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Electrical Calculations



Eight rules to follow:
  1. Standard Size of Transformer (IEEE/ANSI 57.120)
  2. Standard Size of Motor (HP)
  3. Approximate RPM of Motor
  4. Motor Line Voltage
  5. Motor Starting Current
  6. Motor Starter
  7. Impedance of Transformer (As per IS 2026)
  8. Standard Size of Transformer
1. Standard Size of Transformer (IEEE/ANSI 57.120)
Single Phase Transformer
Three Phase Transformer
5KVA, 10KVA, 15KVA, 25KVA, 37.5KVA, 50KVA, 75KVA, 100KVA, 167KVA, 250KVA, 333KVA, 500KVA, 833KVA, 1.25KVA, 1.66KVA, 2.5KVA, 3.33KVA, 5.0KVA, 6.6KVA, 8.3KVA, 10.0KVA, 12.5KVA, 16.6KVA, 20.8KVA, 25.0KVA, 33.33KVA
3KVA, 5KVA, 9KVA, 15KVA, 30KVA, 45KVA, 75KVA, 112.5KVA, 150KVA, 225KVA, 300KVA, 500KVA, 750KVA, 1MVA, 1.5MVA, 2MVA, 2.5MVA, 3.7MVA, 5MVA, 7.5MVA, 10MVA, 12MVA, 15MVA, 20MVA, 25MVA, 30MVA, 37.5MVA, 50MVA, 60MVA, 75MVA, 100MVA

2. Standard Size of Motor (HP)
Electrical Motor (HP)
1, 1.5, 2, 3, 5, 7.5, 10, 15, 20, 30, 40, 50, 60, 75, 100, 125, 150, 200, 250, 300, 400, 450, 500, 600, 700, 800, 900, 1000, 1250, 1250, 1500, 1750, 2000, 2250, 3000, 3500 and 4000

3. Approximate RPM of Motor
HP
RPM
< 10 HP
750 RPM
10 HP to 30 HP
>600 RPM
30 HP to 125 HP
500 RPM
125 HP to 300 HP
375 RPM

4. Motor Line Voltage
Motor (KW)
Line Voltage
< 250 KW
>440 V (LV)
150 KW to 3000KW
2.5 KV to 4.1 KV (HV)
200 KW to 3000KW
3.3 KV to 7.2 KV (HV)
1000 KW to 1500KW
6.6 KV to 13.8 KV (HV)

5. Motor Starting Current
Supply
Size of Motor
Max. Starting Current
1 Phase
< 1 HP
6 X Motor Full Load Current
1 Phase
1 HP to 10 HP
3 X Motor Full Load Current
3 Phase
10 HP
2 X Motor Full Load Current
3 Phase
10 HP to 15 HP
2 X Motor Full Load Current
3 Phase
> 15 HP
1.5 X Motor Full Load Current

6. Motor Starter
Starter
HP or KW
Starting Current
Torque
DOL
<13 HP (11KW)
7 X Full Load Current
Good
Star-Delta
13 HP to 48 HP
3 X Full Load Current
Poor
Auto TC
> 48 HP (37 KW)
4 X Full Load Current
Good/ Average
VSD
0.5 to 1.5 X Full Load Current
Excellent
Motor > 2.2KW Should not connect direct to supply voltage if it is in Delta winding




Electrical Thumb Rules For:
  • Cable Capacity
  • Current Capacity of Equipment
  • Earthing Resistance
  • Minimum Bending Radius
  • Insulation Resistance
  • Lighting Arrestor
  • Transformer
  • Diesel Generator
  • Current Transformer
  • Quick Electrical Calculation
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 Distance = 18 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.
Lighting Arrestor
Arrestor have Two Rating:
  1. MCOV=Max. Continuous Line to Ground Operating Voltage.
  2. Duty Cycle Voltage. (Duty Cycle Voltage > MCOV).
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.
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.
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).



Quick Electrical Calculation


1HP = 0.746 KW
Star Connection
1KW = 1.36 HP
Line Voltage = √3 Phase Voltage
1Watt = 0.846 Kla/Hr
Line Current = Phase Current
1Watt = 3.41 BTU/Hr
Delta Connection
1KWH = 3.6 MJ
Line Voltage = Phase Voltage
1Cal = 4.186 J
Line 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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