When do you use a power factor other than 0.8 for a three-phase power source?

When do you use a power factor other than 0.8 for a three-phase power source?

• A. For all three-phase sources
• B. When the load is purely resistive
• C. When generating at low voltage
• D. When otherwise specified

Answer: (D)

The key idea is that 0.8 is a common default rating for three-phase equipment, but the actual power factor to use is whatever the system or equipment specifies. You would apply a power factor other than 0.8 only when the design, grid code, or device nameplate calls for a different value. That’s why the correct approach is to follow “when otherwise specified.” In practice, a purely resistive load does have a power factor of 1, which is different from 0.8, but the decisive point here is that the source’s operating PF should match the specification on the equipment or system you’re using. The other scenarios—generating at low voltage or applying a universal rule to all three-phase sources—don’t mandate a PF change by themselves.

The key idea is that 0.8 is a common default rating for three-phase equipment, but the actual power factor to use is whatever the system or equipment specifies. You would apply a power factor other than 0.8 only when the design, grid code, or device nameplate calls for a different value. That’s why the correct approach is to follow “when otherwise specified.”

In practice, a purely resistive load does have a power factor of 1, which is different from 0.8, but the decisive point here is that the source’s operating PF should match the specification on the equipment or system you’re using. The other scenarios—generating at low voltage or applying a universal rule to all three-phase sources—don’t mandate a PF change by themselves.