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| mttee2:
I am given a task to convert a Machine's Switchboard from Relays to PLC and improve it's performance. I am still new in this field and facing some high amps motors. I searched google and only received brief explaination on wiring standards not how to choose the right wire size. What is the safety factor practiced? At what temperature the copper wire will melt? What is the resistivity on standard copper wire? ??? |
| GeekGirl:
Any decent rule book (in Au we use the AS3000) for wiring in structural systems eg house, office etc. For in equipment wiring you really need to look at the cable charts for the cable you wish to use, as the current capacity is not derated (as it is for structural). All suppliers have a brochure (some even have pdf's on their site) that lists the ratings of the cable, it may be able to handle say 100A but if you need UL compliance then it may only do 80A etc. If you need a very low voltage drop then you will need a larger cable than otherwise needed for the current capacity. |
| Erik:
Perhaps this site can give you some information: http://www.powerstream.com/Wire_Size.htm |
| Simon:
if your lookig to play very safe and have it last years go for 5-8 A/mm2 |
| Zero999:
Deciding the crossectional area for a high current cable isn't always easy. It depends on many factors: current, maximum acceptable voltage drop, conductor material, ambient temperature, maximum temperature rating of electrical insulation and surrounding materials, the number of neighbouring conductors in the conduit/sheath and the thermal insulation of the surrounding material. For a start answer the following questions: How much current are you drawing? Give the typical DC/RMS figure, not the surge. What's the maximum acceptable voltage drop? Are the cables well ventilated? Low voltage high current cables are, in many ways more dangerous than high voltage low current cables because the amount of heat given off can easily start a fire. It's very easy to get it wrong, for example you buy a 25m length of PVC insulated 1.25mm² mains cable which is rated to 13A. You put a plug on one end and a socket on the other end to make your own extension lead. It's too long for some applications so you coil it, you then use it to power a heater rated at 13A and after a couple of hours of being left on the cable melts and catches fire. The cable should have been derated when coiled because the heat couldn't escape causing it to become dangerous. You should have used a heavier duty cable with a higher temperature rating and larger crossectional area if you wanted to coil it and power a 13A heater. A good section of the IEE wiring regulations is devoted to choosing the right type of cable for the job: you should look at the regulations relevant to your jurisdiction. |
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