Electrical Useful Equations
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.
- Short Circuit Level of Cable in KA (Isc)=(0.094xCable Dia in Sq.mm)/√ Short Circuit Time (Sec)
- Cable Voltage Drop(%)=(1.732xcurrentx(RcosǾ+jsinǾ)x1.732xLength (km)x100)/(Volt(L-L)x Cable Run.
Size of Cable according to Short circuit (for 11kV,3.3kV only)
- Short circuit verification is performed by using following formula:
- Cross Section area of Cable (mm2)S = I x√t / K
- Where:
- t = fault duration (S)
- I = effective short circuit current (kA)
- K = 0.094 for aluminum conductor insulated with XLPE
- Example: Fault duration(t)= 0.25sec,Fault Current (I) = 26.24 kA
- Cross Section area of Cable = 26.24 x √ (0.25) / 0.094= 139.6 sq. mm
- The selected cross sectional area is 185 sq. mm.
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 Volts
- 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=12xDia of Cable.
- Minimum Bending Radius for HT Power Cable=20xDia of Cable.
- Minimum Bending Radius for Control Cable=10xDia 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).
- Protection radius of Lighting Arrestor = √hx (2D-h) + (2D+L). Where h= height of L.A, D-distance of equipment (20, 40, 60 Meter), L=Vxt (V=1m/ms, t=Discharge Time).
- Size of Lighting Arrestor= 1.5x Phase to Earth Voltage or 1.5x (System Voltage/1.732).
Transformer:
- Current Rating of Transformer=KVAx1.4
- 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.
- Fault Level at TC Secondary=TC (VA) x100 / Transformer Secondary (V) x Impedance (%)
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 @ 1meter.
- 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).
Others:
- For Sinusoidal Current : Form Factor = RMS Value/Average Value=1.11
- For Sinusoidal Current : Peak Factor = Max Value/RMS Value =1.414
- Average Value of Sinusoidal Current(Iav)=0.637xIm (Im= Max.Value)
- RMS Value of Sinusoidal Current(Irms)=0.707xIm (Im= Max.Value)
- A.C Current=D.C Current/0.636.
- Phase Difference between Phase= 360/ No of Phase (1 Phase=230/1=360°,2Phase=360/2=180°)
- Most Economical Voltage at given Distance=5.5x√ ((km/1.6)+(kw/100))
- Maximum Voltage of the System= 1.1xRated Voltage (Ex. 66KV=1.1×66=72.6KV)
- Spacing of Conductor in Transmission Line (mm) = 500 + 18x (P-P Volt) + (2x (Span in Length)/50).
- Load Factor=Average Power/Peak Power
- If Load Factor is 1 or 100% = This is best situation for System and Consumer both.
- If Load Factor is Low (0 or 25%) =you are paying maximum amount of KWH consumption. Load Factor may be increased by switching or use of your Electrical Application.
- Demand Factor= Maximum Demand / Total Connected Load (Demand Factor <1)
- Demand factor should be applied for Group Load
- Diversity Factor= Sum of Maximum Power Demand / Maximum Demand (Demand Factor >1)
- Diversity factor should be consider for individual Load
- Plant Factor(Plant Capacity)= Average Load / Capacity of Plant
- Fusing Factor=Minimum Fusing Current / Current Rating (Fusing Factor>1).
- Voltage Variation(1 to 1.5%)= ((Average Voltage-Min Voltage)x100)/Average Voltage
- Ex: 462V, 463V, 455V, Voltage Variation= ((460-455) x100)/455=1.1%.
- Current Variation(10%)= ((Average Current-Min Current)x100)/Average Current
- Ex:30A,35A,30A, Current Variation=((35-31.7)x100)/31.7=10.4%
- Motor Full Load Current= Kw /1.732xKVxP.FxEfficiency
Quick Electrical Calculation | |
1HP=0.746KW | Star Connection |
1KW=1.36HP | 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 |
Cable Coding (IS 1554):( A2XFY / FRLS / FRPVC / FRLA / PILC)
A | Aluminium |
2X | XLPE |
F | Flat Armoured |
W | Wire Armoured |
Y | Outer PVC Insulation Sheath |
W | Steel Round Wire |
WW | Steel double round wire Armoured |
YY | Steel double Strip Armoured |
FR | Fire Retardation |
LS | Low Smoke |
LA | Low Acid Gas Emission |
WA | Non Magnetic round wire Armoured |
FA | Non Magnetic Flat wire Armoured |
FF | Double Steel Round Wire Armoured |