46 CFR § 183.310 - Power sources.
(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:
(i) The vital systems listed in § 182.710 of this chapter;
(ii) Interior lighting except for decorative lights;
(iii) Communication systems including a public address system required under § 184.610 of this chapter; and
(iv) Navigation equipment and lights.
(2) 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.
(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.