Let's break down the problem step by step: **Given:** - Transformer: 480V primary / 208V secondary - Length of primary + secondary conductors: 18 feet - This is a **feeder tap** situation, following NEC 240.21(C). - Primary OCPD: 125 amps - Find: **Minimum ampacity for the secondary conductors?** --- ## Step 1: Identify the Relevant NEC Rule This is a transformer secondary tap, so we use **NEC 240.21(C)**. For a **feeder tap**, specifically a **10-foot tap**, the requirements are: - Secondary conductors do **not exceed 10 feet** (ours is 18 feet, so not this one). - For a **25-foot tap** (NEC 240.21(C)(6)): - The secondary conductors **do not exceed 25 feet**. - The ampacity of the secondary conductors is **not less than 1/3 of the rating of the overcurrent device protecting the primary of the transformer** (125A in this case). - The secondary conductors terminate in a single overcurrent protective device or set of fuses that limit the load to that conductor ampacity. Our total length is **18 feet** (less than 25 feet), so the **25-foot tap rule** applies. --- ## Step 2: Calculate Minimum Ampacity **Ampacity must be not less than 1/3 of the rating of the OCPD protecting the primary.** \[ \text{Minimum Ampacity} = \frac{1}{3} \times \text{Primary OCPD} \] \[ = \frac{1}{3} \times 125 \text{A} \] \[ = 41.666... \text{A} \] Round to the nearest whole number: \[ \text{Minimum Ampacity} = 42 \text{A} \] --- ## Step 3: **Final Answer** **The minimum ampacity for the secondary conductors is \(\boxed{42~\text{amps}}\).** --- ### **Summary Table:** | Step | Value | |------|-------------------------------| | Primary OCPD | 125A | | Tap length | 18 feet (under 25-foot rule) | | Fraction required (NEC 240.21(C)(6))| 1/3 | | Calculation | 125A × 1/3 = 41.67A | | Rounded | 42A | **Final Answer:** > **42 amps** is the minimum ampacity required for the secondary conductors.