Master Electrician Practice Exam

What is the minimum size of copper conductors with THHW insulation required for a building with a demand load of 90,000 VA?

1. 250 kcmil

2. 500 kcmil

3. 750 kcmil

4. 1000 kcmil

The correct answer is: 500 kcmil

To determine the minimum size of copper conductors with THHW insulation required for a demand load of 90,000 VA, we refer to the National Electrical Code (NEC) standards and the ampacity charts based on conductor size, type, and installation conditions. First, one must calculate the full-load current (amps) using the formula: \[ \text{Current (I)} = \frac{\text{Power (P)}}{\text{Voltage (V)}} \] For example, if we assume a standard voltage level of 480V for three-phase systems, we can calculate: \[ I = \frac{90,000 VA}{480 V} \times \sqrt{3} \approx 108.1 A \] This is the current that needs to be supported by the copper conductors. Next, we need to consult the ampacity tables for THHW insulated copper conductors to determine the appropriate conductor size that can handle this amount of current. Based on the NEC and the ampacity of retrofitted copper conductors under suitable installation conditions, a 250 kcmil conductor typically has an ampacity that can deal with loads well below 108.1 A, particularly for normal temperature ratings,