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
- Transformer
- Diesel Generator
- Current Transformer
- Quick Electrical Calculation
- Contactors Types
Thumb Rules
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
- 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
- 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.
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.
Lighting 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.
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
- 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, 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).
Contactors Specifications
Type of Contactor
| Type | Application |
| 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) |
Contactor Coil Types
| Coil Voltage | Suffix |
| 24 Volt | T |
| 48 Volt | W |
| 110 to 127 Volt | A |
| 220 to 240 Volt | B |
| 277 Volt | H |
| 380 to 415 Volt | L |
a few more Rules
Size of Capacitor for Power Factor Correction
- 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
Voltage Limit (As per CPWD & KEB)
- 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
Insulation Class
Insulation Temperature
Class A 105°C
Class E 120°C
Class B 130°C
Class F 155°C
Class H 180°C
Class N 200°C
Standard Voltage Limit
| 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 |
Standard Electrical Connection
| 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 |
| Above 2500KVA | 66KV |
Electrical Load as per Sq.ft Area
| 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 | |
Contracted Load in case of High-rise Building
| 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 Ex: 200Flat=200×11=2.2KW |
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
Star Connection
- Line Voltage = √3 Phase Voltage
- Line Current = Phase Current
Delta Connection
- Line Voltage = Phase Voltage
- Line Current = √3 Phase Current