Understanding the 10-Foot Secondary Conductor Rule in the 2023 NEC: Section 240.21(C)(1)

The 10-foot secondary conductor rule, as outlined in 240.21(C)(1) of the 2023 National Electrical Code (NEC), permits transformer secondary conductors to be installed without overcurrent protection at the transformer itself, as long as specific conditions are met. This rule applies when secondary conductors leave the transformer enclosure or vault and are protected downstream at a panel.

What Does the 10-Foot Rule Require?

To comply with 240.21(C)(1), the following conditions must be satisfied:

• Conductor Length: The conductors must not exceed 10 feet in length.
• Ampacity: The conductor ampacity must be at least equal to the product of the primary-to-secondary voltage ratio and 10% of the primary overcurrent device rating.
• Overcurrent Protection: The conductors must terminate in a single overcurrent device.
• Physical Protection: Conductors must be installed in a raceway or otherwise protected from physical damage.
• Proximity: The OCPD must be installed as close as practicable to where the conductors emerge from the raceway or enclosure.

Example Scenario

• Transformer: 75 kVA, 480V to 208V three-phase
• Primary OCPD: 125A breaker
• Secondary Conductors: 2 AWG copper, rated 115A at 75°C
• Conductor Length: 8 feet
• Load: 100A panel
• Secondary OCPD: 100A breaker in the panel being fed (not located at the transformer)

Application:

1. Conductor Length: The secondary conductors are 8 feet long, satisfying the 10-foot requirement.
2. Ampacity Calculation:
   • Voltage Ratio: 480 ÷ 208 = 2.31
   • 10% of Primary OCPD: 0.1 × 125A = 12.5A
   • Required Ampacity = 2.31 × 12.5A = 28.88A
   • Installed conductor ampacity: 2 AWG copper rated at 115A — exceeds 28.88A and is compliant.
3. OCPD Location: The breaker is in the panel fed by the transformer — satisfies the \"termination\" requirement.
4. Physical Protection: Conductors are in a raceway — compliant.
5. Proximity: The OCPD is located as close as practicable to where the conductors emerge — compliant.

Conclusion: This installation complies with 240.21(C)(1). Because the secondary conductors leave the transformer enclosure and are not protected at the transformer itself, the 10% voltage ratio rule applies — and the installed conductors meet or exceed the required ampacity.

Key Takeaways

• 240.21(C)(1): Applies when secondary conductors leave the transformer enclosure and are not protected at the transformer.
• 10% Rule: Required ampacity = (Primary Voltage ÷ Secondary Voltage) × 10% of the primary OCPD rating.
• OCPD Location: Breaker may be in the panel being fed, as long as it is as close as practicable to where conductors emerge.
• Conductor Protection: Physical protection with raceways is required.

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Originally authored by J. Harris, Edited by P.Abernathy on 6/9/2025