Stumped

[N3092A]

I don't claim to have any electrical knowledge. However, I'm trying to figure out why the IPC calls out 18 gauge wire for the nav lights throughout the airplane. The circuit is protected by a 10 amp fuse. According to the charts I have found, 18 gauge wire is only sufficient for continuous loads of 3 amps or less. If we are trying to protect the wiring with our fuses/breakers, the numbers don't add up.
The reason I ask is because I'm in the process of rewiring and switching to breakers, and I'm confused. The landing light wiring is another similar story. I must be missing something.

Thanks for any guidance.

'53 B model
25736

[ronjenx]

AC 43.13-1B, chapter 11, available for download on the FAA web site, has all the information regarding wire types, amp/temperature rating, circuit breaker/fuse protection, etc.
What I just read there does not agree with what you posted regarding 18 gage wire capacity.

[Jimmy M.]

Check this site for wire ampacity chart.......................# 18 wire is good for 10 amps

http://www.rowand.net/shop/tech/wirecapacitychart.htm

[n2582d]

Here is the chart from AC43.13.

Wire Size Chart.pdf

In yellow is load for the nav light circuit. The 7512 bulb is 14V/26 watt/1.85 amp. The 1512 bulb is 14V/21 watt/1.5 amp. (For an amazingly exhaustive history of wingtip nav lights on the 170 see George's input here .) So I would concur with the previous guys in that 18 ga. wire is adequate. I would agree with you on the landing light circuit though. Harlow said his landing light wire gets hot if left on for any length of time here. Anybody else experience this? The aircraft before s/n 25373 had both 100 watt lights wired to one 14 ga. wire. See the blue line on the graph for the landing light circuit. It looks like the wire should be at least 12 ga. if not 10 ga. It seems to me the SPDT switch for this circuit is also too small for the current load. An easy solution might be to use a double pole switch in place of the single pole switch (AN3027-1 in place of the AN3021-1) and jumper the two poles together. In this post Miles points out that it's the inrush current that shortens the life of the landing lights. I wonder if adding an inrush current limiter or thermistor would extend the life of the landing/taxi bulbs and switch. These things provide high resistance when cold and low resistance when hot thus limiting harmful high inrush current yet hardly limiting current flow once hot.

[N3092A]

Thanks for the help guys. After reviewing 43.13, it seems that 18 is sufficient for 10 amps. I guess I am confused on why it is a 10 amp fuse, when the lights only pull a max 2 amps a piece. I guess because of the wire length. Good point on the landing light wires. My airplane has 2ea. 14 gauge wires, one for each bulb. However, it's just one wire from the breaker to the switch. Seems like the landing lights should be using a relay.

Anyway, thanks again.

[Bruce Fenstermacher]

I guess I am confused on why it is a 10 amp fuse, when the lights only pull a max 2 amps a piece.

Because the fuse is not protecting the bulbs it is protecting the wire going to the bulbs. I know I have trouble remembering that sometimes as well.

[ronjenx]

From 43.13-1B:

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 component
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 timecurrent
characteristics, and to avoid excessive
voltage drop. Refer to section 5 for wire rating
methods.

[GAHorn]

...see George's input here .) ....

And if you are truly concerned about amps and current draw, don't miss http://www.cessna170.org/forums/viewtop ... led+lights