This tells you that you could use a cable with a rating of 4.17A or above, however it is good practice not to design a circuit operating at the upper end of the cable's rating and so you should select a cable with some additional capacity. If we wanted to wire up a light that we know has a power rating of 50W, then using I = P/V the current draw would be 50W/12V = 4.17A. You might find it useful to read our Electrical Circuit Basics article for using the equation I = P/V where the following example is given: Although fuses are used in the circuit to protect the cable, the cable itself should be of an adequate rating to prevent this over-heating occurring under normal circumstances. If it is not capable then it is likely to result in the cable becoming hot and potentially catching fire.
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The following are some points you should consider when selecting cable for a particular application:Įach component or appliance connected to a circuit will have a current draw associated with its operation and it is important that the cable supplying power to these is capable of carrying the normally expected current, plus a margin of safety. This is the resistance of the conductor expressed in Ohms per metre ( W/m) and is important when determining voltage drop (see below)Īdditional specifications may include a working temperature range and resistance to certain chemicals such as acids, fuels, oils etc. This is the diameter including the insulation and usually expressed in mm. So 28/0.30 means that there are 28 strands, each with a diameter of 0.30mmĮxpressed in Amperes (Amps or A) and is the maximum continuous or 'working' current that the cable can safely carry. This can often lead to confusion so just remember that the main specification for a cable will be its conductor's cross-sectional area and cable will never be referred to by its diameter alone.Įxpressed as the number of conductors of a given diameter. You will sometimes see cable described as 1mm or 2mm cable without the ² sign but it is important to note that this does not mean the diameter of the cable. The same applies when crimping the conductor to a terminal - make sure all strands are contained within the crimp or the current carrying capacity will be reduced.Ĭables are generally specified using the following properties: PropertyĮxpressed in mm² and describes the tota l cross sectional area of the copper conductor. This will reduce the total cross-sectional area of the conductor at that point and consequently also reduce the current carrying capacity of the cable. Tip : Be careful when stripping the insulation from a stranded cable that you don't accidentally remove any of the copper strands. The difference in cross sections is shown in the diagram below: This type of cable is (unsurprisingly) known as 'stranded' cable and provides much more flexibility, which means improved resistance to work hardening making it better suited to use in vehicles. This problem is overcome by manufacturing the core from many small diameter strands of copper wire to make up the desired cross-sectional area, rather than using a single wire.
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This work hardening causes the metal to become more brittle which could, over a long period of time, cause a stiff, solid conductor to crack and fail. The reason for this is that copper, although quite ductile, is susceptible to 'work hardening' when subject to vibration and mechanical shock, such as is experienced when installed in a vehicle. You will have noticed that cable used in vehicle electrical systems is very flexible in contrast to the cable you would find in the the walls of your home, which is fairly stiff.
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