Thorp Air Command - T18.net

So I tore down the thorp completely to rewire it and build a new panel. As I'm getting things back together I'm noticing a lot of things that don't seem to be up to snuff.

One of the things I'm confused at (or mostly want confirmation) is the size of the wire from the battery to the panel (bus) and the alt B lead to the 60amp CB in the panel.

I have a 60a alt with 60a CB in the panel. Based on the wiring loads it says I need a 9g wire minimum. Currently mine has a 10g wire for each of those and the last 8" of the battery wire actually is spliced to a 12g wire. 10g shows that it can only handle 55 amps current.

So the next step up is to run 2 8g wires through the firewall. One for the alternator and the other for the main bus. The 8g supports 73amps which is overkill. The plane has lived this long with the 10g.

Is everyone else running 8g alt and bat-bus wires as well? It seems like a nightmare to fit an 8g wire to the back of my circuit breaker panel and finding an 8g ring terminal that is small enough for the little circuit breaker.

Also, does anyone put a fuse at the source of these wires before they are run through the engine compartment and to the panel? The CB is all the way to the panel so that wire could short anywhere from the source to that piece (such as the firewall) which could cause a fire. It might trip the CB but the wire would still be grounded and have power through it even with the CB tripped.

Thanks for the help! Hopefully I'll have my bird back in the air in the next month.

Jeff

Checking my wiring diagram, 55 amp alt, I have 8 awg wire from the alt to an 60 amp ANL (fancy fuse) on the firewall then to the ammeter shunt and then to the battery, no CB. My battery is located behind the pass seat. My main bus is 12 awg wire from the ammeter shunt to the panel, max load on the panel currently less than 10 amps.

Add up the loads for your panel and size the wire accordingly, you don't need an 8 awg wire if you don't have the load. The alt to battery needs to be sized for the max output of the alt.

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Bruce Finney
N18JF T-18C #262
Yelm, WA USA

seems like I used a 6AWG. I followed Bob Knuckoll's Aeroelectric Z11 diagram.

I'll look up that wiring diagram Ryan.

Bruce, is your ammeter shunt on the panel? I'm uploading my wiring diagram and it shows on the front side of the shunt a 10g then on the back side of the 60amp circuit breaker it has me running another 10g to the switched end of master solenoid. That seems like it cuts my alt to battery connection if I pop that 60amp breaker.

If I did that in 8g to the shunt then from the CB to the battery master I would need to pull 2 8g wires through the firewall. Is your set up that way or does yours go from the firewall fuse over to the battery for charging?

I'm not sure which part would be more of a bothering to me: mounting a bulk fuse on the firewall that can split off to the ammeter then over to the battery for charging or running 2 8g wires up to the panel.

That diagram for the "generic" airplane wiring is quite complex compared to my system. Almost a little intimidating. I might have to re-work this whole setup I have done so far.

Here are a couple of aircraft alternator systems that may help but the called out wire sizes may not be correct for your application.

NOTE: The Prestolite drawing does not include the starter switch leg.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Jeff,

Thanks for those! Much more simple to understand.

So basically in order for the ammeter to show correct draw for the plane is to place it between alt and the entire system. Since mine is mounted in the panel I will have to have an 8g wire to the panel then another 8g back to the firewall to the battery.

I was trying to reduce large wires through the firewall and behind the panel by going alt->fuse->bat->ammeter->bus but that would only show amp draw from the bus and not the draw from battery charging as well.

Is it critical to show amp draw from the battery? I want to do it correctly although I don't want to do it "because that's how we have always done it". My space behind the panel is quite limited and it would be better if I only needed to attach 1 large wire to the back of the panel instead of 2.

Thanks for all the help! Fitting all this in back there has been a challenge for me. Right now I also need to find a way to consolidate ground posts for all the electronics without having to use butt connectors to extend wiring from the electronics.

The two gurus in the experimental world are Bob Nuckolls and Greg Richter. (Aircraft Wiring for Smart People. ~ A Bare-Knuckles How-To Guide by Greg Richter.) Nuckolls is big on engineering the system to death. Richter is big on simplicity. They are mortal enemies.... but they each have very good points. You should read both. Personally, I lean towards simplicity, so I like Richter.

Like Bruce alluded to, the "modern" way to do it is to take the B wire to an ANL fuse on the outside of the firewall, and then go to the bus. Loads of info on the net.

Doing a load analysis is a worthwhile exercise. The method is outlined in the AC43.13. Easy to do on a spreadsheet. 60A should be plenty on a VFR airplane. Mine was ok for day/night on 35A.

As an aside, my electrical system was protected by a 50A CB. (On a 35A alternator.)

D

If your ammeter uses an external shunt, the shunt can be mounted anywhere convenient for the larger cables and smaller wires routed from the shunt to the ammeter. I ended up doing that with the PA-20 I use to own.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Mine is an old school panel mount. It's a basic t-18 without any flashy electronics.

You can't necessarily tell a difference between a ammeter with a build in shunt and one without by looking at the face. The terminal posts are different sizes and a gauge with an internal shunt is heavier. The shunt is for stepping down the aircraft voltage to millivolts so the gauge can use it without burning up. That is about the extent of my knowledge on the subject and only know that because I ordered an ammeter from a salvage yard that didn't need a shunt only to find out after it was installed that it actually did.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

oh, I didn't know anything about that. I assumed an external shunt type gauge would be newer design. This is the original from 1978 and had the wire directly from the alternator to the back of the gauge when I pulled the panel. I assume that means it doesn't have a shunt.

Correct, it won't have an external shunt.

_________________
Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

You can always replace the old style gauge with one that uses a shunt.

Here is my alt wiring diagram.

The alternator and the ANL are in the engine compartment, every thing else is behind the firewall. The shunt is mounted on the battery case, which is behind the pass seat, along with the master contactor and the starter contactor. You will note that there is an ammeter along with the shunt, the ammeter is the old style with an internal shunt. I didn't want a hole in the panel when I installed the Dynon EMS and left the old ammeter there. It also makes a convenient tie point for the main bus. The ammeter shows panel load, the EMS shows system load, charge or discharge.

_________________
Bruce Finney
N18JF T-18C #262
Yelm, WA USA

Looks good.

I went ahead and did it like my main diagram previously but used an 8g wire instead of 10g. I'm a little overwhelmed with the rebuild and my business so I just want to make it simple and get flying again. Never again will I tear something down to fix things! This has been a 4 month frustration that has another couple months left.