## 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 |

Rholand Forescal

Sir please tell me why 1hp single phase motor is 7ampere in single phase motor chart in ampere. But if we use the formula in ohms law to get ampere in single phase I = hp / VxExPF we only get only 4.23 ampere when voltage is 230v power factor is 0.9 and i used 0.85 efficiency.. Please teached me how to prove the electrical thumb rules.

Isuru

what is the difference between “Cu Wire Current Capacity” and Cable “Cable Current Capacity”?

gurpreet

Plz tell me how to calculate size of generator set for a specific submersible pump set i.i for specific hp motor? Thanks alot

Swapnendu Sil

pls tell me sir what cable needed for 32amp industrial socket and how we can determine if we need aluminium cable or cu cable or need 3 phase or single phase in general?

Gopi nath

Super sir

rudramurthy

7/16 cable size meanes what is the sqmm cable size in 3 phase supply 30kva load

Balu B Nair

wonderful informative portal….!!!

Pradeep Rathi

A perfect compilation. Thanks Jignesh. (Author)

shah faisal

how i find cable for a suitable load like a if load is 23 which cable is-perfect for this load and how i find by formula ?

MD kashif

Plz tell me how to tabulate the armoured cable size for a particular load.

gurjit

i am living in FLAT WITH 3 BED ROOMS ONE DRAWING ROOM AND HAVE 5 ACs 2 TR AND WASHING MACHINE INDUCTION COOKER AND MICROWAVE WITH GEYSERS 25 LITERS IN BED ROOMS AND KITCHEN AND THE LOAD GRANTED IS 8KW.

THE EARTHING WIRE FROM MCB BOARD ON FIRST FLOOR TO METER ON GF IS OF 1 MM SQUARE .

IS THIS ENOUGH OR SHOULD I REPLACE WITH HIGHER CAPACITY

Rehan

No Its not enough

If you are having a single phase meter use 1 no of 4 sq mm Cu wire

and if you are having a 3 phase meter use 2 nos of 4 sq mm wires

ankesh mishra

Tell me the short cut for calculating load for a flat

Deepak Kumar

Please tell me how to find current rating of the conductor

mohan dass

Can we use 2 run of 25 mm sq instead of 50 mm sq cable of Cu. If yes then tell me full details and If not then tell me also full details.

manish chovatia

How many distance between transformer and diesel genrator as per ie rules

sowmya

I have a current transformer with 1 main meter outside the building and a control panel having 5 CT meters with black round coils inside the building. I want to know, A) when current pass through the Transformers, will the current be reduced or be the same and after that B) when current pass through the CT meters with black round coils, will the current be reduced or be the same. C) in the above process will the reading of the main meter be same or different, from the total reading of all the 5 CT meters.

swapneswar pradhan

What is the simple formula for electrical, current , voltage

swapneswar pradhan

Thanks sir u help for electrical problem .

omar

we known low pf increase line current so does it increase kwh reading in metering…but pf varation is related also actually totally in reactive part…so what is the impact of pf in metering.

Bassey Friday

Please .I have a 500KVA what % of load should I load it considering the fact that max load is about 700Amp

Etop Inyang

Is your generator sets for emergency supply or prime supply? For Prime you can load up to 100% and 70% for emergency generator sets.

mdmukhter

I want to take part in to the technical debates and weekly technical discussion.

abdelsalam mikky

How diesel engine frame should be grounded (I think shall be two earthing diagonal points ) according to NFPA 70 (NEC), could you please provide the supporting standard cod of this

Edvard

Every Genset has predefined points to ground engine frame. Don’t forget to ground gas reservoir if it exist.

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?

kalpesh

Because in 3 phase 4 wire system only unbalance current pass from the nutural so current is never higher then half of the phase current,so purpose of aluminium saving its samller size then phase in power cable of higher size

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

neeraj kumar

thanks 4 d info it was really

help full we all must share

these things so we could get best of

knowledge

Joe C.

Please I have some business proposals for the author.

Please get in touch with me at jescil@hotmail.com

Thanks

Annu

Thanks Dear sharing Such valuable data

nmr

sirs, for aluminium cables and wires current carrying was not indicated in the thumb rules.

aakifnouman

Thank You For This and for those u cant understand this is only a thumb rule just to understand and remember

sumitfloyd

On the section current capacity of Equipments, it is written

# 1 Phase Motor draws Current = 7Amp per HP.

# 3 Phase Motor draws Current = 1.25Amp per HP.

I am not quite clear on these points. Please clarify. Does it mean if we have 3HP single phase motor then it will draw 21Amp current throughout its operation or during its starting?

kriloter

These rules could be a little bit tricky unless you don’t know further connections …

Not all of them are aplicable in a different distribution systems worldwide, especially constants like •Current Rating of Transformer = KVA x 1.4, etc.

I suggest author should notice the conditions and input values in which are these rules aplicable and users should modify them according to a specific distribution system.

alam

well said mate.!

shaijal100

pls tell me the difference between wire and cable.You mentioned different capacity for cu wire and a cable. You mean cable with aluminium conductor or something else..?Pls tell me..

kriloter

cable is bunch of wires :-)

manjunatha_pm

Please give me the solution for below mentioned problem..

Actually in our plant we having two 500 kva DG set and One dg is new one and other one is old …efficiency is different, so when switch on both and synchronizing.. some problem is occurring and i am not able to synchronizing. .. please give any solution for this problem

my mail id is manjunathapm8@gmail.com

srinivas subramani

Dear friend, just find the settings configuration in PCC2100 relay for parameter settings:)

ChAliGhafoor

kindly inform frm where did u get this useful information?