The wire size depends on the Voltage, Current flow (amperage and whether it is a Continuous or Intermittent flow), Fuse size (if appropriate), Length of the Circuit, Ambient Air Temperature around the wire, and whether the wire is in the free air (good cooling) or in a bundle of other wires (which limit cooling of the wire). This is starting to sound complicated. It also depends on the aircraft operating altitude, less than 20,000 feet, so that's easy anyway. I had to go back to a 1976 revision of AC65-15A, Figure 11.8 to find out that Intermittent means in use for a "maximum of 2 minutes". You can do something like installing a placard which says to limit use of the landing light to 2 minutes or less, if needed, to keep the wire to a reasonable size for a load where it could be, but doesn't need to be, on continuously.
Using the Simplified Diagram all this breaks down into circuits for:
- Engine Ignition - No Fuse
- Master Switch Solenoid - No Fuse
- Starter - No Fuse
- Power Bus - No Fuse
- Generator - 15 amp fuse with 12 amp Generator
- Navigation and Cockpit Lights, Plus Landing Light Motor - 10 amp fuse
- Landing Light Bulb - 25 amp fuse
- Electric Turn and Bank - 10 amp fuse
- Radios - 10 amp fuse
- Grounding
Before we jump into the details, one issue I'll deal with later is to assure there is adequate ground (bonding) between the all the various parts of the plane and engine. Without a path for the return current flow to the battery (negative ground) all the rest of this is useless.
- Engine Ignition
The Engine Ignition circuits just ground the magnetos so there in no significant current flow. These were easy to identify in the pile because they were shielded to suppress radio interference. They're 18 gauge wire.
- Master Switch Solenoid
The Master Switch does not turn on the power directly. It operates a solenoid which connects the battery positive cable to the starter and from there to the fuses. The coil in the solenoid is connected to the Battery Positive cable post with a short jumper wire. When you operate the Master Switch it grounds the other end of the coil to operate the solenoid. This means this circuit is active Continuously when the Master Switch is on. Great but how much current does this coil draw? No one seems to know or care. Cessna Service Bulletin SB 65-89 allows the R-57 solenoid to be replaced with an S-1579A2 which Aircraft Spruce sell. I've written to them for an answer for their solenoid. Until I find the truth I've estimated it at 0.5 amps. The total circuit length is about 12 feet so a 16 gauge wire will do. It's in a bundle of 3 wires in the rear of the firewall at 70 degrees F (20 C) so we're still good with 16 gauge. In fact a 16 Gauge wire this long, etc. could handle up to 6 amps so we're not close to a problem.
- Starter
I've read a lot of discussions on the 124/140 forum about the starter cable. I have an old parts catalog showing the various Mil Spec fabric jacketed wires and cable made by Prestolite. They give a dimension over the insulation for each to make identification easier. The cable I have measures 0.353" diameter which is the diameter for 4 gauge Cable. Number 2 gauge is 0.424" and 6 gauge is 0.294". Because it's a stranded cable you can't just measure the wire diameter. You have to measure the diameter of the strands and calculate the total cross section area to determine the cable gauge. My old cable has 7 bundles of 19 wires (7x19). Each wire strand measures 0.018" in Diameter (25 gauge). This makes it an ASTM B173 Class H rope lay up creating a 4 gauge cable. There was some discussion on the 120/140 forum about whether this cable was a 3 gauge cable. There technically is such an animal. If it's made with the same 7x19 layup it uses 24 gauge (0.021") strands. There is no doubt my old cable is a 4 gauge cable. There is no way to know if it's the original factory cable but it's age and everything says it probably is.
Assuming this is a 4 gauge cable let's look at the load to see if this works. The cable is 12 feet long. So how many amps does the starter draw? According to Delco-Remy Service Bulletin 1M-180, Cranking Motor #1109656 draws 65 amps when tested at 11.35 Volts, turning 1200 RPM with No Load. If it's locked (held so the rotor can not spin) it draws 450 amps when tested at 3.9 volts and should produce at least 60 Ft. Lbs. of torque. The estimates I've read for a normal cranking load vary from 150 amps to 300 amps. I think 150 amps would be a reasonable estimate for a hot, well worn engine. I also think 300 amps would be a reasonable estimate for a tight fitting cold engine. The starter is also limited to 30 seconds of operation to avoid overheating, so it's definitely intermittent operation. Per AC 43.13-1B you need to limit the wire to a 1 volt drop for an intermittent 14 volt circuit. Including 1 ft. from ground to the battery and 1 ft. from the battery to the solenoid, this line is 14 ft. long. The formula is easy E=IR (Voltage drop = Current flow x Resistance). The resistance of a 4 gauge wire is 0.28 ohms / 1000 ft. (per AC 43.13-1B, Table 11-9). Therefore at the maximum drop of 1 volt a 4 gauge cable will be carrying 255 amps. I think I'll stick with the 4 gauge cable. If we change to a 2 gauge cable, it could handle 397 amps. but it would be much stiffer to route through the various bends ( as discussed on the 120/140 forum). The new 4 gauge cable from Aircraft Spruce (Mil-W-22759/16-4) is a much stiffer cable. It is wound with 19 bundles of 7 wires (19x7). The math is simple, they both use 133 wires of 25 gauge, but the stiffness is amazingly different. The old cable bends very easily by comparison, just like extra flexible control cable which is also a 7x19 cable.
- Power Bus
The starter cable also provides power to the rest of the airplane's electrical systems. From the battery cable stud on the starter a wire runs to the Ammeter and on from there to the Fuse Holders, effectively the 14 volt Bus. This wire powers everything except the Master Solenoid and Starter. When everything is running and all radios transmitting this creates a 14.9 amp load, which is less than 80% of the Generator output. I realize the normal load is lower than when the radios are transmitting but I don't have all those loads. When the landing light is on this load jumps to 34.1 amps. To cover these loads this wire needs to be a 10 gauge wire.
- Generator
To keep the battery charged the Generator provides up to 20 amps. The original Generator (Delco-Remy #1101876) only provided 13 amps at 15 Volts (per D-R Bulletin IG-185 pg 4). The armature circuit for the generator was fused with a 15 amp fuse. Nothing shows a higher amperage fuse for the 20 amp Generator (Delco-Remy #1101890) or a different size wire to handle the higher current flow. In fact nothing shows the 20 amp generator was actually used on the plane. The airplane Type Certificate Data Sheet A-768 shows only the small generator. The problem is that the generator is part of the engine Type Certificate. It's Data Sheet E-233 shows 4 generators Delco-Remy Models 1101876 (15 Amp ?), 1101890 (20 Amp), 1101879 (25 Amp), and 1101898 (35 Amp). The 15 and 20 amp units both weigh 10 lbs. so there is no weight and balance change and the field winding on the 20 amp generator draws slightly less current 1.58 to 1.67 amps vs 1.62 to 1.69 amps for the 12 amp (per D-R Bulletin 1G-185).
The real difference comes with the Armature and Battery circuits. The Battery circuit to the Voltage/Current Regulator is the line with the 15 amp fuse. This is also the line which feeds current from the generator back to the 14 volt bus to keep the battery charged. The generator field winding is powered from the Regulator. After much research I found that on later planes Cessna used a 15 amp fuse with the 12 amp generator, a 20 amp fuse with the 20 amp generator and a 35 circuit breaker with the 35 amp generator. Per AC 43.13-1B Table 11-3 you need at least a 12 gauge wire with the 20 amp Fuse, so that's what I'll use for all the generator circuits. I'll also relabel the fuse holder to show the correct 20 amp fuse.
If anyone has a 120 or 140 with a generator larger than the 12 amp model I would check the fuse, wire gauge and weight and balance to make sure all of it is correct.
- Lights
The lights are easy. The electrical loads and wire lengths, etc. are such that the 10 amp fuse requires heavier wire, 18 gauge, so that's what I'll use.
Tail Nav. Light - Bulb 1777 - 1.52 amps at 20 feet
LH Nav. Light - Bulb 1512 - 1.5 amps at 22 feet
RH Nav. Light - Bulb 1512 - 1.5 amps at 21 feet
Panel Lights - 6 Bulbs 1826 parallel wired - 0.72 amps at 4 feet
Dome Light - Bulb 1826 - .12 amps at 8 feet
Landing Light Motor - Up or Down - 4 amps at 15 feet
The landing light bulb is wired directly to it's own 25 amp fuse. Because of available wire sizes Table 11-3 does not show a 25 amp fuse. It jumps to a 30 amp fuse which requires a 10 gauge wire so that's what I'll use. Again the other factors don't require it to be heavier.
- Electric Turn and Bank
The electric Turn and Bank gauge is on a 10 amp fuse which requires 18 gauge wire. The parts manual shows a 2 amp circuit breaker between the fuse and the instrument. Geoff, my electrical engineer friend says 10 amps would fry it, so I'll find a place to install the breaker. There doesn't seem to be a hole so it may not have had it originally.
- Radios
The radios originally had a 15 amp fuse. The radios I plan to install call for a 10 amp fuse so I'll change it to that and relabel it. I'm using wire sizes for the radios per the installation instructions.
I think I have all the wire and connectors so it's time to repair wires and assemble the new harnesses.
