How to size cables, MCBs/fuses & starters for a motor-driven machine?
How to size cables, MCBs/fuses & starters for a motor-driven machine?
(OP)
Hello! members,
I have a 5.5kW, 3-phase lathe to hook up, and am wondering how I can choose properly-rated cables, MCBs or fuses, and starter for this job.
Can you share your knowledge, or direct me to or even provide suitable technical documents to help me in this regard?
Many thanks in advance!
PS. our power supply is 230v/400v system.
I have a 5.5kW, 3-phase lathe to hook up, and am wondering how I can choose properly-rated cables, MCBs or fuses, and starter for this job.
Can you share your knowledge, or direct me to or even provide suitable technical documents to help me in this regard?
Many thanks in advance!
PS. our power supply is 230v/400v system.





RE: How to size cables, MCBs/fuses & starters for a motor-driven machine?
Have a look at the Siemens, ABB, or Telemecanique contactor catalogues. All have a chart which shows the recommended contactor / overload / protection to assemble a type-tested starter for a given nominal motor power.
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If we learn from our mistakes I'm getting a great education!
RE: How to size cables, MCBs/fuses & starters for a motor-driven machine?
Rafiq Bulsara
http://www.srengineersct.com
RE: How to size cables, MCBs/fuses & starters for a motor-driven machine?
Rafiq Bulsara
http://www.srengineersct.com
RE: How to size cables, MCBs/fuses & starters for a motor-driven machine?
As a rule of a thumb I'll take 2 A per kW then 11 A as rated current.
The breaker could be 10-16 A. If the cable length from supply panel [or MCC] is less then 50 m the cable may be 3*4 sqr.mm copper, but the grounding wire has to be 6 sqr.mm copper wire. For 100 m cable length 3*6 sqr.mm will be necessary.
RE: How to size cables, MCBs/fuses & starters for a motor-driven machine?
RE: How to size cables, MCBs/fuses & starters for a motor-driven machine?
RE: How to size cables, MCBs/fuses & starters for a motor-driven machine?
Thank you for the kind support, which has been really helpful!
I've searched for manufacturers' technical information, but somehow haven't found the sheet showing how to select a suitable fuse/MCB for protection of cable to the lathe.
If you happen to have these info available, or know about these links on website, I am keen to get them if you don't mind.
Many thanks in advance!
RE: How to size cables, MCBs/fuses & starters for a motor-driven machine?
If the motor started star delta approx 2.5 x FLC (but the current inrush on the delta transition can be problematic when using mcb's).
You can look at the tripping curves of the protective device to find the correct size.
Once the size of device is selected, select the cable size that will carry that current and be of suitable size to prevent excessive volt drop if the cable is to be run over any distance.
In the UK all the relevant charts are in the IEE Wiring regulations.
RE: How to size cables, MCBs/fuses & starters for a motor-driven machine?
Rated current: 11.3A
Fuse for DOL start: 25A
Cable sized on thermal rating for XLPE, surface installation (because I don't know if volt-drop is going to be a problem or not): 4mm^2 minimum
You might look at a manual motor starter which incorporates a breaker, overload and contactor in one assembly.
Is this a home job? If it is in the UK then you might want to read Part P of the Bulding Regs.
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If we learn from our mistakes I'm getting a great education!
RE: How to size cables, MCBs/fuses & starters for a motor-driven machine?
Thank you very much for the contribution of your knowledge!
I think I have pretty much got the answers, and will share with you about how it goes later.
Cheers!
Norman