chin_k wrote: September 11th, 2018, 12:10 pm
Blue, how big is the cable in the photo above?
Do you mean the photo I posted of the original wiring in my Chinook? That right-hand cable (that goes to the separator) was 6AWG. I'm sure it was the original wiring on mine, as when I bought it from the original owner it still had original things wrapped in plastic and etc. In other words, no-one had changed anything, much less started re-wiring anything.
Here is a photo of the Surepower separator on the fenderwell (taken while I was planning my surgical extrication of it). It's the same (red) 6AWG wire size there. The wire came over to the separator from the Ford start battery (mine went over the air cleaner housing near the rear of the hood), then from the separator down in the vicinity of the steering .... whatever you call it (rod?) to the driver's side frame rail and then back along that until it cut over to the battery box below the water heater.
If yours is 2AWG, then perhaps it has already been changed? It's usually a bit hard to see, but there will be strings of numbers on the side of the insulation and at some point (always the hardest one to eyeball), it will say xAWG, wherein "x" is the wire gauge.
chin_k wrote: September 11th, 2018, 12:10 pmIf it is #2, what size fuse can I put on it? I also need to get the right fuse holder too. I think mine is stock, and I will check the other wires along the way to make sure they are #2 or larger per your suggestion.
So there are four factors in determining the "ampacity" of a wire, which is what in turn lets you know what size fuse or breaker you can run. One is the gauge (size). The second is the heat rating. "Good" wire is usually rated to 105ºC, but often wire is only rated to 90º or even 75º. I think everything Chinook that I removed was 105ºC. Third is how hot the area where the wire will pass through is. There is "normal" and "engine space." Fourth is whether the wire is run singly, in a small bundle, a medium sized bundle, a large bundle, or conduit (larger bundles de-rate the wire). All four of these are basically all about heat in one form or another.
So if you look at an ampacity table for 2AWG that's rated at 105ºC, not in a bundle, and passes through an engine space, you see that the ampacity chart says 178.5 amps. So ideally you would fuse it at 178.5 amps or lower. BUT, our V-10 will blow a fuse that small when starting (and this wire is also used for self-jump-starting). I will have to check but I think my buddy with the 2AWG wire started trying smaller fuses and ended up at either 200 or 225 amp fuse (I can check). Now how could he do that with an ampacity of only 178.5? Well there is a guideline that if you have to you can fuse up to I think it is 50% over the rating (I'd have to look this up). And too, fusing is mostly about protecting against a short circuit. So it would likely blow pretty quickly and so perhaps this is why it's okay to go over if you have to. I try not to just to keep it simple. But that was easier for me as the 1/0AWG, 105ºC, engine space, not bundled is rated for 242 amps. I think I have it fused at 225 amps (has never blown even when jump starting).
There is one other thing that needs to be considered when you are fusing directly to a battery bank, and that is the fuse holder/fuse AIC rating. Sorry if this is starting to sound complicated, but I can't just not mention it.
AIC stands for "ampere interrupt capacity." Basically what it boils down to is that the fuse body and fuse holder have to be rated to a certain level or else if you have a short circuit the fuse won't blow safely like it should, but rather the high current will just melt or blow up the fuse holder and then you have a really bad situation. This is not about the fuse size, but rather the sturdiness of the fuse body and the fuse holder.
I'll give you an example, using Lifeline batteries because I have access to their numbers. But similar size/type batteries should be around the same.
Of course this specific wire we are talking about goes from one battery bank to the other, so it has to be fused on both ends (because you fuse at the current source but this wire is different in that there is not a source and a load, but TWO sources (start battery and house battery bank).
So my house bank is three Lifeline Group 31+ sized batteries. Each battery has a short circuit rating, meaning how many amps can surge out of the battery all at once if you have a short circuit situation (why you are fusing to begin with). Each of these Group 31 AGM batteries can put out 4,257 amps. Yes, that's not a typo! So my house bank of three batteries could put out about 12,750 amps at one time. Sobering. Many common fuses have ampacity ratings of between 1,000 and 5,000 amps. Obviously those would not work. The Blue Sea MRBF (meant to fuse batteries and does a nice job) is still only rated to 10,000 amps AIC. So I could not use one of those on my house bank. I had to use a "Class T" fuse that is rated to 20,000 amps. However I could (and did) use a Blue Sea MRBF type fuse on the end that connects to the Ford start battery. I don't know the exact short circuit rating on that one but going on the general size and type it is likely around 3,500 amps max. Thus the MRBF is fine and sits nicely on the terminal post (I put marine terminal post adapters on the start battery).
I don't know what size your house battery bank is, so I can't say exactly what the short circuit amp rating is. I think flooded cells may not be rated as highly as AGM's (and AGM's are not nearly as high as lithiums), but if you had, say, two Group 27 AGM (common), then you'd be looking at something like 6,750 amps and could use the MRBF on both ends if you wanted to.
Blue Sea's catalog lists the AIC rating for all of their fuses and fuse holders (and breakers) and many of them are common types sold by them and others (Mega, MRBF, Maxi, ATO, ANL, etc.)
Hopefully this doesn't just sound like a bunch of mumbo-jumbo. If you have a general idea of what you want to do I could help point to specific fuse holders, wiring supplies, and etc. you could use (if desired).
on the math.
