# Urgent: Cable size calculation

Discuss Urgent: Cable size calculation in the Commercial Electrical Advice area at ElectriciansForums.net

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#### Walid_Abid

Welcome to ElectriciansForums.net - The American Electrical Advice Forum

Good afternonn,
I am new member in this forum and it's my first participation that I hope will not be the last!
I'm a mechanical ingeenier and I'm looking for the complete and correct manner or formula to determine and calculate a cable section in a three phase circuit.
It will be very helpful to give a detailed calculation with detailed terms in such circumstances (I am not known in electrical calculation, but I have to make it during a present project):

Motor power: 8kW,
voltage: 415 V,
Nominal current: I= 22 A
cable length = 50m
ambiant temperature: 50°C

Walid

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#### telectrix

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OK. to carry 22A you would select 2.5mm cable (27A) assuming no derating factors . then calcculate the volt drop.(mV/m x L x A)/1000 i.e.(15 x 50 x 22)/1000 = 16.5V. this is in excess of the 5% limit, so the n select a larger size of cable. 4mm should give a VD of (9.5 x 50 x 22)/1000 = 10.45V which value is acceptable.

then there are correction factors for ambient temp. but as 4mm will safely carry 32A, you can assume that it's adequate.

edit:but on top of that, there are other factors to consider. MCB rating and type, circuit impedance. it's not a simple matter of just current and voltage.

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#### Walid_Abid

thanks
just I don't understand exactly:
- how did you find that the cable section should be 2.5mm?
- in the volt drop, (15 x 50 x 22)/1000 = 16.5V, what is 15 means?

would you please explain all calculation steps since the beginning?

#### telectrix

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post deleted. due to brain fart.

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#### darkwood

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You really need the input of an electrician onsite (read forward to understand what i mean) also 8Kw motor at 22amps dosn't sound like a standard motor size or rating, can you give more info?

Also cable sizing shouldn't be worked on volts drop first in this case but the protective device for the cable that can handle the start up current of the motor.

Has motor got soft start or VSD
What are the installation methods of the cable
More info regarding the motor as its values dont seem to tally to the standard (a 7.5Kw would be 14.4amps)
What is the motors duty? is it an high inertial load...
I wouldn't even attempt to work cable size out without knowing the protective device needed to at the front end which is selected with regards to the characteristics of the motor and its start-up

Your 22amp motor would require a 63amp (c) mcb up front which clearly would require a cable with a higher csa, i would suggest you get a qualified electrician in as designing said installs can be very indepth and usually requires alot of calcs to ensure V.D. isn't exceeded, cable rating exceeds its protective device, nuisence tripping is eliminated etc etc

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#### malcolmsanford

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Also remember Tel you can exceed the 5% VD on a motor due to it's start up mate 525.102, gives details

#### darkwood

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Can i also add if you do employ the services of an electrician then ensure s/he's experienced with motors and machinery as this type of install goes beyond the standard learning of electricians, what you need is an electrical engineer to design your install, its hard to give exact advice because too many parameters come into play when installing supplies to machinery etc, many factors would be picked up with a site visit that may not be given within this thread and could be crucial to install.

Ps forgot my manners.. Welcome to the forum and can i ask where you are based as you said good afternoon and it was only aprox 10am here in the rainy UK

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#### Walid_Abid

ah ok!
no problem for the manner.
waw, I miss this kind of whether just now!!
I'm Tunisian and I am working in the Kingdom of Bahrain: just now, it's approx 1pm with app 43°C and 70% of humidity

#### darkwood

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Uk at the moment is very well air-conditioned the natural way with months of heavy rain and flooding so pretty much the other end of the scale...

Your front end fusing methods regarding your query will change the scope of the install totally, lets 'assume' you are using HRC fuses up front GM rated (allows for motor inrush) then you can pretty much fit 32amp fuses thus meaning Tel's post wont be far off the mark but this would need ambient temp calcs adding as you are clearly Hot over there amonst other calcs, again this is ballpark based on the assumption its a standard motor with standard start-up characteristics.
I hope you can start to appreciate now that if we dont ask the right questions and receive all the info we need then you may get an incorrect answer that could be unsafe if followed.

Also can i stress we are following BS 7671 here in UK and Bahrain will hopefully have its own set of regs but may differ, you really need the knowledge of a local electrical engineer who is familiar with local standards.

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#### darkwood

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Lenny... these links give you a basic easy to follow (for those in the trade) cable calcs, but reading my earlier posts in this thread im trying to explain that if the front end device is say an mcb (BS60898) then these calcs will give you a wrong answer as specific guidelines from MCB manufacturers have tabled values with regards to motors, transformers and inductive lighting, if the OP is correct in that the FLC is 22amp for the motor then a MCB rated value of 63amp type (b) or (c) will be required to avoid nuisance tripping, although 25amp (d) would also be adequate but this leaves issues with meeting requirements with ELI.
Standard form cable calcs should not be applied when supplying inductive loads until you attain the nature of the front end device.

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#### Guest123

I replied to his other duplicated thread which didn't have half the info of this one.

#### darkwood

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I replied to his other duplicated thread which didn't have half the info of this one.
Soz Len did see the merge sign but was unaware of other thread contents but that aside you have drafted a nice little cable calcs page which will come in handy for most with standard requirements .

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