Cable and mcb sizing

pierce · Jan 20, 2016

What is the proper way to size this up.
7.5 kw 3 phase + n motor in a dry cleaning machine.
Is it 7500/400x1.732= 10.82 amps per phase
= 2.5 mm2 in trunking method 3, 21amps current carrying capacity
Do i have to oversize the cable for inrush current? Or is 2.5 ok?
Thanks.

jackhammerJIM · Jan 20, 2016

calculation is good .

You need to take length of run into account . Don't have regs to hand but depending on length of run and reference method i would have thought 1.5mm would suffice ...

You need to take inrush current into account for sizing the type of over current device not cable size . The inrush will be over a short period of time .

telectrix · Jan 20, 2016

2.5mm is OTT. 1.5mm is the appropriate size unless volt drop is excessive.

Darkwood · Jan 20, 2016

Is the 7.5Kw the max demand of the dry cleaner or the rating of the motor? Your opening post is a bit unclear.

pierce · Jan 20, 2016

Thanks lads the run is 27 meters, 16 amp type c be ok. On 2.5
1.5 ccc is 11.5 amps mcb sizing would be a problem couldnt put 10 amp mcb if flc is10.8 per phase?

ian.settle1 · Jan 20, 2016

Don't forget to take grouping factor into account depending upon the number of different circuits that may be in the trunking

davesparks · Jan 20, 2016

TP and N motor?

You need to use the various factors in your calculation, not just jump straight to the tables. Also you don't appear to have included power factor or motor efficiency in you calculation.
With motors you do need to ensure the supply mcbs is big enough to avoid nuisance tripping on startup.

You are only providing fault protection with the MCB as the motor overload will handle overload protection,

LeeH · Jan 20, 2016

davesparks said:
TP and N motor?

Unlikely, but maybe a break.

Darkwood · Jan 20, 2016

pierce said:
Thanks lads the run is 27 meters, 16 amp type c be ok. On 2.5
1.5 ccc is 11.5 amps mcb sizing would be a problem couldnt put 10 amp mcb if flc is10.8 per phase?

Without clarifying the info I requested then we cannot answer your query, like has been said - machines with motors need careful consideration and circuit design to ensure you don't end up with nuisance tripping, motors size is crucial here unless the installation instructions specify front end device.