2 Determining Conductohehshdvdvshshr size.pptx

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1. Selecting cross-sectional area of conductors (copper conductors) 1.0 (0.8) sq. mm for flexible cables of copper conductors for extra low voltage (Bell, …..) 1.5 sq. mm for cables and insulated copper conductors for lighting circuits; 2.5 sq. mm for cables and insulated copper conductors for socket out lets; 2.5 sq. mm also can be for lighting circuit Selecting of protective device and conductor size should be decided scientifically, reasonable and supported by procedure of selecting of ACB rating and cable size.

2. Selecting Automatic Circuit Breaker (ACB) and Conductor (Cable) Size of a load Maximum Circuit Loading - For loads other than motors, the total load on consumer's service, feeder or a branch circuit shall be 80% .of the circuit rating.

3. Selecting ACB and Conductor (Cable) Size of a load Ampacity – it is current carrying capacity of conductors. Depending on the type of cable, different types of cables or conductors they have different values of ampacity. (Table B1-B30 EBCS).

4. Correction Factors Group Correction Factors ,cg, – If the number of wires which are installed in the same conduit, truck or tube, its ampacity decreases (because of heat) . For this reason we should have to consider group correction factor of a conductor (Table A1 – A3 of EBCS). Ambient Temperature Correction Factors ,ca , - The ampacity of a conductor also depends on the temperature of the place. For this reason we should have to consider Ambient Temperature Correction Factor, ca, of a conductor. (Table A4– A6 of EBCS).

5. Procedure of Selecting Cable Size STEP 1: Determi n e the Design Current , Ib For Single phase pure resistive design or circuit, the design current, Ib : I b =P/V For Single phase with inductance design or circuit, the design current, Ib : I b =P/(V* pf ) For Three phase circuit, the design current, Ib: I b =P/( √ 3* V* pf )

6. Procedure of Selecting Cable Size STEP 2: Select the rating of the protection , In Rated Current of Protective Device Against Over Current, In – is t he nominal current or current setting of the device protecting the circuit against over current. In The total load on consumers service or branch circuit shall be 80% of the circuit rating. The rating of the protection (ACB,...), In, should be greater than or equal 1.25 times design current, Ib In > or = 1.25 Ib

7. Procedure of Selecting Cable Size STEP3 : Select the relevant correction factors (CFs) Group Correction Factor (cg) – When 2 or more than two conductors are installed in the same PVC or conduit, we should have to consider group correction factor(cg). Ambient Temperature Correction Factor,(ca) – It is a correction factor due to temperature of surrounding area

8. Procedure of Selecting Cable Size STEP 4 : Determine tabulated current carrying capacity, It, of conductor or cable Tabulated Current carrying capacity will be determined by the following expression dividing the value of rating of protection, In by the relevant C orrection F actors cg & ca I t = In/ (cg*ca) . Tabulated current indicates the minimum value of current that the cable should be carry.

9. Procedure of Selecting Cable Size STEP 5: C hoose a cable size to suit It To select conductor or cable size, we sholud have to consider that Design circuit (dc, single phase, three phase) Type of cables (Single core, multi core, with or without sheath, armoured or non-armoured, ...) Installation method (in an insulated wall, on enclosed in coduit on a wall or ceiling, on cable tray,.......) Iz > or = It

10. Procedure of Selecting Cable Size STEP 6: Check the voltage drop . The size of conductors shall be, such that the voltage drop between the supply terminal and. fixed current-using equipment shall not exceed 4% of the nominal voltage of the supply line, when the conductors are carrying the full, load current. [EBCS p96]. If the voltage drop is greater than 4%, select the next size of conductor and check the voltage drop

11. Procedure of Selecting Cable Size SUMMARY 1) Determin e the Design Current , Ib 2) Select the rating of the protection, In 3) Select the relevant correction factors (CFs) 4) Determine tabulated current carrying capacity, It, of conductor or cable. 5) C hoose a cable size to suit Iz 6) Check the voltage drop.

12. EXAMPLE 1 Select cable size and Protective Device of stove (Injera Mittad ) , P = 3.5 KW , V = 220 V. c a =1, c g = 0.8 , Type of conductor: Single Core, non armoured. Distance between MDB and load is 20 m. Installation type in closed in wall (flush)

13. EXAMPLE 2 15 lamps which have 100 W each are installed in a 230V single phase circuit, select ACB and conductor size. c a =0.91, c g = 0. 8 Distance from MDB to the last lamp = 25 M Type of cable Single Core, non sheathed, non armoured.

14. EXAMPLE 3 Select cable size and Protective Device of water heater with the following data, P = 2 .5 KW , V = 230 V. c a =0.91, c g = 0. 8 Distance from MDB to heater = 40 M Type of cable Single Core, non sheathed, non armoured.

15. EXAMPLE 4 Select cable size and Protective Device of An Electric Furnace which has the following data, P = 1.6 KW , V = 220 V. c a =0.91, c g = 0.85 Length from MDB = 70 M Type of cable Single Core