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How to size cables, MCBs/fuses & starters for a motor-driven machine?

nc96 (Electrical) (OP)
12 Nov 09 21:38
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.
ScottyUK (Electrical)
13 Nov 09 2:11
Most of the controlgear manufacturers have sizing guides for breakers / fuses and contactors. Cable should be sized based on the MCB or fuses protecting it, subject to the volt-drop at the motor terminals during starting being acceptable.

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.

If we learn from our mistakes I'm getting a great education!

rbulsara (Electrical)
13 Nov 09 7:51
Local codes may also govern minimum sizing. They do in North America and NEC world.

Rafiq Bulsara

rbulsara (Electrical)
13 Nov 09 7:52
or maximum rating for overcurrent devices.

Rafiq Bulsara

7anoter4 (Electrical)
14 Nov 09 1:41
First of all I agree with ScottyUK and rbulsara.
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 copper, but the grounding wire has to be 6 copper wire. For 100 m cable length 3*6 will be necessary.
cjhut (Electrical)
14 Nov 09 11:22
I agree with the above, but as another note, in the United States, we passed new laws requiring personal protection equipment from electrical arc flash. It's very cumbersome facesheids and clothing. Alot of money and time was spent in studying the effects of short circuits and the associated fault currents. If I were to choose between fuses or circuit breakers, it would always be fuses. If you don't know what your available fault current is, you could size it for "Low Peak" fuses. This would protect personnel and equipment, in some cases from damage. I beleive if you go to Bussman's website, they have some pretty cool clips showing a fault with a breaker, and then with a low peak fuses.
davidbeach (Electrical)
14 Nov 09 12:15
Drunk the Bussman Koolaide have you?  A current limiting fuse with fault current below the current limiting point is a terrible device to use, can be extremely slow.  There is no one size fits all solution.  I wish Bussman would be far more truthful in their promotional material; fuses have their uses but they are not the end all and be all of overcurrent protection.
nc96 (Electrical) (OP)
17 Nov 09 18:21
Hello! All,

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!

jameselect (Electrical)
18 Nov 09 7:51
Typically for a 5.5kw motor the motor FLC would be 11 amps. The motor could be expected to draw 6 x flc when starting D.O.L so you would need to choose a protective device that would not trip for say 10 secs at 66 amps.
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.
ScottyUK (Electrical)
18 Nov 09 15:43
Ok, from my ready reckoner chart for a 'typical' 4-pole induction motor: 5.5kW machine, 0.82pf, 400V, 86% efficient.

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. wink

If we learn from our mistakes I'm getting a great education!

nc96 (Electrical) (OP)
5 Dec 09 5:23
Hello! All,

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.



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