Cable Sizing 2.2kw motor

[Conross]

Need a bit of advice about a motor rewire i need to do.

There is a 2.2kw motor which is supplied from an MCC with about a 30m ish run using a 4mm 4 core SWA. This then goes to an atex junction box located half a meter from the motor. Then the cable goes from this box to the motor itself.

For the last half meter does this need to be sized the same i.e 4mm or will i get away with using 2.5mm for this half meter run (Is this bad practice). Not sure if the cable sizing calc applies to just the cable or the whole circuit.

2.2KW motor draws 5a FLC @ 415v

 

[westward10]

The final connection should be included in your calculations but if volt drop is okay with the 4.0 cable I shouldn't think a short section of 2.5 will make much difference.

 

[Michael J]

Need a bit of advice about a motor rewire i need to do.

There is a 2.2kw motor which is supplied from an MCC with about a 30m ish run using a 4mm 4 core SWA. This then goes to an atex junction box located half a meter from the motor. Then the cable goes from this box to the motor itself.

For the last half meter does this need to be sized the same i.e 4mm or will i get away with using 2.5mm for this half meter run (Is this bad practice). Not sure if the cable sizing calc applies to just the cable or the whole circuit.

2.2KW motor draws 5a FLC @ 415v

That set up is standard in a industrial application, for example, take a 3 phase,415V, 2.2kW motor fed from an MMC a distance of 150m you would use a 6mm to satisfy the allowable volt drop.
The problem being is that the 6mm may not fit the motor terminal box.
So a few meters from the motor they install a field isolator or other method of connection so as to down size to 2.5 or 1.5
then you can gland into the motor terminal box.

 

[bomjac]

From memory, the volts drop on starting the motor should not exceed 10%, and starting time should generally be <3.5s. You cable needs to be sized to allow for this.

So your voltage drop calculation has to be for the starting current (12.5A or 30A depending whether Y or delta connected). The local connection to the motor would be ok in 2.5mm given that it is so short.

It's not uncommon to see motors wired 2 or even 3 cable sizes up to the rated amps.