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Cable Sizing Problem


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my exams is tmr and i do not understand how to do this please help... thanks..





B1(B) A spray painting factory receives a three-phase 400V 50 Hz supply for its daily operation. It has recently bought a three-phase star-connected blower rated at 9kW and pf of 0.8 and it is to be wired in single-core pvc-insulated copper conductor cables. It is installed in metal trunking (35m), and grouped with two other similar circuits. The ambient temperature is 350 C. Determine:




(i) the design current, hence the nominal rating of the circuit breaker


Standard circuit breaker rating: 6A, 10A, 15A, 20A, 25A, 30A, 40A, 50A. (Ans: Ib = 16.24, In = 20A)



(ii) the suitable size of the cable (Ans: 6mm2)



(iii) the actual voltage drop and check whether the cable size selected can meet the voltage drop requirement assuming half of the allowable voltage drop is allocated for this 35m run. Re-select cable size if necessary. (Ans: Vd = 5.4V, acceptable as it is less than half of 4% of 400V)



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