A newly installed 120/208 V, three-phase, four-wire Wye service for a commercial building includes several non-linear loads (e.g., LED lighting with switch-mode power supplies, variable frequency drives). The electrician routinely measures current and finds excessive current (approaching 150% of rated phase current) flowing in the neutral conductor, even when phase currents are balanced. What is the primary cause of this anomalous neutral current?

Question: A newly installed 120/208 V, three-phase, four-wire Wye service for a commercial building includes several non-linear loads (e.g., LED lighting with switch-mode power supplies, variable frequency drives). The electrician routinely measures current and finds excessive current (approaching 150% of rated phase current) flowing in the neutral conductor, even when phase currents are balanced. What is the primary cause of this anomalous neutral current?

Answer options:

• An intentional ground fault occurring at the service entrance.
• Reversed phase rotation causing a negative sequence current. ✅ High harmonic currents, particularly third harmonics, not cancelling in the neutral.
• Improper bonding between the main service panel and the building's metallic piping.

Correct answer: High harmonic currents, particularly third harmonics, not cancelling in the neutral.

Explanation: In a balanced linear three-phase system, the fundamental currents cancel in the neutral. However, non-linear loads generate harmonic currents, especially odd triplen harmonics (3rd, 9th, 15th, etc.). These triplen harmonics are in-phase in the neutral conductor of a Wye system and add up rather than cancel, leading to significant and potentially overheating neutral currents, even with balanced phase currents.