- The NEC Section 240.21(C)(6) "25-foot tap rule" requires the secondary conductors to be 25 feet or shorter.
- Condition I requires the ampacity to be calculated based on the primary OCPD and transformer ratio.
- Condition II requires the ampacity to be at least the rating of the load-side overcurrent device.
- Condition III requires physical protection (raceway).
- Therefore, all three conditions are necessary, making (b) the correct answer. The final answer is \(\boxed{b}\).
Explanation
- Analyzing the Problem Requirements
The National Electrical Code (NEC) Section 240.21(C)(6), commonly referred to as the "25-foot tap rule" for secondary conductors fed from a transformer, specifies the conditions under which these conductors do not require overcurrent protection at the point of supply (the secondary terminals). To ensure safety and prevent fires, the code mandates strict requirements. We need to evaluate three specific statements:
I. Ampacity \(\geq \left[ (\text{Pri OCPD} \div 3) \times (E_{\text{pri line}} \div E_{\text{sec line}}) \right]\).
II. Ampacity \(\geq\) rating of the device at the end.
III. Installation in a raceway or otherwise protected from physical damage.
- Evaluating the NEC Requirements
- Statement I represents the calculation for the minimum ampacity of the tap conductors based on the primary side overcurrent protective device (OCPD), adjusted by the transformer turns ratio. This is a fundamental requirement for sizing transformer secondary taps to ensure that the secondary conductors are protected by the primary OCPD.
- Statement II addresses the situation at the load end of the 25-foot tap. The secondary conductors must have sufficient ampacity to handle the load, which is defined by the overcurrent device (breaker or fuses) at the termination point.
- Statement III concerns the physical environment of the conductors. Since these conductors are protected only by the primary-side device (as they are a "tap"), they are susceptible to damage. The NEC explicitly requires them to be enclosed in a raceway or otherwise protected from physical damage.
Upon reviewing the NEC standards for 240.21(C)(6), it is confirmed that all three conditions—the calculated ampacity based on the primary side, the ampacity relative to the load-side protection, and physical protection—are required.
- Conclusion
Based on the analysis of NEC Section 240.21(C)(6), all three statements (I, II, and III) are required conditions. Therefore, the correct combination of these requirements is found in option (b). The final answer is (b).
Examples
Think of the electrical system in a large building as a network of highways. The transformer is a major supply station, and the tap conductors are smaller secondary roads leading to individual panels. Just as you need traffic signs, speed limits, and guardrails (physical protection) on these roads to ensure safety when the main highway controls traffic flow, the NEC requires specific ampacity calculations (Statement I & II) and protective measures (Statement III) to ensure these electrical "side roads" do not overheat or become damaged, which could lead to electrical fires. Following these rules is essential for any electrician to ensure the safety and reliability of a building's power distribution system.