Another ACR Wiring Question (dual banks, lots of potential amps)

[deminimis]

Howdy,
Time to order up parts for a Blue Sea ACR install. Combining camper batt bank (4 for 420 a/hr total) and dual truck batts bank (alternators' output is 440amp max). ACR will be mounted in engine compartment and connection b/t camper and truck will be via an Anderson connector. I will not fuse truck batts to ACR, but I will fuse the camper batts before making the long run to the ACR. Thinking I'll manage with 1/0, looking at charts (camper to ACR). A 250 amp fuse would protect the 1/0 wire, but it seems underkill for this application. I mean, I could, theoretically, see some serious amps coming from the truck to the camper when inverting and running, say a microwave, air compressor, or whatever, or should I whollop my house batts and the alternators take up the charge. Inverter and camper batts wired with 4/0 and a 400 amp T fuse, not that it really matters here. Running 4/0, and even 2/0, is not an option for this install (camper bank (banks, really) is in a slide, so wire needs more flexibility, plus the near 90 degree turn the wire has to make to enter the camper). I may run 2 gauge from the camper bank to the fuse as it will be a short run (and is good and flexible for the slide situation). So ol' wise ones, do you think a 250amp fuse and 1/0 is going to be okay, overkill, or underkill? Can't tell you the exact length of the run, but the wires will pop up just behind the headache rack and will enter from underneath the area of the sewer hose compartment (about mid length of the upper box). Anderson connector will be forward of the upper box. Let's call it 20', to be safe. Old pic, but the only one showing the proposed wire routing:

If you don't think the 250amp fuse 1/0 guage is the right combo for this, your recs (and why)?

Thanks.

 

[DiploStrat]

Run the Numbers

I have 250A of alternators feeding 600Ah of AGM batteries.

Running the numbers on your rig, you get this: http://www.calculator.net/voltage-d...ce=20&distanceunit=feet&amperes=200&x=39&y=13

-- 200A
-- 20 Feet (one way)
-- 14v

With a single 1/0 AWG cable you can expect a voltage drop of about 0.8v which means that you will only deliver about 13v to the batteries. (The Anderson connecter, good as it is, will only make this worse.) 13v is below float level for AGM, so you will have little charge.

As the amp flow drops, the voltage drop decreases, so at 100A you will get up to 13.6v, or right about float level for an AGM (http://www.calculator.net/voltage-d...ce=20&distanceunit=feet&amperes=100&x=26&y=13)

This is why I run 2x1/0 AWG. At 200A you get the same 13.6v; that is, twice as much amperage.

So basically, if you run a single 1/0 AWG you will be trowing away much (most?) of your charging capacity.

13.6v will charge a battery that is down to, say, 12.5v and the voltage will rise as the charge rate drops to, say 50A, so you will, finally, complete the charge. It will just take hours longer.

Cable size translates grossly to charge speed.

 

[deminimis]

Glad you saw this and jumped in. I wasn't clear. I do plan on running two cables. I'll do a negative busbar in the engine compartment and a second near the batts if I need to go with a smaller feed from the batteries due to the slide (I'll try it with 1/0 first, but I've got to consider the possibility the 1/0 won't play nice in the slide). I'll probably go back to my old familiar 4/0 for the engine compartment from batts to the acr and busbar. That way the smaller wire doesn't come into play until leaving the engine compartment. I did think long and hard about 2/0 cabling, but I just don't think I can manage the turn into the camper with that (and not have clearance issues with the top box, etc.). So, you're saying if going with the 1/0 feeds, I should go with only a 200amp fuse and not a 250? As an aside, and I hadn't really thought about it, but suppose I could go 2/0 from engine compartment to the Anderson connector (that's the long run, so it woudl further cut down on losses) and then 1/0 into the camper. Thanks!

 

[deminimis]

Actually, disregard the 4/0 in the engine compartment. Thought I had some extra, but I'm plum out. I'll just stick with whatever I run for the majority of wire (be it 1/0 or 2/0).

 

[DiploStrat]

Copper is copper. Look at the link I gave you; use 2x1/0 AWG. If a 1/0 cable can make the bend, two can. 1/0 AWG is 50mm2; 4/0 AWG is 100mm2. I am effectively running 4/0 cable. You can too, just run a pair of 1/0 AWG for the sections where you must make tight corners.

Wire is run in a circuit. There is no point in putting 4/0 in one place and 1/0 in another; the net result will be the resistance of the 1/0 section.

Fuse to the melting point of the wire or any device on the circuit, whichever is lower.

 

[adam88]

If I may join in... Diplo... The reason you use 2x1/0 versus a single 4/0 is the size of the 4/0 being too big right? Otherwise a single 4/0 would be best solution? (Just wondering as I read this...)

 

[DiploStrat]

Several Reasons

Copper is copper, so, in one sense, it is better to think about the cross section of copper required to move the desired amperage at the desired voltage, rather than to talk about the wire gauge. Wire gauge is merely the way that the copper is sold.

That said, Verkstad is exactly correct; a given wire is made up of a number of strands, the wire gauge mere describes how many are wrapped within a given bundle of insulation.

As a practical matter, it is often easier to deal with several smaller wires than with one large one. With allowances for the increased resistance of connectors, etc., copper is simply copper.

In the case of my Chevrolet, the vehicle is equipped with two alternators, each connected to a starter battery, one on each side of the vehicle. (The two alternator/battery pairs are not separated.) Initially, I ran a single 1/0 cable from one starter battery to the camper batteries, but that cable got hot when the camper batteries were deeply discharged. So we then went to one 1/0 cable from each of the starter batteries to the relay. (I have no idea if this has lower resistance than simply doubling up on one of the batteries; it is certainly more "elegant.") From the relay we could have run a single 4/0 cable to get the same cross section, but again, as Verkstad noted, it was easier to route and terminate a second 1/0 cable than to wrestle with a 4/0 cable.

 

[deminimis]

I also came across this helpful chart (there are tons out there). The 1/0 I'll be using has a temp rating of 105C. (Same with 2ga I'll use for the slide if I need to). So, the 1/0 wire can handle up to 285amps and 210amps for the 2ga.

 

[AndrewP]

You likely know this, but the current a wire an handle safely, does not address the voltage drop that occurs in a long wire run. It only addresses a safe current that can flow indefinitely in a wire without heating the wire and melting it's insulation.

But, yes, a 250amp fuse protecting 1/0 copper wire should be fine.

What you propose to do with 1/0 gauge will work, the "voltage drop" will only limit charging speed and not ultimate battery voltage. The voltage drop will self regulate to some degree the current flow into the batteries. Likely not a huge deal, and you certainly have an overkill alternator situation!

Do you know how many amps at say 14v your house bank can accept? If you have this number in hand, it makes it easy to figure wire size. If the bank at 1/2 charge can only accept 100 amps (and this would by my guess) then 1/0 will be plenty and you can forget about bigger wiring.

Diplo's post above oversimplifies this point. If your voltage dropped from say 14v to 13v, there would be little current flow, so in reality, the voltage drop would be much less, and the voltage will come up to some threshold that allows current to flow, and will self regulate if that current is less than what can be accepted by the batteries. As the batteries continue to charge and internal resistance increases, current flow will be even less, allowing voltage to rise even higher, until the batteries are fully charged at full charging voltage as set by your voltage regulator.

I totally agree that wire size (assuming safe within the maximum current that will flow) limits charging speed, not charging adequacy to regulated voltage.

I am missing something about why you need so much alternator capacity? Even 250 amps is huge, and 440 amps is off the charts. I'm guessing you could run your system, and fully charge all the batteries with just a 200 amp alternator. Generally battery acceptance is more of a limiter than alternator output, depending of course on the size and type of your house batteries.

 

[deminimis]

Thanks. I have dual 220 amp alternators on my big Ram. Run[ing] what you brung.