## Electrical Thumb Rules For:

- Cable Capacity
- Current Capacity of Equipment
- Earthing Resistance
- Minimum Bending Radius
- Insulation Resistance
- Lightning 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.**

### Lightning 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 |

THANKS FOR SHARING THE THUMB RULES

Dear parmar ji,

You are a great person, of Modi vision, make in India, you are making our indian technicals lerning strong.

Thanks for best effort.

useful for engineers those who works in projects

Perfect

NICE AND PRACTICALLY APPLICABLE METHODS INVOLVED ….THANKS

i learned many things by your post. thank you .

Dear sir,

how to make calculate , cable size 3cx70 sqmm copper pvc insulated double gi strip armoured,

pl clear this point the conductivity of armour shall not less then 75% of the conductivity of larg core

how many copper strip use for the conductivity maintain.

What would be current carring capacity of cable in 3phase system?

Very good info. for all engineers

Dear sir , need overload current carrying capacity for MV cable with trefoil and flat touching condition with different hours.

Generally 1 /2/3/4/6/8/10/12/24 hour run with 60% / 75%/90% & 30% overload condition with respectively above mentioned time durations so please how can calculate in easy format.

Need Cable current carrying capacity for 3 Phase Cable

can we use 2 x 16sq.mm x 4core and 1 x 10sq.mm x 4core power cable instead of 1 x 35sq.mm x 4core cable for the connectivity of Diesel Genset (60 KVA) to ATS panel 20 feet apart.

Please advice…

2x16mm2 4-core cable which is 2 run of 16mm2 4-core cable is more than enough to connect 60kVA generator to ATS panel

giving that the generator current = 86.6A

We Have a powerloom factory…with each one has 330 watts 3 phase induction motor voltage -220 freq-50hz

we have used 4 sq mm for mainline and 2.5sqmm for each motor for 12 motors.

Can We Connect that same 4 sq.mm for another 12 motors or we have to add 4 sq mm wire…Help me with your knowledge

YES.

We bundled conductors instead of single conductors for ac transmission to reduce skin effect.

If you provide two cables 4Cx35 Sq mm for one cable 4Cx35 Sq. mm then the system is sufficient and addition of 4Cx10 Sq mm will make it extra strong.

Yes you can

Hi Sir,

In a substation we use different connectors, like, bus support, couplers, body & cap connector etc, I would like to know what each component is rated for with respect to ampacity. How can we calculate that, is there a way to do that.

Excellent and neat write up, keep the great work !!

3phase 40A energy meter 17Nos in one apparetment.

How much amps switch rating and transformer is required?

Typically the local utility provides transformers rated up to 1250kVA For maximum connected load of 150kW.

Power factor*KVA=KW

0.8*1250=1000KW

But here answer is 150KW

How it will calculate?

Sir, one of my client wants to take connection having his required load 70 KW. In fact I have maximum feeder capacity in the existing panel 100 AMP TPN,MCCB-16 KA. Please suggest me the Feeder specification, so that I can install it in my panel in the spare box. also please eleborate the formula to select.

Thank you sir. This is very needful to us.

Sir,

I need to know the load calculations of

2core 1sqmm al ar cable = Amps

1sqmm cu cable = Amps

Tor electrical calculation should we take 1HP= 735.5W or 746W

1HP=746W

Dear sir

Twin core flat aluminium pvc insulated cable when we are supply current one core than earthing second core why ?

Dear sir

I need your expertise ON TRIPOING OF 220KV EHV transmission line, during lightening.

Need your help. Can I have your contact number please.

Very very nice ..very very useful informations &Calculations..thank u very much

For 4Cx 6sqmm cu at cable, you can go with 4Cx 25 sqmm Al AR XLPE cable. And for 3Cx4sqmm you go with 16sqmm if the length is less than 80 to 100mts.

load calculation of3.5*300 sq mm al. cable

For single core house wiring to 1.5 sq. Mm. How much current take ?

2800KW, 12Amps 1 phase AC load and distance between incoming supply source to load point is 1500mtr. Cable laid in under ground and HDPE pipe. now can you please suggest cable size and voltage drop calculation

How to select ct burden when current 1000A and voltage 433 v for metering and capacitor panel realy operating purpose.

Regards

Avinash Lahane

M-9225305991

i want to replace 4c*6sqmm copper armed cable with aluminium armed cable can u suggest the thickness of aluminium cable .and also i want to replace 3c*4sq mm copper with aluminium can u please suggest the thicknesses of aluminium cable

How to calculate the full load current of a 3 phase submersible,440V?

Sir

What are the details of motor installed in washing machine to be found out to select strength of wire ie. 2.5 s.qm or 4.5 sq.m or 1.5 sq.m wire to give power supply?

whether 15 amp.plug and socket can be powered with 1.5 sq.m flexible wire already given for 5 amp socket in the boar or more than 1.5 sq. metre strength wire to be used?

your kind reply will be useful for me.

Somasundaram N

Dear Sir

Can We use 4mmx4core copper armoured cable for 10HP motor coupled with 60LPM hyraulic pump. Application is for chopping/cutting of scrap sheet after blanking

pls see size of cable. i think it should be higher. you can follow chart for “current vs size of cable”. also different chart for Cu and Al is there

Can I use 25x2x0.75sqmm twisted,shielded cable instead of 50×0.75sqmm shielded cable?

Application :- I need to transmit 230VAC power from stepper drives to motor.

SIR,

FOR 100 KW SOLAR PLANT CAN I USE 4C*70 SQ.MM(COPPER) TELL ME ITS ENOUGH.FROM ACDB TO EXISTING PANEL

Hi fella,

I came across this website and got lost in wire Vs cable hurricane :) Later in comments I’ve seen cables AKA “bunch of wires” explanation. Where I got stuck is, according to your statement

“Cable Current Capacity = 4X Size of Cable in Sq.mm, Ex. For 6 Sq.mm = 4×6= 24 Amp.”

Yet in the mean time, I’ve witnessed mine sq.mm wire stood against the current capacity of around 28A for nearly 30minutes!!! Its details are Unsheated,Copper made,No. of wires/Wire dia metre=85/.30. So I’m expecting 6xSize in sq.mm instead of 4XSize in sq.mm. So I want you to clearly tell me what does wire Vs cable means?

its really helpful i am very thank full to you for your this kind of act.

hi

finaly i found nice web site to improve my knowledge. i have to know if i communicates RS485 data with 1.5sq mm 2 core twisted wire without sheld and run the wire 60 mtrs what will happen.

pls advice

Here we using the load 3.6Amps and the same we are using the wire 2.5Sq.mm.If it is ok or we need to change.

Hi,

We have the Main Panel MCCB of 400A 4P and the load is 120kVA. The client has provided raw power using 4 core 95Sqmm Aluminum Armoured cable. Please let me know whether the raw power cable is fine for our load?

Also what is the load 95sqmm can take?

Hi,

According to load the cable size is good enough, and the cable can carry up to 220A current which means 120Kw or 140KVA

Thanks. Very effective information.

Please give me a clear idea how can I find the contactor rating & over load relay rating for Star-delta Starter. Give me this example for 30kW motor.

Very useful make PDF and publish for download

Please correct 2.5×4 = 9Amps to = 10Amps.

in cable current capacity thumb rule.

Intresting and educative forum for power engineers

This forum is indeed amazing quite educative.

Keep up the good work.

Thank you.

Sir, How do the GEB pole lines are identified for phase , neutral and earthing. Reason for my asking is all the lines ( Either 3 or 5 for single and 3 phase ) are same and there is no distinction. Recently in our street light pole tuble light connection, an electricians has given some wrong connection by joining 2 wires either in phase or in neutral. Because of this many of our street people have lost TV, Fridge, AC, Digital wall clock etc. The GEB officials said that Munipal contractor has done the mistake while rectifying the defect in the pole street light. This is the reason why want to know for easy identification of these Phase, Neutral and Earthing wires of GEB Pole. pls. explain with sketch or drawing for easy understanding.

Dear Sir,

Would you please advise how can I connect 2 numbers of single phase 3000VA 230V 50 HZ UPS

in Parallel for redundancy… Appreciate your early response

Thanks and regards

Joseph Puduvala

Hi,

These single phase ups can be connect in parallel only when there Output voltage and frequency are same. so please communicate with service provider to set voltage and frequency of both ups on same value and then your can synchronize the ups.

Regards

Vinay kumar

It’s awesome page..I love to visit the page but there are no answers posted for all the questions asked by the Engineers..And I would love to join this group on LinkedIn or FB to gain some knowledge about the EEE.Thank you so much guys for your time and patience.

Anyone please suggest me a book or a site to learn from basics to the core subject of Electrical Engineering including the calculations.

.You guys TC!!

Current capacity of aluminium cable

Hi i have a question

we require 100KVA DG as part of the factory setup can anyone explain how much capacity of transformer will be needed, is there a possibility to determine the transformer capacity based on the DG or let me know if you need the machinery list thanks in advance

Hi,

To calculate capacity of Transformer,you have to calculate the load of plant and also consider extension of load in future. then maximum load should be less than 80% of transformer capacity.It does not depend on your D.G availability.

Thanks

Vinay

In a 3 phase motor we use 10 mm cable one phase taking 40 amp for example how much total current of this motor

Can you give motors equations for Induction motors, DC Motors ect.

What is equivalent size of cable will be used for earthing of DG sets and Transformer instead of cu /GI strip

Is considered the same cross sectional area or how to calculate

Esteemed sir

I want to know how to calculate current carrying capacity per square mm Arial bunched cable.

Sir, please give details about how to bend an earth cable to be connected to diesel generator of 25 KVA. Give me an idea how to bend it.

Dear Sir,

I want to know what is required AC cable size (in sqmm) for solar on grid 3phase 12.5kW inverter for inverter to ACDB?

4cx16 sqmm Cu.

We have infrared lamp resin curing plant & paper dryers. Total load is 330 KW. Connected with neutral and 3 phases i.e.each phase will have a load of 110 kw. Current 500 amp. What should the cable size and 3.5 core or 4 core. We will prefer copper.

how can we have check the copper nd aluminium cable size according to load via formula or trick ?

this is very urgent for me, pls help me sir ASAP.

Dear Sir,

your above mentioned information is very useful.But please mention the load capacity of aluminium cables also..for 30sqmm copper cable we can put a load of 120A??

Gd evening sir

Pls told me sir

How will be calculate cable size in mm^2

Pls told me a short trik

For example which size of cable should be use for 2500amp rating braker

Pls sir as soon as possible rply me

Thanks a lot sir

this helping me more……..

i just want solution for this. if i want to pass current of 1600 Ampere so what size of cable should i select?? cable size should in MM^2

It should be 400 Sq mm

Sir, can you say that how a dc current of 2.5 amps will flow through a wire of resistance 60 ohms? input is 36 volts 11 ah battery

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.

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

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

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?

Super sir

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

wonderful informative portal….!!!

A perfect compilation. Thanks Jignesh. (Author)

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 ?

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

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

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

Tell me the short cut for calculating load for a flat

Please tell me how to find current rating of the conductor

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.

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

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.

What is the simple formula for electrical, current , voltage

Thanks sir u help for electrical problem .

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.

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

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

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

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

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

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

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.

minimum 6 mtrs

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?

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

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

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

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

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

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

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

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.

What is the difference between line and phase?

line to line voltage means voltage between 2 line

and Phase voltage means voltage between 1 line and neutral

thanks 4 d info it was really

help full we all must share

these things so we could get best of

knowledge

Please I have some business proposals for the author.

Please get in touch with me at [email protected]

Thanks

Thanks Dear sharing Such valuable data

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

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

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?

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.

well said mate.!

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..

cable is bunch of wires :-)

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 [email protected]

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

kindly inform frm where did u get this useful information?