IET logo
 
IET
Decrease font size
Increase font size
Locked Topic Locked
Topic Title: urgent: cable size calculation
Topic Summary:
Created On: 14 July 2012 10:19 AM
Status: Post and Reply
Linear : Threading : Single : Branch
1 2 3 Next Last unread
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 14 July 2012 10:19 AM
User is offline View Users Profile Print this message



WalidAbid

Posts: 1
Joined: 14 July 2012

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

thank you in advance
Walid
 14 July 2012 10:53 AM
User is offline View Users Profile Print this message



perspicacious

Posts: 7172
Joined: 18 April 2006

Welcome Walid

I expect others will be asking you for details of the overcurrent device and the earth fault loop impedance at the start of your proposed circuit.....

Regards

BOD
 14 July 2012 11:03 AM
User is offline View Users Profile Print this message



John Peckham

Posts: 7401
Joined: 23 April 2005

BOD

In addition the cable type, the installation method, motor method of starting, overload protection for the motor, frequency of starting, starting current and the power factor. Is the motor power input or output and efficiency if the later?

-------------------------
John Peckham

http://www.astutetechnicalservices.co.uk/
 14 July 2012 11:57 AM
User is offline View Users Profile Print this message


Avatar for Legh.
Legh

Posts: 3478
Joined: 17 December 2004

I would hazard a guess that pf = 0.87lag

edit:
or is it 0.5 lag

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 14 July 2012 12:37 PM
User is offline View Users Profile Print this message


Avatar for Dave69.
Dave69

Posts: 445
Joined: 16 July 2011

sounds like you need to buy a better motor if its rated at 8kW and has a FLC of 22 amps

you need to know the motor duty, type of cable, how it will be installed (in ducts with other cables(on tray with other cables)) etc. is it protected my motor rated fuses or an MCB.

I would say slap in a bit of 6mm2 swa and to hell with the clacs
 14 July 2012 09:29 PM
User is offline View Users Profile Print this message


Avatar for slittle.
slittle

Posts: 3482
Joined: 22 November 2007

Dave,

That's either known as experience or finger in the wind design.

Either way, I like your thinking


Stu
 15 July 2012 09:56 AM
User is offline View Users Profile Print this message



perspicacious

Posts: 7172
Joined: 18 April 2006

John

Started simple so that OP wouldn't be scared off by subsequent posters

Regards

BOD
 15 July 2012 11:43 AM
User is offline View Users Profile Print this message



Jaymack

Posts: 4617
Joined: 07 April 2004

Originally posted by: WalidAbid
voltage: 415 V,

An old motor then or are you outside of the U.K.?

ambiant temperature: 50°C

Is this also the ambient temperature of the motor? If so, what is the method of cooling?, ducted ventilated or not? Derating may be applicable, or a larger frame size is required.

What is the pull out torque requirement for the method of starting, (I'd imagine that this is D.O.L.), and the permissible volt drop on starting, at the lowest tolerance of voltage?

Regards
 15 July 2012 03:46 PM
User is offline View Users Profile Print this message


Avatar for kj scott.
kj scott

Posts: 2144
Joined: 02 April 2006

It's a simple cable calc, why go in to irrelevant motor characteristics, to confuse the OP; or is it prove you know it?

-------------------------
http://www.niceic.biz
 15 July 2012 03:57 PM
User is offline View Users Profile Print this message



Jaymack

Posts: 4617
Joined: 07 April 2004

Originally posted by: kj scott
It's a simple cable calc, why go in to irrelevant motor characteristics, to confuse the OP; or is it prove you know it?

L.O.L. I've had experience with 'consultants' coming a cropper on motor applications. It needs a proper engineering calculation, not the usual simplistic wiring regulations. The voltage drop is critical to motor starting w.r.t. the torque requirements of the load, and economical design! Ducks in a row! Quack, Quack.

Regards
 15 July 2012 04:11 PM
User is offline View Users Profile Print this message


Avatar for kj scott.
kj scott

Posts: 2144
Joined: 02 April 2006

I don't know about ducks, more like peacocks.

-------------------------
http://www.niceic.biz
 15 July 2012 04:24 PM
User is offline View Users Profile Print this message



Jaymack

Posts: 4617
Joined: 07 April 2004

Originally posted by: kj scott
I don't know about ducks, more like peacocks.

L.O.L. Can't show your feathers in a good light? How many motor circuits have you designed?

Regards
 15 July 2012 06:03 PM
User is offline View Users Profile Print this message


Avatar for kj scott.
kj scott

Posts: 2144
Joined: 02 April 2006

Enough to know how to, what's your problem.

-------------------------
http://www.niceic.biz
 15 July 2012 08:26 PM
User is offline View Users Profile Print this message



Jaymack

Posts: 4617
Joined: 07 April 2004

Originally posted by: kj scott
Enough to know how to, what's your problem.

You talking to me? You started it!

And elephant time ..... What is achieved by testing for Zs, at every 13A socket outlet, on an E.I.C.R.?

Regards
 15 July 2012 08:31 PM
User is offline View Users Profile Print this message


Avatar for rocknroll.
rocknroll

Posts: 8835
Joined: 03 October 2005

Cable calculations come in two parts and there are no ducks or techno-bullsh!t.

Part One (cable calculation and selection)

1. Calculate the design current for each circuit applying any diversity factors.

2. Select type and nominal rating of protective device.

3. Determine and apply correction factors.

4. Select cable.

5. Calculate voltage drop and check for compliance.


Part two (shock protection and thermal constraints)

6. Check circuit complies with shock protection.

7. Check that circuit complies with thermal constraints.

Worked example for you to play with;

A 400V 50Hz three phase motor with an output of 7.5kW, power factor of 0.8 and efficiency of 85% is to be wired using 500V light duty three core mineral insulated p.v.c. covered cable. The length of the run from the HBC protecting fuses is 20m and for half this run, the cable is to be clipped to wall surfaces. For the remainder it shares a cable tray touching two similar cables across the top of a boiler room where the ambient temperature is 50 degrees C. Select an appropriate BS88 fuse rating suggest a size of cable that may be appropriate and determine voltage drop.

Notes:
1. Efficiency will have an effect on the input power required. ** later in post.
2. For three phase systems, Ib = Power / √ 3 x line voltage x power factor.
---------------------------------------------------------------------------
(Ib) = calculated load or design current after applying any diversity factors.

Ib = (kW x 100 / √ 3 x UL x Cos 0) x (100/%)

Ib = (7.5 x 1000 x √ 3 x 400 x 0.8) x 100/85)

Ib = (13.55) x (1.18)

Ib = 15.9A
----------------------------------------------------------------------------
(In) = size of nearest appropriate fuse.

In = 16A BS88 fuse.
----------------------------------------------------------------------------
Correction factors for ambient temperature (Ca) and grouping with other cables (Cg)

(Iz) is the minimum current the cable will need to carry.

Iz = In / Ca x Cg

Iz = 16 / 0.67 x 0.81

Iz = 29.48A
----------------------------------------------------------------------------

In this case the cable selected from the appropriate tables in BS7671 was a MI cable of 4.00mm ² with rating of 37A and a voltage drop of 9.1 mv/A/m, in order to ensure the voltage drop is within the limits 4% of 400V (400 x (4 / 100) = 16 volts, the calculation for this case is;

Vd = mv/A/m x Ib x cable length / 1000

vd = 9.1 x 15.9 x 20 / 1000

vd = 2.89V

So the cable and fuse comply.
----------------------------------------------------------------------------

Your question does not require shock protection and thermal constraints to be examined so there is no need at the moment to progress to stage two.

Lets examine the information you have given;

Motor power: 8kW,
voltage: 415 V,
Nominal current: I= 22 A **
cable length = 50m
ambiant temperature: 50°C
No power factor or any other information is given so dont cook up numbers to suit, stick to the information supplied.


The Nominal current ** will be the nameplate current or FLC so as I noted above the efficiency will have an effect in the input power required so to find Ib you will have to do two calculations.

Ib = (kW x 100 / √ 3 x UL x Cos 0) x (100/%)

Ib = (8 x 1000 / √ 3 x 415)

Ib = 8000 / 717

Ib = 11A

Now at the moment the design current is given as 11A and in your information you have a nominal current of 22A so we need to work the efficiency and do the calculation again.

Efficiency = calculated or design current / nominal (FLC) current x 100
Efficiency = 11 / 22 x 100
Efficiency = 50%

Therefore:

Ib = (8 x 1000 / √ 3 x 415) x (100 / 50)

Ib = 8000 / 717 x 2

Ib = 22A

So we can safely say that the motor has an efficiency of 50%.



No ducks or peacocks were harmed in the application of this simple task.

I am on the train at the mo and do you know that this is only the third train I have been on since Xmas that has bloody Wi-FI so I am just killing time with this post, on the continent Wi-Fi on every train is standard.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 16 July 2012 at 10:39 AM by rocknroll
 15 July 2012 09:47 PM
User is offline View Users Profile Print this message



Jaymack

Posts: 4617
Joined: 07 April 2004

Originally posted by: rocknroll
Cable calculations come in two parts and there are no ducks or techno-bullsh!t.

It's only one methodical electrical calculation, but the pre-requisite is getting the mechanical requirements correct! i.e. Torque and starting duration etc.! After that it's straightforward. Quack, Quack!

Regards
 15 July 2012 10:29 PM
User is offline View Users Profile Print this message



Zs

Posts: 2842
Joined: 20 July 2006

Welcome to the forum Walid. Eventually you will get a straight answer. Rock & Roll seems to have provided it for you but keep asking. It is, in fact a fairly straightforward flow of a set of calcualtions and just a process. If you're really stuck then I'll scan and send you some similar ones from my scruffy notes but I reckon you've got it now. But do ask if you have not covered Rock's section 2, it is really important if this is to be a real installation.

Rock, the last time I was looking at electrical stuff on a train was notes for GN3 on the way into Savoy Place. I found myself totally distracted looking over the shoulder of the woman next to me. Her train-fodder was so enviable and was a detailed study of harmonics in stringed instrument playing. All bearing a Royal Academy of Music logo. I tend not to talk to fellow passengers so I just did a few glances.

Towards the end of the journey she spoke to me and said she had been doing the same and how much more interesting she found my pages to hers. We damned nearly swapped and went to each other's meetings

I wonder who was looking over your shoulder?

Zs
 15 July 2012 11:08 PM
User is offline View Users Profile Print this message


Avatar for rocknroll.
rocknroll

Posts: 8835
Joined: 03 October 2005

Originally posted by: Zs

Welcome to the forum Walid. Eventually you will get a straight answer. Rock & Roll seems to have provided it for you but keep asking. It is, in fact a fairly straightforward flow of a set of calcualtions and just a process. If you're really stuck then I'll scan and send you some similar ones from my scruffy notes but I reckon you've got it now. But do ask if you have not covered Rock's section 2, it is really important if this is to be a real installation.

Rock, the last time I was looking at electrical stuff on a train was notes for GN3 on the way into Savoy Place. I found myself totally distracted looking over the shoulder of the woman next to me. Her train-fodder was so enviable and was a detailed study of harmonics in stringed instrument playing. All bearing a Royal Academy of Music logo. I tend not to talk to fellow passengers so I just did a few glances.

Towards the end of the journey she spoke to me and said she had been doing the same and how much more interesting she found my pages to hers. We damned nearly swapped and went to each other's meetings

I wonder who was looking over your shoulder?

Zs


LOL Afraid from Newbury to Paddington I was the only one in the carriage apart from the lady who serves the free coffee and biscuits and she was too busy lying across the seat at the end talking on her mobile. Bless her!!!

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 16 July 2012 02:18 PM
User is offline View Users Profile Print this message


Avatar for rocknroll.
rocknroll

Posts: 8835
Joined: 03 October 2005

Here we go some smarta$$ bounced this to top without revealing themselves.

I have to take credit for being the only one to answer the OP's question without putting them through the usual technical hoops that are created on this forum, I must say before I posted I did look around for someone who looked like they had gone through some elementary education to check my calculations, I noticed a young chap clearing tables, picking up rubbish and checking the toilets who seemed a viable candidate, anyway, after some checks he remarked "I thought being an electrician was technically challenging and akin to rocket science but its not really is it, its quite easy, how do I become an electrician?", I directed him to a competent training provider in the Croydon area who specialises in creating electricians in 5 days to which he thanked me for and went on his merry way whistling 'things can only get better'.

I thought it was very nobel of me not only to answer the first poster who I feel is somewhat scared off by you lot now, but also found a future sparky to help you as you are so busy, and you never know may work hard and become an accomplished designer, just looking after the future.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 16 July 2012 at 05:18 PM by rocknroll
 16 July 2012 03:01 PM
User is offline View Users Profile Print this message


Avatar for kj scott.
kj scott

Posts: 2144
Joined: 02 April 2006

It would appear that the OP has already run off in despair.
Your calculation example is a good effort to start the ball rolling, but is incomplete with too many assumptions.
Shock protection has to be considered, as does motor duty cycle; I think BOD's approach was a good way, starting gently. We don't know if this is study; or a real world calculation yet.
btw,

It x Correction factors = Iz

As opposed to

In / Correction factors

-------------------------
http://www.niceic.biz
IET » Wiring and the regulations » urgent: cable size calculation

1 2 3 Next Last unread
Topic Tools Topic Tools
Statistics

See Also:



FuseTalk Standard Edition v3.2 - © 1999-2014 FuseTalk Inc. All rights reserved.