(for example : 1sqmm, 4sqmm, 35 sqmm) i need a formula to calculate current by knowing the area of the cable?
The area of a cable is the CMA (circular mil area). The attached link shows calculations for conductors like house wiring. Ampacity varies not only by CMA but material and temperature.
This is not a simple answer. The amount of current that a cable can carry depends on the amount of voltage drop that you can tolerate (mostly an issue in long cables) and the temperature rating of the insulation used on the cable ( the higher the temperature rating, the higher the current that you can run in the cable) and the ability of the cable to loose the heat that is generated from the I^2R drop in the cable. For example, if a cable is in open air with no other heat sources near it, it can accomodate more current before compromising the insulation. If a cable is in a conduit with other cables that are generating heat, then the conductor needs to be increased in size to decrease the heat. The ampacity of the cable is also affected by the duty cycle of the current that the wire is carrying. If the duty cycle of the current is 10% then the wire can carry much more current compared to a current of 100% or continuous. Having said all this, there are tables of ampacities for various sized wires.
Hi Sherlock, You need a copy of BS 7671 The IEE Regulations. By the cable sizes mentioned you must be in the UK. The current carrying capacity of a conductor is a function of its Cross Sectional Area, Length, Method of installation, Is it grouped with other circuits, Ambient Temperature and fuse or CB type. (these are called multipliers) For larger cables the power factor of the load must also be considered. The basic formula assuming no multipliers are used is Volt Drop per ampere per metre X length of run X Design current / 1000 Volt drop is from the tables in BS 7671 Length of run is the cable length design current is the current the cable will have to carry in normal use 1000 is because Vont Drop per Amper per Metre is given in millivolts
