Monday, June 13, 2016

Electrical Thumb Rules You MUST Follow (Part 1)

Cable Capacity

  • For Cu Wire Current Capacity (Up to 30 Sq.mm) = 6X Size of Wire in Sq.mm
    Ex. For 2.5 Sq.mm = 6×2.5 = 15 Amp, For 1 Sq.mm = 6×1 = 6 Amp, For 1.5 Sq.mm = 6×1.5 = 9 Amp
  • For Cable Current Capacity = 4X Size of Cable in Sq.mm, Ex. For 2.5 Sq.mm = 4×2.5 = 9 Amp.
  • Nomenclature for cable Rating = Uo/U
  • where Uo = Phase-Ground Voltage, U = Phase-Phase Voltage, Um = Highest Permissible Voltage

Current Capacity of Equipment

  • 1 Phase Motor draws Current = 7Amp per HP.
  • 3 Phase Motor draws Current = 1.25Amp per HP.
  • Full Load Current of 3 Phase Motor = HPx1.5
  • Full Load Current of 1 Phase Motor = HPx6
  • No Load Current of 3 Phase Motor = 30% of FLC
  • KW Rating of Motor = HPx0.75
  • Full Load Current of equipment = 1.39xKVA (for 3 Phase 415Volt)
  • Full Load Current of equipment = 1.74xKw (for 3 Phase 415Volt)

Earthing Resistance

  • Earthing Resistance for Single Pit = 5Ω, Earthing Grid = 0.5Ω
  • As per NEC 1985 Earthing Resistance should be < 5Ω.
  • Voltage between Neutral and Earth <= 2 Volt
  • Resistance between Neutral and Earth <= 1Ω
  • Creepage Distance18 to 22mm/KV (Moderate Polluted Air) or
  • Creepage Distance = 25 to 33mm/KV (Highly Polluted Air)

Minimum Bending Radius

  • Minimum Bending Radius for LT Power Cable = 12 x Dia of Cable.
  • Minimum Bending Radius for HT Power Cable = 20 x Dia of Cable.
  • Minimum Bending Radius for Control Cable = 10 x Dia of Cable.
  •  

Insulation Resistance

  • Insulation Resistance Value for Rotating Machine = (KV+1) MΩ.
  • Insulation Resistance Value for Motor (IS 732) = ((20xVoltage (L-L)) / (1000+ (2xKW)).
  • Insulation Resistance Value for Equipment (<1KV) = Minimum 1 MΩ.
  • Insulation Resistance Value for Equipment (>1KV) = KV 1 MΩ per 1KV.
  • Insulation Resistance Value for Panel = 2 x KV rating of the panel.
  • Min Insulation Resistance Value (Domestic) = 50 MΩ / No of Points. (All Electrical Points with Electrical fitting & Plugs). Should be less than 0.5 MΩ
  • Min Insulation Resistance Value (Commercial) = 100 MΩ / No of Points. (All Electrical Points without fitting & Plugs).Should be less than 0.5 MΩ.
  • Test Voltage (A.C) for Meggering = (2X Name Plate Voltage) +1000
  • Test Voltage (D.C) for Meggering = (2X Name Plate Voltage).
  • Submersible Pump Take 0.4 KWH of extra Energy at 1 meter drop of Water.

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