Charging system (generator) wire size.

[Bruce Fenstermacher]

I'd like to rewire at least some of my 170A for a few reasons not the least being some of it is 61 years old. The early planes came with smaller generators of 12, 15 and 20 amps. Later 170B models had 25 amp generators. From memory my electrical bus fuse is labeled for either a 20 or 25 amp fuse but I can't remember which. In any case it would indicate that the factory wiring was not adequate for the 35 amp generator that could be installed and what I'd like to wire for.

The early IPCs don't have wire size or length but the later B model IPC does. One piece of information missing is for what size generator was the wire size used intended or what size generator could they accommodate. I believe from IPC and TCDS research the 170Bs for the most part came with a 25 amp generator so the wire size can carry at least that load.

A review of the wire sizes used in the last series of B model serial numbers 26996 and on indicates the following. The number 7 wire from the generator field terminal to the master switch is 70" long and 18 gauge shielded wire. Wire 136 from the master switch to the voltage regulator field terminal is 50" long and also 18 gauge shielded. Wire 129 from the voltage regulator battery terminal to the main bus is 72" long and 10 gauge. Wire 146 from the generator to the voltage regulator armature terminal is 22" long and 10 gauge. Wire 131 from the main bus to the amp meter is 33" long and 10 gauge. Wire 130 from the amp meter to the starter relay terminal is 54" long and 10 gauge. Of course the main bus is a wire between each fuse holder (in my A model) and that I'd expect to be 10 gauge.

So my first question is, in that last paragraph have I missed any wires from the generator/battery/charging circuit who's gauge would be dependent on the system capacity?

I know that if I brought my aircraft up to the standard outlined above I'd be good for a 25 amp system but I want to be good for a 35 amp system that is approved by a source other than Cessna. Sources to include this forum, the TIC170 Electrical System Service Manual and AC 43.13 1B table 11-9, indicate that the 10 gauge wires outlined above should be increased to 8 gauge to handle the 35 amp generator.

But figure 11-2 seems to indicate that at the length of the wires used 10 gauge wire would be more than adequate and even 12 gauge wire would be good enough to carry the load. I've placed red dots on the chart to indicate how I'm reading it.

So my second question is do I need to upgrade to 8 gauge or is 10 gauge OK.

My third question is what about the 18 gauge wires outlined in my earlier paragraph. Should they be up sized from 18 gauge to 16 or 14 gauge or is 18 gauge still adequate in their application in a 35 amp system.

[lowNslow]

It looks like your original wire sizes are fine for a 35A generator. The field control circuit should have I believe a 5A fuse or circuit breaker, unfortunately, the circuit diagrams do not show of the fuses. Are you also going to convert to CBs instead of the fuses?

[Bruce Fenstermacher]

Karl the wire item numbers, lengths and sizes in my post above are not necessarily what is in my A model but is what the IPC says should be in B models 26996 and on. I'm not going to CBs at this time just bringing the wiring to a known standard. My partner and I don't know what is installed now and with all this airframe has been through it's likely to be anything but what should be there.

[lowNslow]

Karl the wire item numbers, lengths and sizes in my post above are not necessarily what is in my A model but is what the IPC says should be in B models 26996 and on.

That's true, but since we're talking about items that are just on the other side of the firewall I'd wager the lengths are almost identical.

[Bruce Fenstermacher]

Karl the wire item numbers, lengths and sizes in my post above are not necessarily what is in my A model but is what the IPC says should be in B models 26996 and on.

That's true, but since we're talking about items that are just on the other side of the firewall I'd wager the lengths are almost identical.

We are thinking alike.

[GAHorn]

Wire gauge/size is dependant upon several factors, all of which are related to heat and the materials used. The reason one might see similar gauges used on differing amperage circuits is likely related to materials and bundling. The charts used by many resources are based upon wires in free air...not wires in a bundle, which retain/share heat and which might be compromised. (An excellent example is the battery cable to the starter which is too small for the amperage carried...except for the fact it's not in a bundle but is in free-air and that it is for short-duration useage only.)

No. 8 wire is what should be used for 35 A circuits in bundles (anywhere wires are collectively strapped to each other.)

The generator FIELD wire can remain UN-altered because it only provides a ground for the field thru the regulator and Master Sw. (For the same reason it is not provided a seperate fuse/c.b.)

[Bruce Fenstermacher]

George can I assume your answer of 8 gauge wire is based solely on table 11-9. That for this application using paragraph 11-19 which describes the process to figure current carrying capability using formulas which I believe figure 11-2 shows, is just over kill considering the negligible difference of the weight and cost of 8 gauge verses 10 gauge wire. My inquiring mind wants to know for educational purpose as clearly 8 gauge wire would meet or exceed requirements.

The number 7 wire from the generator field terminal to the master switch is 70" long and 18 gauge shielded wire. Wire 136 from the master switch to the voltage regulator field terminal is 50" long and also 18 gauge shielded. Wire 129 from the voltage regulator battery terminal to the main bus is 72" long and 10 gauge. Wire 146 from the generator to the voltage regulator armature terminal is 22" long and 10 gauge. Wire 131 from the main bus to the amp meter is 33" long and 10 gauge. Wire 130 from the amp meter to the starter relay terminal is 54" long and 10 gauge. Of course the main bus is a wire between each fuse holder (in my A model) and that I'd expect to be 10 gauge.

So to be clear George you would use 8 gauge wire everywhere 10 gauge is called for in this list and that the field wire from the generator to master switch (7) and master switch to field terminal on the voltage regulator (136) can remain 18 gauge.

[lowNslow]

No. 8 wire is what should be used for 35 A circuits in bundles (anywhere wires are collectively strapped to each other.)

While it certainly doesn't hurt to use #8 wire especially if you are replacing it anyway, it is not required. Cessna specifies #10 wire on the early 172s and 150s with 35A generators.

The generator FIELD wire can remain UN-altered because it only provides a ground for the field thru the regulator and Master Sw. (For the same reason it is not provided a seperate fuse/c.b.)

Correct. I was thinking of the alternator system where the field is protected.

[cessna170bdriver]

Wire gauge/size is dependant upon several factors, all of which are related to heat and the materials used. The reason one might see similar gauges used on differing amperage circuits is likely related to materials and bundling. The charts used by many resources are based upon wires in free air...not wires in a bundle, which retain/share heat and which might be compromised. (An excellent example is the battery cable to the starter which is too small for the amperage carried...except for the fact it's not in a bundle but is in free-air and that it is for short-duration useage only.)

No. 8 wire is what should be used for 35 A circuits in bundles (anywhere wires are collectively strapped to each other.)

The generator FIELD wire can remain UN-altered because it only provides a ground for the field thru the regulator and Master Sw. (For the same reason it is not provided a seperate fuse/c.b.)

George is absolutely right, I wouldn't use anything smaller than 8 awg for 35 amps in ANY application. I consider 30 amps to be the limit for 10 awg. Even if you can find a way to route the wire without bundling it with others so the self-heating wouldn't be an issue, you might still run into voltage drop problems.

[lowNslow]

George is absolutely right, I wouldn't use anything smaller than 8 awg for 35 amps in ANY application. I consider 30 amps to be the limit for 10 awg. Even if you can find a way to route the wire without bundling it with others so the self-heating wouldn't be an issue, you might still run into voltage drop problems.

It certainly doesn't hurt (and if I was replacing it I would use #8) - but it is not required by AC43-13 or Cessna.

[GAHorn]

...So to be clear George you would use 8 gauge wire everywhere 10 gauge is called for in this list and that the field wire from the generator to master switch (7) and master switch to field terminal on the voltage regulator (136) can remain 18 gauge.

Yes, that would be my recommendation. I do not believe the 11-2 graph allows for interpolation...the next highest value is therefore recommended.

[..., it is not required. Cessna specifies #10 wire on the early 172s and 150s with 35A generators.
...

Keep in mind that mfr's are always willing to lean towards the minimum requirements in the interest of saving weight and cost.

[n2582d]

Several years ago I flew with a guy who owned an avionics shop in San Carlos. To help preclude radio noise he recommended using shielded wire for the alternator cable. I would imagine the same should help when using a generator rather than an alternator. He also said to attach an additional #8 ground wire between the alternator and a bolt attaching the regulator.

[LBPilot82]

To help preclude radio noise he recommended using shielded wire for the alternator cable. I would imagine the same should help when using a generator rather than an alternator. He also said to attach an additional #8 ground wire between the alternator and a bolt attaching the regulator.

I recently completely rewired my A model. I think that as long as everything is properly grounded, there is no need for shielded wire to the alternator. It certainly can't hurt but the cost of any wire gets pretty high if you add shielding. I also ran #8 from the alternator to the regulator. I have absolutely no problem with any radio noise. My two cents.

Bruce, I sometimes tend to get carried away with things, but why not just rewire it all. It seems be a pretty big project but it doesn't take that much time and you know the end product will be right. Might be a good winter project. After all, just because it may LOOK to be a top contender in the RAT plane catagory, doesn't mean it IS underneath.

Richard

[CAVU Mark]

I have been having some intermittent charging issues and am planning for circuit breaker installation for a group of three fuses: generator, stall warning and nav. lights. Current fuses installed are: 30A, 15A and 20A respectively - ouch yes I know the stall warning and nav. light fuses are high. Measured current is 0.8A and 4.8A. I will be installing a 2A and 7.5A circuit breakers for those positions.

The generator is the one I am unsure of. It is, I believe I have an original Delco unit with a fairly low capacity. The wire on the back of the 30A fuse looks like 12 to 14 gage, certainly not 10. It is also of the woven insulation type with some fraying and original looking. Any recommendations on the capacity of the generator and for the new wire size and circuit breaker rating? Currently I am planning for a straight replacement of 30A as I found. Yes, I am also skeptical of all the fuses now. Thank you.

[Bruce Fenstermacher]

Mark,

Look at the part number for on the generator tag then reference here for it's capacity. I'd also look for the stamped part number on the voltage regulator to make sure you have the correct one (or at least one in the correct range for the generator). The wire you describe sounds about right for the stock installation.

The Turn and Bank is usually tied to a self resetting Klixon breaker along with the stall warning as I recall. This is a funny round cylinder mounted just above and center of the fuses behind the piano keys on '52 and earlier aircraft. Don't know if it is in the same spot on later aircraft.