USCG Exam PracticeElectrical fundamentals — DC/AC, motors, distribution
Under 46 CFR §111.51-3, for an AC system with aggregate generating capacity below 1500 kW, what multipliers are used to calculate the MAXIMUM asymmetrical short-circuit current?
- A. 8 times rated generator currents plus 3 times rated motor currents.
- B. 10 times rated generator currents plus 6 times rated motor currents.
- ✓ 10 times rated generator currents plus 4 times rated motor currents.Correct
- D. 8 times rated generator currents plus 4 times rated motor currents.
Why C is correct
46 CFR §111.51-3(b) specifies that the maximum asymmetrical short-circuit current for an AC system is 10 times the aggregate normal rated generator currents plus 4 times the aggregate normal rated currents of all motors in operation. The 8×/3× multipliers apply to the average asymmetrical value per §111.51-3(c), and the 10×/6× multipliers apply to DC systems per §111.51-3(a).
Cited:46 CFR §111.51-3