Can you always use a higher gauge wire? Does over gauging wires affect the fuse choice?

[sawmonandnatalie]

If a higher gauge wire fits in the terminals, is it ok to always go with the highest gauge possible? Let's say the expense is not the concern.

For example, let's say I need only an 8 AWG between the charge controller and battery bank, and I decide to use a 4/0 AWG wire. Is that bad?

Does over-gauging the wires affect what fuses we would need to use for that line? Or the fuse choice is only related to the load on the line (allowed max current)?

 

[Dzl]

If a higher gauge wire fits in the terminals, is it ok to always go with the highest gauge possible? Let's say the expense is not the concern.

For example, let's say I need only an 8 AWG between the charge controller and battery bank, and I decide to use a 4/0 AWG wire. Is that bad?

I can't think of any reasons that oversizing, even drastically, would create problems beyond practical considerations (max wire size your components terminals will accept, flexibility of the wire, cost), but I should note that I'm not very experienced and not an expert.

I think this is one of those situations where (reasonable) overkill is a good thing.

Does over-gauging the wires affect what fuses we would need to use for that line? Or the fuse choice is only related to the load on the line (allowed max current)?

It affects it in the sense that in many cases you could use a larger fuse if you use a larger wire. I'm not sure if there are any effects beyond that.

 

[boondox]

As noted above there is no problem electrically. Mechanically there can be issues but you would have to have an extreme example like you give above.

As to fuses, it depends on what the fuse is for. If it is to protect a device then use the specified size fuse. If it is to protect the wiring only you can rate the fuse according to the wire.

 

[WA5IDX]

Here's something to consider - If you use a common Buss Bar for your power Distribution a "larger" gauge conductor "should be used" since the combined loads to/from the Battery bank could be higher than the Charging Current from your Charge Controller.

In my RV I used a 1.5" x .25" buss bar for Power distribution ( both Positive and Negative) and a #6 from my Charge Controller to the Buss bar since it was limited to 50A. The Bus Bar has 4/0 cable to the Battery Bank. I have a DC Power Panel ( breakers) that is feed with a #8 from the Bus Bar. The Inverter is feed with 2/0 from the Bus Bar, etc.............

 

[Hedges]

Larger gauge wire is almost always better or just as good.

One possible exception is where the source (e.g. lithium battery bank or utility transformer in an industrial neighborhood) can deliver more short-circuit current than you can deal with.

In the event of a fault which shorts power to ground/return, a fuse or breaker must be able to interrupt that current safely. The balance of the system (wires, switches, breakers) carrying the fault current need to hold together long enough for the fault to be cleared. If fuse or other components don't, they can blow apart in an arc-blast, an electric explosion that causes impact damage, hearing damage, and severe burns.

Where smaller gauge wire (or longer length of wire) helps is its resistance. That can drop what might have been 50,000 amps from a bank with multiple strings of lithium batteries to 20,000 amps or less, which a class-T fuse can handle. Or, 20,000A from a single string lithium battery bank to what a lesser OCP device could handle.

Where someone has a fuse that isn't rated for what we estimate their battery can deliver, I would suggest considering where the cables first pass through a metal bulkhead or otherwise have the possibility of shorting, and calculate cable resistance.

One guy here experienced such a failure first-hand. It appears a busbar had bad contact. It was fortunately covered with a lid (on a golf cart) when he shorted a wire outside. The fault went through a fuse (which I don't think opened), but the busbar exploded. No injury, and apparently just one cell damaged by arcing to its terminal. But material blasted around. If the busbar hadn't gone first he did have a good fuse.