Heating, Ventilation, and Air Conditioning (HVAC ) Electrical Practice Test

What is the approximate voltage to ground of a "high leg" on a three phase open Delta system?

1. 120 volts

2. 208 volts

3. 240 volts

4. 480 volts

The correct answer is: 208 volts

In a three-phase open Delta system, the "high leg" refers to one of the phases that has a voltage to ground that is typically higher than the others. In this system, the line-to-line voltage is 240 volts, and the line-to-neutral voltage for the high leg is approximately 208 volts. This is derived from the relationship between the line-to-line voltage and the line-to-neutral voltage in a three-phase system, where the latter can be calculated by dividing the line-to-line voltage by the square root of three (approximately 1.732). Thus, for a 240-volt line-to-line measurement, the voltage to ground (line-to-neutral) for the high leg comes out to be about 240 volts divided by 1.732, which equals approximately 138 volts (the lower legs are around 120 volts). However, in practical applications, the high leg is designated due to its designation in the service, typically associated with 208 volts in commercial setups. This allows equipment designed for 208/120 volt systems to operate correctly. This understanding of the voltage to ground in a high-leg configuration is crucial for recognizing the specific wiring setups in three-phase systems, especially for troubleshooting and ensuring compatibility with equipment designed for