## 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 |

madhu

How to calculate Cable sizes for the Below in thumb rule,

01.Single core cable above 30Sqmm

02.Three phase cable size.

Pls send it with a clear Example

rahul

what is maximum distance between VCB panel and transformer at 11 kv voltage supply using HT cable of size 3cX95sqmm. Is it ok that transformer is installed at 150 meter away from VCB panel.

ali

minimum 6 mtrs

JOYDEEP KUNDU

SIR I JUST WANT TO KNOW THAT IN 3PHASE 4 WIRE ELECTRICAL DISTRIBUTION SYSTEM WHY THE RADIUS OF NEUTRAL WIRE IS LESS THAN OTHER WIRE?

balu

Dear sir,

Please send me your mobile number because I talk personally with you so I request you please give me your mobile number

Thanks&Regards

Vagh Balu R

7698260140

Huj

Dear Jebalu, are ju crazy. There is inernet, no personalities!

Arnel

what is the meaning of 2×6+4 for wire sizing?

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