Mike Dolt Enterprises
Mike Dolt Enterprises
User: kwn, Location: USA
Asked on: Friday, August 8, 2025 at 4:36 pm EDT
Answered on: Friday, August 8, 2025 at 4:56 pm EDT
Question: Greetings All;I'm wondering about the affect on compliance with Article 240.21(C)(4) when installing a Transfer switch between a transformer and a remote electrical building (MCC inside building).Case 1:The station has a customer owned transformer located approx 1000 feet in from the road, where the utility meter and service disconnect switch are located. There is an electrical building with an MCC inside located another 500’ in from the transformer. As per NEC Article 240.21(C)(4): “Outside Secondary Conductors of Unlimited Length”:Secondary conductors located outside a building or structure can be of unlimited length, without overcurrent protection at the point they receive their supply, if they’re installed as follows:The conductors are protected from physical damage within a raceway or manner approved by the authority having jurisdiction.The conductors terminate at a single overcurrent protection device that limits the load to the ampacity of the outside secondary conductors.The terminating overcurrent protection device for the outside secondary conductors is part of the building feeder disconnect.The building feeder disconnecting means is readily accessible either outside the building or nearest the point of entrance of the outside secondary conductors.The installation complies with these 4 conditions:The transformer secondary conductors are run in galvanized rigid conduit from transformer to the electrical building, approx 500’ away.The transformer secondary conductors terminate in an incoming feeder circuit breaker which is the input of an MCC in the electrical building.The terminating overcurrent protection device for the outside secondary conductors is a circuit breaker which is the disconnecting means for the electrical building.The building feeder disconnecting means (incoming feeder circuit breaker in this case) is readily accessible and is located inside the electrical building at the wall where the transformer secondary conductors enter the building.Case 2:What then If we install a Transfer switch near to the transformer and wired between the secondary terminals of the transformer and the incoming feeder circuit breaker in the MCC in the electrical building ? :The transformer secondary conductors are the same size as the conductors from the output terminals of the transfer switch to the incoming feeder circuit breaker in the electrical building/MCC.A generator, with it’s own circuit breaker (same or lower rating for loads being protected by the generator) is installed and wired to the backup input terminals of the Transfer switch.In this case, the installation complies with NEC Article 240.21(C)(4) as follows:(1) The transformer secondary conductors from the transformer to the transfer switch, and from the transfer switch to the electrical building are run in galvanized rigid conduit.(4) The building feeder disconnecting means (incoming feeder circuit breaker in this case) is readily accessible and is located inside the electrical building at the wall where the transformer secondary conductors enter the building.However does this comply with NEC Article 240.21(C)(4) (2) and (3) ?The actual transformer secondary conductors terminate at the input terminals of the transfer switch, not at a single overcurrent protection device that limits the load to the ampacity of the outside secondary conductors. The output terminals of the transfer switch then terminate at the incoming feeder circuit breaker at the MCC in the electrical building.My question then is if a transfer switch is installed (as described above) between a transformer and an incoming feeder circuit breaker in an MCC located in the electrical building, does this comply with Article 240.21(C) ? or do we need to install a disconnect switch or circuit breaker between the transformer and the transfer switch ?Thanks for your comments.
This is a great question, and your breakdown is very helpful. Let's analyze the impact of adding a transfer switch on compliance with NEC 240.21(C)(4).
The Core Issue: The Definition of "Terminate"
The crucial point revolves around the word "terminate" in 240.21(C)(4)(2): "The conductors terminate at a single overcurrent protection device..."The NEC doesn't explicitly define "terminate" in this context. Therefore, interpretation is key, and AHJ opinions can vary. However, the intent of 240.21(C)(4) is to ensure that these long, unprotected outside secondary conductors immediately feed into an overcurrent device that will protect them at their ampacity.
Analysis of Case 2 with the Transfer Switch
With the transfer switch in place, the transformer secondary conductors do not directly terminate at an overcurrent protective device. They terminate at the transfer switch's terminals. The overcurrent protection (the breaker in the MCC) is now downstream of the transfer switch.
Arguments for Non-Compliance
Interruption of Direct Protection
The transfer switch introduces a break in the direct protection scheme. If a fault occurs on the conductors between the transformer and the transfer switch, there's no overcurrent device to clear it. The MCC breaker won't see this fault directly unless it's severe enough to reflect back through the transfer switch and the generator's own overcurrent protection (which might be slower to react).
Potential for Overload
While you mention the generator breaker is sized appropriately, the transfer switch itself introduces a point where the conductors could potentially be overloaded before the MCC breaker would trip. For example, if someone connects a load to the generator that exceeds the ampacity of the secondary conductors between the transformer and transfer switch, this condition might persist for a time until the MCC breaker trips.
Interpretation of "Terminate"
Most interpretations would conclude that "terminate" in this context means a direct connection to the overcurrent protection device without an intervening piece of equipment. The transfer switch breaks that direct connection.
Arguments for Possible Compliance (and Why They're Weaker)
The MCC Breaker Eventually Provides Protection
One could argue that eventually, the conductors are protected by the MCC breaker. However, this weakens the intent of 240.21(C)(4) and introduces the potential for an unprotected fault.
Redundant Protection
In the case of a fault that causes a higher current than the generator can supply, the generator's breaker should trip. This could be presented as a form of redundancy.
Conclusion and Recommendation
Based on the prevailing interpretation and the intent of 240.21(C)(4), installing the transfer switch without an overcurrent protection device at the transformer secondary terminals is likely a violation of the NEC.Therefore, I strongly recommend installing a suitably sized overcurrent protection device (circuit breaker or fused disconnect switch) on the secondary side of the transformer before the transfer switch. This would: Ensure compliance with 240.21(C)(4) by having the conductors "terminate" at a protective device that matches their ampacity. Provide immediate overcurrent protection for the conductors between the transformer and the transfer switch. Improve the overall safety and reliability of the system.
How to Size the Overcurrent Protection Before the Transfer Switch
Conductor Ampacity
Determine the ampacity of the secondary conductors running from the transformer.2.
Overcurrent Device Rating
Select an overcurrent device (breaker or fuses) with a rating no higher than the conductor ampacity. (You may be able to go slightly higher based on the next higher standard overcurrent device rule, but it's generally best to match the conductor ampacity.)3.
Coordination
Ensure coordination with the downstream MCC breaker. The transformer secondary overcurrent device should trip faster than the MCC breaker for faults occurring on the conductors between the transformer and the transfer switch.
Important Considerations
Consult with the AHJ
Discuss this design with your local electrical inspector or authority having jurisdiction (AHJ) before proceeding. They have the final say in interpreting the code.
Transfer Switch Rating
Ensure the transfer switch is rated for the full load of the secondary conductors and the available fault current.*
Coordination Study: A proper coordination study is highly recommended to ensure that all overcurrent devices (transformer secondary breaker, generator breaker, and MCC breaker) are coordinated to minimize unnecessary outages.By adding the overcurrent protection at the transformer, you eliminate the ambiguity and significantly improve the safety and compliance of your installation. Good luck! Flag for review