I want formula to calculate cable size as per load given in
kw & amp.I searched many sites but didn't right answer.Plz
reply me asap.
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Answer Posted / er. naushad ashraf
Dear See 1 eg. of Cable Designing
From - 5 MVA power T/f LV side
To - 11 kV LA’s
• RMS Symmetrical Short Ckt current = 17.39 kA (As per Tender spec.)
• Fault Clearing Time = 1 sec
• Min. Cable Size = (17.39x1000xsq root 1)/94= 185 sqmm.
• Min. Cable Size = 1-3C 185 sqmm.
• Cable Length = 0.06 Km. (Assumed).
• Ifl = (5000x1000)/(1.734x11000x0.9x0.95) = 307 A.
• Derating Factor = Ca X Cg = .88x1 = 0.88
• Cable sized considered = 3C X 185 sqmm. (XLPE+Al).
• Cable Resistance (Ohm/Km) = 0.21
• Cable Reactance (Ohm/Km) = 0.087
• Cable Ampacity = 330 A.
• Cable Derated Current = 330x0.88 = 290.4 A.
• Min. No. of cable required = 307/290 = 1.0586 = 1 No.
• No. of Cable Runs Selected = 1 No.
• % Voltage Drop = (sqrt 3x307x0.06(0.21x0.9+0.087x0.43)x100)/(1x11000)
= 0.066 % (Acceptable)
| Is This Answer Correct ? | 2 Yes | 2 No |
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1. A 380 / 220 V r.m.s. 3-phase 4-wire circuit carries loads of 8 + j8.1 ohm; 4 + j4.1 ohm; and 2 +j2.1 ohmin the three phases respectively. The length of the circuit is 100 m, and the resistivity of the phase conductors is 0.0005 Ω/m. The neutral conductor is of the same size as the phase conductors. a) Select a reference angle 0for the first phase. Then write down the three phase voltages in both polar format and complex number format. b) Make appropriate assumptions, and take temperature effects on resistance is insignificant. Calculate the currents in the three phases. (Note: Determine conductor resistance, and apply Kirchhoff’s Voltage Law) Based on the result, suggest a main switch rating for the circuit. c) Show your preferred direction of neutral current in a diagram, and then determine its value. (Note: Apply superposition principle, and Kirchhoff’s Current Law) Use its magnitude to show that in this circuit, the unbalanced load may increase the voltage drop in some of the phases. (Note: Also observe the common volt drip in neutral) State your assumptions, if there are any. d) Suddenly the neutral wire of the circuit is broken. Estimate the new currents in the first phase, i.e. Phase L1. State your assumptions, if there are any.
