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PPS Installation and Operating Manual
Rev. C.2 (January 19, 2018)
When planning your electrical system, assume the alternator provides
80% of its rated output (in amperes), and therefore your total continuous
load (don’t worry about trim or flaps or other transient loads) should not
exceed 80% of rated alternator capacity.
3.4 Wire Sizes and Circuit Protection
A table below shows wires sizes versus loads for a typical homebuilt-size
airplane. The wire size can be larger than necessary but should not be
smaller. Circuit breakers (and fuses) protect the wiring, not the device. If
the breaker is too large, then the wire may overheat and fail. If too small,
then the device may fault (breaker trips) because it draws too much
current.
Most kit aircraft companies and avionics companies provide
recommendations for sizing wires and breakers. You can use these
recommendations. Or, you can borrow or purchase an ammeter (typically
under $50 at Radio Shack, etc.) measure the current draw of each
electrical device and then determine the sizes yourself. When you know
the current draw for each device, use the chart below to size the power
wires. For simplicity, the wiring harnesses available from Vertical Power
use the wire sizes in the table.
Up to (amps)
Use wire size (AWG)
5 A
20
10 A
18
15 A
14
80 A
4
300 A (starter)
2
Data signal
22
Recommended wire gauges
3.5 Grounding - IMPORTANT
Many people think that the power, or positive, wire is the most important
wire to provide electricity to a device. The electrical ground is just as
important as electricity must flow the entire path from the power source
to the device and back to the source. The ground wire must be the same
wire gauge or a larger diameter (smaller gauge number) as the wire that
provides power to the device.
A ground loop is when electricity takes two different paths, and each path
has a different resistance. Ground loops are most noticeable in aircraft
audio equipment, and can produce a variety of problems, most notably
unwanted noise.
Several options for grounding your system are provided below. Choose
the one that best fits your needs. Keep in mind that more wiring means
more weight (although likely negligible). Also, note that in all the
examples below the avionics grounds are kept together.
•
Option 1: Run a ground wire from each and every electrical
device back to a common grounding point, typically a ground bus
on the firewall.
•
Option 2: Run ground wires from all the avionics to an
intermediate grounding point, then run a larger wire from the
local ground bus to the firewall ground. Run wires from all the
other devices to the firewall ground.
•
Option 3: Run the ground wires from the avionics to the firewall
ground, and run the other ground wires to a local ground (a metal
part of the airframe located near the device).
The GND Stud connection to the PPS shall be ≤ 2.5 mΩ.
Failure to meet this requirement will damage the PPS and
lead to an unsafe condition.
