Currently I think the wire they used was only 8AWG at the most, and I have an 80 amp inline fuse near the battery.
8 AWG wire is really too small for 1500 Watts at 3 feet. Whoever put the 80 amp fuse in is an idiot. You fuse for the wire, not the load, and an 80 amp fuse would be for 10 AWG cable.
And my fuse seems to get brown and burned, and finally blowing(after a while).
Shocking.
But that could be because they used a crappy rubbery fuse holder, and the wiring melted everything and fused things together. (I've replaced that already)
No, the crappy rubbery fuse holder had nothing to do with it. It was the melting wiring that did it. Consider yourself lucky.
So here are my questions...
With a 1500 watt inverter, needing about 3-5 feet of wire, should I run 4AWG minimum, or if everything will accept 0, just use 0?
4 AWG is what three feet of cable to a 1500 Watt inverter calls for, and at 3-feet will allow for a 1% or less drop in voltage (standard is about 3%, so anything less than that is good). You could certainly use larger wire between the batteries and the inverter, but the inverter will never draw more than 1500 Watts, so larger cable (and lugs) is just a waste of money, as at that length larger wire won't give you any real advantage of a lower voltage drop, either. If you go more than four feet, which you really shouldn't do, use larger cable, though.
Also, I know they say 1500 watts, from 12 volts, (dividing the two) would be around 125 amps. So, should I use at least a 125 amp fuse near the battery? Do they come that large?
Oh, they come that large. I've got a 400 amp and a 300 amp Class-T fuse in my system. But again, you don't fuse for the inverter, you fuse for the wire. Always.
For 4 AWG wire, the "wire ampacity" (the RMS electric current which a wire or cable can continuously carry while remaining within its temperature rating) is 135 amps. But that's a rough ballpark, as the actual ampacity depends on its insulation temperature rating, the electrical resistance of the cable material, the ability to dissipate heat (which depends on cable geometry and its surroundings, like is the cable in the engine compartment or wrapped up in some way), and ambient temperature. But generally, you can count on 135 amp ampacity for a cable with a 90 degree insulator, which is what you want. (If you go with a cable with a 75 or 60 degree insulator, you'll need to bump it up to 2 AWG wire, instead).
You generally want to fuse the cable at 150% of its ampacity, to prevent the fuse from constantly blowing when the current is at or near max current. In the case of 4 AWG wire and 135 amp ampacity, a 202.5 amp fuse is the absolute max fuse you should use. Since the inverter draws a maximum (sustained) 125 amps, then it needs to be at least that much so fuse isn't constantly blowing, as well. Best bet is a 175 amp or 200 amp fuse.
If you have a large battery bank (and I certainly hope you do if you are constantly maxing out a 1500 Watt inverter) you should probably use Class-T fuses because of their high arc interrupt rating (the arc smothering capability). The Arc Interrupt Rating is at what amperage a blown fuse
stays blown. In other words, if the current is too high and blows the fuse, and if the current is
way too high, it can blow right past the blown fuse and just reconnect on the other side, completing the circuit. That would be bad.
CNL fuses have an Arc Interrupt Rating of 2500 amps.
ANL and ANN fuses have an Arc Interrupt Rating of 2700 amps.
You're probably thinking, "I'll never use that many amps," and let's hope you're right, but if you have a short you could easily have that many amps flowing through the cable. But, it is the
combined Cranking Amps of all the batteries in the bank that's key here. Batteries have two types of cranking amps, CA (Cranking Amps, sometimes called Marine Cranking Amps) and CCA (Cold Cranking Amps). CA, or Marine Cranking Amps, is how many amps the batteries can provide for cranking at 32 degrees. CCA is how many amps at zero degrees. In all cases, you're going to have more than that at 70 or 80 degrees, for sure.
If you have four batteries in the battery bank, each with, say, 800 Cranking Amps, that's 3200 Cranking Amps that will be fed through the cables into a short. If not smothered completely and quickly, that's enough amps to blow the bottom out of a truck or a van (or a boat, which I have witnessed). An ANN, ANL or CNL fuse cannot interrupt that arc if such a short were to blow the fuse (and it would).
Class-T fuses have an Arc Interrupt Rating of 20,000 amps. You need a
really big battery bank to blow through a blown Class-T fuse.
So, fuse for the cable, in this case a 175 amp or 200 amp fuse, and then choose the type of fuse based on the Arc Interrupt Rating your battery bank requires.
Class-T fuses are expensive, but it's cheap compared to losing your ride.
And I'm also wondering about putting in a slightly smaller, maybe 100 amp breaker inside near the inverter. Good idea to do this? Can I be pointed in the right direction to see how I would install a breaker like this?
What's the matter? Got the "I'm sick and tired of replacing that fuse" Blues?
If you wire and fuse it up correctly, circuit breakers aren't necessary at all, but it certainly won't hurt. You could use circuit breakers instead of or in addition to fuses. Just be sure to use DC and not AC circuit breakers. Get a circuit breaker designed for use on a boat. You need to select a circuit breaker for the wire, not the inverter, same as you would a fuse. So you'll need a minimum of a 150 amp circuit breaker, something like a Blue Sea
185-Series,
187 Series, or
285-Series circuit breaker.
