Motors

[norv]

Got a job to do tomorrow for another Electrician. I need to put in a 3ph supply to a Dist board for a Wood crusher thing. The run is about 50metres & he has given me 10mm 4 core SWA to do the job. It will be supplying x4 3KW motors & x1 6KW motor. Is this up to the job. Am I right in saying the motors will be drawing approx 45amps ?

 

[steptoe]

is that the loading per phase!?!?!?!?!?

or in total?

assuming in total then personally I would think possibly pushing it(no calcs done)

I would deffo put something bigger in, without even working out loadings /volt drop etc.

but if your only doing what your told then so be it.

 

[norv]

Thats in total. 3KW per motor

 

[norv]

Just checked & 10mm 4 core can take 73 amps so it should be ok.

 

[steptoe]

remember that you will prob be using type C protection and will have a lot lower Zs to ashere to, also the motor start up could be quite significant, deffo would want to be throwing in a bit of 16mm at least IMHO.

 

[green-hornet]

10mm 4 core is more than enough in my opinion, providing that it is 415v and not 230v!

 

[Andy™]

Just checked & 10mm 4 core can take 73 amps so it should be ok.

just because a cable is rated at X amps doesnt mean it can run at X amps. you need to take VD and a few other things into consideration.

 

[steptoe]

I would just be concerned about the Zs, especially if it is a TN S install, could be hard getting the final circuits to comply if they are quite a bit from source, as OP would suggest by his sub main length.

 

[steptoe]

25A type C is only something like 0.92 if I remember rightly,

assuming a decent Ze of maybe 0.45 then that doesnt allow much for 50m + final circuit=0.47 , not a lot, I dont have my book handy, can some look at the resistance of 10mm copper please?

 

[Andy™]

Just checked & 10mm 4 core can take 73 amps so it should be ok.

dont know where you get that value from

BRB says 10mm 4 core 3 phase SWA can take 62A ref method E (free air, perforated tray). only 58 & 50A for other methods.

VD being 3.8mV/A/m

dont forget that motors will have a large start up current and may need larger cable to cover start up VD

and C25 is 0.92 ohms

 

[green-hornet]

If its 230 volts

Required Cable Size 16 mm

Voltage Drop 7.39 Volts.

Percentage Drop 3.2%

Current Load 59.1 Amps

Max Cable Load* 94.0 Amps

If its 415v

Required Cable Size 4 mm

Voltage Drop 16.4 Volts.

Percentage Drop 3.9%

Current Load 32.8 Amps

Max Cable Load* 42.0 Amps

But i would not like to go with 4mm. :coat

 

[steptoe]

Thats in total. 3KW per motor

I need to pay attention more,,,,, http://www.electricianforum.co.uk/talk/images/smilies/Blushing.gif http://www.electricianforum.co.uk/talk/images/smilies/Blushing.gif http://www.electricianforum.co.uk/talk/images/smilies/Blushing.gif

so only 1Kw per ph per motor then, + 2Kw for other motor, so only 6Kw total.

not much at all really.