Picking Wire

Possibly the most important decision to make when building a wiring harness is the wire itself.  Choosing a wire involves knowing the environment the wire will be used in, how much current it is expected to carry, how long of run that current needs to travel, what types of load or signal is going to pass through it, how the person installing or serving the product in the future will identify the wires for certain components and much more.  

Material.  The first decision that is likely to be made is to choose a wire material or type.  There are many choices of wire and you can start to narrow down what you need by starting with if you need a single conductor wire, or if you need some sort of specialty wire/cable such as a twisted pair (for differential signals such as CAN Bus), or maybe a standard impedance cable such as a coax cable, etc.  While typically cables will not be included inside a wiring harness assembly, it is possible to do so. To simplify things we’ll assume you’re picking individual conductor wire for your project, and if any specialty cables are to be used, you’ll understand the requirements those must meet.

For automotive wiring harnesses PVC (Polyvinyl Chloride) wire is typically used for general purpose wiring.  PVC will typically be good for -40degC to +85degC and have pretty good abrasion resistance as long as it is properly secured, and any places where there is movement (door jams, etc) thick rubber strain reliefs and protectors are used.  PVC wire is also typically available in two insulation thicknesses, standard (GPT), or thin (TWP). Thin insulation can be used in tight spaces or in connectors without a lot of extra room for insulation. GPT is most common insulation type.

The next most common automotive wire material is XLPE (Cross Linked Polyethylene).  XLPE wire is often times used in engine compartments, motorcycles, and other powersports and agricultural applications due to its higher temperature rating of -40degC to +125degC.  It is also more abrasion resistant than PVC and will hold up better in areas where chemicals or moisture is present. XLPE can resistant many types of oil, grease, gasoline, acids, and solvents.  This wire is also available in 3 thicknesses of insulation SXL, GXL, and TXL (Standard, thin, extra thin respectively).

If your application has special requirements there are many other types of wire available such as PTFE (Teflon), Silicone, Marine Wire (PVC), Fiberglass, etc.  Typically these wires would be used when you need very high temperatures, extreme flexibility, or special environmental considerations. PTFE for example can handle very high temperatures (-90degC to +260degC, is very abrasion resistant, can handle many chemicals, and is required for many military, aerospace, and commercial applications.  If you have any questions about what type of material you should be considering feel free to reach out to us for help.

Color and Marking.  Colors aren’t just about looking good when it comes to wiring harnesses, colors fill an important role in identifying individual circuits to make finding and fixing problems in the future far easier.  If a vehicle has a problem in the future and the circuit must be traced back the service technician can refer to a wiring schematic of the vehicle and see what color wires are involved in the circuit he or she is troubleshooting to make finding them much easier.  Usually these wires are buried in the vehicle and wrapped in tape or some other covering material making it impossible to track a connection by any other method. There are usually around 12-14 standard colors, but the wire vendor can often also stripe the wire in another color to produce dozens of combinations so that every circuit can be an individual color combo that is unique to it alone in the vehicle.  Additionally wires can also be marked with labels, printed on, or laser etched to further identify them as is required in some industries such as aerospace.

Size.  Wire can be produced in a huge variety of sizes and it is important to size your wire correctly to ensure safety, correct operation, and to keep costs down.  Wire is not sized by the outside dimension of the finished wire and cable, but rather by the cross sectional area of the conductor only (the metal part inside the insulation).  This is important to understand if you are choosing your wire to fit into an existing application or replace a portion of existing wiring. You must measure the actual conductor, not the outside of the wire.  As mentioned in the materials section, the size size wire may be available in 2 or 3 insulation thicknesses. Using a wire that is too large for a given application will cost more due to more copper (or other conductor) being used.  Sizing a wire too small will cause electrical friction (think about a pipe that is too small), this results in higher resistance, and thus result in a higher voltage drop across the wire, and a lot more heat. The electrical resistance of different wire sizes is easy to obtain and by simply using Ohms law you can calculate the voltage drop at different current levels as well as the power loss (Voltage drop * Current) that the wire will have to dissipate.  If for example you calculated that a particular size wire will cause a 60W power loss, think about grabbing an old fashioned 60 w incandescent light bulb after it has been on a while, you’ll get burned! The same thing can happen in a wire and it will self heat and if that temperature gets higher than the rating on the insulation the insulation will begin to melt off and pose a fire, electrocution, and burn danger. There are three common wire sizing standards, AWG, SAE, and Metric (cross sectional area).  Another size consideration would be the thickness of the insulation, which is usually dictated by the dielectric withstand it is rated for. Or put more simply, the voltage potential you can have on either side of the insulation without an arc burning through. Most automotive wire types are rated to withstand 300v or 600v, which is plenty for an automobile that typically has around 12v in most circuits. If dealing with AC power from a building, or a high voltage circuit like a spark plug wire, you will need to pay far more attention to the insulation thickness and type for a given application.

As you can see picking wire isn’t as easy as stopping by the local home depot and grabbing a spool of “blue”, but we’re here to help if you need help designing or manufacturing your wiring solution so don’t hesitate to contact us for help!

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