QUICK ELECTRICAL CALCULATION

MACHINE CURRENT CAPACITY


  • Single-phase Motor draws current = 7 A / HP.
  • Three-phase Motor draw current = 1.25 A/ HP.
  • Full load current of Three-phase Motor= HP x 1.5.
  • Full load current of Single-phase Motor= HP x 3.
  • No Load current of Three-phase motor = 30% of full load current.
  • KW rating of Motor= HP x 0.746.
  • Equipments Full load current = 1.39 x KVA (3-Phase 415 V)
  • Equipments Full load current = 1.74 x KW (3-Phase 415 V)


ABOUT CT


 

  1. Rtaio - I/O Current ratio.
  1. Burden(VA):- Total burden including pilot wire.(2.5, 5, 10, 15 and 30 VA)
  1. Class:- Accuracy requird for operation (Metering - 0.2, 0.5, 1 or 3; Protection- 5, 10, 15, 20, 30).
  1. Nomenclature of CT :- Ratio, VA Burden, Accuracy class, Accuracy limit Factor.  Example- 1500/5, 15 VA 5P10 (Ratio- 1500/5, burden- 15 VA, Accuracy class- 5P, ALF - 10)
  1. As per IEEE metering CT :- 0.3B0.1 rated metering CT is accurate to 0.3%, if the connected secondary burden if impedance does not exceed 0.1 ohms.
  1. As per IEEE Relay(Protection) CT:- 2.5C100 Relay protection CT is accurate within 2.5% if the secondary burden is less than 1.0 ohm(100volts/100A)  

CURRENT CARRYING CAPACITY OF CABLE


  • For Copper Wire current capacity (30 sq.mm.) = 6 x wire size in sq.mm.   Example:- 4 sq.mm. wire = 6 x 4= 24 A, and 1.5 sq.mm. wire = 6 x 1.5 = 9 A.
  • For Copper Cable current capacity = 4 x cable size in sq.mm.     Example:- For 10 sq.mm. cable = 4 x 10 = 40 A.
  • Nomenclature for cable rating = Uo/U. where, Uo= Phase-ground voltage, U= Phase-Phase voltage, Um= Highest permissible voltage.


INSULATION RESISTANCE

  • Insulation resistance value for rotating machine = (KV +1) M-Ohm.
  • Insulation resistance value for motor(IS-732) = {(20 x Voltage (L-L))/(1000+(2 x KW)}.
  • Insulation resistance value for equipment's (<1 KV)= Minimum 1 M-Ohm.
  • Insulation resistance value for equipment's (>1 KV)= KV 1 M-Ohm per 1 KV.
  • Insulation resistance value for panel= 2 x KV rating of the Panel.
  • Minimum Insulation resistance value (domestic)= 50 M-Ohm/No. of points.(All electrical points with electrical fitting & plugs). Should be less than 0.5 M-Ohm.
  • Minimum Insulation resistance value (Commercial)= 100 M-Ohm/No. of points.(All electrical points with electrical fitting & plugs). Should be less than 0.5 M-Ohm.
  • Test voltage (AC) for Megger = (2 x Name plate voltage)+1000.
  • Test voltage (DC) for Megger = (2 x Name plate voltage).
  • Submersible Pump take 0.4 Kwh of extra energy for 1 meter water drop. 


BASIC EARTHING RESISTANCE


  1. Earthing resistance for single pit = 5 M-Ohm, Earthing Grid = 0.5 Ohm.
  1. As per NEC 1985 Earthing resistance should be <5 Ohm.
  1. Resistance Between Neutral & Earth <= 1 Ohm.
  1. Voltage Between Neutral & Earth <= 2 Volt.
  1. Creepage distance = 18 mm to 22 mm/ KV(for medium air pollution)
  1. Creepage distance = 25 mm to 33 mm/ KV(for highly air pollution)

Basic Electrical
1HP
0.746 KW
1KW
1.36 HP
1WATT
0.846 Kla/Hr
1WATT
3.41 BTU/Hr
1KWH
3.6 MJ
1Cal
4.186 J
1Tone
3530 BTU
1Kcal
4186 Joul
1KWH
860 Kcal
1Cal
4.183 Joule


  1. STAR CONNECTION: - Line voltage = √3*Phase voltage, Line current = Phase current.
  2. DELTA CONNECTION: - Line voltage = Phase voltage, Line current = √3*Phase current.