CEP Power Systems & Distribution Flashcards

When calculating voltage drop for a single-phase circuit, which formula is correct?

Answer: VD = 2 × K × I × D / CM

For single-phase circuits, voltage drop = (2 × K × I × D) / CM, where K is resistivity, I is current, D is one-way distance, and CM is conductor area in circular mils.

What is the primary reason transformers used in distribution systems are rated in kVA rather than kW?

Answer: Transformer losses depend on current (copper loss) and voltage (core loss), not power factor

Transformers are rated in kVA because their heating losses are functions of current and voltage magnitude, independent of the load's power factor.

A 3-phase motor draws 40A at 460V with a power factor of 0.85. What is the approximate real power consumption in kW?

Answer: 27.2 kW

P = √3 × V × I × PF = 1.732 × 460 × 40 × 0.85 ≈ 27,144W ≈ 27.2 kW.

In a delta-wye transformer, where is the neutral point created?

Answer: On the wye secondary side at the common connection of the three winding ends

In a delta-wye transformer, the three secondary windings are connected in a wye configuration, and the common junction point creates the neutral.

What is the function of a ground fault protection (GFP) system required by NEC 230.95 on service disconnects rated 1000A or more at 150V to ground or more?

Answer: To detect low-level ground faults that could cause arcing fires before the main overcurrent device operates

NEC 230.95 GFP detects arcing ground faults at current levels too low to trip the main overcurrent device, preventing destructive fires in high-current services.

Which harmonic order is most commonly generated by 3-phase, 6-pulse variable frequency drives (VFDs) and causes the greatest concern for distribution transformers?

Answer: 5th harmonic (300 Hz)

6-pulse VFDs primarily generate 5th and 7th order harmonics, with the 5th harmonic (300 Hz) having the highest magnitude and being most problematic for transformers and capacitor banks.