HOLT Electricians Practice Exam

What is the minimum size THHN conductor required for a 5 horsepower 230-volt, single-phase motor?

1. No. 8

2. No. 10

3. No. 12

4. No. 14

The correct answer is: No. 10

To determine the minimum size THHN conductor for a 5 horsepower (hp) 230-volt, single-phase motor, we can refer to the National Electrical Code (NEC) guidelines. A 5 hp motor operating at 230 volts draws a specific amount of current, which can be calculated or looked up in standard motor tables. Generally, a 5 hp motor will draw approximately 24 amps at 230 volts. When selecting the appropriate wire size, it is essential to consider not only the current draw but also the temperature rating and the type of insulation of the conductor. According to NEC Table 310.16 and considering continuous operation and permissible voltage drop, the minimum conductor size for a motor of this rating is heavily influenced by its load characteristics and the applicable ampacity requirements. THHN wire, which has a temperature rating of 90°C for the insulation, allows for a decreased size due to its higher thermal tolerance. When applied to the specifics of this situation, the No. 10 conductor can handle 30 amps when adjusted for ambient temperature and particularly common conditions found in residential and commercial wiring. This provides an adequate safety margin for a motor that requires around 24 amps. Therefore, the correct choice reflects the standard practices