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Wire Sizes and Fusing
Wire Sizes and Fusing:
There is a maximum amount of current that each size wire can carry safely before suffering either long term, or in some cases RAPID deterioration.
The technical term for this is its AMPACITY, which is a made-up term from the two words Ampere Capacity.
In addition to wire size, it is also partially dependent on the temperature rating of the insulation on the wire.  Wire with insulation rated for 90 degrees Celsius has a higher ampacity rating than the same size wire with insulation rated for 60 degrees Celsius does.  Bundling a bunch of wires together calls for de-rating the ampacity, because the wires in the center of the bundle can’t shed heat like they could if they were run singly.

The length of the wire also affects how much current a wire can safely carry,  Every size wire has a certain amount of electrical resistance PER FOOT, and the more feet of wire there is, the more the resistance adds up.  More resistance causes the voltage to drop.  In simple terms, here's how that works:

Every device - fan, light, water pump, refrigerator, etc. need a certain amount of power to run.  That power is expressed in WATTS.  Now watts is simply the voltage times the amperage.  If you had a fan that needed 100 watts to run, a fully charged battery, at 12.6 volts, would need to provide 7.93 amps.  (100 / 12.6 = 7.93)  If you'd already used a lot of power, and the battery only had 12.2 volts, that would be 100 / 12.2 = 8.19 amps

Now, if your fan were located 1 foot away from your 12.6 volt battery, it would probably see all 12.6 volts, and it would draw 7.93 amps.  If it were 100 feet away, the resistance in the wire might cause enough voltage drop that - even though the battery were at 12.6 volts, the fan would only see 12.2 volts, and hence would draw 8.19 amps instead of 7.93 amps.

Now, admittedly, the difference between 8.19 and 7.93 is really not all that significant.  But where this principle really comes into play is when you are wiring up items like large inverters or 12 volt fridges.  Resistance and voltage drop are the reasons that you always keep the wires between the battery and the inverter as short as possible, and use as big a wire as possible.  There is a hell of a difference between a 3 foot section of 2/0 cable and a 10 foot section of 2 gauge cable.

But as a practical matter, the distance inside our vans is small enough that we can usually just ignore the voltage drop tables. 

I personally would just use 12 gauge wire for almost everything (except inverters, solar panel controllers and battery chargers) and not worry about the length or the resistance.
Anyway here’s a simple table of fuse sizes matched to wire gauge size that’s close enough for government work, as they say.
For 14 gauge wire, it’s 15 amps
For 12 gauge wire it’s 20 amps
For 10 gauge wire it’s 30 amps
For 8 gauge wire it’s 50 amps
For 6 gauge wire it’s 80 amps
For 4 gauge wire it’s 125 amps
For 2 gauge wire it’s 200 amps
For 1 gauge wire, it’s 250 amps
For 1/0 gauge wire it’s 325 amps
For 2/0 gauge wire it’s 400 amps
You can ALWAYS put a smaller fuse on a wire safely.  For instance, If I were installing a 12 volt fuse block for my house circuits, and it was rated fora maximum of 100 amps, I would run 4 gauge wire(125 amps) from the battery to the fuse block, and fuse it close to the battery with a 100 amp fuse.

That said, it's almost always best to fuse a wire with the max fuse for that wire.  In most cases, the fuse is there to protect the wire, and not the item it is powering.
If you have a 12 gauge wire going to a radio that has a built in 3 amp fuse, the 3 amp fuse is there to protect the RADIO from too much current.  You would still put a 20 amp fuse on the positive wire.  That's there to keep the wire from overheating and starting a fire if the insulation on it were to become damaged and the wire were to short against the chassis.  In that case, the wire would draw LARGE amounts of current, and the resistance would cause it to get Red Hot VERY quickly.  With a fused wire, the fuse blows and shuts it down before that can happen.

You could, of course, fuse that wire with a 3 amp fuse, or even an intermediate fuse like 5 or 10 amps.  But fuses have electrical resistance, too, and the larger the fuse, the less resistance there is.  Plus, it makes it easy to add other items to the same circuit in the future.

There are regular fuses, fast blow fuses, and slow blow fuses.  Most fuses are regular fuses, and they are perfectly adequate for protecting wire from short circuits.  Fast blow fuses are used to protect electronic units like radios, etc.  While a regular fuse would blow, it would allow enough dangerously high current to flow long enough to damage delicate internal electronic components.  Slow blow fuses are used on circuits with electric motors.  Because of inertia, starting a stopped motor turning requires more current than it needs to keep running after it's already turning at speed.  A motor, on startup, can draw two or three times as much current as it needs when it is running.  A slow blow fuse tolerates that short term high current, but will still blow if the high current goes on too long.


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