6 awg will do and you you need wiring protection there
Pete's got it right, but
6 awg would be plenty, either 4 or 6 would work, but from the alternator to battery or main buss 6 awg works. (see size chart below) There is no one way to wire. The only down side with oversize wire is weight and harder to wire. In this case one wire will not add much total aircraft weight, but it is about 4.5 ounces more or 1/4 lb. It adds up.
I agree you may not need 50 amps continuous wiring but if you alternator can produce it you should wire it accordingly.
Not sure about the 10-12 awg, it will work, but I would stick with 6 awg, may be 8 awg. I don't see an advantage of dropping the main buss major feed wire down to 10-12 awg. Here is a rough guide:
awg Current Range (amps)
4..........68-133
6..........50-97
8..........38-71
10.........17-32
12.........13-25
14.........10-18
16..........7-14
18..........6-12
20..........4-9
22..........3-6
(depends on continuous load, length, bundled or not and max volt drop and wire temp allowed, see wire chart below)
As far as fuse protection,
YES there's circuit protection. You are trying to protect the battery. Some run the alternator output, the battery lead or "B-lead" direct to the battery -OR- more traditionally to the main battery buss. (see diagram below)
If you run the B-lead to the main battery buss direct, you usually go thru a circuit breaker (mounted in the instrument panel) and than the main buss. This is the traditional or aircraft standard way, aka cessna, piper..... van's wiring kit.
If running alternator B-lead direct to the battery than you'll typically use a big fuse, typically a "Buss Man" (name brand) chassis fuse. The fuse is located near the battery (usually fwd firewall). This method has been made popular by the Aeroelect book. Although non standard, it does simplify wiring (see below) and the use of a fuse may not be critical since the alternator should not routinely or really ever blow the fuse under normal Ops, as long as the fuse is sized right. If it blows that is it until you get to the ground, there is no resetting in flight.
Either way works but I prefer a pullable CB, panel mounted, especially if you have an internal regulated alternator. This allows a manual disconnect if ever desired. Also if the alternator pops the CB you do have the opportunity to reset. With a fues you can not reset and must carry a spare fuse or two around.
The wire between battery and main buss is often NOT fused. Why? I don't know, it seems as critical as any. I guess it's run is short and well protected? Also this line is off the battery master relay (contactor). If you have a short you can turn the master off. (see diagram below) You could use a fusible link like automotive wiring, which is not a bad idea.
A
fusible link is a sacrificial short piece of wire (about 6" long) that is several wire gages lower, at least two. It's covered in a fiberglass fabric sleeve. If the wire shorts, the fusible link will smoke, melt and protect the aircraft. The fiberglass insulation keeps the link from catching near by items on fire until it opens. Obviously this needs to be on the fwd side of the firewall since it puts out smoke. There are high amp in-line automotive fues readily available now, so that's also an option. In line fuse's maxed out at 25-30 amps, but now there are MAXI in-line fuse (blade ATC type) in the 50-60 amp range. They are physically larger than they lower rated fuses, but still flat. They are also made by "Buss Man".
Here are the two typical routing variations for alternator wiring
When selecting wire size you can choose on basis of temp rise, voltage drop or both, depending on the length of the wire run and power (current and voltage). This chart assumes a fixed voltage drop (0.50 for a 14 v system) and max temp rise of 20C.
Note: the chart is for continuous; it does not apply to starter wire that must take up to 300-320 amps, but only for an in-rush or surge. 2 awg works well for most applications, here is a link to a SkyTec tech sheet for sizing the starter wiring:
http://www.skytecair.com/Wiring_Experimental.pdf
WHERE TO PUT THE AMMETER?
I agree on putting the ammeter on the B-lead between the alternator and where it connects to the aircraft wiring (battery or battery buss) is most useful. This can be confusing for builders, since
there are two ways to wire the ammeter. Even factory planes wire the ammeters differently. Which way is better?
The preferred one for me, is to place the ammeter on the B-lead like you plan, so you get the load on the alternator, which is most important. It is how hard is it working and this is where all the power comes from with the engine running and alternator on line. The only time we are on battery power normally is on the ground.
The other way is with the ammeter between the battery and battery buss. This measures if the battery is charging or discharging. Most of the time with the engine running, the battery is charged, so the ammeter reads almost nothing, zero 99% of the time. Some might say they want to know if the battery is discharging. Well even if you don't have a battery ammeter, there are a few ways to tell if the battery is draining or charging indirectly.
1) The battery is discharging if voltage is around battery voltage, 12.6 volts, not the normal 14.3 +/- when the alternator is working.
2) If you know the standard draw of your aircraft electrical system, you'll know if the alternator is producing a sufficient output to meet the demand. So you can assume the battery is charging or charged.
3) Right after start, before you turn on all electrical items you'll see a higher alternator load at first, as the battery is charging. The alternator load drops as the battery becomes fully charged. If the battery is in good conditon this takes less than a minute to a few minutes, to get the battery back to near full charge. From then on the battery is just along for the ride as a back-up and handle short surges. If you notice excess alternator load is high for an extended time, post engine start, it could be an indication the battery is not in good conditon. Know thy battery.
If you do loose the alternator you should have figured out ahead of time what your approx electrical demand is, for your essential items. From that you should know how long you can fly. The battery voltage will give you an indication of how fast it is draining and how long it will last. Most electrical items stop working when voltage drops to low. Good idea to have an idea what that voltage is for this non normal alternator failed conditon. VFR with a magneto mechanical fuel pump engine (aka Lycoming) it's not a big deal. IFR it could be a big deal.
