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Topic Title: Cable size calcs for motor please help!! Topic Summary: Created On: 21 April 2013 01:38 PM Status: Post and Reply 
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21 April 2013 01:38 PM


hi basically i am doing my final year project on my degree and i have some confusion.. i need cable sizes worked out via calculations
i have a 18.5Kw motor 400v 960RPM i didnt get a power factor but have sourced a good estimated one by finding a very similar motor and used that so working of 0.85 for power factor i have a run of 30M and will be 3 core in SWA cable for my calcs i have been getting really confused how would i go about calculating the needed cable size ive donea few calcs but i feel there wrong and i need to get it done asap please help many thanks ps motor is star connected 



21 April 2013 02:19 PM


My motor current chart gives 35 A per phase.
That said you've got start up to consider. What sort of sizes have you come up with ?? Logically 4mm would probably deliver 35A over that sort of run (without getting my books out on a Sunday afternoon) but you've got motor start currents to consider which could be pretty high. Stu 



21 April 2013 02:26 PM


You should find working examples in this book that will help you on your way;
http://www.teammccallumuk.com/...anced7thEdition.pdf 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!"  



21 April 2013 02:30 PM


ill be using a VFD to control so start up current is okay,
I have worked out 1 set of figures like so: due to start Ip=Il IP=18500/400 = 46.25 how ever in catalouge for 400v full load it give 36.31 so i used that instead Input power = v x i x PF x sqrt 3 / 1000 input power = 21.38 anyway using current from 18.5kw smallest protection 40A fuse = In ambient temp @40 in tables SWA calculated as 70 degree PVC?? 0.87 = Cg in another table this is 1 circuit all in its own cable therefore Ca=0.8 Iz = In/Cg x Ca Iz= 57.47 Using another table rating for sheathed multicore PVC cable (3 core) 10mm be suitable with volt drop of 3.8 mc/a/m however i done a quick look at the input power 21.38 calc and its even higher im really confused i will also have to calculate a meter of 4 of these motors in parallel... (using VFD to power 4 motors in parallel for long travel system) hope this helps im so confused! 



21 April 2013 02:38 PM


thanks rocknroll ill take a look




21 April 2013 02:59 PM


Should have added on my post, 4mm would be ok for a resistive load at that length....
It depends as well whether you are designing for good engineering or for a budget. From experience I'd be looking at a larger cable than the maths gave me. My last 22kw floating pump is running on 25mm flex because sometimes you need a bit more mechanical strength as well as the electrical characteristics. Stu 



21 April 2013 03:11 PM


ahh cheers for the info ! also would a 3 core swa go down as single cores with 3 circuits enclosed or ?? i cant really find the info for this either and its more for engineering over budget as its for my final year project sorry just alot of confusing factors on cable sizing in the past its been done by engineers on site etc




21 April 2013 04:54 PM


all sorted now people thanks for help i almost had it right first time was just out! many thanks again




21 April 2013 05:41 PM


My motor current chart gives 35 A per phase. That would be the FLC for the nameplate power. And for the input current at full load? Regards 



21 April 2013 06:38 PM


You'll want the power factor and some formulae.
It was Sunday afternoon and I was being lazy Stu 



21 April 2013 06:40 PM


IP=18500/400 = 46.25 This is the Output power. Output power = Input power × Efficiency, but you know that already as a budding engineer. Correct = where the input power is in kW. What means meter? (sic) The application appears to be an overhead gantry crane or a stack reclaimer, with 4 motor drives on rails. (Thinks, how would you avoid crabbing?) The input current results for one motor, are multiplied by 4 for the total. Regards 



21 April 2013 06:43 PM


You'll want the power factor and some formulae. Not so, it's a lot simpler, you'll just need the efficiency. Regards 



21 April 2013 06:58 PM


hi basically i am doing my final year project on my degree and i have some confusion.. i need cable sizes worked out via calculations i have a 18.5Kw motor 400v For an engineering exam especially, you need to use the correct units! i.e. kW and V in this case. Regards 



21 April 2013 08:30 PM


The caps have been in wrong place on this post few times jay my bad the Ip was for phase current and Il line current , also I've just realised I've only worked my cable sizes out for output power too doh!! Been too stressed today with Uni ! Sort it out tomorrow to input powers




21 April 2013 09:19 PM


also I've just realised I've only worked my cable sizes out for output power too doh!! Been too stressed today with Uni ! Sort it out tomorrow to input powers Just divide the output power by the efficiency. Regards 



21 April 2013 10:14 PM


Will do jay
Thanks 



21 April 2013 11:04 PM


Assuming your 18.5kW is input power (if not and it is output you will need to know the efficiency) then Ib the design current =
18500/(400 x route 3 x PF) = 18500/( 400 x 1.732 x 0.85) = 31.42A. Select circuit protection In. A 32A Type C MCB would be theoretically OK but sensible design would be 40A for some head room. If the VFD provides overload protection then Iz = Ib/any rating factors. You have not mentioned any rating factors so Iz = Ib. If the VFD does not provide overload protection then Iz = In/rating factors. Selecting XLPE cable (90 degrees) but using PVC cable data for 70 degrees in BS7671. So assuming Iz = Ib nearest cable size for PVC SWA is 4mm for 3 core on tray (35A). Then check volt drop = 30 x 9.5mV/A/m = (30 x 31.42 x 9.5)/1000 = 8.95V. That is acceptable as max = 5% = 20V. Max Zs for 40A BSEN 60898 MCB = 0.57 ohms. R1 + R2 for 30m 4mm XLPE SWA @ 20 degrees = 0.36 ohms x 1.2 =0.43 ohms at 70 degrees. So if your Ze is less than 0.14 ohms (Zs R1 + R2) then you are Ok. For a fault current of 7kA the energy let thro for a 40A Schneider MCB IS 23,000 A squared seconds. The thermal constraint for the cable ( S squared x K squared ) is 21 squared x 51 squared for the steel CPC. = 1147041. So the thermal constraint is ok as I squared t is less than S squared x K squared. So after all that Stu you were correct with 4mm!  John Peckham http://www.astutetechnicalservices.co.uk/ 



22 April 2013 09:33 AM


So after all that Stu you were correct with 4mm! And for allowable starting V.D.? Regards 



22 April 2013 09:53 AM


Assuming your 18.5kW is input power From the OP  "i have a 18.5Kw motor" Regards 



22 April 2013 10:28 AM


thanks for info jay and john
the overload i have looked into and sorted that also now, but the motor manufacturers full load current has been given as 36.31A so this si the figure i will be using.. the size i have calculated is 10mm^2 does this seem right? many thanks 



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Cable size calcs for motor please help!!

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