Electrical wire size theory

I need help wrapping my head around wire sizeing and need help with a couple questions.. why is it recommended to run a large guage wire such a 10 guage through a fuse such as a 3 or 5 amp size. I know larger diameter wire can carry more voltage but seems that going through a fuse with a filament the size of a couple wire strands would choke the power down and restrict the flow of electricity.
Another question, why run 10 guage to a unit with 22 guage supply at the unit?
I was having voltage drop on my humminbird units so I decided to run a dedicated fuse block just for electronics and ran an 8 guage wire to my fuse block and ran 10 guage to all my electronics. I did this because that's what is recommended, but why exactly? The 8 guage I ran may have been a little over kill but it's what I had on hand when I upgraded the trolling motor wiring to 6 guage.

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It is all a matter of voltage drop. Smaller wires having higher internal resistance. Voltage sensitive electronics require lower resistance wire. As for the fuse filament, it is such a short span of the wiring it does not have much impact at all with total resistance in a circuit. Same as with the short run of smaller wire running up to the head unit

DSJ said it well, but let me try a different approach. What is happening is the 20 guage wire will have more resistance and it limits current (amperage) flow in the wire. The newer head units typically require more current, and that is why it is recommended to run a dedicated 8 to 10 guage wire. The larger wire can properly carry the necessary current. And as DSJ indicated, the fuses/breakers are not only a short span of distance, but they are typically made of material that can also carry the necessary current.

I was trying to think of it as water flowing through a 2 inch pipe and pushing it through a 1/2 inch pipe. You decrease the volume but increase the pressure. Just didn't know how that related to electrical.

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In order to perform at their peak the new electronics are nore sensitive to the incoming viltage. It is usually low voltages that cause problems in electronics. In an electric circuit if you have a 100 watt load. If you supply it at 10 volts it will draw 10 amps. If your voltage is reduced to 5 volts it will draw 20 amps of current

In a “perfect” DC wiring scenario ... :

DC wiring would start at the battery at 2awg - and then 2ft from the battery change wire gauge to 4awg - and then 6ft from the battery change wire gauge to 6awg - and then 8ft from the battery change wire gauge to 8awg, and then 10ft from the battery change wire gauge to 10awg, etc etc etc -> onto the completion of the circuit at the device ...

Recommendations of oversized wire gauge is simply because “voltage is constant” (over the wire run length) —- but “amperage decreases (continually)” over the wire run length ... and no current wire product offers that same distinct properties ...

Note : “amperage” is not the same as “voltage” ... voltage thru 2awg wire vs 20awg wire (over the run length) will be the same ... amperage decreases (over the run length) ...

I’m not an electrical engineer but I think I got that right ...

Rickie

Quote:

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Originally Posted by rnvinc

In a “perfect” DC wiring scenario ... :

DC wiring would start at the battery at 2awg - and then 2ft from the battery change wire gauge to 4awg - and then 6ft from the battery change wire gauge to 6awg - and then 8ft from the battery change wire gauge to 8awg, and then 10ft from the battery change wire gauge to 10awg, etc etc etc -> onto the completion of the circuit at the device ...

Recommendations of oversized wire gauge is simply because “voltage is constant” (over the wire run length) —- but “amperage decreases (continually)” over the wire run length ... and no current wire product offers that same distinct properties ...

Note : “amperage” is not the same as “voltage” ... voltage thru 2awg wire vs 20awg wire (over the run length) will be the same ... amperage decreases (over the run length) ...

I’m not an electrical engineer but I think I got that right ...

Rickie

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Voltage cannot stay constant if current (amperage) is changing. Ohms law states if voltage increases, current (amperage) decreases, and vice-versa. They are directly proportional. Now resistance of wire on the other hand does infact increase over the length of wire. The longer the wire, the higher the resistance. This is why it is best to use tinned copper wire in marine applications because it has less resistance than standard copper wire, albeit less corrosive too.

As far as wire length and guage are concerned, there is no need to increase wire guage size until you get to the 100 ft mark of wire length, then you go up one guage size. 200 foot would be 2 wire gauge sizes, 300 foot would be 3 wire guage sizes, etc.

Get a fluke meter and add the ac/dc current (amperage) clamps and you can see what is happening over the length of wire.

Two things determine wire sizing: Desired maximum voltage drop and how much heat the insulation can handle. Voltage drop is determined by the length and size of the wire. So a short segment of fuse does drop more voltage per foot, but the fuse is less than an inch long, so it doesn’t affect voltage nearly as much as 20ft of small wire.

Thanks everyone. I think I grasp it a little better.. it was just that part of running large wire through a small filament fuse that had me scratching my head. I probably went a little over kill on the install but better than the alternative.

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One major factor that most people don't consider is human error. Most issues I see are caused by abrasion/kinks, improper termination, loose/dirty connections. The reason bigger is often better is because it can withstand some of these human errors and still perform well. just my 2 cents. You will never regret your decision to upsize.

Many problems with boat electronics come from when cranking the big motor and the associated current drop and too small of a battery.