## 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:**

- 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 |

R Knight

What do you reckon on SWA 3 phase and Neutral Cable size?

Circuit 1

primary fuses, 16A and 20A = 36A

However, connected load is 9kVA and 3kVA = 17.3A and average consumption is 3kVA and 2kVA respectively, giving ~ 8A

Circuit 2

Primary fuses, 40A, 16A, 16A and 16A = 88A

However, nominal current is 33A, 6A, 8A and 11A, giving ~ 58A

Circuit 2

11kw fan with a 32amp supply

on Cable run of 150m

kushvendra tyagi

If the load requirement is 1200 kva , what kind of independent feeder should be considered , 11th or 33 kv

Glenn

Hi need clarification to choose between 3 core, 3.5 core and 4 core for cable selection application. I know we choose 3.5 core and above in case of unbalanced loads any advice on making selection more clear is greatly appreciated

Thuong.nguyen

1 Phase Motor draws Current = 7Amp per HP.

Full Load Current of 1 Phase Motor = HPx6

I think full load current of 1 phase is too high, for examples, in catalog of manufacture, current of 1 phase motor 4HP only about 12A. Moreover, please clarify why percent of draw current and full load current is small (7/6), as i know that factor is 3/1.( In my country, there used 220V for line voltage and 380V for phase voltage). The same question for 3 phase motor.

Muhammad Bilal

What is the difference between line and phase?

Ganesh Kaneriya

line to line voltage means voltage between 2 line

and Phase voltage means voltage between 1 line and neutral