Cable Sizing Calculations?

Hi,

Cable sizing is not as simple as many want it ot be. There are several factors to consider before selecting a cable:

1. What is the load current?
2. What is the starting method (DOL/S-D/VFD/Soft-starting/...) in the case of a motor load?
3. Length between load and supply?
4. Ambient temperature?
5. Is the cable laid in ducts and if yes, what ducts ( covered/underground/wall-mounted/...)?
6. Nature of the area (corrosive substances/possible water/moisture ingress/...)?
7. Power factor?

Cables have specific max temperatures and I believe the extremes should be considered depending on what insulation is used.

Regards,

Mbalu

My dear friend Mr.Mabalus advice is right you need all the inputs listed by him before cables are sized (you may mesup every thing going by thumbrules)and you can getin tuch with M/s ABB who has the software called DOCWIN (Design optimisation on computers)avilable free and very useful for electrical design as it comes with electrical CAD you need to contact ABB nearest to your location

best of luck

murali

Murali,

I did a search for DOCWIN and DOC WIN and DOC Design Optimization and I could not find the software. Can you please tell us the internet link to this free software?

Thanks,

Mike

Dear Friend,

etap is free demo soft ware. Pls find link below.

Hi

I think the easiest way is to look at the fuses for the motor. The cable shall not only be able to stand up to the motor current, it shall be able to stand up to a short circuit at the motor.

In EU we have rules for this, i.e. if the fuse is 16 A, the cable must bee 2.5 mm². It is not depended of what kind load, but the load must bee lower than the fuse value.

Anders

Hi

This is not only depends on the fuse rating suppose the load is far away from the control panel the wire size will be varing

Regards

Momin.S

Yes there is. The is a Manual That I Use for this its Called ' THe "AC43-13b Acceptible Methods and Standards" Its an FAA Publication. Its the holy bible for aviation technicians and airplane designers. I Can Email the apprpriate charts and technical info if you want. I Work with aircraft and other electrical equipment quite abit. The AC43-13b is the one manual that I always have open if the maintenace manual doesn't have the info that Iam looking for at time.

Here is my email Address

jparcjr@techie.com

The National Electric Code Handbook will quickly tell you what size conductors, the type of insulating properties of the conductor insulation to use for the current (amperage) draw of the motor. It will also tell you what size motor starter and what size fuse to protect the starter with. Additionally, it will tell you the correct size conduit to place the conductors in. In terms of voltage drop, there is a formula to use for 3 phase applications, which can also be found in the NEC handbook. A new addition of the handbook comes out every three years, the most current one being the 2005 edition.

In addition to the NEC you can contact Square D salesmen or other Motor suppliers and request a Motor Data Calculator which will tell you the exact information that the NEC does with one exception. The voltage drop calculation that must be done to determine if the wire size must be increased due to the distance between power supply and motor.

The motor data calculators are free from the suppliers generally.

I agree, JLD - I see that vince02_brad could be from the UK, but wherever he's from, you should use your local code of practice to determine cable sizes and types. In most countries a registered electrician of some sort would have to install or at least commission it anyway, and they normally have such books/manuals available. Mbalu touches upon all the relevant issues in his reply too.

Your cable manufacturer will have a book you can get and look up. The allowable voltage drop will be set by local regulations but the data in mv per ampere metre in the book will allow you to calculate the required sizing. Olex's little orange book is a good source of information.

Hi,

Here is some approximate calculation for your reference You need to be good in IEC standards and actual field conditions. (IEC 60364-5-52)

1) Calculation of Grouping Factors

a) Air temperature - Refere IEC table 52.14

b) Grouping factor - Refere IEC table 52.20

Overall derating factor = C = a x b

Motor Load current Ifl = 15 / (415 * Eff * Pf * Sqrt(3) )

Cable Ampacity =Ifl /C

Depending on the cable ampacity select the cable size from IEC Table 52.5

2) Voltage drop calculations:

2.1) Running voltage drop

Refer standard tables for the selected cable size and find out R & X / Km

Now, Z = R * cos (Phi) + X * sin (Phi)

% V Drop = 100 * Ifl *Z * sqrt(3) * (L/1000) / 415 , Where L is the cable lenght

Allowable votlage drop generally is 5%

2.2) Starting voltage drop

same as above, Instead of Ifl use Istarting, Instead of Full load Pf Use Starting Pf (Cos (phi))

Allowable voltage drop is generally 10%

Hope this helps you

Npower

Hi

How are you?

I gone to u r formulas ..................can u give me exact or apprpximate K value for cable size calculation.

plz mail me : sahmed@emcouae.ae

Best regards

syed

k values differ depending on the material of the conductor, generally speaking 115 is used for copper.

However i would not advise cable calculation if you are not a qualified electrical person because there are so many factors to consider which havent been mentioned.

1. Where are you getting your power from, generator or mains?

2. What is your altitude?

3. Is your power supply network large enough to cope with the extra demand.

4. What starting method does your motor use.

5. How long is the run.

6. What type of motor is it.

7. What type of protection is being used for the cable supplying the motor?

8. How will the cable be run? Through what materials?

9. What type of conductor are you using and why?

10. What insulating material covers this conductor.

11. Is the motor to be installed in a special location?

12. Is the cable grouped with other cables, and if so how many and what type?

13. Ambient temperature of the environment?

14. Type of environment for cable selection.

15. Conductor material could depend on method of installation.

There are far to many variables to do this properly by chance, BS7671 is a good guide

hi frnd..

wat s that K value (K denote which factor) in calculation of cable size? My Mail Id is ktva361_eee@yahoo.co.in

Hi,

In your formula multiplying by sq rt 3. Is it applicable for delta? Kindly suggest and mail me

Thanking you

Pradeep

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DHAYANANDHAN.S

Hi Npower and others,

That was quite detailed and useful. But I have a doubt, depending on the number of cores selected (ie 3 runs of single core or 3-core) how does these formulas change in selecting the cable.

Also if you use DOL or VSD or SD should I correct the starting current according by a factor of 6 or 1.5 or 4. Pls suggest.

Is this sizing applicable for only cable sizing from MCC to motor or can it also be used for generator to switchboard or transformer to switchboard.

Appreciate your clarification.

Thanku

VS

I have given the way to calculate cable size based on my understanding from IEC.

Is this voltage drop calculation correct? Appreciate your comments.

__________________________________________

Cable Selected: 3 Core × 35 mm²

Cable Data:

Cable Resistance @ 20°C = 0.6270 Ω/km

Cable Reactance @ 50 Hz = 0.0860 Ω/km

Cable De-rated Current = 83.1 A

Cable Length = 0.200 km

Motor Full Load = 55 kW

Voltage = 440 V

Power Factor = 0.85

Efficiency = 0.89

Full Load Current = 1000 × kW / √3 × V × p.f. × effy.

= 1000 × 55 / 1.73 × 440 × 0.85 × 0.89 = 95.4 A

Minimum no. of cable runs required = 1.1 ie 2 runs

Volt Drop = √ 3 × IFL × L × ( R CosÆ + X SinÆ ) × 100 / n × V

= 1.73 × 95.4 × 0.2 ( 0.627 × 0.85 + 0.086 × 0.527 ) × 100 / 2 × 440

= 2.17 % Maximum Allowable Volt Drop 2.5% Feeder to Motor (Running)

Maximum Starting Current (Multiple of I FL) = 6

Starting Power Factor = 0.3

Locked Rotor Starting Current Ist= 6 x Full Load Current (Direct On Line)

= 6 x 95.4

= 572.4 A

Volt Drop (Starting) = √ 3 × IST × L × ( R CosÆST + X SinÆST ) × 100 / n × V

= 1.73 × 572.4 × 0.2 ( 0.627 × 0.3 + 0.086 × 0.954 ) × 100 / 2 × 440

= 6.09 %

Maximum Allowable Volt Drop 15% Feeder to Motor (Starting)

Therefore Recommended number of cables 2 Runs of 3C × 35 mm²

__________________________________________________

But I have some doubts:

If I am using Star-Delta starter, how would this selection be changing. I guess it would affect only the voltage drop at starting. So should we take this into account or just ignore starter type and treat it as DOL?

Also in S-D, we have 2 runs of cables, so how do I change this calculation, should I treat each run separately with a starting current factor of 4.5 times instead of 6 times (used for DOL)?

Appreciate your valuable comments.

VS

The resitance values used in teh voltage drop are based on 20 deg C.

Whereas the ambient temperature and the operating temperature of the conductor will be different. It could be 70deg C or 90deg C or 105 deg C depending on teh insulation type.

Resitance at the operating temperature must be considered for voltage drop calculations.

- Ramesh V R

As per your data,

3Cx35 sq.mm de-rated current is 83.1A but FLC is 95.4A. So you decide to run 2cables. Why you need to run 2 cables for 1 motor? Why not just increase the cable size to 3Cx50 sq.mm for easy termination in the motor terminal rather than 2x3Cx35 sq.mm

Thanks & Regards,
Mwady

You can. But in a project we try to optimize on the sizes of cables to a few sizes only in order to minimize the cost. Moreover if the load gets bigger beyond the normally available cable size of 240 sqmm, this illustration helps.

in this calculation u have given resistance and reactance value

but when i m starting cable sizing without knowing which size of cable is coming .

how can i put that data in voltage drop formulla

Regards,

jayesh

The power factor cannot be any more than 1. It relates to the phase angle between the voltage and current. I would not recommend that you do these calculations unless you have some understanding of what you are doing.

Pls read the comment carefully. It never talked about powerfactor being more than 1.

hi frnd

I gone through ur formulas very nice it is . From the obtained values ,how we calculate the cable Sq.mm .Is the formula will applicable for all the electrical devices?

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hi frnd...

In your formula multiplying by sq rt 3. Is it applicable for delta? Kindly suggest and mail me

thank u

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Hi,

This is a small comment to "Ramesh V R" reply.

You are right, the max. core temperatur dep. on insulation (70°C for PVC, 90°C for XLPE, etc.). But you can save a hell of money, if you calculate and consider the real temperatur of core for your cable design. Specially for long cables it is very helpful. .

RT Nha que

Why not invest in a cable calculation program. They do all the technical stuff mentioned here such as cabletemperature versus XLPE or PVC cable and a bad power factor. See for instance cable++ on www.cablecalculation.co.uk

Yes, There is calculation for cable sizing. As you ask what care should take :

1) While workout LT Cable size you have to calculate 1st FLC of that motor.

2) Select cable from cable catalogue.

3) Derate that cable current froim derating factor (assumeing you are aware to work out derating factor)

4) Now calculate starting and running voltage drops. if that voltage drops are within permissable limit then calbe is suitable from view point of % voltage drop.

British Standard 7671.