Electrical Thumb Rules For:
- Cable Capacity
- Current Capacity of Equipment
- Earthing Resistance
- Minimum Bending Radius
- Insulation Resistance
- Lightning 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.
Lightning Arrestor
Arrestor have Two Rating:
- MCOV=Max. Continuous Line to Ground Operating Voltage.
- 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 = 0.746KW | 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 |
Related electrical guides & articles
Perfect
NICE AND PRACTICALLY APPLICABLE METHODS INVOLVED ….THANKS
i learned many things by your post. thank you .
Dear sir,
how to make calculate , cable size 3cx70 sqmm copper pvc insulated double gi strip armoured,
pl clear this point the conductivity of armour shall not less then 75% of the conductivity of larg core
how many copper strip use for the conductivity maintain.
What would be current carring capacity of cable in 3phase system?
Very good info. for all engineers
Dear sir , need overload current carrying capacity for MV cable with trefoil and flat touching condition with different hours.
Generally 1 /2/3/4/6/8/10/12/24 hour run with 60% / 75%/90% & 30% overload condition with respectively above mentioned time durations so please how can calculate in easy format.
Need Cable current carrying capacity for 3 Phase Cable
can we use 2 x 16sq.mm x 4core and 1 x 10sq.mm x 4core power cable instead of 1 x 35sq.mm x 4core cable for the connectivity of Diesel Genset (60 KVA) to ATS panel 20 feet apart.
Please advice…
2x16mm2 4-core cable which is 2 run of 16mm2 4-core cable is more than enough to connect 60kVA generator to ATS panel
giving that the generator current = 86.6A
We Have a powerloom factory…with each one has 330 watts 3 phase induction motor voltage -220 freq-50hz
we have used 4 sq mm for mainline and 2.5sqmm for each motor for 12 motors.
Can We Connect that same 4 sq.mm for another 12 motors or we have to add 4 sq mm wire…Help me with your knowledge
YES.
We bundled conductors instead of single conductors for ac transmission to reduce skin effect.
If you provide two cables 4Cx35 Sq mm for one cable 4Cx35 Sq. mm then the system is sufficient and addition of 4Cx10 Sq mm will make it extra strong.
Yes you can