SWA Cable Size Query

Darkness · 25 Jan 2011

I have a motor which will run for 10-15mins each day and I need to connect power to it. The motor is 230v / 2100w and the length of run is around 75m from CU to motor but I will get around 85m or so to be sure.

The cable size calculator at TLC states 4mm swa. I know its only a guide but do you think I should go for 6mm to be on the safe side?

Nothing else will run of this cable, only the motor so I need to lay the cable into the trench that I have dug.

Cheers

flameport · 26 Jan 2011

What is the starting current of the motor?

Darkness · 27 Jan 2011

Marked on the motors panel when I last checked it was 14.4A. I will get a better look at it tomorrow to be sure.

Basically if it is 14.4A, I need to pick a cable to handle that amp?

ban-all-sheds · 27 Jan 2011

Think about what might happen if you don't.

SgtTrojan · 27 Jan 2011

With a 4mm² the volt drop is very high tbh. It calculates out to 11.88 volts where the maximum permissible is 11.5 volts. The R1+R2 will be 0.69 Ω so assuming you have a decent Ze shock protection shouldn't be a problem but the volt drop is.

75 metres is a huge run. A 6mm² will lose 7.88 volts and will see an improvement in disconnection time. A termination for a cable of this size is still a 20mm gland so ensure you have a sufficient cable entry and enough room to terminate it (assuming you choose to purchase this size). I'd question the TLC calculator at this point too.

ban-all-sheds · 27 Jan 2011

I'd question the TLC calculator at this point too.

Why?

SgtTrojan · 27 Jan 2011

Darkness said:
The cable size calculator at TLC states 4mm swa.

ban-all-sheds · 27 Jan 2011

Sorry - I thought you meant you'd question it in respect of the figures for 6mm² which you posted.

Darkness said:
The cable size calculator at TLC states 4mm swa.

I still wouldn't question it.

I would, however, question the use of the running current to size the cable rather than the starting current....

bernardgreen · 27 Jan 2011

What type of motor is it and what sort of mechanical load has it when it starts ?

Does it reach normal running speed rapidly or does it accelerate slowly to normal speed.

Marked on the motors panel when I last checked it was 14.4A. I will get a better look at it tomorrow to be sure.

That seems a bit low and very precise for a 2 Kw motor's short duration start up current. Our 1 Kw pump motor pulls over 15 amps for a few cycles when it starts.

Basically if it is 14.4A, I need to pick a cable to handle that amp?

The impedance of the cable must be low enough that the motor is not undervolted ( by voltage drop along the cable ) during start up. Under volt is not a serious problem for most motors but for some if can be a serious problem.

For some types of motor a 10% undervolt at start up increases the time the motor takes to reach full speed and the windings have to carry the start up current during this time. There may be a limit to the time the motor windings can carry the start up current before they overheat.

Also is there a need to consider an isolator at the motor end of the cable to prevent the motor being started remotely when the equipment is being serviced ?

Darkness · 27 Jan 2011

I couldn't get at the motor today to take all the details down but its a motor to drive an auger of short length. These motors are low rpm so once they start, there is no speed build up like a fan if you know what I mean.

The motor comes with a 15A Start, Stop/Reset control box.

tomathy · 27 Jan 2011

Curiosity demands that I ask what the auger is for...

Darkness · 27 Jan 2011

From wood pellets to animal meal.

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SWA Cable Size Query

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