Short Attention Span Do's and Don'ts

Do's:

* Use a proper rated wire. If you are not sure, get larger wire. An 8 gauge copper stranded wire will be okay for 250 watt applications if it is no longer than 10 feet. Fuse it to 20 amps.
* Use a proper rated fuse suited to the wire you are using. If it is a main power cable, fuse it within 18" of the battery for the total expected load and fuse each device you connect to it to the load the device requires.
* If you are not sure how to wire up our system, ask someone who does. Even if you have to pay for the advice or even someone else to do it for you. It may be cheaper than replacing parts of your CarPC or your whole car.
* Check and double check your wiring before powering your system for the first time.
* Put electrical tape or sleeving (heatshrink) over connections and terminations so your wiring can't short out to the car body.
* Secure wiring and hardware properly.
* If your wiring is going through a hole in the body of you car, protect it with a grommet or some sleeving.
* Use connectors, crimps or solder wires together where possible.


Don'ts:


* DO NOT RUN YOUR SYSTEM WITHOUT A MAIN POWER FUSE. This fuse should be no more than 18" from the battery.
* Don't rush to get your install finished. This is when most mistakes can occur. Especially when it is late at night.
* Don't cut corners. You will most likely have to redo your wiring because you have tried to finish it quicker or cheaper. This ties in with the point above.
* If you are using a soldering iron to solder wires together, be carefull where you place the soldering iron. It will burn through carpet and interior plastics before you know it. They can also give you really bad burns

Whys

Fusing - On main power cables (such as those that run from the battery to the PC power supply), you need TWO fuses. The first fuse should be very close to the battery -within 18". This fuse is designed to prevent a FIRE if your main power cable should short to the body or a ground point. Because main power cables tend to be quite thick, they will heat up, then ignite the insulation and anything nearby if they happen to short out. This is BAD. To prevent that, use a fuse (distribution blocks often have them built in) that will blow before the wire gets hot. Some people use circuit breakers, but make sure they are DC and rated for the proper load if you decide to use them.

The second fuse is placed near the actual electronics. Most often, several wires are branched off of the main power cable to things like the PC, USB hubs, LCD power and so forth. EACH of these devices should be fused in accordance with their power rating - LOWER than the main fuse for the power cable. (E.G. a 1 amp LCD should be fused at 1 amp). The reason for this is that the main power cable fuse may be rated for say, 30 amps of power, and may not blow if something small like the thin wires to the USB hub or motherboard experience a short circuit. In that case, the small wires themselves will become the fuse, particularly if they short against something inside the dash. If not fused properly, the wires will heat up until they break the circuit. However, in doing so they will also cause the insulation around them to heat up, possibly burst into flames, or worse, may weld themselves to the item they are shorting to. FUSE APPROPRIATELY.

Wire gauge - using appropriate gauge wire. Although wires are designed to conduct electricity, there is still a certain amount of resistance per foot of, say, copper wire. That resistance tends to be higher when an item draws a lot of power through the wire and it generally shows up as heat, which is why an overloaded wire can cause smoke or fire. In general, thicker wires have lower resistance and can carry more power. When wiring for 12 volt systems in a car, many people don't recognize that the lower voltage wires in a DC 12 volt system are much thicker than the AC wires you are used to seeing in your home. Therefore, wires in a 12 volts system tend to be a LOT thicker than you think. A good rule of thumb is if in doubt, increase the thickness of the wire. There is no penalty to using too thick wire, while the penalty for using too thin wire is a FIRE.

In addition, that resistance translates into a performance issue that is very important for car PC installers - voltage drop. If you use too thin a wire, the voltage from the battery to the PSU will drop. For example, a 14 gauge wire (a wire like a common light duty extension cord you might have around the house) connected to a 12 volt battery over a 20 foot run under 15 amp load will have a voltage drop of almost 1 full volt (from 12v at the battery to 11.02 at the power supply end). When you start the car, the voltage will drop even further -possibly so far that many DC-DC power supplies for car use cannot compensate and your PC will cut off while starting. Using a thicker wire makes a difference. In the example above, if we replace that 14 gauge wire with an 8 gauge wire, the voltage drops to only 11.6 volts over the same 20 feet. Stranded wire is superior to solid wire and of course the type of conductor makes a difference as well. If we used aluminum on that 8 gauge wire, the voltage would drop to 11.24 volts. (How do we know this? We're using a calculator like this one)

For the same reasons as stated above, don't assume that just because your in car power supply came with 12 gauge wire on it that you can simply extend that wire for 15 feet and be safe. Calculate how much power you need the wire to carry, how many feet it must travel, and use a wire gauge chart to determine the appropriate gauge. As a rule of thumb, 8 gauge copper wire will USUALLY suffice for most application in the 250 watt range over a distance of 10 feet. The voltage at the end of the wire will be approximately 11.7 volts. Fuse it for 20 amps.


Determining proper wire gauge / fuse size / battery size alternator size

This is to be used as a guide only. If you are not sure what to make of all the numbers in the following tables, consult an auto electrician, car audio installer or a car audio shop.
BD Batteries power calculator
Clang4l power calculator



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