Electrical Thumb Rules(Part 1).
July 27, 2013 65 Comments

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=PhaseGround Voltage, U=PhasePhase Voltage, Um=Highest Permissible Voltage

Current Capacity of Equipments:
 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 Volts
 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=12xDia of Cable.
 Minimum Bending Radius for HT Power Cable=20xDia of Cable.
 Minimum Bending Radius for Control Cable=10xDia of Cable.

Insulation Resistance:
 Insulation Resistance Value for Rotating Machine= (KV+1) MΩ.
 Insulation Resistance Value for Motor (IS 732) = ((20xVoltage (LL)) / (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=
 (1) MCOV=Max. Continuous Line to Ground Operating Voltage.
 (2) Duty Cycle Voltage. (Duty Cycle Voltage>MCOV).

Transformer:
 Current Rating of Transformer=KVAx1.4
 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 (PhasePhase)
 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 @ 1meter.
 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 
Very useful information
Thanks
excellent for this words
Jignesh it is very usefull for all engineers.
Man, you are the best..I never knew most of these rules.
Long live Jignesh!
3 PHASE MOTOR DRAWS CURRENTS – 2.5 AMP/hp.if it is 230 v
and 1.5A/HP If it is 440 volt .
Can you also please add formulas too..
Read Tile of this Page.This is THUMB RULES so There is no any formula/References/Code For that !!!
This is used for Quick calculation…
sir ihave 500kva dg ..iwant units/litreand 25% load only
Really the topics is very useful to me
VERY USEFUL AND HANDY INFO
very imp info
Dear Jignesh
I am regularly getting updateds from this websites. It is very useful.
Can you please reply me for following query.
As per Indian standrad : Emergency standby duty generator ( for
example : 1250 KVA,415 V) can be loaded for full load (i.e 1250 KVA
constant or vairiable load ) in case of
main power source outgae for 3 days. If any supporting IS number is
available please advise.
In case Alternate relevant international standard is clarifying this
query please advise.
With best regards
Shailesh G Patel
it is very useful for electrical engineer and thank you very much Mr jignesh
Very useful in short. Thanks
great refferences, i joined your page on facebook and followed your blog by email
Sir please help me… My question is, what is the suitable stater and breaker for 400kw,1440rpm induction motor…
Thanks for this rule of thumbs. It is really usefull for short assesment.
Thanks yery useful for all electrical engineers in the world
hie sir help me here’a single phase load consists of 1. 12kw of lighting & heating at unity pf.2. 8kw of motors at 0.8pf lagging & 3. 10kva of motors at 0.7 pf lagging.calculate (a)kwt (b)kvart (c)kvat (d)overall pf and the total supply current at 240v.my main wory is how to come with 30.3total kva.13.1total kvar &overall pf pliz help!
as a site engineer this info is very helpfull to me ..
what are electrodynamic stresses?
yes
It’s useful info. thanks
THIS INFORMATION IS VERY USEFUL FOR EVERY ENGINEER AND ELECTRICAL MAINT. PERSON.
THANKS JIGNESH.
_ DHARMENDRA KUMAR
so greatful job you had done.
Jay jay garvi gujarat
thank u sir…………………………….
Thank you sir
thank you for all your hard work in putting this together….Greetings from West Texas
in what case voltage reading between ground and neutral up to 230 at distribution transformer L T side .The transformer ratio is 33 kv / .415 kv .
Already given in the Blog !!!!! (< 5V)
thank you very much sir………
All the three phase are generate in the power plant…Sir plz tell me about the neutral generation…
Plz reply soon.
Dear Sir
i am working in residential building my one flat load is 15 kw so how to calculate diversity factor.
regards
Sumit Sharma
sir
thanks for sharing such a information
Thank you sir, i am never know about this thumb rules.
Dear Mr. Jignesh
■1 Phase Motor draws Current=7Amp per HP.
■3 Phase Motor draws Current=1.25Amp per HP
Can you explain the above information, please?
Dear Alwin..
This is “THUMB RULE”..This is Quick and handy calculation which is near to mathematical calculation or it’s equation.
Dear Mr. Jignesh,
Kindly use reference and standards. This ‘ll help us to improve the knowledge about standards and it ‘ll also induce to search about the standards.
Paramasivan Tsp (My FB id)
Normally I always indeed to indicate Standard and mention for the same..But where it is not possible,it is not mentioned..
as in the case of “THUMB RULE”
what about alluminium cable?
Dear sir,
I m gating regular mail Its a very use full information from for me
Thank you
Dear
Which rule have you followed for cable current carrying capacity?
thank you for all the information!!!
Dear Sir,
Thanks a lot for such technical information which will help to solve day to day workings.
It is reallya most valuable and sound as well as perfect calculations ..
Please be in touch by mail,
I shall be grateful if send me such an electrical related informations,calculations etvc.
Dear sir,
Thanks for your lot of Technical information.
One our customer in UK.Their asking for Below ground earthing calculation used BS (british standard).But I have IEEE80 format Ex sheet. Their not accepting IEEE80. Hence.Hence Kindly send the BS format for below ground earthing.
This is very useful for Us.
Regards
V.vasantha kumar.
Dear Sir,
can you prepare thumb rule based on DEWA regulations.
Gud information for public knowladge.. Thanks lot.
In domestic wiring, if neutral line is shotred with housing earth line, What problem we can face? Whether it will effect on KWH meter reading? Is there any chances of getting my electronic equipments damage?
Excellent information. LIVE LONG DEAR ENGR JIGNESH. GK
WHAT IS 10X RULE IN VOLTAGE DIVIDER CIRCUIT
Sir
its wonderful knowledge is here
i m very thankful to u . Jagnesh ji
Sir please suggest
can we use elcb in horizontal mounting?
Thank you sir..!
What is the minimum cable size that I can use for AL cable in building distribution. Please suggest me the reference standard also.
Thumb rule for line loss for 300 square mm cable
Cu wire current carrying capacity is wrong. If you take 25 sq.mm cable then according to you sir it should be 25*6 = 150 amp. But in the catlogue of polycab its showing 108 amp. How there is so much of difference?
What is this 6
very very important knowledge for us ,thank you sir.
Hello sir….
How to make the this kind thumb rule based on the reference document???
sir i want know about detail manner each thumb rule or otherwise you mention standard number…sir… example u use copper current rating calculation=6* size of sq.mm…why it’s used 6….???
32 amp industrial socket what cable or wire we need? what about cu or aluminium cable ?
For cable current capacity example calculation is wrong
very useful information thanks
Very usefully
very useful Thanks
NICE SIR
useful sir
Very useful thanks