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Topic Title: One Tank, Two Immersions
Topic Summary: Wired from two phases using one cable
Created On: 16 January 2013 08:23 PM
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 16 January 2013 08:23 PM
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primo

Posts: 439
Joined: 10 January 2008

Any opinions on this set up?

This is a two element direct hot water cylinder and has been wired using one cable taking 2P&N to the tank with one phase connected to each element with the neutral looped between them. The elements share the same enclosure for the purposes of wiring.

Although this is in a commercial environment the cylinder is manufactured for domestic and MI's state that it should be wired using 2x 1.5mm flex, although they probably didn't consider it for a commercial environment.

I haven't trawled the regs but I can't see anything wrong with this set up but it just doesn't feel right?!
 16 January 2013 08:57 PM
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DOUGIE1000

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what size of cable used?

-------------------------
Dougie
Power Plus Electrical.co.uk

My mission is to live as long as possible......so far so good!
 16 January 2013 09:23 PM
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primo

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2.5mm I am told, only seen pics so far. Fed from 16A TP&N socket.
 16 January 2013 09:38 PM
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DOUGIE1000

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521.8.2, 528.1 for a start then there's overload of the 2.5mm

-------------------------
Dougie
Power Plus Electrical.co.uk

My mission is to live as long as possible......so far so good!
 16 January 2013 09:40 PM
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Martynduerden

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Twin heater units are normally night & boost -so you can only ever have one connected.

If the MI's require two flex connections then I guess your not going to get past 3.14 in the regs (number from memory???)

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Martyn.

Only a mediocre person is always at their best



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 16 January 2013 09:48 PM
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primo

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Originally posted by: DOUGIE1000

521.8.2, 528.1 for a start then there's overload of the 2.5mm


Does 521.8.2 apply here, only one circuit but supplying two elements.

Re: 528.1, each core should be insulated up to 400v

Would the 2.5mm be overloaded? Isn't the neutral in this situation be the same as the neutral in any three phase circuit?
 16 January 2013 09:50 PM
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primo

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Originally posted by: Martynduerden

Twin heater units are normally night & boost -so you can only ever have one connected.



If the MI's require two flex connections then I guess your not going to get past 3.14 in the regs (number from memory???)


Agreed with the MI's, but electrically?

Yes, it is meant for night and boost but you could still have both elements on at the same time even if wired conventionally, you would be able to turn on the peak element while the off peak element was running.
 16 January 2013 10:00 PM
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Martynduerden

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Originally posted by: primo

Originally posted by: Martynduerden

Twin heater units are normally night & boost -so you can only ever have one connected.

If the MI's require two flex connections then I guess your not going to get past 3.14 in the regs (number from memory???)


Agreed with the MI's, but electrically?

Yes, it is meant for night and boost but you could still have both elements on at the same time even if wired conventionally, you would be able to turn on the peak element while the off peak element was running.


Not strictly true the MI's and indeed the DNO normally require that both elements cannot be simultaneously connected, in practice this is normally archived with an off peek controller (cannot remember the unit) Honeywell I think.

Electrically I'm not sure there is a problem assuming the cable is 6kw capable.

-------------------------
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Martyn.

Only a mediocre person is always at their best



www.electrical contractors uk.com
 16 January 2013 10:13 PM
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DOUGIE1000

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Originally posted by: primo

Originally posted by: DOUGIE1000



521.8.2, 528.1 for a start then there's overload of the 2.5mm




Does 521.8.2 apply here, only one circuit but supplying two elements.



Re: 528.1, each core should be insulated up to 400v



Would the 2.5mm be overloaded? Isn't the neutral in this situation be the same as the neutral in any three phase circuit?



its 2phase so its 2 circuits, 2 CPD's

-------------------------
Dougie
Power Plus Electrical.co.uk

My mission is to live as long as possible......so far so good!
 16 January 2013 10:15 PM
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DOUGIE1000

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Originally posted by: primo

Originally posted by: DOUGIE1000



521.8.2, 528.1 for a start then there's overload of the 2.5mm




Does 521.8.2 apply here, only one circuit but supplying two elements.



Re: 528.1, each core should be insulated up to 400v



Would the 2.5mm be overloaded? Isn't the neutral in this situation be the same as the neutral in any three phase circuit?



Tell you what.....ill switch on the 6kw of heating elements and you disconnect the neutral while live then ?

-------------------------
Dougie
Power Plus Electrical.co.uk

My mission is to live as long as possible......so far so good!
 16 January 2013 10:16 PM
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Martynduerden

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Originally posted by: DOUGIE1000

Originally posted by: primo

Originally posted by: DOUGIE1000

521.8.2, 528.1 for a start then there's overload of the 2.5mm


Does 521.8.2 apply here, only one circuit but supplying two elements.

Re: 528.1, each core should be insulated up to 400v

Would the 2.5mm be overloaded? Isn't the neutral in this situation be the same as the neutral in any three phase circuit?


its 2phase so its 2 circuits, 2 CPD's


Or 1 DP CPD? 3 phase isn't 3 ccts?

-------------------------
Regards

Martyn.

Only a mediocre person is always at their best



www.electrical contractors uk.com
 16 January 2013 10:18 PM
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DOUGIE1000

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Originally posted by: Martynduerden

Originally posted by: DOUGIE1000

Originally posted by: primo

Originally posted by: DOUGIE1000

521.8.2, 528.1 for a start then there's overload of the 2.5mm


Does 521.8.2 apply here, only one circuit but supplying two elements.

Re: 528.1, each core should be insulated up to 400v

Would the 2.5mm be overloaded? Isn't the neutral in this situation be the same as the neutral in any three phase circuit?


its 2phase so its 2 circuits, 2 CPD's


Or 1 DP CPD? 3 phase isn't 3 ccts?


Agreed however depends what Primo says how its protected at source,

-------------------------
Dougie
Power Plus Electrical.co.uk

My mission is to live as long as possible......so far so good!
 16 January 2013 10:20 PM
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primo

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Originally posted by: DOUGIE1000

Originally posted by: primo



Originally posted by: DOUGIE1000







521.8.2, 528.1 for a start then there's overload of the 2.5mm








Does 521.8.2 apply here, only one circuit but supplying two elements.







Re: 528.1, each core should be insulated up to 400v







Would the 2.5mm be overloaded? Isn't the neutral in this situation be the same as the neutral in any three phase circuit?






Tell you what.....ill switch on the 6kw of heating elements and you disconnect the neutral while live then ?


Okay, I'll admit I haven't gone back to the books and looked at the maths, but wouldn't the neutral current at any one pint be the same as in a single phase circuit as the elements will be pulling out of sequence as on different phases?

It is fed from a 16A TP socket, dedicated circuit with TP MCB.
 16 January 2013 10:24 PM
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DOUGIE1000

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Quote for remedial works would be my advice

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Dougie
Power Plus Electrical.co.uk

My mission is to live as long as possible......so far so good!
 17 January 2013 07:39 AM
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perspicacious

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"in practice this is normally archived with an off peek controller (cannot remember the unit) Honeywell I think."

Has no one looked at it for you Martyn?

Regards

BOD
 17 January 2013 09:29 AM
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AJJewsbury

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its 2phase so its 2 circuits, 2 CPD's

Or 1 DP CPD?

3-phase circuits can be legitimately protected by fuses, so I don't see an objection to two single-pole MCBs in this case in principle.

The circuit conductors are protected by the combination of the devices (while either one would protect against L1-L2 faults, protecting N against overload or faults requires both to be present) - so I think the definition of a circuit 'protected by the same protective device(s)' - with the option for plural on the devices - still holds.
- Andy.

- Andy.
 17 January 2013 12:26 PM
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broadgage

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I grant that this does not "seem right" but cant see any actual harm or danger.

Is the flex and the cable suitable for 400 volts between cores ? almost certainly it is as all common types are rated for at least 500 volts between cores.

Can the heating elements be fully isolated for servicing ? yes they can, pull out the plug.

Is the circuit protected against small but long term overloads ? almost certainly it is, presuming 16 amp OCPD and 2.5mm cable.
 17 January 2013 02:30 PM
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slittle

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Originally posted by: DOUGIE1000

Originally posted by: primo



Originally posted by: DOUGIE1000







521.8.2, 528.1 for a start then there's overload of the 2.5mm








Does 521.8.2 apply here, only one circuit but supplying two elements.







Re: 528.1, each core should be insulated up to 400v







Would the 2.5mm be overloaded? Isn't the neutral in this situation be the same as the neutral in any three phase circuit?






Tell you what.....ill switch on the 6kw of heating elements and you disconnect the neutral while live then ?


With no neutral connection you would have two phases across the two elements in series. ok so the end of the neutral conductor that you've got in your hand may be rather more above earth than you would like it but I'm struggling to see that either element would object.

That said, I'm stuck at home suffering from a nasty case of man flu so it's possible i've missed something.

Stu
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