Electrical Thumb Rules You MUST Follow (Part 1)

Electrical Thumb Rules You MUST Follow (Part 1)

Electrical Thumb Rules For:

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

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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)

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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)

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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.

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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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Lighting Arrestor

Arrestor have Two Rating:

  1. MCOV=Max. Continuous Line to Ground Operating Voltage.
  2. Duty Cycle Voltage. (Duty Cycle Voltage > MCOV).

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Transformer

  • Current Rating of Transformer = KVA x 1.4
  • Short Circuit Current of T.C /Generator = Current Rating / % Impedance
  • No Load Current of Transformer =< 2% of Transformer Rated current
  • Capacitor Current (Ic) = KVAR / 1.732xVolt (Phase-Phase)
  • Typically the local utility provides transformers rated up to 500kVA For maximum connected load of 99kW,
  • Typically the local utility provides transformers rated up to 1250kVA For maximum connected load of 150kW.
  • The diversity they would apply to apartments is around 60%
  • Maximum HT (11kV) connected load will be around 4.5MVA per circuit.
  • 4No. earth pits per transformer (2No. for body and 2No. for neutral earthing),
  • Clearances, approx.1000mm around TC allow for transformer movement for replacement.

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Diesel Generator

  • Diesel Generator Set Produces = 3.87 Units (KWH) in 1 Litter of Diesel.
  • Requirement Area of Diesel Generator = for 25KW to 48KW = 56 Sq.meter, 100KW = 65 Sq.meter.
  • DG less than or equal to 1000kVA must be in a canopy.
  • DG greater 1000kVA can either be in a canopy or skid mounted in an acoustically treated room
  • DG noise levels to be less than 75dBA at 1 meter.
  • DG fuel storage tanks should be a maximum of 990 Litter per unit. Storage tanks above this level will trigger more stringent explosion protection provision.

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Current Transformer

Nomenclature of CT:

  • Ratio: input / output current ratio
  • Burden (VA): total burden including pilot wires. (2.5, 5, 10, 15 and 30VA.)
  • Class: Accuracy required for operation (Metering: 0.2, 0.5, 1 or 3, Protection: 5, 10, 15, 20, 30).
  • Accuracy Limit Factor:
  • Nomenclature of CT: Ratio, VA Burden, Accuracy Class, Accuracy Limit Factor.Example: 1600/5, 15VA 5P10  (Ratio: 1600/5, Burden: 15VA, Accuracy Class: 5P, ALF: 10)
  • As per IEEE Metering CT: 0.3B0.1 rated Metering CT is accu­rate to 0.3 percent if the connected secondary burden if imped­ance does not exceed 0.1 ohms.
  • As per IEEE Relaying (Protection) CT: 2.5C100 Relaying CT is accurate within 2.5 percent if the secondary burden is less than 1.0 ohm (100 volts/100A).

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Quick Electrical Calculation

1HP = 0.746KWStar Connection
1KW = 1.36HPLine Voltage = √3 Phase Voltage
1Watt = 0.846 Kla/HrLine Current = Phase Current
1Watt = 3.41 BTU/HrDelta Connection
1KWH = 3.6 MJLine Voltage = Phase Voltage
1Cal = 4.186 JLine Current = √3 Phase Current
1Tone = 3530 BTU
85 Sq.ft Floor Area = 1200 BTU
1Kcal = 4186 Joule
1KWH = 860 Kcal
1Cal = 4.183 Joule

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About Author //

author-pic

Jignesh Parmar

jiguparmar - Jignesh Parmar has completed his B.E(Electrical) from Gujarat University. He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 12 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Assistant Manager at Ahmedabad,India. He has published numbers of Technical Articles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.



24 Comments


  1. rahul
    Oct 15, 2014

    what is maximum distance between VCB panel and transformer at 11 kv voltage supply using HT cable of size 3cX95sqmm. Is it ok that transformer is installed at 150 meter away from VCB panel.


  2. JOYDEEP KUNDU
    Oct 06, 2014

    SIR I JUST WANT TO KNOW THAT IN 3PHASE 4 WIRE ELECTRICAL DISTRIBUTION SYSTEM WHY THE RADIUS OF NEUTRAL WIRE IS LESS THAN OTHER WIRE?


  3. balu
    Oct 01, 2014

    Dear sir,
    Please send me your mobile number because I talk personally with you so I request you please give me your mobile number

    Thanks&Regards
    Vagh Balu R
    7698260140


    • Huj
      Oct 03, 2014

      Dear Jebalu, are ju crazy. There is inernet, no personalities!


  4. Arnel
    Sep 22, 2014

    what is the meaning of 2×6+4 for wire sizing?


  5. R Knight
    Sep 21, 2014

    What do you reckon on SWA 3 phase and Neutral Cable size?

    Circuit 1
    primary fuses, 16A and 20A = 36A
    However, connected load is 9kVA and 3kVA = 17.3A and average consumption is 3kVA and 2kVA respectively, giving ~ 8A

    Circuit 2
    Primary fuses, 40A, 16A, 16A and 16A = 88A
    However, nominal current is 33A, 6A, 8A and 11A, giving ~ 58A

    Circuit 2
    11kw fan with a 32amp supply

    on Cable run of 150m


  6. kushvendra tyagi
    Sep 17, 2014

    If the load requirement is 1200 kva , what kind of independent feeder should be considered , 11th or 33 kv


  7. Glenn
    Sep 03, 2014

    Hi need clarification to choose between 3 core, 3.5 core and 4 core for cable selection application. I know we choose 3.5 core and above in case of unbalanced loads any advice on making selection more clear is greatly appreciated


  8. Thuong.nguyen
    Jul 12, 2014

    1 Phase Motor draws Current = 7Amp per HP.
    Full Load Current of 1 Phase Motor = HPx6
    I think full load current of 1 phase is too high, for examples, in catalog of manufacture, current of 1 phase motor 4HP only about 12A. Moreover, please clarify why percent of draw current and full load current is small (7/6), as i know that factor is 3/1.( In my country, there used 220V for line voltage and 380V for phase voltage). The same question for 3 phase motor.


  9. Muhammad Bilal
    Apr 15, 2014

    What is the difference between line and phase?


    • Ganesh Kaneriya
      Jul 18, 2014

      line to line voltage means voltage between 2 line
      and Phase voltage means voltage between 1 line and neutral

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