The Cessna 120-140 Association Forum

I would appreciate help on wiring on the Grimes Landing Light. I restored the light itself viewtopic.php?f=12&t=1137&start=80, so that wiring is not an issue and is safe and functional. However, there are old wires that are original and in good shape but I would rather not use as they are 73 years old and the wire couplers do not seem to match the lamp.

I have had people refer me to charts and talk all kinds of amp and length stuff but that might as well be like asking me to play the violin. I may make it squeek a sound but it will most likely not be right. I have little electrical experience or knowledge.

The motor itself (13VDC) uses the cloth covered wires marked <-A 39-> and <A 40->. They appear to me to conduct 4 amps to operate the up and down movements of the motor. I don't know for sure. That is what I have found on a blog of a Cessna 140 restoration. Its fuse on the panel is a 10 amp shared fuse.

The wire marked <-A 43-> is to the bulb, a GE Sealed Beam, part No. M4522 (250 Watt, 13 Volt, PAR 46 type - 4522). I am guessing it draws 20 amps based on Neil Wright's info I found here. The fuse is 25 amps.

I am really wanting to order the correct MilSpec wire to replace these old wires. Does anyone have any clue what gauge these are and / or what I could replace them with? Please, no charts. I have them, to include the A&P FAA charts that tell length & amps, etc., but that is like showing me Chinese. The part numbers on these old wires meant something. They had to have had a gauge and a rating. All I am trying to figure out is how I can get 25 ft.(?) of wire for each and replace these old wires so I can close the wing. I estimated I needed about 15 ft. from the lamp through the mount then rerouted along the wing spar grommets to the wing root. The originals appeared to be connected at that point to the receiving wires that went to the bus / panel (estimated at another 10 ft.)

Has anyone out there replaced these wires and been signed off--or signed off yourself if an A&P--some replacement wires for a standard Grimes 3905 Landing Light? And if so, what did you use?

Thanks as always for any assistance. Steve

Steve:
At the risk of sounding like a smart a$$, who is going to sign off this work? That's who you should be asking. In the words of Dirty Harry, "a man's got to know his limitations."

To determine the wire gauge you need to know the length of the run from the buss, through any switches and circuit protection devices, and out to the load, as well as the current draw of the load. In addition, you have to take into consideration the ratings of the circuit protection devices. They need to blow before the wire starts smoking. So, if you have a 25 amp fuse for the lamp, you need wiring for which a 25 amp fuse is appropriate, and that can carry the 20 amp load. Likewise, if the fuse for the motor is 10 amps, you need wiring to match.

So, if your estimate of 25 feet is correct, based on the fuse ratings, the minimum size for the lamp is 10 awg and 18 awg for the motor. Since the landing light (and the motor) is an intermittent load, you can get away with #10 for the lamp but you should use #8 if you intend to leave it on for long periods of time. You can use #18 for the motor circuit as long as the load doesn't exceed 5 amps.

Thanks John. This is why I am asking because I do know my limitations on electrical and don't want to screw it up.

I have several A&Ps, a Tech Counselor and a sign off I can get help from but everyone keeps asking me what wire do I have. I am in a 'do loop.' What wire do I need?

Another pilot on the association's FaceBook page said he used #18 and #10. The main A&P who checks my work suggested a MilSpec #20 wire for the motor switches. I guess #18 would be safer? I have a line on that now and want to make sure. Your info on #8 for the lamp sounds like it will work regardless. While I intend to run an LED for flight, I also want to wire it to use both, in case I, or someone else, were to place the old bulbs back in.

Appreciate the help. Steve

Steve,

The A39,A40,and A43 are number designations that Cessna gave them and are referenced to the IPC wiring diagram that you posted.
Another page in the IPC will tell you where those wires go from and to. For example A39 is "Wire-Landing light to wingroot".
They have nothing to do with wire size and load carrying capacity.
If it were me I would run #10 for the lamp and #18 for the motor.
I think #8 for the lamp would be to heavy and not necessary. Do Not buy any from the hardware store but go look and it and compare the sizes to see what I mean.

Good luck.

Rick

A #20 wire for the motor might carry the current, but the fuse doesn't provide adequate protection. #18 is the smallest wire you can use with the 10 amp fuse. #20 will smoke before the fuse blows, if it blows at all. There is no wiggle room.

I agree that #8 is probably overkill for the lamp, but if you want every last lumen out of it that's the way to go. #10 is not sufficient for a continuous load at the current.

Any wire you use must meet the appropriate mil spec. If you order it from somewhere like Aircraft spruce you'll be OK.

Thanks guys. I finally have enough info and have ordered MilSpec wire from Aircraft Spruce. Very much appreciate the feedback. Steve

Suggestion.....unless youre really going for originality, replace the bulb with LED.

I’d keep wire same size so the next knucklehead doesn’t start a fire.....but a few amps for LED vs 25ish for incandescent is remarkable .... will be more reliable and save alternator/generator, etc.

But if you’re doing a restoration, then do what you gotta do!

Jon

My 2c.

To get an LED that is as bright as the original 250W bulb will cost you hundreds of dollars, and don’t talk to me about Ebay and Amazon bulbs.
Cheap LED’s have all kinds of issues from throttling the brightness way down to keep heating in check to generating lots of electrical noise, so much that it will render your radio ineffective.

USCG advisory on cheap LED’s
https://www.dco.uscg.mil/Portals/9/DCO% ... 091109-630

Now good quality bulbs like Whelen won’t have those issues, but a great many, even ones that hold themselves out to be aircraft bulbs do.

So unless you do a lot more night flight than most, the stock incandescent is fine, it’s worked for 73 years.

Better living through technology for 500 please.

Back then they weren’t running modern transponders with adsb and gps plus other gizmos off the main bus like iPads etc.