Cable Sizing for Motor Feeder

So, what is wrong with using the applicable national electrical standard, such as British Standard 7671?

1 and 2. Why should starter / controller selection change the LENGTH 125 mtr ?

Please clarify.

Good question!

The very first thing is once located - distance between starter and motor will remain fixed (will not change) unless either starter or motor are re-sited. Regarding cables you need calculate resistance and drop of voltage in the cable due to distance and size of cable and if this is more than 10% (Some standards allow up to 15%), better go for a higher size of the cable to reduce resistance.See that voltage regulation permitted by local law is not fouled. One good idea is to use copper cables but it will cost high and at some locations law may not permit it. Consider these aspects and decide cable size and not the length , which remains fixed once located, notwithstanding the size of motor and type of starers you use.

Refer EC&M website

A DOL-started motor can take a starting inrush of 6 to 8 times the rated full-load current. Star Delta staring will be 1/3 of that. This inrush current will have some bearing on the cable sizing, whatever the source of information.

Reputed controlgear manufacturers give complete details of all equipment required for starters (contactors, relays, SCPDs, and cables) based on motor rating. You can refer to any catalogue for that.

If the control cables are also very long, you may want to check the effect of (a) voltage drop due to cable impedance and (b) non-switching-off by the stop button due to cable capacitance. There are excellent papers about this on the web. However, since you mention MCC and the motor being far away, this may not be a factor.

As M.R.Iyengar already said, the cable length stays unchanged. What, I think, you meant it is: if the voltage drop has to be calculated -or limited- for DOL start or Star/Delta .The current at star connection will be 1/3[ as kvsridhar already said], but the torque will decrease in the same proportion. If it is an usually motor Design B as per NEMA MG-1 or Design N as per IEC 60034-12 at rated voltage the minimum starting torque will be 1.3 [NEMA MG-1 Table 12-2 for 30 hp [22 kw] 2 poles, IEC 60034-12 Table 1- 2 poles for 16<Pn<25 kW], then in star starting case Tlock=1.3/3=0.43*Trated.

If the voltage drop -from the rated- will be 10% then Tlock=0.9^2*0.43=0.348*Trated.

Now, it depends on the required load starting torque. A pump or a ventilator could manage the case, but a lift or other constant torque installation could not.

I am in agreement with 7anoter4 & kvsridhar regarding the current to be taken for calculations of the drop of voltage. Besides If the largest available cable size ( may be with the user or in the vicinity market or the one motor terminal box can accommodate doesn't sort out the issue then comes the questions of multiple runs. This while takes care of current handling also reduces resistance because of parallel runs. But one must consider the connecting the cable into the motor terminal box. The Glands size may have to be considered.