Fuse size based on Cable/wire size, right?

[pnishr]

Hey all. I've committed the cardinal sin... converted to 12v. AND I LOVE IT!! I did it the appropriate way with the Roscommon instructions. So, now I am getting read to re-install the driver's side alternator, and run a second battery bank (2 more optima yellow tops in parallel). I'll be running a 2500w inverter off of that. I've already got the cables and appropriate 250A fuse from the batteries to the inverter. SO, here is my question:

From what i understand, fuse selection is primarily a function of cable guage selection. You want the fuse to blow instead of cables to overheat/burn. Cable guage selection is generally a function of amps and distance it will be travelling.

In the roscommon instructions, they suggest using 8ga wire, with a 50A fuse in line to the second set of batteries. I'll be running the batteries in the back of the M1009. So, based on all the charts I've found online, I'm being very safe by choosing 4ga cable. As to the fuse size... shouldn't I want to use as large a fuse (within a very conservative margin) as possible, in order to charge those batteries faster? And given my 4ga cable, wouldn't a 70amp fuse still be quite safe (given a 125-150a rating on 4ga cable over 15ft), but allow more current? This is the one place I feel like straying from the Roscommon instructions. If I'm beefing up the cables, I should be safe in increasing the amperage for the fuse. Also, looking at one of these for said fuse. Resettable Circuit Breaker Cooper Bussmann

Any and all help is appreciated. I know everybody castigates the 12v'ers. But, I did it because it just makes more sense for my needs/wants of the truck. And since this is really more of GENERAL electrical/safety question, it shouldn't matter.

Oh, also, electrical stuff is not my forte, so try to keep answers as layman-y as possible. I've worked my way through issues from wiring diagrams on more than a couple of vehicles now, and I do own a good multimeter, but... well, most of what I know is self-taught on the topic, so go easy on my ignorance.

Please let me know if this post is more appropriate in another forum area.

 

[SCSG-G4]

Batteries will only take a charge at a given maximum rate. Go with the standard fuses, as they are protecting more than the wire. If they pop, there's something wrong that needs to be fixed right away.

Think of it as a water pipe, the bigger the pipe, the more water can flow. But the battery is like a spigot that lets water flow in or out. The voltage is the 'water pressure' in this example. So you tell me, what good is a 10 inch pipe of water, when the valve is only half an inch. Charging the battery is a chemical reaction, which generates heat - put too much amperage in too fast and you have a battery explosion, not a fast charge.

 

[pnishr]

Excellent answer/analogy. Thanks, I'll stick with 50Amp and 4ga.

 

[Skinny]

From what I understand, you need a wire that is capable of carrying 100 amps of 12 volt current to the back batteries?

If so, you would need 6 gauge which is good for 23 feet. I would probably go with 4 just to be one size larger which keeps voltage drop to a minimum. I have to disagree with the fuse size. If your alternator is capable of 100 amps, then you need a fuse of 100 amps. Even though your batteries may not charge at a rate near that, what if you load the batteries which pull draws the alternator above 50 amps? The fuse would blow.

 

[pnishr]

From what I understand, you need a wire that is capable of carrying 100 amps of 12 volt current to the back batteries?

If so, you would need 6 gauge which is good for 23 feet. I would probably go with 4 just to be one size larger which keeps voltage drop to a minimum. I have to disagree with the fuse size. If your alternator is capable of 100 amps, then you need a fuse of 100 amps. Even though your batteries may not charge at a rate near that, what if you load the batteries which pull draws the alternator above 50 amps? The fuse would blow.

You mean like running a high draw accessory off the inverter while the engine is running, right?

And yes, from all the charts I had looked at, I knew 4ga would be somewhat overkill, which I prefer.

 

[Recovry4x4]

No worries. You didn't convert to 12V, you deleted the 24V option.

 

[MarcusOReallyus]

You mean like running a high draw accessory off the inverter while the engine is running, right?

And yes, from all the charts I had looked at, I knew 4ga would be somewhat overkill, which I prefer.

Somewhat overkill is good. Massive overkill is not. 4 ga would be overkill, but not massively so. If you are JUST charging the batteries, 6ga would be fine. If you think you might also be drawing from the alternator, maybe 4, but is that likely with the batteries right there at the back next to the inverter?

I get disgusted with kids on 4x4 forums who think you aren't doing it right unless you run 00 cable for a 6 foot run.

As for the fuse, it needs to be LOWER than the current capability of the least capable device on the circuit. I would not fuse it at 100 amps with a 100 amp alternator. You don't want to draw maximum current from your alt - that will shorten its life.

 

[TechnoWeenie]

As for the fuse, it needs to be LOWER than the current capability of the least capable device on the circuit. I would not fuse it at 100 amps with a 100 amp alternator. You don't want to draw maximum current from your alt - that will shorten its life.

Stupid advice... Really stupid.

1. Fuses are slow blow, or fast blow. Most fuses need to exceed their rating by %200 or MORE to 'instantly' blow.

2. Do not use circuit breakers, in a dead short, dealing with high amperage, you're likely to fuse the breaker, welding the contacts together.

3. Running fuses that are rated for LESS than the device, will cause fuses to constantly blow, and at worst, will cause thermal breakdown of the fuses, leading them to be useless, and fusing themselves TOGETHER in the event of a catastrophic failure (direct short).

By your statement, if I have an LED light and a 2500W inverter on the same circuit, I should put a 1A fuse on the line, for the LED light... The inverter in STANDBY uses more than that, and the fuse would kick INSTANTLY.

Familiarize yourself with Ohms law, wire resistance, voltage drop, etc. before making such comments. I'm sure you mean well, but giving someone BAD electrical advice can mean them burning down their rig.



Step 1 - Calculate maximum current draw, then add 20%. So, alternator would be 100A +20% = 120A

Step 2 - Calculate maximum distance, and add 10%, rounding up to the next foot. 4ft run = 5ft run, 10ft run = 11ft run, etc

Step 3 - Calculate the wire you need, based on the amperage and cable length. Example. 120A @ 13.8VDC, 4ga wire, @ 5ft, would drop .3 volts, or 2.17%, meaning you'd have 13.5VDC at the terminating point. Being that it's only drawing 100A, we're good.

Step 4 - Calculate the maximum current draw of highest device, and fuse @ 10% above the rating. Ie a 2500W inverter would draw 181 Amps @ 13.8VDC, at full tilt, so fuse at 200. USE A FAST BLOW FUSE! Thermal breakdown can occur when consistently running a fuse at its rated limit.

Step 5 - Fuse wires CLOSEST to the point of which power is SUPPLIED. ALWAYS FUSE AS CLOSE TO THE BATTERY AS POSSIBLE.

Step 6 - In long runs, fuse at the battery AND at the terminating point of the run.

Step 7 - (optional) fuse ground leads on end user equipment. A bad body/block ground could have high current devices trying to find a ground THROUGH your radio/inverter/equipt. I've seen it happen.

Step 8 - (optional, but preferred) If using multiple devices, drop your large gauge wire into a distribution panel, with fused outputs. This makes troubleshooting and repairing simpler, and prevents damage to EACH device in the circuit. Ie if you connected a 20A radio to a circuit with a 200A fuse, and the radio shorted out, it wouldn't stop smoking until the 200A fuse kicked, which would be a L O N G time.... Which leads me to the next point.

Step 9 - ALWAYS FUSE THE EQUIPMENT/DEVICE. Don't care if it's a 2A LED dome light, or a miniature fan. FUSE IT.

 

[Skinny]

I think the misunderstanding here is the size of the alternator fuse. A 100amp alternator needs a 100amp fuse or something with a buffer say 120amp fuse. The crucial part is that the charge wire from the alternator needs to be capable of supporting 120amps, the rating of the circuit protection device. Ideally the wire would have a buffer as well as you would not want it to heat up at full current draw. The alternator life will shorten if at max load all the time but by nature will never over produce amperage under normal operation. You could put a load tester on it and crank it right to the rated output without harming anything.

As far as wiring aux circuits especially inverters which can pull some serious amperage under full load, if you don't understand it don't wire it. Your Cuck Vee does you no good in a big ball of flames.