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Topic Title: sunbed
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Created On: 14 October 2013 11:59 PM
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 14 October 2013 11:59 PM
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SKElectrical

Posts: 911
Joined: 01 February 2009

6 months ago, I wired a 10mm on a 50A mcb for a single phase sunbed. It has since been replaced with a 3 phase sunbed.
This 3 ~ sunbed comes with NO manufacturer's instructions. The flex to it has 5 x approx 1.5mm csa.
Can I just connect up to the unfused isolator I originally installed 6 months ago ie no protection for the new sunbed's flex?

As an idea I suppose I could replace the unfused single phase isolator with a 3 pole 16A fused isolator.

Your thoughts?
 15 October 2013 05:37 AM
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leckie

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Do you mean you are going to connect it to the single phase supply that you have already installed or am I misunderstanding what you mean?

The 5 cores are going to be three phase, neutral and earth. If you were to link the three phases together you would have the combined loads all going through the 1.5 neutral core so you can't do that.

Sounds like you need to establish the load to each phase and install a suitable TPN supply. Isn't the manufacturers name and model number on the equipment so you can google for information? Or a rating plate?
 16 October 2013 01:13 AM
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peteTLM

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Does the premises even have a 3p supply?

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 16 October 2013 10:04 AM
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impvan

Posts: 826
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I had to convert a 3ph stand-up unit to 1ph last year - the maker supplied a kit (which didn't include the flex!) which involved wiring the lamps into three banks and installing time delay relays to start them in sequence.
 16 October 2013 08:18 PM
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SKElectrical

Posts: 911
Joined: 01 February 2009


Does the premises even have a 3p supply?


Yes


...wired the lamps into three banks, installing time delay relays to start them in sequence.


Interesting.




Do you mean you are going to connect it to the single phase supply that you have already installed or am I misunderstanding what you mean?


Yes the single phase supply into a 3 pole isolator, linked out across the 3 poles, each pole has a 16A fuse to the load side.



The 5 cores are going to be three phase, neutral and earth. If you were to link the three phases together you would have the combined loads all going through the 1.5 neutral core so you can't do that.


Wouldn't the current in a 3 phase scenario still go down the neutral?
 16 October 2013 08:47 PM
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peteTLM

Posts: 3233
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A new piece of cable from a suitable 3p mcb in the DB, and a 3pole (i prefer 4 pole as they usually come like that anyway) isolator adjacent to the sunbed would be the way to go.

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Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
 16 October 2013 08:55 PM
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OMS

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Wouldn't the current in a 3 phase scenario still go down the neutral?


It would, but the phases would be displaced by 120 degrees so the resultant vectors would reduce the effective neutral current to potentially zero if totally balanced

In your case you have 3 single phase loads with no displacement so the neutral current will equal the sum of the single phase currents - if 1.5mm2 is big enough for the line, you'll need 3 x 1,5mm2 for the neutral

Regards

OMS

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Failure is always an option
 16 October 2013 10:24 PM
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SKElectrical

Posts: 911
Joined: 01 February 2009

A new piece of cable from a suitable 3p mcb in the DB, and a 3pole (i prefer 4 pole as they usually come like that anyway) isolator adjacent to the sunbed would be the way to go.


There is 3 phase incoming supply luckily just 5 metres away. Currently, there is no 3 phase dist board.



... phases would be displaced by 120 degrees so resultant vectors would reduce the effective neutral current to potentially zero if totally balanced



In your case you have 3 single phase loads with no displacement so the neutral current will equal the sum of the single phase currents - if 1.5mm2 is big enough for the line, you'll need 3 x 1,5mm2 for the neutral


Okay, well I need to consult my text books. I never gave it much consideration when first taught this - I thought it only applied to pure 3 phase loads ie motors
 16 October 2013 10:34 PM
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AJJewsbury

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pure 3 phase loads ie motors

Look deep enough and most 3-phase loads look like three single-phase loads side-by-side - e.g. individual windings in a three-phase motor have just two wires.
- Andy.
 17 October 2013 11:06 AM
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OMS

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Put it this way - lets say the sun bed is 8kW.

That would be 32A single phase and thus the supply cable would need to carry 32A on the phase and 32A on the neutral.

If it was a 3 phase machine, then we could say that it's effectively 2.67kW per phase - ie 10.6A /phase - and if balanced there would be zero neutral current.

So you are planning on connecting up 3 lots of 2.67kW loads supplied individually down 3 wires - but go back to our single phase example above and you'll have 32A running through the neutral - but on a small cable - ie the same size as the line which is only supplying 2.67kW per core.

Make sense ?

Regards

OMS

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Failure is always an option
 17 October 2013 02:06 PM
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SKElectrical

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yes I asked a tutor at college today who explained it.

From what I can make out... if I go down the single phase route I would have to increase the neutral (of the sunbed flex) csa by 3 times.


As for the -ve and +ve currents he described...... :-(
 17 October 2013 10:03 PM
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kj scott

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Its not only the neutral current that will be x3.

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 18 October 2013 10:22 PM
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peteTLM

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Originally posted by: SKElectrical

A new piece of cable from a suitable 3p mcb in the DB, and a 3pole (i prefer 4 pole as they usually come like that anyway) isolator adjacent to the sunbed would be the way to go.




There is 3 phase incoming supply luckily just 5 metres away. Currently, there is no 3 phase dist board.












If it doesnt have a 3p DB, does it have a 3p meter? Their want of this sunbed is going to be expensive

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Lack of planning on your part doesn't make it an emergency on mine....

Every man has to know his limitations- Dirty Harry
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