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To know how much current a load is drawing, you must know Ohm's Law. You must also need to know what the voltage is and the resistance of the load.
IMPROVED ANSWER 1 / DATE:25.05.10:
Before discussing this question, I would like to share few things regarding the information, I got from the webs. When I search the different kind of answers for this question, I finally satisfied from sir Kevin Boone's (UK) explanation. I can share those information.
Topic is selection of cable, selection of protection device for a circuit etc.
Several steps are there, we can see one by one:
Step 1: Find the nominal current for the given load. For eg., if you choose 5kw load, the nominal current for such load is 5000/230 = 21.7 ~ 22A.
Step 2: From the nominal current, we can choose a MCB of rating 30A. (should be greater than the nominal current, so it could be next available MCB)
Step 3: Selection of cable should withstand the nominal current. In which one should keep in mind that we should consider the type of installation of the cable. Because the rating of current for the cable can be vary according to the installation. For eg., installing the cable inside the wall or trunk or any other strong insulation or through the open air. The cable which installed open in the air can withstand more current when compare to the cable which installed inside the wall. So for simplicity, we can assume the cable is installed inside the wall (generally this kind of installation is followed by most of the engineers). When I go through the manufacturer table 4mm 2 cable installed inside the wall can carry current up-to 25A. So it is suitable for our load.
Step 4: Now for our 5kw load, 30A MCB & 4mm 2 size cable are selected. But keep in mind the surrounding temperature should be consider. Since the current rating for the cable could be vary to the temperature. Normally the installation is designed for 35 degree to 40 degree Celsius. Let us consider the ambient temperature is 35 degree. So under 35 degree Celsius the 4mm 2 cable's correction factor is 0.94 (IEE wiring Regulations 2001). So the current rating of the cable under 35 degree Celsius is 25 * 0.94 = 23.5A. It is satisfied to withstand for our 5kw load.
Step 5: Voltage drop should be taken in account. Normally voltage drop could be up-to 9.2 V (4% from the rated voltage) is allowed. Let us assume from the distribution board to the load, the total cable length is 20m. From IEE wiring Regulation table 4D2B: the resistance of line & neutral per meter is 0.011 ohms. So for 20m length it will be 0.011 * 20 = 0.22 ohms. The voltage drop is 22A * 0.22 ohms = 4.84V. So it is well suited under 9.2V. Hence our selection is correct.
CONCLUSION: For our selected load 5kw, 4mm 2 cable with 30A MCB is the good choice.
NOTE: Comments, Correction, modifications, improved answers, related questions & answers are most welcome.
What else can I help you with?
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