Fuse for radio

Bigger wire has less resistance per length. A fuse is basically a wire designed to clear when overloaded.
Fuses and breakers are really to protect the wire from overloads.
A 12ga wire is good for a max load of about 20A. We won't get into free air load vs. in a bundle or conduit (it is rated at less).
Sooo.....a 12ga wire can carry about 20A load. So the fuse would be equal or less so you don't overload the wire and start a fire.
A smaller fuse on a big wire can then help protect the equipment.
Small fuse (amp/A rating) is the same as a lower load. The good thing is it clears when overloaded but no fire.
Downside is voltage drop since it is not designed for a big load.
So a big wire, then fuses that protect the equipment within reason. This gives max power (less voltage drop) to the fuse.

Hope I didn't muddle things.....

On 7/15/19 9:04 PM, Charlie M. (UH & 002 owner/pilot) wrote:

Fuses and breakers are really to protect the wire from overloads.
A 12ga wire is good for a max load of about 20A. We won't get into free air load vs. in a bundle or conduit (it is rated at less).
Sooo.....a 12ga wire can carry about 20A load.

No, it can carry a whole lot more than that. Residential electrical
codes limit it to 20 amps to minimize I*R drops, where the length of the
wiring could reach into hundreds of feet. The wiring will actually
handle over 200 amps before it starts to melt.

Fuse resistance and voltage drop - Yep, each fuse size has a different resistance and thus a voltage drop. Who knew??

Small values of fuses (and breakers) have a bigger/worse voltage drop. For the widely used Bussman brand fuses here are some examples (at a 1A load)*;

1A 0.190Vdc drop
2A 0.078Vdc drop
3A 0.045Vdc drop
5A 0.024Vdc drop

So bigger is certainly better and 3A and higher is recommended.

Thanks, John

*Reference: http://www.cooperindustries.com/cont...AGC_Series.pdf

On Tuesday, July 16, 2019 at 5:36:33 AM UTC-5, kinsell wrote:
On 7/15/19 9:04 PM, Charlie M. (UH & 002 owner/pilot) wrote:

Fuses and breakers are really to protect the wire from overloads.
A 12ga wire is good for a max load of about 20A. We won't get into free air load vs. in a bundle or conduit (it is rated at less).
Sooo.....a 12ga wire can carry about 20A load.

No, it can carry a whole lot more than that. Residential electrical
codes limit it to 20 amps to minimize I*R drops, where the length of the
wiring could reach into hundreds of feet. The wiring will actually
handle over 200 amps before it starts to melt.

Not to be pedantic but ... FAA Advisory Circular AC43-13-1B defines the maximum current along 20ft of 12ga mil-spec (Tefzel) wire across 14Vdc as ~12amps (fig 11-2 continuous current flow) or ~22 amps (fig 11-3 intermittent current flow).

Reminder that this is DC, not AC, current we are talking about. Edison pushed DC systems but was beat out (trounced) by Tesla/Westinghouse and their long-haul AC systems due to exactly what we are talking about here. Just sayin'. ;-)

Thanks, John

References:
https://en.wikipedia.org/wiki/War_of_the_currents
https://www.faa.gov/documentLibrary/...-1B_w-chg1.pdf

Thanks.....

My comment was not, "what can it carry before becoming a lightbulb", but what is acceptable for continuous load at a voltage.
Basics......small wire carries low load.....fuse keeps wiring from igniting.
Big wire carries larger load and/or less voltage drop.
Fuse/breaker keeps from a wiring fire.

Huge wire is a minor weight penalty.
Huge wire may be a PITA to run/fit/connect.
Huge wire does NOT "overwork" a device.....the device only uses what it needs until failure...thus.....a fuse or breaker.

Would anyone here use a 12ga jumper cable when car has a dead battery?!?!
Well.....off in neverland, cold, rainy, etc......I would try rather than quibble.
Going to welding cable with suitable clamps would likely yield a better result.....

Have a great day....

UH - I am with you on this. My pedantic comments were needlessly complicated for soaringjac's simple initial request. RAS at its best in the weeds.

Light bulbs? Welding cables? What a thought. ;-)

- John

From now on, please all peeps label me as Charlie.....I wish nothing negative to "UH" if I do a FUBAR. I know him and fly his ships at times, I do not speak for him.

Whatever......while a 12ga wire "may" handle a lot more than 20A, for how long, what insulation (yes, I am sorta versed from PVC/MTW through Tefzel and above....;-)).
As stated before, big wire from battery to buss is great.....
Fuses are basically the resistance and should fail before wire becomes a "heating element/light bulb".,...
So, specking (SP) a low amp fuse can/will drop voltage to devices.

To others......do you want to use a 12ga jumper cable to get home?, or maybe 4O welding cable to handle the current?
High load WILL roast a small cable/wire. Short very high load may pass, not our issue.....we want to know the downside of very small fuses/breakers versus voltage supplied to devices....
My thought.....big wire, great connections, protection (fuse or breaker...), and have fun.