3-Phase Cowboys?

[Sparks13]

Hi all,

I was sent to a job the other day to put in a supply for a new 3-phase motor. Our projects department decided instead of installing a new circuit from the DB which was situated 10 metres away wanted me to cut an existing three phase 32A SWA supply, feeding another motor and install a junction box to spur off of this supply to feed the motor.

Now personally I don't agree with this set up of spuring off single/3phase (with the exception of lighting and ring mains where BS7671 allows), especially as the junction box was hidden within a service duct. Now I am not too sure as to how this corresponds with BS7671...

Have you guys got any comments as I'm not too sure where I stand with refusing to do works that don't comply with the conventional set-up.

Many thanks,

Peter

 

[Paultrim123321]

It might be toward BS60204 Machinery Standards and in particular isolation of equipment/machinery etc. If feeding motors I imagine the protective device isn't going to like the fluctuating loads and possible high inrush currents in the event both motors start instantaneously.

 

[davethsparky]

This could be a bit of a minefield for you!

There is a regulation that requires the circuits to be divided so that one going down creates the minimum disturbance to the rest of the installation.

There is the fact that the existing supply you are to spur off from is likely to have been designed to be the right size for what it is currently feeding and not be a size or two bigger than necessary just in case you want to add something else to it later.

Nobody further down the line will be very happy if they turn off the supply to machine X and machine Y suddenly stops working too.

 

[Bobster]

Was quite common in the past, especially in German built panels for 2-3 motors supplies in the same mcc to be fed from the same bottle fuses. Usually 63A.

The motors have to have seperate thermal overloads or equivalent. I don't like the practice personally.

 

[Engineer54]

Personally, i dont know how you're going to cut into an existing installed cable, insert a junction box, spur of that cable for a new motor position?? Not unless there is enough slack on the existing SWA cable to re gland with enough tail length into a junction box!!
That's also if the existing cable is suitable to supply both of these motors. One would have thought the the existing cable and OCPD would had been sized and installed to suit the existing motors requirements, rather than for sizing for an additional unknown motor size as well... lol!!

Jesus, how much time and money is going to be saved here, by piggy backing another motor onto an existing motors supply cable. It's saving a £1 too far, as far as i'm concerned, whether it's acceptable or not, which in this instance i doubt very much if it is acceptable!!

 

[Knobhead]

 

[Engineer54]

Was quite common in the past, especially in German built panels for 2-3 motors supplies in the same mcc to be fed from the same bottle fuses. Usually 63A.

The motors have to have seperate thermal overloads or equivalent. I don't like the practice personally.

Yes, but those motors would have been for use on the same machine or system, and outgoing cable(s) would have been sized accordingly!! You wouldn't see an ''additional'' motor supply piggy backed off an existing motor supply cable...

EDIT.... They would also be under the control of that MCC panel, EG, sequential stating, or flip flop arrangement etc, which is not what's being described here by the OP!!

 

[ackbarthestar]

Interesting ......
No extra circuits, no extra paperwork, no testing, no doubt !

Seems to me a cheapskate method of keeping costs down...

 

[Farmelectrics]

New cable new circuit for me

 

[Darkwood]

I seen worse in house sparkies making demands like this even went to one wanted help connecting a motor he had started rigging up a temp motor of a star delta starter and was unsure about which winding was which so i went out and FACE-PALM he'd only tried to wire up a DC shunt motor up as a AC induction motor because it had 6wires..... i was in shock and given the plate expressed field and armature voltages too, never understood those plates he said OMG and they pay people like this to drink tea all day :stupid: