Let's break down the problem step by step: **Given:** - Single phase transformer - 2-wire 480V primary, 120V secondary - Secondary conductor: 1/0 AWG THW copper - Terminations rated 75°C - Need to determine the **maximum standard size primary OCPD** allowed to protect the **secondary conductors**. --- ## Step 1: Find the ampacity of 1/0 AWG THW copper at 75°C From **NEC Table 310.16 (2023)**: - 1/0 AWG copper, 75°C column = **150 amps** --- ## Step 2: Determine the maximum allowed OCPD for secondary conductors **Per NEC 240.4(B):** - Next higher standard OCPD is allowed if the ampacity does not correspond to a standard OCPD size and if certain conditions are met. **Per NEC 240.6(A):** - Standard sizes: 100, 110, 125, 150, 175, 200, etc. **Per NEC 240.21(C):** - For transformer secondary conductors, the OCPD shall **not exceed the ampacity of the conductor**. --- ## Step 3: Apply the "next higher size" rule Ampacity = 150A Standard OCPD sizes above and below 150A: 125A, **150A**, 175A. Since **150A is a standard size**, and matches the ampacity, **the maximum standard size OCPD is 150A**. --- ## Step 4: Final Answer **Final Answer:** **150** --- ## **Summary Table** | Conductor Size | Ampacity (75°C) | Max Standard OCPD | |----------------|------------------|-------------------| | 1/0 AWG THW Cu | 150A | **150A** | --- **Input only the number:** **150**