USCG Exam PracticeElectrical fundamentals — DC/AC, motors, distribution
For a DC system with aggregate generating capacity below 1500 kW, 46 CFR §111.51-3 requires the maximum short-circuit current to be calculated using specific multipliers. If the aggregate rated generator current is 400 A and all motors in operation have an aggregate rated current of 150 A, what is the assumed maximum short-circuit current?
- A. 3,200 A (8 × 400 A + 3 × 150 A = 3,200 A + 450 A = 3,650 A — rounded to nearest hundred)
- ✓ 4,900 A (10 × 400 A + 6 × 150 A = 4,000 A + 900 A)Correct
- C. 4,600 A (10 × 400 A + 4 × 150 A = 4,000 A + 600 A)
- D. 3,650 A (8 × 400 A + 3 × 150 A = 3,200 A + 450 A)
Why B is correct
46 CFR §111.51-3(a) specifies that for a DC system the maximum short-circuit current is 10 times the aggregate rated generator currents plus 6 times the aggregate rated motor currents: (10 × 400) + (6 × 150) = 4,000 + 900 = 4,900 A. The 10×/4× multipliers apply to the AC maximum asymmetrical value per §111.51-3(b), and the 8×/3× multipliers apply to the AC average asymmetrical value per §111.51-3(c).
Cited:46 CFR §111.51-3