OCPD size (100 amps in this case) determine the actual current demand.
• Accurate Ampacity Calculations: Secondary conductor ampacity must satisfy both the connected load and the formula in 240.21(C)(2) for field installations.
• Conductor Protection: Secondary conductors must be installed in raceways to meet NEC physical protection requirements.
• Proximity to Transformer: OCPDs must be as close as practicable to the point where secondary conductors emerge from the raceway.

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