46 CFR §120.310

1. (a)
   1. (1)Each vessel that relies on electricity to power the following loads must be arranged so that the loads can be energized from two sources of electricity:
      1. (i)The vital systems listed in § 119.710 of this chapter;
      2. (ii)Interior lighting except for decorative lights;
      3. (iii)Communication systems including a public address system required under § 121.610 of this chapter; and
      4. (iv)Navigation equipment and lights.
   2. (2)Except as provided in § 120.312 of this part, a vessel with batteries of adequate capacity to supply the loads specified in paragraph (a)(1) of this section for three hours, and a generator or alternator driven by a propulsion engine, complies with the requirement in paragraph (a)(1) of this section.
2. (b)Where a ship service generator driven by a propulsion engine is used as a source of electrical power, a vessel speed change, throttle movement or change in direction of the propeller shaft rotation must not interrupt power to any of the loads specified in paragraph (a)(1) of this section.