selecting a cable

[Farmelectrics]

advice please need to select a cable for the following not sure with the calculation for the starting current 30kw motor 400v full load current 52amps but 358amps starting current ive selected a 10mm cable how do you allow or what calcs do you apply for starting currents

 

[Pete999]

advice please need to select a cable for the following not sure with the calculation for the starting current 30kw motor 400v full load current 52amps but 358amps starting current ive selected a 10mm cable how do you allow or what calcs do you apply for starting currents

What Type and Size OCPD are you intending using?

 

[Farmelectrics]

that depends on the size of the cable I'm not to sure how to allow for the starting current hence why I'm asking for help

 

[telectrix]

for a 52A load your OCPD needs to be > 52A., i.e. next size up is 63A (or 60A if a fuse can be used). then the cable needs to be selected acording to the design load ( Iz> Ib ). if ref method C or E, then 10.0mm will do. however your query is as to whether the 10,0mm cable will withstand the start-up current. IMO, as this is for a very short duration, I'd say yes. better minds may disagree.

 

[Lucien Nunes]

The cable will need to be adequate for:
Running voltage drop
Starting voltage drop
Running CCC
Starting CCC
Loop impedance

Running CCC and voltage drop are exactly like any other load. Maximum permissible starting voltage drop will depend on application, mode of starting and supply impedance. For a load of this size it is worth checking with the manufacturers as to what it will accept. Too much starting VD on a fan will just increase the runup time by a second or two, whereas too much on a conveyor supporting a static load might prevent it starting at all. Starting CCC is often satisfied once everything else is - it won't be taking the 358A LRA all the way up to speed - but start duration and frequency are also functions of the application. This will inform the choice of OCPD and hence confirm that the cable will satisfy disconnection time again allowing for the possibly significant contribution from external EFLI.

TL;DR: Check the starting duty. If the cable is long or CSA requirement seems excessive, seek advice from a motorhead.

 

[MJPD29]

How is the cable ran? Clipped direct etc. Also length of cable and other factors to be considered

 

[DPG]

How is the cable ran? Clipped direct etc. Also length of cable and other factors to be considered

I think he's OK on that, it's just the large starting current compared to normal running.

 

[Kidsolo]

Just work it out on your full load current of 52 amps

 

[marconi]

If the motor application is a low starting torque application and the motor can be connected star then delta, you might consider using a star-delta motor contactor instead of a direct-on-line one. This would reduce the starting current in the lines by a third but the starting torque is reduced by about a quarter to a third too which may not be suitable. So starting current would become 358 x 1/3 = 120A. This might make a worthwhile cost saving if it is a long run. Also less of a disturbance to the mains supply.

 

[FarmersFriend]

advice please need to select a cable for the following not sure with the calculation for the starting current 30kw motor 400v full load current 52amps but 358amps starting current ive selected a 10mm cable how do you allow or what calcs do you apply for starting currents

Also consider the environment the cable I’d installed in, if ifs a fan in a grain drier ambient temperature could be high, if it’s running through a milking parlour or slurry it will be subject to strong chemical attack. All farm cables seam to be eaten or farmerised (abused?) so install the toughest you can!

 

[Farmelectrics]

thanks for the advice its a direct on line starter and agree will tel 10mm is adequate I think

 

[Spoon]

thanks for the advice its a direct on line starter and agree will tel 10mm is adequate I think

You can't pick a cable size without knowing what breaker/fuse size you are using.
What size are you using?