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How to Find out Number of Sub Circuits in Electrical Load?

Sub Circuits

Disadvantage of single electrical circuit

  • The supply to the load circuit is given through single switch as shown in the Figure A. 
  • The main disadvantage of such wiring is that if there is short circuit, fuse burnt out and supply is totally cut off. 
  • If there are large number of points through main distribution board, large voltage drop in the line resulting low voltage available at last point of the load. 
  • This problem is eliminated by making small sub circuit of wiring in the building. These small circuits are called as sub circuits.
various load connected to distribution board

Why Sub circuit is necessary in the electrical wiring?

  • When there is a fault in one sub circuit, that part of sub circuit is isolated and remaining part of the load gets supply. 
  • Less voltage drop due to sub circuit.

How to calculate sub – circuit?

Light and Fan sub circuits
  • A light, fan and 5 Amp three pin socket load is considered in a sub circuit.
  • There are maximum 10 points in light and fan sub circuit as per IE ( Indian Electricity ) rules. 
  • The maximum load for single light and fan sub circuit should not exceed 800 W. 
  • If there are maximum 10 number of fans allowed for separated fan circuit if any. 
  • There are minimum two sub circuit is kept in the residential housing therefore if there is fault in one sub circuit, other sub circuit does not affected and load get supply though other sub circuit.

Power sub circuit
  • The 15 Amp three pin socket load is calculated in the power sub circuit. It includes water heater, AC, refrigerators etc. 
  • The maximum load for power sub circuit should not exceed 3000 W. If the power load exceed above 3000 W, separate power circuit is taken.
A residential house consists of following loads. Find out number of sub circuits.
No. of T5 tube light 48” = 20  
No. of ceiling fan = 8
No. of Three pin ( 5 amp ) socket = 10
No. of Three pin ( 15 amp ) socket = 6 
Solution :

Type of Electrical load

T5 Tube light 48”

Ceiling fan

3 pin socket ( 5 Amp )

3 pin socket ( 15 Amp )


Light and Fan circuit sub circuit load :
( 20 * 28 ) + ( 8 * 60 ) + ( 10 * 100 )
560 + 480 + 1000
2040 Watt
Number of Light and Fan sub circuit = 2040 / 800 = 2.55 = 3
Power sub circuit load : ( 6 * 1000 ) = 6000 Watt
Number of Power sub circuit = 6000 / 3000 = 2
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