Which Wires and How Long?

[oside trailer]

As I've slowly graduated from a simple electrical system (comprised of a car battery and a cheap power inverter) to a full-blown 12V system (with two 6V deep cycle batteries, a 500W converter/charger, 700W inverter with fuse box and 6 separate circuits) it has become clear that careful consideration needs to be given to the wires that will support this matrix.

The new 6V batteries are wired in a series (and to the converter/charger) using 2 gauge battery cables that are 28" in length. But now that the batteries are going to move to a tongue box, I'm needing some additional length to reach the converter/charger (inside the trailer), and to run from the batteries to the fuse box.

Battery cables from 'switch-to-starter' found at your local Pep Boys are the only ones that are long enough and feature eyelets on each end, but the terminal connector is too large in diameter to fit either my inverter or my fuse box. It seems as though wiring everything together is going to require customizing these cables with different diameter connectors. Is this ok? And should I be limiting the length of these cables?

Last thing I need is to have wires start overheating...

[TPMcGinty]

I'm going to use some 8 gauge multi stranded wire to run the power from my battery to my converter and back. It should be plenty.

[brian_bp]

I see what you mean: J.C. Whitney and USAhardware.com list battery cables up to 6 feet... but they have a post-style connector on one end, not the lugs for studs on both ends that we need and the "switch-to-starter'" cables have. "Switch-to-starter" cables come up to 36" long, but they have identical big holes in each end.

In addition to the automotive battery cables, RV stores carry cables for exactly our purpose. The last store I was in still only had them to 26" long, and the usual RV approach seems to be to bring 12V circuits to very near the battery so no single long large-gauge cable is used. I have seen several circuits all terminated right on the battery positive terminal, making for one messy installation and no large cable... that would not be my choice. I think the heavy cables in the RV store are typically used as the jumpers between paralleled or series batteries in a set, not to connect them to the converter or other equipment.

The different sizes of studs at each end would not concern me, as long as each connection is sufficient for the expected current... but it does suggest custom assembly.

Custom-building certainly seems reasonable to me. I would likely use welding cable (because it is flexible, has a reasonable cover, and is available) but depending on local sources nicer stuff might be available; personally, I would check with my favourite industrial supply house. Welding cable is usually just black.

The gauge of cable is likely to be determined by efficiency (thick enough to not lose too much precious energy) than by safe current-carrying capacity, so there should be no problem with cable or connector capacity.

Looking at the way connectors are crimped (and not sealed) on most ready-made battery cables, I think we're all capable of doing as well. On the other hand, commercially assembled crimped connections in this size are squeezed from all around in a tubular terminal when they are crimped, not placed in a U-shaped terminal end and pinched closed from one side like typical small automotive crimped fasteners. I would look for bolt-on terminals or threaded compression connectors (I don't know the official term for them) like the QuickCable 1&2 ga lug compression connectors.

If you have time (to wait) and money (for custom work), I see that you can get custom cable assemblies made to order, with your choice of cable and ends. That link is for a solar energy business, and certainly the solar crowd would be big users of heavy-gauge cable connections to threaded battery posts.

[Dale M.]

Just back things down a bit and use smaller gauge wire and crimp on your own connectors..... Custom made cable in any size from 6 to 8 or 10 gauge wire is probably large enough to do your wiring needs and you can make your own cables custom length...

Steer away from "auto parts" stuff and look real hard at "electrical" wiring and connectors.....

Look a bit at these sites and you will see that there is a multitude of connectors and wire that will suit your needs....

http://www.delcity.net/
http://www.waytekwire.com/
http://www.tiewraps.com/

Really suggest you get paper catalog so you can sit at your leisure so you can "stare and compare"...

Dale

[Alphacarina]

Just back things down a bit and use smaller gauge wire and crimp on your own connectors..... Custom made cable in any size from 6 to 8 or 10 gauge wire is probably large enough to do your wiring needs and you can make your own cables custom length...

I agree

You don't need heavy gauge cables unless you're running a starter off those batteries. For what you're doing, 10 gauge eveywhere would probably suffice but I think I would go with 8 gauge for the run from the tongue box back to your power distribution panel just because it's a longer run and it supports everything you're running all at one time

Don

[Leon]

I would go with something closer to 6ga to the inverter. At 700 watts, it draws close to 60 amps which is too much for 10ga and maybe even 8ga.

[TPMcGinty]

I would go with something closer to 6ga to the inverter. At 700 watts, it draws close to 60 amps which is too much for 10ga and maybe even 8ga.

Isn't it closer to 6 amps? Isn't the formula Current = Power / voltage or am I wrong?

[brian_bp]

I would go with something closer to 6ga to the inverter. At 700 watts, it draws close to 60 amps which is too much for 10ga and maybe even 8ga.

Isn't it closer to 6 amps? Isn't the formula Current = Power / voltage or am I wrong?

The formula is right... but this is the 12 V (input) side of the inverter, not the 120 V (output side of the inverter).
700 W / 12 VDC = nearly 60 A from the battery to the inverter
700 W / 120 VAC = more like 6 A from the inverter to the AC stuff

Inverters are not perfectly efficient, so the power input to the inverter (at about 12VDC) is always somewhat more than the power output from the inverter (at 120VAC), so that means even more current on the input side.

[Dale M.]

If one uses TW wire type #8 wire is good for 60 amp...

http://www.wiktel.com/standards/ampacit.htm

If you want to use a wire with better grade of insulation, it can carry more current.

Also in industry I was trained in they have a saying "if you are unsure of specific wire size, go to the next larger size"...

Dale

[oside trailer]

so, i'm going to go with 6ga from the battery to the inverter, but what about to the fuse box?

nothing on that circuit ought to draw anything more than 15amps.

[brian_bp]

...I would look for bolt-on terminals or threaded compression connectors (I don't know the official term for them) like the QuickCable 1&2 ga lug compression connectors...

I checked with my favourite local industrial supply house today, and those QuickCable compression connectors (in the size for 1 & 2 gauge cable) are less than $7 each. Or $30 each with a shiny finish and the Monster brand name at an auto sound shop, if you want to impress people with your wealth (the main purpose of Monster branded components). There is also a smaller 4-ga and 6-ga sizes, and larger sizes (1/0 to 4/0), with 3/8" and 1/2" stud holes.

I'm now convinced that these connectors are my choice for high-current connections (as long as the threaded stud size is 5/16", 3/8", or 8mm), such as supply to the 1000W inverter I have not yet installed. In smaller sizes (such as 6 ga), I'm sure that a simple crimp connector works fine, too.

Also, if the different stud sizes happen to be 3/8" and 1/2" (they're likely smaller than that) QuickCable has ready made battery jumpers (and heavier than maybe needed) cables with those two different terminal ends.

[BPFox]

I would go with something closer to 6ga to the inverter. At 700 watts, it draws close to 60 amps which is too much for 10ga and maybe even 8ga.

Actually with the right wire, 10 ga is plenty big enough to handle 60 amps. If it makes you feel better, go with 8 ga wire. I know "bigger is better" but at some point it is just costing you money. Keep in mind here folks that most TD and TTT wiring runs are far shorter than most other electrical applications. Just take a tip from your tow vehicle. My Chevy Trailblazer has power everything and that includes heated seats. This vehicle uses far more dc power than most any trailer will and you won't find anything running on 6 ga wire. (the exception being the power feed to the large fuse block but that provides power to several circuits in the 40 to 60 amp range, not just one.) After all we are just trying to power up a small 700watt inverter here not the
space shuttle.