In panelboards, where the voltage between busbars is 150 volts and the bars are opposite polarity and held free in air, the minimum spacing between the parts is ___.

In panelboards, where the voltage between busbars is 150 volts and the bars are opposite polarity and held free in air, the minimum spacing between the parts is ___.

• A. 1/2 in
• B. 11/4 in
• C. 2 in
• D. 3/4 in

Answer: (D)

The spacing is about preventing arcing between exposed energized parts in air. When busbars are opposite polarity and held free in air, the air gap must be large enough to withstand the voltage difference without an arc forming. For a 150-volt difference, the required air clearance is three-quarters of an inch. That size provides a safe margin under normal conditions, including humidity and dust. A half-inch gap is not enough for this voltage, while a much larger gap (1.25 inches or more) would be unnecessarily bulky for panelboards. The two-inch option is simply over-sized for 150 V.

The spacing is about preventing arcing between exposed energized parts in air. When busbars are opposite polarity and held free in air, the air gap must be large enough to withstand the voltage difference without an arc forming. For a 150-volt difference, the required air clearance is three-quarters of an inch. That size provides a safe margin under normal conditions, including humidity and dust. A half-inch gap is not enough for this voltage, while a much larger gap (1.25 inches or more) would be unnecessarily bulky for panelboards. The two-inch option is simply over-sized for 150 V.