Power leads to fuse blocks

Hello again everyone.

I'm planning the basic electric system using "The Aeroelectric Connection" and the Advisory Circular as my main references. I'm a little fuzzy on how the apply some of the formulae from the AC for wire sizing. I think if you go very strictly by the AC, to run about 55 amps from the battery to the distribution busses I need something like a 2 AWG cable, though most of Mr Knuckolls' drawings run around 4 awg cable.

The big fat cables seem like they'll be a bear to work with. Maybe split the loads up and run several smaller cables through the firewall?

Worst case - highest demand on the power will be close to 55 Amps - if I have all the lights on, pitot heat, lowering the flaps, boost pump on, and transmitting on a com. Battery up front on the firewall.

Opinions on the heavy power lines?

I think AWG 6 covers 54A from Bob's book.

AWG 8 may work if shorter, relatively open air run.

Also your max loads may be of short duration (flap motor only seconds).

PP 8-9 of the 'Connection R12 discusses overload considerations

Depending on your setup, you will likely not have more than 20-30 amps max load. Fuse sizes are rated for the wire, not the associated device so adding up those values will give you a high number.

My setup, with everything turned on uses < 20A. LED lighting helps a bunch here.

Without knowing what you will power on the bus and its distance from the power source, it is not possible to make a recommendation.

Base the wire size on the continuous load, not intermittent loads.
Many of Bob N's drawings use 6 AWG to feed the main power bus.
2 AWG is overkill.

I have a 60 amp fuse protecting a 6 ga feed wire that goes from my master solenoid through the firewall to my main power bus. A 50 amp fuse will protect an 8 ga wire. Make sure you protect the wire passing through the SS firewall with a thick grommet held still with high temp RTV, or some sort of heavy duty snap bushing. The edge of the thin SS firewall will eat through the insulation on that fat, stiff wire quickly if it's not held firmly and allowed to vibrate - and it will melt something, maybe you.... the fuse forward of the firewall will melt at 60 amps, but the initial surge prior to that could cause problems. If you have a marine supply source near you, you could get one of their power cables. A little more expensive, but they are high quality (usually), with safe tefzel insulation and more wire strands, which make them much more flexible.

Typical is to use 2 AWG for the large current drawn by the starter for a short time period. Then a smaller cable #6 or #8 can be used from the alternator depending on the alternator size.

Van's standard drawing for the RV-10 is a #2 cable from the battery to the master relay (both located in the tail cone), then a #2 from the master relay to the starter relay on the firewall and from the starter relay to the starter. Then they run a #8 from the starter relay on the battery side through an ammeter shunt to the main bus bar, and a #8 from the 60 amp alternator output to the main bus bar. They do call for this #8 cable to be in open air rather than conduit to help dissipate the heat during times when the alternator is running at full load such as when charging a nearly dead battery.

That said, I'm running a #6 cable from my 60 amp alternator to allow the cable to be bundled, and because it will be a fairly short wire so the larger size won't cause much of an impact. I'm running a #10 wire from my standby 40 amp alternator.

I may be a bit conservative on my wire sizes, but as a registered professional electrical engineer with 40+ years experience in industrial design & construction, I learned a long time ago that it is better to be a little conservative on wire sizing.

Hope this helps a bit?

DRMA said:

.... I'm running a #6 cable from my 60 amp alternator to allow the cable to be bundled, and because it will be a fairly short wire so the larger size won't cause much of an impact...

Click to expand...

That's what I did.
I also ran a #2 AWG for power to the starter. It's the only place on the plane I used #2 wire.

Thanks!

I appreciate the feedback.

I did spend the afternoon studying AC 43.13 and with your suggestions I'm comfortable running an 8 ga wire to the main bus. That will be a lot more manageable.