The less distance between your inverter and batteries, the better. You'll have to route some massive 4⁄0 (pronounced 'four-ought') AWG cables (¾ inch diameter) between the inverter and batteries. In my installation, I have about 5 feet between my inverter and my closest battery. Unfortunately, there is also about a 6 to 7 foot cable run between my two house batteries, which are separated by the right rear wheel well. Roadtrek had wired these two batteries in parallel using 6 AWG wire. I removed this smaller cable and replaced it with 4⁄0 cable. Thus, I needed a total of 25 feet of cable. I bought 28 feet at $7 per foot = $196. One thing they don't tell you is how difficult it is to find 4⁄0 cabling. I managed to buy the 4⁄0 cabling from a local welding supply shop. You may have to mail order it. Xantrex recommends X-Flex cable from www.cobrawire.com. The finer the copper strands within the cable the better since it makes the cable more flexible and easier to work with.
4/0 Welding Cable
I also bought 25 feet of flexible split wire loom to cover the cable. The ¾ inch loom fits the cable perfectly. Since all the other battery cable on my Roadtrek also has split wire loom covering it, I thought I'd be consistent. In hindsight, I am very glad I did this. In the process of pulling the cabling into position, it got pulled by rough edges in several locations. The wire loom protected the cable from abrasions. It will also protect the cable from any areas of potential rubbing, although I tried to minimize this by firmly attaching the cable at multiple points. I found it helpful to tape the loom about every foot so the cable cannot come out when positioning it. Wherever the cable passes through a wall, such as passing through the van floor, or through the side wall of the battery compartment, I used a 1 inch cable strain relief clamp (the kind the electricians use on house wiring when it enters a junction box). This protects the cable and serves as an anchoring point. ¾ inch split wire loom is not as easy to find as the more typical ½ inch. Radio Shack carries it locally by me as does WalMart.
You should always run your cable with the positive and negative wires parallel and directly adjacent to each other. You can use plastic ties to tie the two cables together as much as possible. The reason for this is to avoid generation of magnetic fields, which can lead to interference with electronic devices. High currents running through the cables will generate strong magnetic fields. By running the positive and negative cables adjacent to each other, these magnetic fields (which are in opposite directions in the + and – cables) will cancel each other out.
Use high quality copper lugs at the ends of your cables. I bought most of my cable making supplies from John Drake at www.solarseller.com. This included 4⁄0 straight copper lugs, which are tinned. I crimped these onto the wire using the Brute Hammer Crimping Tool. This tool worked extremely well and was easy to use. Cut your cable to the correct length using a hacksaw. Then strip about 1 inch of insulation off the ends of the cable. I used the QuickStrip Cable Stripper, recommended by John Drake. It definitely was worth the money and did a great job stripping the insulation from the end of the cable. It has a little knife blade sticking out from the end. You can adjust how far it sticks out with a screw on the opposite end. You then place the spring loaded hook over the cable so that the knife penetrates the cable insulation. Now turn the entire tool around the cable so that the knife is pushed around the circumference of the cable. I found that it worked better if I pushed it toward the open end of the hook rather than pulling it toward the closed end of the hook. You can tell if the knife is adjusted too far out, because you can feel the knife blade hitting the wire strands. You don't want to knick the copper. It doesn't matter if the insulation is not cut completely through. Just take a pair of pliers and wobble the cut end of the wire insulation back and forth a few times. The insulation will tear the rest of the way at the cut line.
Brute Hammer Crimping Tool - see scale on side that lets you know how far you have to pound the punch down for any cable size
Here is a 2 AWG lug and cable in place, ready for crimping (actually I cheated and didn't strip this wire - photo op only)
Once you have the ends of the cable stripped so that bare wire will fit all the way into the lug, place the lug containing the cable into the hammer crimping tool so that the spade portion of the lug is toward the bottom (see photo). Adjust the position of the lug so that the punch will be centered on the crimp-able portion of the lug. I found that the spring loaded tool nicely tends to hold the lug in the correct position. Now, with the tool sitting on a solid concrete floor or equivalent, start whacking the top of the crimping tool with a small sledge hammer. The firmer the hits the better. There is a scale marked on the side of the tool that will show you when you have pounded the punch down far enough into the lug. I found it typically took 8 to 12 solid hits. These lugs are tough. I first thought I would get a more controlled crimp by using a vise instead of a hammer to press down the punch. My small vise was unable to make even a small indentation in the lug. The sledge hammer worked much better and was more convenient. You end up with a very tight crimp.
Hint: For some reason, once I stripped the insulation off the end of the cable, the copper strands seemed to expand out a little. This made it extremely difficult to get the bare wire stands into the copper lug, which was a tight fit. Some of the strands just didn't want to go into the lug. Part of my problem may have been due to my using welders cable, which has very fine copper strands. After some frustrating moments, I figured out a solution. After cutting the insulation, I would pull it only partway off the cable, so that about ¼ inch remained holding the wire strands together. I then placed a small plastic tie around the copper strands just below the insulation and pulled it tight. Now when I completely removed the insulation from the wire, the strands stay tightly together. This makes it easy to place the stripped cable end into the lug opening. Once I got over half of the stripped wire length into the lug, I would cut the tie off the cable and push the wire the rest of the way into the lug.
Once the lug is crimped onto the end of the cable, cover the base of the lug and at least 1 inch of the insulated cable with heat shrink tubing. I used a 2 inch length of heat shrink tubing for each connection. The 1 inch diameter heat shrink tubing I used (obtained from John Drakes site) has an adhesive inside it. When you shrink the tubing with a heat gun, it will also melt the adhesive, forming a secure tight firm covering. I used red heat shrink for my positive cables and black for my negative cables. I also used red or black tape to wrap the ends of my split wire loom where it ended near my connections. I have no question as to which cable is positive or negative. The finished connections look very professional and much better than the cable connections used by Roadtrek.