14 CFR § 25.1310

Each installation whose functioning is required for type certification or under operating rules and that requires a power supply is an “essential load” on the power supply. The power sources and the system must be able to supply the following power loads in probable operating combinations and for probable durations:

Loads connected to the system with the system functioning normally.

Essential loads, after failure of any one prime mover, power converter, or energy storage device.

Essential loads after failure of—

Any one engine on two-engine airplanes; and

Any two engines on airplanes with three or more engines.

Essential loads for which an alternate source of power is required, after any failure or malfunction in any one power supply system, distribution system, or other utilization system.

In determining compliance with paragraphs (a)(2) and (3) of this section, the power loads may be assumed to be reduced under a monitoring procedure consistent with safety in the kinds of operation authorized. Loads not required in controlled flight need not be considered for the two-engine-inoperative condition on airplanes with three or more engines.