Thumb Rules for Electrical Fitment
mandatory for any project
list of thumb rule listed herewith
- Cable Capacity
- Current Capacity of Equipment
- Earthing Resistance
- Minimum Bending Radius
- Insulation Resistance
- Lighting Arrestor
- Diesel Generator
- Current Transformer
- Quick Electrical Calculation
- Contactors Types
- 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
- 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 for Single Pit = 5Ω, Earthing Grid = 0.5Ω
- 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)
Thing to Know
- As per NEC 1985 Earthing Resistance should be < 5Ω.
- Creepage in the design of electrical and electronics equipment clearance is the air distance between two electrical contacts or conductors. and conductor less than that distance will result in arc.
- 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 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.
Thing to Know
- 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.
- Arrestor have Two Rating:
- MCOV=Max. Continuous Line to Ground Operating Voltage.
- Duty Cycle Voltage. (Duty Cycle Voltage > MCOV).
- 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.
Thing to Know
- 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 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.
Nomenclature of CT: Ratio, VA Burden, Accuracy Class, Accuracy Limit
- 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:
Example: 1600/5, 15VA 5P10 (Ratio: 1600/5, Burden: 15VA, Accuracy Class: 5P, ALF: 10)
Thing to Know
- 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).
|AC1||Non Inductive Load or Slightly Inductive Load|
|AC2||Slip Ring Motor, Starting, Switching OFF|
|AC3||Squirrel Cage Motor|
|AC4,AC5,AC5a, AC5b,AC6a||Rapid Start & Rapid Stop|
|AC 5a||Auxiliary Control circuit|
|AC 5b||Electrical discharge Lamp|
|AC 6a||Electrical Incandescent Lamp|
|AC 6b||Transformer Switching|
|AC 7a||Switching of Capacitor Bank|
|AC 7b||Slightly Inductive Load in Household|
|AC 5a||Motor Load in Household|
|AC 8a||Hermetic refrigerant compressor motor with Manual Reset O/L Relay|
|AC 8b||Hermetic refrigerant compressor motor with Automatic Reset O/L Relay|
|AC 12||Control of Resistive Load & Solid State Load|
|AC 13||Control of Resistive Load & Solid State Load with Transformer Isolation|
|AC 14||Control of small Electro Magnetic Load (<72 VA)|
|AC 15||Control of Electro Magnetic Load (>72 VA)|
|110 to 127 Volt||A|
|220 to 240 Volt||B|
|380 to 415 Volt||L|
a few more Rules
- For Motor
Size of Capacitor = 1/3 Hp of Motor ( 0.12x KW of Motor)
- For Transformer
- < 315 KVA 5% of KVA Rating
- 315 KVA to 1000 KVA 6% of KVA Rating
- >1000 KVA 8% of KVA Rating
- 240V < 5 KW (single Phase)
- 415V <100 KVA (three Phase)
- 11KV <3 MVA
- 22KV <6 MVA
- 33KV <12 MVA
- 66KV <20 MVA
- 110KV <40 MVA
- 220KV >40 MVA
Class A 105°C
Class E 120°C
Class B 130°C
Class F 155°C
Class H 180°C
Class N 200°C
|Voltage||IEC (60038)||IEC (6100:3.6)||Indian Elect. Rule|
|ELV||< 50 V|
|LV||50 V to 1 KV||<=1 KV||< 250 V|
|MV||<= 35 KV||250 V to 650 V|
|HV||> 1KV||<= 230 KV||650 V to 33 KV|
|EHV||> 230 KV||> 33 KV|
|As per Electrical Load Demand|
|Up 6W Load demand||1 Phase 230V Supply|
|6W to 100KVA(100KW)||3 Phase 440V Supply|
|100KVA to 2500KVA||11KV,22KV,33KV|
|Sq.ft Area||Required Load (Connected)|
|< 900 Sq.ft||8 KW|
|901 Sq.ft to 1600 Sq.ft||16 KW|
|1601 Sq.ft to 2500 Sq.ft||20 KW|
|> 2500 Sq.ft||24 KW|
|For Flats :100 Sq.ft / 1 KW|
|For Flats USS /TC: 100 Sq.ft / 23 KVA|
|For Domestic Load||500 watt per 100 Sq. foot of the constructed area.|
|For Commercial||1500 watt per 100 Sq. foot of the constructed area|
|Other Common Load||For lift, water lifting pump, streetlight if any, corridor/campus lighting and other common facilities, actual load shall be calculated|
|Staircase Light||in KW = 11 * No of Flats /1000
Quick Electrical Calculation
- 1HP = 0.746KW
- 1KW = 1.36HP
- 1Watt = 0.846 Kla/Hr
- 1Watt = 3.41 BTU/Hr
- 1KWH = 3.6 MJ
- 1Cal = 4.186 J
- 1Tone = 3530 BTU
- 85 Sq.ft Floor Area = 1200 BTU
- 1Kcal = 4186 Joule
- 1KWH = 860 Kcal
- 1Cal = 4.183 Joule
- Line Voltage = √3 Phase Voltage
- Line Current = Phase Current
- Line Voltage = Phase Voltage
- Line Current = √3 Phase Current