Wiring 3-phase machines

[Fredxxx]

I have two lathes, where I need to cable these up. It's only going to be on very rare occasions that both will be used at the same time. I would be happy to make this a rule.

They have 3hp motors which I'm assuming will have a current rating of rather less than 5amps.

Because the cable run is going to be around 100ft/30m I was going to use a single 2.5mm2 x 4 SWA Armoured cable to both machines so as to limit voltage drop during start-up. I have no idea what sort of starting current I should expect for a DOL motor, ie one without any soft start capability, although some articles suggest 5-7 times.

I was intending to use a 16A MCB, type B or C. At the moment there is no RCD protection.

Would this be reasonable?

[Bob Minchin]

I have a modern 3 phase TEFC motor here 4 pole 1.5 hp rated 3.2a @415v So 5amp might be a little light.

16a type C would be advisable to avoid nuisance tripping. I'm assume that each motor has its own NVR and thermal relay to protect it so the MCB is simply protecting the cable. I'm sure that 2.5mm will work adequately but might not meet the regulations fully. Depends if you need it signing off?

[Fredxxx]

I was assuming 3hp = 2.2kW So current 2,200 / (230 x 3) = 3.1A / phase Assuming a worst case pf of 0.7 gives a current of 4.55A / phase.

And I thought add 10% for tolerance to give a nominal 5A / phase.

Each has a No Volts Release though to be honest I'm less sure about thermal protection.

There is minimal documentation for each machine, apart from the installation "should be carried out by a competent electrician, and all wiring should be of a permanent character". There is no requirement of overload protection! Also just that the NVR "should be checked from time to time".

I'm also assuming a Zs based on the SWA 4 x 2.5mm2 of 7R/km for each line and for armour resistance, so at a length of 30m Zs should be around 0.5, low enough for a disconnection time of < 0.4s with virtually any MCB type.

[ARW]

Normally provided in the DOL relay.

[Fredxxx]

Thanks, because these machines are not to be ever left running unattended, incombustible, and are virtually indestructible thermal protection wasn't something I was particularly worried about.

[Bob Minchin]

Well you really ought to be as the MCB won't protect the motor.

[Fredxxx]

In principle I agree, however:

If the motor is mechanically overloaded then it will/should be obvious to competent user of the machine.

If the motor starts taking an overcurrent for any other reason then it's probably toast anyway.

[The Other Mike]

Cost of motor 100 quid or more vs 30 quid for an overload relay?

If you want to undersize the cable for just one machine and ensure only one machine is in use at a time then provide just a single socket outlet (IEC 60309)

[Andy Wade]

You haven't allowed for the efficiency being less than unity. 2.2 kW is the mechanical output (P2), typical efficiency is ~85% for that sort of motor size, so input (P1) will be nearer to 2.6 kW. PF ~0.8 maybe, so use 3.3 kVA for the FLC calculation. 3x 5A, isn't far off the mark. It should be on the rating plate in any case. Go for a 3x C10 MCB, and you might want to think about using 4 mm^2 cable, given the longish run.

That might imply PTC devices built into the motor?

[John Rumm]

Is that 3hp mechanical output, or 3hp electrical input? (if its the output you may need to revise the load up a bit). The motor ought to have the actual electrical specs on its plate.

Yup that's normally the kind of multiple spoken of - say 9x as a top limit. Its only really an issue when selecting the fault current trip rating of the circuit breaker. Type C devices are typically rated at 10x In, so a C10 will allow for 100A of irush.

Yup... I might go for 4mm^2 SWA (do you need four core?)

[Fredxxx]

Hmm, why 4mm^2?

I was considering 4 core as these machines were originally powered by an inverter, and the lights were independently powered. A possibility is to add a 13A socket close by, where I would need the neutral conductor.

Many thanks.

[John Rumm]

Only that once its there you will probably find extra uses for it, coupled with the thought you may end up wanting both machines at once for some reason. The incremental cost of the cable is cheap compared to the time and effort of sticking it in.

Yup fair enough, and see my comment above ;-)

[Martin Crossley]

Or a bit of logic (with auxilliary contacts on the contactors if necessary) in the start and emergency stop circuit to lock the other machine out once one is started.

Martin.