K

Kato's 8

Well Known Member
Good morning!
Quick question: Does it matter where in a line a fuse such as a in-line fuse is placed? Im running a wire direct from battery to a cig adapter 12V and I want it fused. Does it matter in this case where the fuse is placed? Should it go nearest the battery or nearest the plug. I dont see why it would matter but maybe there are some comments on this. I tried to post this in electrical section but must of lost it. But I always get good help from rv8 viewers.
Thanks!
 
Current is constant throughout a series circuit. I doesn't matter where you put the fuse.
 
The fuse is there to protect the wire - so ideally you would want it as close to the power source as possible (battery in this case) so that you can minimize the chance of a short circuit (wire abrasion, etc) between the battery and the fuse which would cook the wire.

I've got a similar setup going into my 9A right now, I've got a dedicated wire for it fused at the battery.
 
As close to the power source as possible. All wire between the source (here, the battery) and the fuse is unprotected and a fire risk.

BTW, select wire size and fuse amps to match the maximum capacity of the power socket...those air mattress pumps pull a lot of juice!
 
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Your protecting the wire!

The purpose of the fuse is to protect the wire from over heating. The fuse should be placed near the source of power.
Having a fuse at the far end of the circuit, like at the cigarette plug does nothing if the wire shorts to ground behind it.
Tim
 
In that case, I've got one for y'all. I'm putting a heavy lighter plug in the back of the 10. It's gonna be hot-bussed with a fuse or breaker. It's purpose is to provide 12V for back seat toys AND provide a port for battery charging. Bidirectional current. Where would you fuse it? In the middle?
P.S. Do you breathe in or out when you putt?
 
aerhed said:
In that case, I've got one for y'all. I'm putting a heavy lighter plug in the back of the 10. It's gonna be hot-bussed with a fuse or breaker. It's purpose is to provide 12V for back seat toys AND provide a port for battery charging. Bidirectional current. Where would you fuse it? In the middle?
P.S. Do you breathe in or out when you putt?
Click to expand...

Mel said:
Current is constant throughout a series circuit. I doesn't matter where you put the fuse.
Click to expand...

This^ with 1 caveat:

The less of a straight line your outlet is from your "hot" battery buss, the more wire you have to install; that's more weight and more overall circuit resistance (albiet very small, but with many circuits, it adds up), and more places for wiring problems to occur.

Since (on GAA light aircraft) your battery "hot" buss is usually just a flat bar connected to the + side of your circuit breaker panel, why not centrally locate this CB or fuse holder on your CB panel? Then use the mininium length of wire you can and still comply with acceptable aircraft wiring practices. <an old Navy avionics tech>
 
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No, the "hot bus" ends at the battery contactor. That's why it's always hot, duh. Just kiddin'. Anyway, in this case it will derive from the downstream post on the battery ANL to the 1006 bulkhead, up to the overhead console and to the socket. I'll toss a breaker right next to the plug and rate it for the socket. Probably a 6 foot run of 14ga and a 10 amp breaker. Quick connection for moderate charging and power for shop work.
 
aerhed said:
In that case, I've got one for y'all. I'm putting a heavy lighter plug in the back of the 10. It's gonna be hot-bussed with a fuse or breaker. It's purpose is to provide 12V for back seat toys AND provide a port for battery charging. Bidirectional current. Where would you fuse it? In the middle?
P.S. Do you breathe in or out when you putt?
Click to expand...

Here is the fuse panel I put in for my rear power outlets, also provides power to the seat heaters, and the ANR headsets power module---black item to the right of the fuse block.

The wire feeding the fuse block has a 30a inline fuse right off the battery, on the switched side of the main contactor.

For your bidirectional setup, you would put the wire on the battery side of the main contactor.

P5110057.jpg


Here is what I did to accommodate battery charging without having to remove the baggage bulkhead. Both pigtails have an inline fuse.

P4190047.jpg


P4190048.jpg


P4190049.jpg


Here is the way I mounted the utility power outlet, and the ANR power outlet.

10%2520interior%2520for%2520Abby%2520013.JPG


Hope this helps.........
 
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aerhed said:
... Probably a 6 foot run of 14ga and a 10 amp breaker. Quick connection for moderate charging and power for shop work.
Click to expand...

That sounds a bit hot. I'd think for that application, you'd use something like 18ga and 2.5A. Especially to limit charging current. I wouldn't be trying to jump start a weak A/C battery through a cigarette lighter. But, what do I know. I've been wrong before....
 
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A wire will become a fuse at a certain current. Smaller wires if shorted don't create too much heat when they melt, but a larger un-fused wire will create a lot of heat when it burns and is very dangerous. I would be very careful about running larger wires any length without the protection of a fuse or a breaker. It might be better to use a fuse at the battery and run a heavier wire to a distribution fuse box where you can then protect the wire runs to each of your loads. There are auto type fuse panels with a small number of fuses that would be simple to install and be much safer. Besides you'll probably come up with something else you want to add later.
 
rapid_ascent said:
It might be better to use a fuse at the battery and run a heavier wire to a distribution fuse box where you can then protect the wire runs to each of your loads. There are auto type fuse panels with a small number of fuses that would be simple to install and be much safer. Besides you'll probably come up with something else you want to add later.
Click to expand...

Take a look at the photo I posted top of this page.

Exactly what you describe.
 
aerhed said:
In that case, I've got one for y'all. I'm putting a heavy lighter plug in the back of the 10. It's gonna be hot-bussed with a fuse or breaker. It's purpose is to provide 12V for back seat toys AND provide a port for battery charging. Bidirectional current. Where would you fuse it? In the middle?
P.S. Do you breathe in or out when you putt?
Click to expand...

In that case I would still fuse it at the battery - because that is the only source of power with ENOUGH power to cook the wire. The external charger will only put out about 10-12 amps and will have it's own internal current/overheat limiter. The charger is not physically capable of cooking the wire but the battery is.
 
Thanks for the inputs. Ill be sure to fuse close to battery..probably just inside firewall in baggage compartment.
I also need to thank Dan for reminding me that I may need to plan the amps and wire size for using a pump for airing up my queen size air mattress. I cant always depend on my wife to blow it up....Right?ha!
Thanks!
Bill
 
Mel said:
Current is constant throughout a series circuit. I doesn't matter where you put the fuse.
Click to expand...

Even though according to Kirchoff's laws the current is constant in a circuit, resistance of individual segments is not. A longer wire has more resistance than a shorter wire hence why there is a relationship between wire gauge, resistance, and current carrying capacity. Since the longer wire's resistance is higher the fuse has to be sized according to the load carrying ability of the wire and still have an acceptable temperature rise for the maximum current of the circuit. So where the fuse is does matter and should always be as close to the source as possible.

Then there is always the practical side of things...if the wire is shorted somewhere before the fuse the fuse is useless in that case.
 
rocketbob said:
.... So where the fuse is does matter and should always be as close to the source as possible.

Then there is always the practical side of things...if the wire is shorted somewhere before the fuse the fuse is useless in that case.
Click to expand...

Then, you get into the definition of the "source".

Since on aircraft, you can't just pull over and check the fuse, you want to have a circuit breaker panel that you can get to in-flight.

Typically, aircraft electrical systems are wired so that the battery is connected to an electrical buss located in the cockpit, within reach of the aircrew. The visible part of this buss is the circuit breaker panel.

Heavy guage wires are used between the battery, electrical buss, and aircraft ground. One reason its better to have the battery foward, rather than in the tail (when CG permits) is to keep the length of this thick, heavy buss wire as short as possible.
 
And the winner is......Airguy. 6 feet of 14 ga. is good for around 25 amps. This will be closer to 4 feet. 10 amp breaker to accomodate most common small chargers with plenty of headroom in the wire. Also, duh, not intended for jump starts. Mike's setup is fine except I just won't have that much going on around that area. Rear headsets and reading lights will feed up the windshield bar and through the overhead.
 
While I agree that any flight-critical loads should either be breakered or have fuses accessible to the crew in flight, there are many auxiliary loads that are not flight-critical and would be just fine to be fused without having in-flight access. I count the OP's application in the non-critical pile, and I have a similar battery-charging circuit in my plane with a non-flight-accessible fuse. If I lose that fuse then I can't draw 12-volt power through my cigarette lighter or 5-volt power through my USB charger - neither of which are critical for flight. It might be a nuisance but it is not a necessity.
 
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