Wire Length to Always Hot Battery Buss Question

[SHORTRV7]

I am studing Bob Nuckolls AeroElectric Connection and I do not understand why he wants the wire to be 6 inchs or less to the Main Battery Buss in Figure z-11. My problem here is that I am installing circuit breakers in my panel instead of Muti-Slot auto fuse blocks and the distance is about 24 inches to where I need to build a Battery Buss (Always Hot) with circuit breakers. Do I up the wire size for the length and Amps or what? I just need some help here to proceed with my wiring.
Thanks now for all the help.
Don Short

[rv8ch]

I believe this rule of thumb is to minimize the risk of sparks if something bad happens, like an off field landing.

The size of the wire should be enough to carry all the expected loads on the battery buss. There is a formula in Bob's book, and nice little table in the Van's manual that gives a good indication of the wire size to use depending on the current expected and the length of the wire.

[w1curtis]

Originally Posted by SHORTRV7

I am studing Bob Nuckolls AeroElectric Connection and I do not understand why he wants the wire to be 6 inchs or less to the Main Battery Buss in Figure z-11. My problem here is that I am installing circuit breakers in my panel instead of Muti-Slot auto fuse blocks and the distance is about 24 inches to where I need to build a Battery Buss (Always Hot) with circuit breakers. Do I up the wire size for the length and Amps or what? I just need some help here to proceed with my wiring.

The reason for the recommended length is because this wire is FULLY unprotected and the longer you make it, the greater your potentially risk. You have to remember that the PRIMARY purpose of circuit protection devices (fuse or circuit breakers) is to protect the WIRE. The secondary purpose is to protect the attached device(s). Battery buss circuit protection devices are usually not on the panel but right by the battery. If you are going to bring an always hot wire back to the panel (no matter what size) and "then" put circuit protection devices after it, you might as well forgo those circuity protection devices since they are mostly useless for their primary purpose anyway. This may be stating it a little over dramatic but is still true. A short circuit of that 24" wire will just burn the wire and surrounding material with nothing to stop it.

Go ahead and install you circuit breakers, but install them *by the battery*. Ask yourself "why do these need to be on the panel" --always on devices are the last thing you want to reset or troubleshoot in the air.

[fodrv7]

Don,
I am not clear on what you are trying to do or maybe we are talking about different things.

Hot Battery Bus.
The ONLY thing on the HOT Battery Bus (on the airliners I flew) was the FIRE BOTTLES, because you never want to switch them off or have them inoperative because they were down stream of a failed switch or solenoid.

Everything else should be the other side of the MASTER SWITCH SOLENOID.

As we don?t normally have Fire Bottles on Rvs, the only thing that will always be live is the cable between the Battery and the MASTER SWITCH SOLENOID. (I suppose you could call that cable, the Hot Battery Bus, but there should be nothing on it, except the Master Switch Solenoid.)
That cable should be short for that reason, as stated by others.

In the event of a fire, selecting the Master Switch OFF, turns off EVERTHING in the aircraft, EXCEPT the cable between the Battery and the Master Switch Solenoid.

So on an RV I don't see why you would have a HOT Battery Bus.
(The above is an Airline Pilots perspective, not and electrical engineers.)

I can't give you an engineer?s perspective as I did not wire my RV, but paid a Pro. to do it. But I can tell you what he did; and he is a disciple of Nuckholls.
It is wired such that with the Master switch OFF, no live wires pass through the Firewall. The wire to the Master Switch is the Solenoid Earth; not the positive.

This is not my forte, so I stand ready to be shot down.

Pete.
PS. Don, ironic surname. Oop! Sorry.

[robertahegy]

That's the way I did it, Peter. It is the way Van's shows on the electrical plans using a Continuous Duty Solenoid to power the bus and starter solenoid.

Roberta

[L.Adamson]

Originally Posted by fodrv7

Hot Battery Bus.
The ONLY thing on the HOT Battery Bus (on the airliners I flew) was the FIRE BOTTLES, because you never want to switch them off or have them inoperative because they were down stream of a failed switch or solenoid.

Everything else should be the other side of the MASTER SWITCH SOLENOID.

In the event of a fire, selecting the Master Switch OFF, turns off EVERTHING in the aircraft, EXCEPT the cable between the Battery and the Master Switch Solenoid.

It's normal to have a clock on the hot side of the master solenoid. But it needs it's own fuse, very close to it's connection.

[w1curtis]

Peter/Roberta,

The purpose of an "always hot" battery buss is for devices that need to be independent of the Master switch. Things such as the clock, electronic ignition, computer, etc. I'm sure even airlines have something similar as there are devices you DO NOT want to be powered off with the main *master* switch. You still want them the wires protected by a fuse or circuit breaker however.

[SHORTRV7]

Most drawings I have seen show a Battery Bus that in my case will provide power to P-Mags, Clock, Alternate Feed for the E-Bus, and Accessory Port. I would prefer to build a bus bar with fusses that I can get to while flying. but it looks like I should just use an Automotive fuse block with a 6 inch wire. Using it would provide me with fuses for the Clock, Alternate E-Bus feed, and the Accessory Port. But ----- The P-Mag requires a 3 amp pullable circuit breaker to test the internal support to keep the Mag working. So from the Automotive Fuse Block HOW do I install such a fuse that I can test the P-Mag preformance???
I really appriciate your resposes to this.
Don Short

[fodrv7]

I can understand the need to have a wire for a clock, which you wish to keep running when the aircraft is hangared.
If so, I would have thought it best to have a SMALL wire- separate to the main power supply- that was ONLY large enough to carry the power for the clock. That way, if it shorted it is less likely to be drawing enough amperage to do any serious damage.
What you should be avoiding is having a LARGE wire penetrating the firewall that you can not take the power off. Otherwise you have negated your 'Firewall'.
You could also fuse it forward of the firewall to protect your clock. If the fuse blew because of a short, I am sure you could live without the clock.

I have Lightspeed igntion, which has it it's own wire, but it still comes of the Master Solenoid so I can isolate it with the Master Switch. I accept the extremely remote possibility that I could have a fire and turn off the master, loose the Lightspeed ignition and have a Magneto failure shortly after. I am willing to risk that.
As I said, I am not an Engineer, but I can't see why your power to this Fuse bus cannot come through the Master Solenoid so that you can isolate it.

The other option 9the one that I used) is to install a small drycell in the cockpit for those itemswith a switch (Avionics Master in my case) to isolate the battery.
If you are planning to install and EFIS (such as GRT) you can use it to power the EFIS during start up- as it should not be on the battery being used to start the engine.

Pete.

[fodrv7]

Originally Posted by robertahegy

That's the way I did it, Peter. It is the way Van's shows on the electrical plans using a Continuous Duty Solenoid to power the bus and starter solenoid.

Roberta

Roberta,
Glad I did it the same as you; you being in the trade.
Pete.