FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 11. Aircraft Electrical Systems | 5. Electical Wire Rating | 11-66. General
9/27/01
AC 43.13-1B CHG 1
SECTION 5. ELECTRICAL WIRE RATING
11-66. GENERAL. Wires must be sized so
that they: have sufficient mechanical strength
to allow for service conditions; do not exceed
allowable voltage drop levels; are protected by
system circuit protection devices; and meet
circuit current carrying requirements.
a. Mechanical Strength of Wires. If it is
desirable to use wire sizes smaller than #20,
particular attention should be given to the me
chanical strength and installation handling of
these wires, e.g., vibration, flexing, and termi
nation. Wire containing less than 19 strands
must not be used. Consideration should be
given to the use of high-strength alloy con
ductors in small gauge wires to increase me
chanical strength. As a general practice, wires
smaller than size #20 should be provided with
additional clamps and be grouped with at least
three other wires. They should also have ad
ditional support at terminations, such as con
nector grommets, strain relief clamps, shrink
able sleeving, or telescoping bushings. They
should not be used in applications where they
will be subjected to excessive vibration, re
peated bending, or frequent disconnection
from screw termination.
b. Voltage Drop in Wires. The voltage
drop in the main power wires from the genera
tion source or the battery to the bus should not
exceed 2 percent of the regulated voltage when
the generator is carrying rated current or the
battery is being discharged at the 5-minute
rate. The tabulation shown in table 11-6 de
fines the maximum acceptable voltage drop in
the load circuits between the bus and the utili
zation equipment ground.
c. Resistance. The resistance of the cur
rent return path through the aircraft structure is
generally considered negligible. However, this
is based on the assumption that adequate
TABLE 11-6. Tabulation chart (allowable voltage drop
between bus and utilization equipment ground).
Nominal
system
voltage
Allowable voltage
drop continuous
operation
Intermittent
operation
14
0.5
1
28
1
2
115
4
8
200
7
14
bonding to the structure or a special electric
current return path has been provided that is
capable of carrying the required electric cur
rent with a negligible voltage drop. To deter
mine circuit resistance check the voltage drop
across the circuit. If the voltage drop does not
exceed the limit established by the aircraft or
product manufacturer, the resistance value for
the circuit may be considered satisfactory.
When checking a circuit, the input voltage
should be maintained at a constant value. Ta
bles 11-7 and 11-8 show formulas that may be
used to determine electrical resistance in wires
and some typical examples.
d. Resistance Calculation Methods.
Figures 11-2 and 11-3 provide a convenient
means of calculating maximum wire length for
the given circuit current.
(1) Values in tables 11-7 and 11-8 are
for tin-plated copper conductor wires. Be
cause the resistance of tin-plated wire is
slightly higher than that of nickel or silver-
plated wire, maximum run lengths determined
from these charts will be slightly less than the
allowable limits for nickel or silver-plated
copper wire and are therefore safe to use. Fig
ures 11-2 and 11-3 can be used to derive
slightly longer maximum run lengths for silver
or nickel-plated wires by multiplying the
maximum run length by the ratio of resistance
of tin-plated wire, divided by the resistance of
silver or nickel-plated wire.
Par 11-66
Page 11-21