NS Dev
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| posted on 8/8/12 at 09:16 AM |
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Electricians - Cable size for 4KW (8A) 3 phase motor load
Hi folks!
Could do with an electrician who knows motor calcs to confirm a cable size for me before I buy it!
I'm installing a 4kw 3 phase motor in an extraction system, the motor plate says 8 amps at 415 volt supply.
Its on a 45m run from the supply board which will house the breaker supplying it. It won't be doing frequent starts so shouldn't need to
allow for startup current when calculating cable size.
It will be run for 25m in metal 4" square trunking, with a couple of other cables, and the remainder clipped to vertical perforated tray.
Ambient temp will be 25 deg c max unless its a REAlly hot day (what's one of those!!)
Basic question is......will 2.5mm do the trick or do I need to up it to 4mm on that run length? While I can calculate the volt drop, I don't
know all the ruddy correction factors, and as usual using google results in lots of arguing americans on forums!!!
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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daniel mason
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| posted on 8/8/12 at 11:55 AM |
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id probaby run a 2.5mm 4 or 5 core swa. if its 8 amps per phase? does it have a neutral?
id run a c type 16amp tripple pole breaker in the dist board
a 3 phase isolator/switch fuse with a 3 phase socket below
then an armour flex with a 3 phase plug on the end to the machine so disconnecting is simple should you ever need to move it.
as long as your 2.5mm cable is protected by a fuse with a lower rating than the cable can carry,and its not tripping breakers it will be fine! eg a
2.5 can cope with 20 amps so if on a 16 amp fuse and machine working fine its all good
[Edited on 8/8/12 by daniel mason]
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rallyingden
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| posted on 8/8/12 at 12:10 PM |
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According to Amtec power software
4Kw Motor FLC 8.23A ( PF 0.8 )
Cable 70Deg C Thermoplastic insulated Armoured in Conduit ............ Size 3C 1.5mm Max length for 8V volt drop limit is 54.12m Actual volt drop
3.82V
Cable rating 18A but in rush current shown as 20A
Based on 400V 3Ph supply and all cable installation methods other than in Infloor troughs & trenches when the cable size goes up to 4mm.
All in all your 2.5mm cable will be fine.
RD 
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NS Dev
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| posted on 8/8/12 at 10:10 PM |
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Thanks folks!!!
Just what I was hoping to hear.....2.5mm is cheap!!
(just to clarify, I used to do a lot of 3 phase installation work in my old job, but I'm not a qualified electrician. All the work I did was
tested by the sparkys before going live, and I always had them to check cable sizes etc before I bought the stuff, i.e. I designed the layouts and did
the donkey work, so that we could cope with the workload with less contractors!!)
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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NS Dev
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| posted on 8/8/12 at 10:17 PM |
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quote:
Originally posted by daniel mason
id probaby run a 2.5mm 4 or 5 core swa. if its 8 amps per phase? does it have a neutral?
id run a c type 16amp tripple pole breaker in the dist board
a 3 phase isolator/switch fuse with a 3 phase socket below
then an armour flex with a 3 phase plug on the end to the machine so disconnecting is simple should you ever need to move it.
as long as your 2.5mm cable is protected by a fuse with a lower rating than the cable can carry,and its not tripping breakers it will be fine! eg a
2.5 can cope with 20 amps so if on a 16 amp fuse and machine working fine its all good
[Edited on 8/8/12 by daniel mason]
Yep 8 amps per phase, no neutral. yep motor on 16a plug and socket already on armoured flex. Will connect that to starter/relay box, then to isolator,
then the long run back to the board in the workshop. Will get a used 16a breaker ordered off ebay (they are square d QOE ones, hard to get new now,
but tough things that don't go wrong like cheap nasty ones now!) only got a 20a d type breaker spare at the mo.
thanks for the help!
cheers
Nat
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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