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Old September 10th, 2006, 12:12 PM
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Chris Davis
94 NAS D90 6.2LS
Join Date: Nov 2003
Location: Colorado
Posts: 2,509
Electrical Wiring (rather long)

I wanted to share my thoughts on wiring with those that may not have thought much about it but are interested in learning more. There are many resources available that are more accurate and exacting, so please take this as just my overview and thoughts.

Let me start off by saying that low voltage wiring could just as easily be called high amperage wiring (given the same amount of power). Basically, Power=Voltage x Amperage, so to get 120 watts we could either multiply 120 volts (home voltage) by 1 amp or 12volts by 10 amps. Wire is sized according to two things: Amperage and voltage-drop. First and foremost is amperage--this is very important as many people think they can use smaller wire for automotive work since it is “low voltage” when the exact opposite is true. You should always use a wire that can carry the maximum amperage possible, and this wire needs to be protected from carrying more than it should by a fuse or circuit breaker (in case of a fault). Voltage-drop is the reduction of voltage in a circuit and is caused by three primary things—wire size, wire length, and connections. The bigger the wire, the more efficiently it will carry electricity and the less the voltage will be affected over it’s length. The longer the wire, the more the voltage will drop by the time it reaches your accessory and it can add up to become significant. Every connection you make also affects your voltage, so use the minimum number of connections you can and make them as secure as possible soldering whenever possible. The lower the voltage of a system, the more voltage drop occurs—that is why utility lines operate at high voltage when transporting electricity across country. Since they use very high voltage +30,000 volts, they can use lower amperage for the same amount of power—and since wire is sized on amperage and voltage drop is less for higher voltages, they can use much smaller wire. But we don’t operate at high voltages, so voltage drop can be a very real concern at 12 volts.

So, you know that your wire needs to be able to handle your maximum amperage—but what about this voltage drop thing? Usually with the short runs in vehicles, we don’t have to worry about it too much, but some electronics and halogen lights want close to 12 volts. If we have a long run, say 20’ of wiring by the time it makes it to the accessory, we may only have 10 or 11 volts if using a small gauge wire (18 gauge) versus 11.7 volts if using 12 gauge. These are not true figures—I am just explaining a concept. You can always measure the voltage at your accessory to see if it is an issue. My point being that sometimes you size the wire even bigger than the amperage says you need to in order to account for voltage drop.

Should I post an amperage chart/wire size chart if you size wire for amperage? Well, that is kind of tricky because not all wire is created equal—so a word about wire. First and foremost, automotive wire has a sheath/coating that resists oil, water, fuel—stuff found in a car. Other wiring may or may not handle this environment, so good quality auto wire is strongly recommended. Also, the type of sheath also plays a role in how much amperage the wire can carry and how hot an environment it can be used in. Basically, the higher the temperature around the wire, the lower the wire is rated in amperage. One manufacturer’s wire can be rated differently than another so it is not prudent to list that here. Here is one chart’s recommendation from the12volt site—I use it as a guideline only. Check your wire’s amperage for yourself and for your own conditions--here is their chart:

Cable Size
Wire Gauge Current Capacity
Amperage (amps)
1/0 350
2 225
4 150
8 100
10 60
12 40
14 25
16 15

Enough about wire. How about connectors? Basically crimp connectors suck. If you have to use them (don't!), at very least use an anti-corrosive compound with them. Beetter yet, just solder the connections. When soldering the connection, first slip a heat-shrink sleeve over the wire and then make the connection and cover/shrink wrap it. Crimp connectors corrode over time and are notoriously unreliable. I have a bunch in my truck and I am slowing but surely removing them all and soldering everything. Try not to lengthen a wire by using a connector/splice—run a new wire instead. Try never to use fork connectors at terminal blocks and accessories—use ring terminals soldered to the wire. When running multiple wires in a loom, try and use different color wires—it makes trouble shooting much, much easier.

Every wire should be fused or circuit breakered in case of a short. This fuse should always be sized for to protect the accessory and should never, ever be bigger than the capacity of the wire. If you have wire rated for 25 amps, but you are powering a 4 amp device, use a 5 amp fuse.

When do you use a relay? Good question. For me, I don’t like to use a switch to control more than about 5 or 10 amps, so when that occurs I use a relay. A lot of time, it is very hard to negotiate and route bigger wires in dash panels and such, so using a small gauge control wire for a relay is a convenient thing. Also, many switches (like your headlight switch!) don’t like to see big arcs every time a load is switched on and off. The arcs cause carbon deposits as well as heat and prematurely kill lots of switches. Good relays are designed to handle these arcs—all the switch has to do is control the relay. Also, when using a relay, you can wire directly from the battery to the relay (through a fuse, of course) and directly to your component thus eliminating all the routing for the switch. This not only saves from voltage drop, it also saves having to use more wire which can (with bigger wire) get expensive.

When do you use circuit breakers versus fuses? I don’t have a great answer on this one but most circuit breakers for automotive use are self resetting which can be convenient, but if they ever fail (none of mine have ever failed) you better have a spare or be prepared to bypass it and run the risk of an unprotected circuit. Replace it as soon as humanly possible if you do. I have my aux lights on a circuit breaker—my logic was if a water crossing cause a fault, it would come back on when safe and I would not have to change a fuse. My electric fans are fused though because I wanted to be able to fix them readily if I needed to. So when do you use one over the other? No clue. If anyone has good advice on this, I am always willing to learn.

Quick connects. Most suck. Most corrode. If you have to use a quick connect, use a watertight version and use silicone grease when assembling. Matter of fact, I use silicone grease on every removable fitting I have from my taillights to my sparkplugs. It does two important things—it protects against corrosion by removing the oxygen and it displaces water. Get a big tube and use it often. Especially on your tailights--those stupid connectors always corrode, get stuck and break and they are ridiculously expensive.

Wire routing: Pay close attention to heat and moving parts. Never put wire in an area where it can get pinched. Think also about hitting rocks and accidents and try and route things like your winch cables in ways that a front end collision would not compromise the cable. I have noticed that mine could get caught between my skid plate and bumper and will correct that very soon. My battery has a 5000Amp short circuit rating, so I really, really don’t want to short that cable. I don’t know what would happen exactly, but I know it would be exciting to say the least. Don’t run wires next to heat sources—wires generate their own heat and by adding even more, you lower the capacity of the wire and it can make the cover brittle and crack/melt. If you have to pass close to something, at least run a heat shield around the wire. Summit Racing and speed shops in your area carry this heat tubing. One manufacturer is Thermo-Tec.

Avoid messy wiring. Just do. Painless Wiring makes nice terminal blocks and wiring accessories. I just picked up a 7-circuit “Circuit Boss” that has a nice water resistant cover and potted base. It has 7 fuses, a few of which can be set in an ignition-on mode. I plan on rewiring my lights, fans, and air compressor through this block to clean up my engine bay. By having clean wiring, troubleshooting can be made much easier. I have spent days looking for bad connections that eluded me before and am trying to eliminate that kind of waste in the future—clean wiring is a huge help.

When running wiring, try and use a wiring loom to protect the sheathing. I use a lot of that plastic split-loom stuff and it works fine. It keeps the wire from chaffing against stuff. It is not essential everywhere, but if you route a wire where it may come into contact with anything, use something.

Lastly, we run power wires all over the place but just as important are your ground wires. A bad ground is just as bad and a lot harder to find than a bad power cable. For large draw components, consider running a separate ground line back to the battery. Winches and large stereo amps are perfect examples of this condition when they should have their own ground. For other things, when using the truck as the ground, make sure you provide a good path. Sometimes just using a ring connector on a bolt or a screw is not good enough but looks like it is. If you are having issues with a component, especially audio components, check your ground first. Make sure you have adequate ground straps from your battery to your chassis, engine, tranny, ect. Bad grounds account for a ton of electrical issues.

I hope this helps some people. If other people can add to it, please do—I pick up good info and incorporate it all the time. I find I constantly find better wiring techniques from others and am always wanting to learn more…
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Old September 10th, 2006, 12:54 PM
DJ Menasco's Avatar
DJ Menasco
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DJ Menasco
Join Date: Sep 2003
Location: Oregon/South Carolina
Posts: 920
Excellent write-up. I think you addressed a lot of the basic issues required for proper automotive wiring.

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Old September 21st, 2006, 10:45 PM
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Neil Steinhagen
1990 Perentie GS
Join Date: Apr 2006
Location: Periscope Depth
Posts: 759
Okay, I have a dual battery setup with battery terminals that take 2-1/0 connections and 1-4 awg connection each. I used the 1/0 connections to parralel the batts and go to the winch. I am planning on using the 4 awg connection to go to an aux fuse block, then start wiring relays for the lights and power inverter. I am pretty savy with a wiring diagram, but where do you guys get the switced power for your relays? My fuse block has 12 slots. My lights, for example, will I have 1 30A fuse for the lights and 1 5A fuse for the switched power? How many amps does a relay take? Should I just dedicate a circuit from the aux fuse block to handle all of the relays, like a 15A circuit?
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Old September 22nd, 2006, 03:28 PM
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Barry O'Mahony
97 D-90 SW LE
Join Date: Dec 2003
Location: Bend, OR
Posts: 281
Basically crimp connectors suck.
Well, yes and no.

Properly done, good pressure (crimp) connectors can be highly reliable. They form "gas tight" connection to the wire, and are corrosion-resistant. For example, in aerospace applicaitons, they are considered to have a lower failure rate than soldered connections.

However, most people don't bother to buy a $100+ crimping tool when they use these connectors in an automotive environment. And without the right tools, it's real difficult to install the connector on the wire in a relia ble, repeatable manner. Those squeeze pliers just don't cut it. And some off-brand connectors themselves are just garbage.
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