Find the total current draw on the circuit. If you are installing a piece of equipment, this information will be in one of two places: the Installation Manual, or in the data tag on the item itself. If there is more than one item on the circuit, you must total up the current draw from each item. Example: Let’s assume your circuit breaker is for navigation lights. There are three lights that each draw five amps. Your total current draw for the circuit would be 15 amps.
Find the wire size the circuit breaker will protect. If this is a new installation, the wire size for the circuit will be specified in the installation manual. If you are adding a circuit breaker to an existing wire, the wire size will be in one of two places: printed on the wire; or found in the wiring schematic in the aircraft’s maintenance manual.
The size of the wire is contained in the identification code printed on the wire or schematic. A typical code is: J14C-20. The wire size for this code is 20 gauge.
Calculate the correct size circuit breaker using the total current draw of the circuit and the wire size. A table of circuit breaker sizes for given wire size and current can be found in the FAA Advisory Circular 43.13-1B, Section 4, Chapter 11, Paragraph 11-49, Table 11-3 (DC Circuit Protector Chart). See the Reference section for a link to this table.
