FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 11. Aircraft Electrical Systems | 4. Inspection of Circuit-Protection Devices | 11-51. Circuit Breaker Usage
9/8/98
AC 43.13-1B
SECTION 4. INSPECTION OF CIRCUIT-PROTECTION DEVICES
11-47. GENERAL. All electrical wires
must be provided with some means of circuit
protection. Electrical wire should be protected
with circuit breakers or fuses located as close
as possible to the electrical power source bus.
Normally, the manufacturer of electrical
equipment will specify the fuse or breaker to
be used when installing the respective equip-
ment, or SAE publication, ARP 1199, may be
referred to for recommended practices.
11-48. DETERMINATION OF CIRCUIT
BREAKER RATINGS. Circuit protection
devices must be sized to supply open circuit
capability. A circuit breaker must be rated so
that it will open before the current rating of the
wire attached to it is exceeded, or before the
cumulative rating of all loads connected to it
are exceeded, whichever is lowest. A circuit
breaker must always open before any compo-
nent downstream can overheat and generate
smoke or fire. Wires must be sized to carry
continuous current in excess of the circuit
protective device rating, including its time-
current characteristics, and to avoid excessive
voltage drop. Refer to section 5 for wire rating
methods.
11-49. DC CIRCUIT PROTECTOR
CHART. Table 11-3 may be used as a guide
for the selection of circuit breaker and fuse
rating to protect copper conductor wire. This
chart was prepared for the conditions speci-
fied. If actual conditions deviate materially
from those stated, ratings above or below the
values recommended may be justified. For ex-
ample, a wire run individually in the open air
may possibly be protected by the circuit
breaker of the next higher rating to that shown
on the chart. In general, the chart is conserva-
tive for all ordinary aircraft electri-
cal installations.
TABLE 11-3. DC wire and circuit protector chart.
Wire AN gauge
copper
22
20
18
16
14
12
10
8
6
4
2
1
0
Circuit breaker amp.
5
7.5
10
15
20
30
40
50
80
100
125
Fuse amp.
5
5
10
10
15
20
30
50
70
70
100
150
150
Basis of chart:
(1) Wire bundles in 135 °F. ambient and altitudes up to
30,000 feet.
(2) Wire bundles of 15 or more wires, with wires carrying
no more than 20 percent of the total current carrying
capacity of the bundle as given in Specification
MIL-W-5088 (ASG).
(3) Protectors in 75 to 85 °F. ambient.
(4) Copper wire Specification MIL-W-5088.
(5) Circuit breakers to Specification MIL-C-5809 or
equivalent.
(6) Fuses to Specification MIL-F-15160 or equivalent.
11-50. RESETTABLE CIRCUIT PRO-
TECTION DEVICES.
a. All resettable type circuit breakers
must open the circuit irrespective of the posi-
tion of the operating control when an overload
or circuit fault exists. Such circuit breakers are
referred to as “trip free.”
b. Automatic reset circuit breakers, that
automatically reset themselves periodically, are
not recommended as circuit protection devices
for aircraft.
11-51. CIRCUIT BREAKER USAGE.
Circuit breakers are designed as circuit protec-
tion for the wire (see paragraph 11-48
and 11-49), not for protection of black boxes
Par 11-47
Page 11-15