Cable size for motor Starting

[dw3101]

I want to start a 2.2kw motor, as far as I can remember its 4x rated current to start?

Problem?

Length of cable run... SWA possibly upto 100m max

What size would you go with.. to also allow expansion, lights, sockets etc..

Obviously motor starting wont be done whilst other things are being used.. (shouldnt be)

Im thinking 16mm 3 core SWA - 25mm swa is getting pricey on reels.. Having said that theres some tempting deals on ebay!

David

 

[supaspark]

Where is this being installed? what supply are you using? ie 240v 3 phase ?? this will affect the size of cable etc. if you could say how many sockets lights etc you want to add on that would help in sorting the requirments out.

 

[dw3101]

240v single phase.. at a mates house / farm.. we are using a barn as a garage.. and have a car lift

say an extra 2kw of load, if theres owt else the motor starting load will deal with that iyswim

 

[ban-all-sheds]

240v 3 phase ??

That's a new one on me.....

 

[mapj1]

The ratio of inrush to run current depends on the motor type, capacitor run could be 3 times, induction start could be 8 times, also it depends if it starts on load or freewheeling, on load of course its much harder.
(most domestic fridges draw ~20A for a brief flicker when they pull up to speed, but less than an amp running. )

However, if the start is short duration (say <0.5 seconds), and side effects like the lights flickering is not a probem, then it is usual to rate the cables based on the running load, rather than the starting load. (No-one fits 20A flex to a fridge....)

 

[supaspark]

it should read ... 240v, 3ph??

thanks for pointing that out

 

[ryanj]

it should read ... 240v, 3ph??

Try: 400v, 3ph.

You aren't an electrician are you?

 

[dw3101]

It will most likely be a capacitor start, its going to start on load AFAIK, its on a hydraulic pump, Depends on the valving arrangement..

Lights dimming isnt the problem, its the voltage dropping so low that the motor wont start.

 

[crystal ball]

it should read ... 240v, 3ph??

Try: 400v, 3ph.

You aren't an electrician are you?

Are you?

Never seen a single phase input inverter drive with 240v 3 phase output, very common in industry, most 3ph induction motors will run on 240/415 3 phase

 

[Stoday]

most 3ph induction motors will run on 240/415 3 phase

That's for star-delta starting. Windings connected in star to minimise starting transient, so you get 230V across each winding, then when motor's run up switches to delta connection and you get the full 400V across each phase's winding.

240/415 V became obsolete over a decade ago. The declaired LV voltage is now 230/400 V in the UK.

 

[pipme]

Disagreements .. What hope for us customers under Point P then ? Will installers and Inspectors agree ?
P

 

[crystal ball]

most 3ph induction motors will run on 240/415 3 phase

That's for star-delta starting. Windings connected in star to minimise starting transient, so you get 230V across each winding, then when motor's run up switches to delta connection and you get the full 400V across each phase's winding.

240/415 V became obsolete over a decade ago. The declaired LV voltage is now 230/400 V in the UK.

No it is not, single phase input thru inverter drive to 240 three phase output to a delta connected induction motor used for speed control by frequency, with digital I/O for external stop start and optional potentiometer for remote speed control, I have fitted and commissioned plenty over the years

As for the 230/240 comment its still 240 in my house 245 actually

 

[Stoday]

most 3ph induction motors will run on 240/415 3 phase
...
No it is not, single phase input thru inverter drive to 240 three phase output to a delta connected induction motor used for speed control by frequency, with digital I/O for external stop start and optional potentiometer for remote speed control, I have fitted and commissioned plenty over the years

As for the 230/240 comment its still 240 in my house 245 actually

I accept that that may be the case for variable speed induction motors, but most 3 ph induction motors are single speed. They are not designed to run on 240V simply because the UK line voltage is 400V.

I said the declaired voltage is 230 V. Your actual voltage may be 245V at a particular time, and this is within the tolerance of the declaired voltage.

 

[mapj1]

Stepping aside from what one calls 3 phases when they are not 3 phases, (experts eh!)
For the motor to not start, the voltage drop will have to be a damn sight greater than the dozen or so volts that the regs would allow under steady state. (fortunately!) To not start at all you probably need the volts to dip to between a half and 2/3 of supply, although such a marginal condition would be really bad news as the starting would be sluggish and the motor could overheat.

If its a compressor, and starting under load, then it will be a bit like the fridge (only bigger currents all round!), but if it is a capacitor type, then the inrush will be perhaps 3 to 5 times running, so its probably OK if you pass voltage drop test with the steady state current with a comfortable margin.A capacitor start peak inrush will be perhaps 3 times running, if the capacitor is not on a centrifugal switch.

- If you are worried about having to re-do it and want a warm confident feeling you could try it out for size on an long reel of cable (try 100m of lighting 1mm2 - ~1.8ohms, so 20v drop on load ) and see if it starts !
(If you do try this, stand by the off switch, and don't run it for more than a minute or two without unwinding the cable - the heat can't get out from the middle.)

 

[dw3101]

The main problem is the transformer supplying the property is rather small (inside information! ) I know the rating, voltage and impediance of the transformer, I havent measured the voltage at the property yet to see what I have to work with.. All I know is the lights are going to dim regardless!