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Calculation of cable size

How to calculate cable size, switchgears and fuses for feeders and equipment?

By Anonymous on 3 July, 2007 - 12:41 am

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By Anonymous on 3 July, 2007 - 4:30 pm

By Ram on 31 July, 2007 - 12:08 am

This is a great site... It's really helpful. Thanks for sharing.

I was also looking for transformer & battery size calculations for a turnkey project. And also what are the paramaters I need to consider for calculations? Appreciate if you could help in these subjects...

Regards,

Ram

By Shahul KM on 3 August, 2007 - 12:31 pm

Hi,

Its very happy to share our technical clarifications and doubts here.

For you clarification of Transfomer Sizing calculation could be followed as

1. Calculate Total Connected Load(KW)(

2. Calc the Total I/P in KW ( Considereing 85% efficiency)

3. Calc Total I/P in KVA (Considering 0.92 Power Factor)

4. Required KVA by mutiplying the Total I/P in KVA by 1.2 times.

Calculation Example

1. Say Total Connected load in KW=1100

2. Therefore Total I/P in kw = 1100/0.86 = 1279

3. Total I/P in KVA = 1390

4. Total KVA required = 1390*1.2=1668

Therefore by considering the future connection you can select 2000 KVA transformer for the above said example.

With Regards

Shahul KM

Elec & Instrumentation

By Abhishek on 17 November, 2007 - 10:04 pm

the calculation explained above is right basically

but actually transformer sizing also depends upon which type of load is connected

if the connected load is motor so its starting current is too high and trasformer has to withstand this starting load

so also consider the 5 to 6 times of full load current of largest connected motor

in our company we are doing this type of calculation.........

if anyone need so i can explain in detail next time

and one more thing i m just GET here, i may be wrong ok? i am giving my id

Regards,

Abhishek

abhi_panchaxari@yahoo.co.in

By saiyu on 17 August, 2008 - 1:38 pm

Dear Abhishek

You are right about the Tranformer sizing considerations. I feel you are much more experience as compared to me , I have just started my job in this field. Kindly can anyone send me some sample Cable and Trasformer sizing calculations. I'll appreciate that so much.

Thanks and Best Regards,

Sai

Email: vitsigns@yahoo.com

By Malik on 15 May, 2011 - 10:01 pm

Hi,

Can I get a cable sizing calculation spreadsheet or something.

Thanks,

Regards,

malik-hassan@hotmail.com

1 out of 1 members thought this post
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Malik... search web for: Electrical Calculators; Electrical Engineering Calculators; Cable Sizing; and Wire Sizing!

Regards, Phil Corso

By robin on 3 August, 2011 - 2:05 pm

By NRO on 18 August, 2011 - 8:37 am

Hi,

Please check the following

http://www.ielectrix.com/?page_id=8

Here is a simple cable calculation per IEC 60364 standard.

Thanx and regards,

NRO

By prince mukhia on 10 September, 2011 - 1:26 pm

Hello Sir

can you explain calculating KW in three phase supply like we have current R=55 Y=78 B=90 and W=root3*VI*power factor so which value of current we have to put either highest or average

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Prince... presuming you are interested in determining the overall power, please supply the following values:

o R-Y Volts.

o Y-B Volts.

o B-R Volts.

o R-Neutral Volts.

o Y-Neutral Volts.

o B-neutral Volts.

o PF associated with Line Volts R-Y and R Amps.

o PF associated with Line Volts Y-B and Y Amps.

o PF associated with Line Volts B-R and B Amps.

o Phase sequence or rotation, R-Y-B or B-Y-R.

Regards, Phil Corso (cepsicon[at]AOL[dot]com)

By prince mukhia on 12 September, 2011 - 12:16 am

Dear Sir

can you please help me to calculate Kilowatt with following data:

R 161A

S 145A

T 117A

Power(KW) ?

Phase to phase voltage =380v

Cosine of phi =0.8

please calculate overall kilowatt

thanking you

regard,Prince

By prince mukhia on 12 September, 2011 - 10:03 am

Dear Sir

> can you please help me to calculate

---- snip ----

actually i need to know only how i will put the value of current while calculating watt because i hav three valur RYB so either i hav to take average of RYB or take or by applying VIcos phi with each phase current

thanking you

regard, Prince

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Prince... in answer to your 12-Sep-11 (00:16) an approximate (I emphasize 'approximate') formula, based upon the data you provided is:

o kW = [Sqrt(3) x Vavg x Irms x Cos(phi)]/ 1,000, where,o Vavg = [( Vry + Vyb + Vbr)/3.0].

o Irms = [Sqrt (Ir^2 + Is^2 + It^2)/3]

I will provide the 'more/ accurate method later. Regards, Phil Corso

1 out of 1 members thought this post
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1 out of 2 members thought this post
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Prince... before replying to your second question, were you satisfied with the answer to your 1st question?

Phil Corso

By ssatta on 14 December, 2011 - 8:46 am

>R 161A

>S 145A

>T 117A

>Power(KW) ?

>Phase to phase voltage =380v

>Cosine of phi =0.8

first of all the load is not balanced. it is not a sign of healthy system.

still we r consider the highest amps i.e. 161

KW=1.732*V*I*Cos phi/1000

=1.732*380*161*.8/1000

=84.7

1 out of 1 members thought this post
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ssatta (Prince)... the current values clearly indicate an ampere unbalance. In fact the Ampere Unbalance Factor (AUF) per NEMA is 36%. Per IEC it is 44%!

Your power calculation, however, is suspect. Using the maximum line-current and a guess-timate that PF is equal to 0.8, is incorrect.

The Unbalance Factor is is quite clear. Secondly, if the phase to phase values were measured at the motor terminals, and they were all 380V, then the motor is faulted!

What are you trying to determine? Can you provide additional detail. How about its nameplate data, to start?

Regards, Phil Corso

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Ssatta (Prince)... further to my 14-Dec-11 post:

1) Using V, A, and PF parameters presented, total power drawn from supply is 43kW.

2) Ignoring PF presented, then total power drawn from supply is 61kW.

Regards, Phil Corso (cepsicon[at]AOL[dot]com)

By BHANU PRATAP on 16 January, 2012 - 6:21 am

DEAR

YOU MUST CHOSE THE HIGHEST CURRENT VALUE TO CALCULATE THE MAX POWER (KW) AND CHOSE THE LOWEST VALUE OF CURRENT FOR MIN POWER (KW).

By Dinesh on 4 February, 2010 - 4:29 am

Dear sir,

im having small clarification in your transformer sizing calculation.

that is why you put 85% efficiency with input kw?

why you used 0.92 power factor?

Regards,

K.Dinesh Raj.

Electrical designer.

By vijay on 10 February, 2010 - 5:15 am

actually when we give some load to electrical equipment it takes some energy as loss that means the electrical parts like coil and core takes some energy some of energy wasted in these parts so if we give some input the output will be low when compare to input. commonly the losses will be in these ranges so that approx we take 85% in total load in calculation. efficiency of 85%. output/input is called efficiency.

and 0.92% is the power factor between voltage and current. it varies depends upon the load.commonly we have to maintain this power factor. if u have any doubt on this just mail my id.

vijayakanth.boopathi [at] gmail.com

By Jhotha on 22 April, 2010 - 6:53 am

Hello,

With reference to your cable sizing calculation, you have written I/P in kVA By 1.2 times. Kindly clarify this statement.

Regards

Jhotha

By s.balasubramanian on 25 May, 2011 - 4:54 am

Dear

The method you have given is not correct, although your answer and answer you get by vector addition will not differ as pf you consider in each equpt is not very different from your general assumption. if you take pf as say 0.9 and then compare your figure with vector addition it will be different to a good value

what you have to do is

1.find kW and Kvar wrt the actual loading of motor

2. add all kws and kVars

3.find kVa by hypotenuse formula for trafo capacity

By Wady on 20 June, 2011 - 6:12 am

Hi,

Just want to confirm, for the sizing of the transformer, is it to consider the total connected load or max. demand? I think for saving of the money, better go for the max. demand.

Any advise from experts? TQ

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Wady... in general, transformer size is never based on "connected" load! Neither will "maximum demand" result in an economical solution, because load profile is extremely important!

Also, because there is no "one size fits all" approach, I suggest you contact several transformer manufacturers. Many are happy to provide you with a "Product Specifier", or the like, to aid you!

If you have a specific application in mind, and you are unable to find a suitable answer on the web, I'd be glad to help!

Regards, Phil Corso

By Sitaram on 23 November, 2011 - 2:44 am

Hello,

Considering total connected load sometimes would be misleading. for example some process lines which consist large AC motors but effective load on the input transformer is negligible as most part of the energy need is taken care by the regenerative function. Hence while selecting the cables and transformer it would be prudent to consider the operating loads at peak performance.

Sitaram

By theb00gyman on 4 April, 2012 - 10:58 am

> Calculation Example

> 1. Say Total Connected load in KW=1100

> 2. Therefore Total I/P in kw = 1100/0.86 = 1279

> 3. Total I/P in KVA = 1390

> 4. Total KVA required = 1390*1.2=1668

> Therefore by considering the future connection you can select 2000 KVA transformer for the above said example.

greetings,

i would like to ask about that factor [1.2] entered in that last equation. what's that factor called and what is it about?

By Sanjay Shirsat on 1 June, 2012 - 4:39 am

How this figure 1390 comes (Total I/P in KVA = 1390)? please clarify.

> For you clarification of Transfomer Sizing calculation could be followed as

> 1. Calculate Total Connected Load (KW)

> 2. Calc the Total I/P in KW (Considering 85% efficiency)

> 3. Calc Total I/P in KVA (Considering 0.92 Power Factor)

> 4. Required KVA by multiplying the Total I/P in KVA by 1.2 times.

>

> Calculation Example

> 1. Say Total Connected load in KW = 1100

> 2. Therefore Total I/P in kw = 1100/0.86 = 1279

> 3. Total I/P in KVA = 1390

> 4. Total KVA required = 1390*1.2=1668

>

> Therefore by considering the future connection you can select 2000 KVA transformer for the above said example.

By nik on 11 February, 2009 - 1:27 am

By kgb on 13 August, 2011 - 2:43 am

>Visit this link, it might help.

>

>http://www.tlc-direct.co.uk/Book/Contents.htm

tnx u for sharing this link. Godspeed!

By Curt Wuollet on 5 July, 2007 - 8:03 pm

In most places, you start with an electrical engineer or master electrician, who knows how to do this. But, there really isn't any rocket science involved. You simply sum up the expected and anticipated loads and size the wires for the current. In most places, the safety factors, allowable temperature rise, conduit fill and the like are regulated so you do your calculations and apply the regulations. In my part of the world, you need to have permits and inspections and even UL approvals to discourage those who don't know how from doing this. The regulators discourage even those who do know how, and provide employment for electricians with safety as the cover story. There is a push in my home state to require that you be licensed to do anything involving wire.

Regards

cww

By Qarsaan on 8 July, 2007 - 10:59 pm

Whenever amps go running down wires they give off surface heat. The amount of heat given off depends on the wire resistance. Bigger wires have less resistance, and hence give of less heat.

The amount of heat given off leads to a temperature increase, whitch in turn heats the wire insulation. If the insulation gets too hot, it melts, leading to cable failure. (If the installation has been designed properly, this will blow a fuse. If the design was wrong, it can start a fire.) The insulation melt-point depends on the material selcted. This is why different cable types can take different amounts of current for the same conductor size.

Sizing of switchgear is the topic of many textbooks. Some of them are very thick.

Fuses are sized so they let enough amps flow down the circuit to service the load, but not enough amps to overheat the cable insulation... which would cause it to fail as mentioned above.

So. First you need to know how many amps the load needs. Next, make sure the fuse size will allow the load to be met. Next, make sure the cable can carry more amps than the fuse.

But... a little knowledge is a dangerous thing. The above is the basic idea only. Cable selection is a mutli-factor task, always best left to an expert. Remember, fuses always blow at the most inconvenient time, (just after the replacement-fuse store has closed for the weekend) and electric shock can be painful.

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Anonymous, following is a list of many factors that must be considered:

o Allowable voltage-drop for normal operating load.

o Allowable voltage-drop for future operating load as determined by motor's Service Factor.

o Allowable voltage-drop for starting condition.

o Type of starting, i.e., full-voltage, reduced-voltage, part-winding, delta-wye, VFD, etc.

o Short-circuit withstand capability for an in-close fault (@ motor starter.)

o Ground or earth-fault capability for a far-end fault (@ motor terminal box.) Ground or earth-return impedance must be included.

o Plastic or rubber sheath resistance to chemical attack.

o Metallic screen or shield material withstand capability for excessive heating.

o Derating factor(s) covering installation, i.e., free-air, direct-burial (dry, wet) in conduit (duct bank, steel, plastic.)

o Derating factor(s) covering conductor material (Cu, Al.)

o Derating factor(s) covering construction, round or sector shaped, 3x1/c, 1x3/c, 3-1/2 core, 4-core, overall sheath and/or shield.

o Derating factor(s) covering insulation type and voltage level

o Derating factor(s) covering termination type (bolt, lug, clamp.)

o If direct burial consider subsidence concerns.

If you need additional info, please feel free to contact me.

Regards, Phil Corso, PE (cepsicon@aol.com)

By Phil Corso,PE on 10 July, 2007 - 10:41 pm

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Anonymous, I forgot to include a very important factor:

o Allowable voltage-drop for motors to be re-accelerated following a voltage interruption.

Regards, Phil Corso, PE (cepsicon@aol.com)

By John on 26 June, 2011 - 10:12 am

Hi there

I did a calculation there recently and my derating factors came to about .4 - is this between over cautious. As you can imagine the size of the cable ballooned. Does that sound right- i'm not 100% thanks

>Anonymous, following is a list of many

>factors that must be considered:

>

>o Allowable voltage-drop for normal operating load.

>o Allowable voltage-drop for future operating load as determined by motor's Service Factor

----- snip -----

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John... it appears your question is directed to my 10-Jul-07 (19:55) reply to Anonymous! It is possible to end up with a derating factor equal to 0.4. However, in order to answer you quantitatively please provide some detail!

Regards, Phil Corso

By Dave Newbury on 12 July, 2007 - 10:50 pm

If you are working to NEC regulations I recommend this inexpensive (but good) software package.

Follow link to get download http://www.mc-group.ca/

Good luck.

Dave

By Steven on 21 November, 2007 - 1:45 am

If you want to work to IEC standards you can use the following free cable sizing tool:

http://www.myelectrical.com/pages/cables/cableSizing.aspx

Steven

By don't know on 22 January, 2009 - 12:34 pm

How to calculate cable size:

cable size[in mm2]=(18*meters*amp)/v*1000

Meter = total length of the cable (both positive and negative)

Amp = total current flow through the cable

v = acceptable voltage drop in volt

By Anks on 10 February, 2010 - 11:35 am

By Calculation of cable size on 22 September, 2010 - 12:42 pm

By surinder on 16 December, 2011 - 1:54 am

>> cable size [in mm2] = 18*meters*amp)/v*1000

> What does 18 stand for?

18 stands for voltage drop (mv/Am)

By fashi on 27 August, 2012 - 9:25 am

>> cable size[in mm2] = (18*meters*amp)/v*1000

> What does 18 stand for?

please clarify ASAP.

if i have this type of specification (below mention), then which size of cable i can use for this DB? (i can use 4c, 70mm2) but i am worry about how can i calculate for gating correct size. please share any formula for calculation.

3Ph.4w, 60hz, 220v, 160 total connected load 58kw.

thanks in advance

By jony on 3 August, 2009 - 7:22 pm

By battler on 3 December, 2009 - 6:33 pm

By rick hawkins on 10 February, 2010 - 11:39 am

you need to follow whatever code book is being used in your area. in Canada we have a Canadian code, amended province by province. there are many changes made in the provincial books, and every rule must be followed. if not, problems can arise. for instance some types of wire are not suitable for underground use, or to be pulled into conduit or to be run free air or not. wire with certain insulation needs to be pulled into machines only, not into conduit (tew f/i cannot be pulled into conduit.)

if all of the rules are not followed you may need to pull out the wire and run it again.

what country/province/state are you in?

rick

By electeng on 16 November, 2011 - 1:53 am

when doing cable sizing calculation, there are several factors to be taken into consideration.. you might want to check this link.. http://electeng.wordpress.com/2011/11/16/cable-sizing-calculation/

By Francis Tang on 27 November, 2011 - 12:47 am

By Paul on 1 December, 2011 - 8:07 pm

> How to calculate cable size, switchgears and fuses for feeders and

> equipment?

A very rough and ready way to calculate cable size is to count the number of cores, divide by the average diameter of cores then multiply the result by 3.14 (Pi)

By crosswind on 23 February, 2012 - 7:39 pm

even my opinion was too late. i would like to share to all guys on this forum on how to size the transformer, busbar, breakers (ACB or MCCB).

In he first place, Engineer can decide whatever he want and believe that the calculation his done is correct. IEC is not telling how much the sfe factor to consider during sizing therefore is up to engineers on how to be safe in sizing with respect to the connecte load.

The most important things to calculate the above items is to know the ELECTRICAL LOAD ANALYSIS(ELA).

Electrical engineer should consider all the possibility in sizing the equipment (diversity factors, Load Factors , KW, Eff, PF, KVA rating)

In designing the system and decided the size of the transformer bus and breaker.the ELA is the key on deciding the proper rating of equipment... Take note and put in mind that the motors will not operate beyond the rated current or equal in rated value (nameplate)! the total connected load will be decided by ELA and sizing the busbar as well the transformer and ACB as well. The short circuit value during fault is based also in larger connected load value and multiplying by 10 (assuming) for easily identify the Ka rating... somehow ETAP software is the best of the best for calculation the Short circuit value and load flow analysis in identifying if the transformer is overload or under load ... friendly user software thumbs up!!

By Quddoos Mughal on 15 August, 2012 - 7:29 pm

By Bob Peterson on 17 August, 2012 - 7:44 am

There are generally well accepted codes, standards, and practices that cover this in depth. Which one(s) would apply to your particular situation depends on many factors including (but not necessarily limited to) geographic location, voltage, and end user preferences.

In general, it is about Ohm's law and how high of a temperature the wire insulation can take before it is degraded.

By Engr Wasim on 19 October, 2013 - 12:04 pm

Here is the simple guide...follow these simple steps...

How to determine the suitable size of cable for Electrical Wiring Installation with Solved Examples (in both British and Si System)

**Moderator's Note:** The forum software places spaces in long urls, please copy the url and remove the spaces before trying to follow the link.

By Kumar on 1 January, 2014 - 7:28 pm

By happy dean on 2 July, 2014 - 8:26 am

How to calculate cc of the cable calculation steps

I know that CC is for fuse factor but i have cables on a tray with a tray underneve the tray so I have to use a factor does this mean i Use the factor as part as the grouping factor because I know that

THE INSTALLATION METHOD AND FUSE FACTOR = CC

iN THE BOOK IT SAYS BURIED DIRECT IN GROUND 0.9 ONE CABLE WITH A BS3036 FUSE 0.725= CC

SO

DOES TRAPEZE BRACKET 2 TRAYS GO PART OF THE GROUPING FACTOR Cg OR PART OF THE CC FACTOR FOR CABLE EQATIONS TO FIND OU RATING OF CABLE It OR Iz