single phase load off 3 phase wiring

[cybermax2]

If the single phase load is 10KW would the incoming live need to be 10sq mm and if it is not would connecting two live phases together be permissible or would rewiring be necessary?

 

[ColJack]

you can't use 2 phases together unless the dishwasher is wired for that..

you CAN however use the 4 cables that are to the 3 phase point ( if it has a neutral ) and re-wire them at the distribution boards into 2 pairs carying a single phase and neutral..

or rather your electrician can.. since this is obviously a commercial installation then your insurance will no doubt sspecify that any electrical work is carried out by a qualified and competent person..

 

[Space cat]

am I correct in assuming the electrician will just put the 3 live phases into the single phase live terminal

Sorry,but what else can you say!

If you actually did that you would be putting a dead short across all three phases - with catastrophic results! To see what's going on here you have to know what a three phase supply is. I suppose you know that, in a single phase supply, the voltage on the live wire alternates from positive to negative and back again 50 times per second. In a three phase supply, all three phase (live) wires do the same thing BUT NOT ALL AT THE SAME TIME.

I find that the easiest way to visualize this is by drawing a circle then putting three, equally spaced spokes in it to represent the voltages. Everything above the centre is positive while everything below is negative. Now watch the wheel go round. As one phase voltage peaks at the top, where are the other two? (Electrical engineers call this wheel a phasor diagram. It's incredibly useful.)

Three phase power has a number of advantages, one of which is that, if all the phases are equally loaded, the currents in the neutral return wires add up to zero. This is why a three phase supply might not have a neutral terminal at all.

Can you connect a 10kW single phase load to one phase of a three phase supply (with a neutral). In theory, yes; if the three phase supply is good for at least 30kW AND if the neutral wire is as big as the phase wires.

And if there's no neutral? Then you'll need a transformer.

would connecting two live phases together be permissible

Can you see from the phasor diagram why this would be a disaster?

 

[cybermax2]

Yes I see when one is positive the other two will always be negative so to connect two together would create a dead short. Thanks very much for the info you've been extremely informative about 3 phase. I was asking as I've got the chance to buy a similar model second hand dishwasher to replace my to replace my broken old one but its single phase. i dont want to pay any extra big bills for electical work and as its on ebay auction I've got to decide tomorrow. so if I've only four wires I've no neutral and as its 10KW 3 phase its not likely to support 10 KW on one phase wire.

Thanks again for your help

 

[plugwash]

If you actually did that you would be putting a dead short across all three phases - with catastrophic results!

More likely just a bang as the breaker trips/fuses blow.

Not that I'd reccomend it.

 

[cybermax2]

The single phase load is 15.5KW so using two of the 6 sq mm phase wires together back to a single phase 32 amp breaker( as there is a neutral there) would work, but am i right to think it would not comply with the regulations in a commercial premises?

 

[ColJack]

make your mind up, first it was 10KW, now it's 15.5KW?

are you sure that the dishwasher cannot be connected to a 3 phase supply?
maybe it's just linked inside like some of the larger range cookers are?

give us a link to the e-bay item and we'll look the specs over..

also considering that 15.5KW is 68A, then I'm guessing that wiring it to 32A breaker isn't going to work..

10KW is 43.5A, so you're screwed there too..

 

[Space cat]

maybe it's just linked inside like some of the larger range cookers are

Now there's a thought. There was a thread on here some time back about a second-hand washing machine with the reverse problem; it was designed for a three phase supply that its owner didn't posess. If I remember rightly, only the heating elements required three phase and they could be reconfigured for single phase use.

If the power rating really is 15.5 kW, this suggests that ColJack is right and it was always meant to have a three phase supply. Problem solved perhaps.

 

[cybermax2]

The dishwasher is a Winterhalter GS502. Hot fill is 10 KW which I thought it was but checked and found it is cold fill which requires 15.5KW. Its going at a possible a good price but its not worth it if I need to replace the wiring. The ebay listing says look up the specs on the net which I did, I couldn't see a reference to being able to convert the washer and as they list two models one single and the other 3 phase my guess is you can't.

So i thought I'd enquire from the experts and it seems the consensus is I would need to rewire.

you've been a great help and I would like to thank everybody for considering this.

 

[Steve]

i've used one of them dishwashers. Crackin pieces of kit. In and out in no time.

 

[ColJack]

I see 2 on there, 1 pass through and 1 corner, which one were you thinking of bidding on?

 

[Steve]

I see 2 on there, 1 pass through and 1 corner, which one were you thinking of bidding on?

I think they have the same electrical spec? "available in corner or pass thru"

 

[ColJack]

I know, but one specifically says it's single phase after someone asked, the other doesn't specify