Wiring an immersion heater

[thedigger]

Hi, just had a new boiler and unvented cylinder fitted. The plumber hasn't wired in the immersion heater (he wanted to surface mount the cables... right through my house) whereas I want to lift the floorboards and put the cable down properly. He asked if I could run the cable from the CU to the cylinder, and he would connect it up.

Trouble is, I don't know what size cable I need. Will 2.5mm T&E be enough? The immersion is 3KW which according to the table here means 2.5 will be well up to the job.

 

[ban-all-sheds]

You need to ask him - his is the name that will be on the certificate as being responsible for the design, and on the declaration of compliance with the Building Regulations (I take it that the plumber is registered to self-certify his electrical work?)

Also there's more to sizing cables than just catering for the load.

• //www.diynot.com/wiki/electrics:mcbtocable
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm
  
  
• http://www.tlc-direct.co.uk/Book/4.3.11.htm
  
  
• http://www.tlc-direct.co.uk/Book/5.3.1.htm
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
  

 

[thedigger]

He shrugged his shoulders and said "flex" which doesn't really fill me with confidence about his electrical ability (if it's any consolation, his plumbing is s*** hot ).

I guess that means he'd be happy putting it on 2.5mm, and he'd happily sign that off. My concern is, he doesn't have to live here and I don't want another fire!

 

[ban-all-sheds]

As I said...

• //www.diynot.com/wiki/electrics:mcbtocable
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm
  
  
• http://www.tlc-direct.co.uk/Book/4.3.11.htm
  
  
• http://www.tlc-direct.co.uk/Book/5.3.1.htm
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
  

 

[bernardgreen]

his plumbing is s*** hot ).

Not much use for the cold water taps.

But seriously

I guess that means he'd be happy putting it on 2.5mm, and he'd happily sign that off. My concern is, he doesn't have to live here and I don't want another fire!

Is he "qualified" to work on non-vented systems which can be a serious hazard if not properly installed ? The safety implications of a boiler that does not shut down or an immersion heater that does not switch off due to a wiring error are far more serious with a non vented system than with a vented system. The vented system creates steam and hot water in the loft ( or where the header tank is ) while the un-vented system can do the same in the airing cupboard or wherever the hot tank is located ( that assumes one of the two safety valves opens when it should, if they all fail the tank can explode )

 

[kai]

In very general terms, it is a B16 breaker at the consumer unit (Assuming a 3kw immersion), then a Minimum of 2.5.mm twin and earth (or heavier depending on distance and installation methods/insulation etc), run from CU to a 13amp DP switched fuse-connection unit beside the immersion cylinder itself.

Between the FCU and the element, use three core heat resisting immersion type flex of 1.5.mm for a 3kw immersion on a 13amp fuse.

Some of you may prefer to omit the 13amp FCU, in that case substitute a 16 or 20amp rated DP switch in place of the FCU.

My preference for the FCU, is that the plumbers servicing the megaflow will find the fuse holder in the FCU as a handy means of padlockable locking off of supply, if a FCU with a lockable fuse drawer is used, such as MK Part No. K330-WHI.

 

[thedigger]

In all honestly, I've heard loads of good things about his plumbing, and the system seems to be installed properly (really neat install, and obviously the parts that are wired in work perfectly). I have no concerns about his plumbing really, I'm just a bit concerned about the laisez-faire attitude to cabling.

The immersion which was previously installed on the hot water tank (which has been removed) also had 2.5mm wiring, it just doesn't really seem that substantial and I'd feel happier with 4mm or 6mm cable.

As for BAS' links, I understand what voltage drop is, but how to I know what the resistance of the cable is? There will be no joins, junction boxes or sockets, just a straight cable from the CU to the immersion, and the run is no more than 15m.

 

[Taylortwocities]

The immersion which was previously installed on the hot water tank (which has been removed) also had 2.5mm wiring, it just doesn't really seem that substantial and I'd feel happier with 4mm or 6mm cable.

Assuming the fuse/cb in the consumer unit is 16 or 20amp then you do not need thicker cable. The immersion is 3KW = 13amp so thicker cable is not required unless the immersion is 100 yards away from the mains inlet.

Why aren't you (he) using the existing immersion heater feed?

Please please pay special attention to Bernard's post above. It is VITAL that the cylinder cannot boil either through a fault with the DHW controls or the immersion heater.
Personally I don't care if his plumbng work is neat. Is his electrical competence up to the task?

 

[thedigger]

Cheers!

 

[JohnW2]

The immersion which was previously installed on the hot water tank (which has been removed) also had 2.5mm wiring, it just doesn't really seem that substantial and I'd feel happier with 4mm or 6mm cable.

2.5mm² cable is more than enough for a 3kW immersion. In fact, in terms of current-carrying capacity, 1.5mm² would be adequate, as would even 1mm² with some cable routings.

... I understand what voltage drop is, but how to I know what the resistance of the cable is? There will be no joins, junction boxes or sockets, just a straight cable from the CU to the immersion, and the run is no more than 15m.

The voltage drop of 15m of 2.5mm² carrying 13A (3kW @ 230V) would be about 3.5V. The Wiring Regs allow a voltage drop of up to 11.5V for a non-lighting circuit.

Kind Regards, John.

 

[EFLImpudence]

As for BAS' links, I understand what voltage drop is, but how to I know what the resistance of the cable is? There will be no joins, junction boxes or sockets, just a straight cable from the CU to the immersion, and the run is no more than 15m.

Assuming there is no insulation under the floorboards this is one of Bas' links.
You will see, as has been said, 2.5mm² cable is more than enough.

You can alter the input amounts to see what happens-

http://www.tlc-direct.co.uk/Technic...230&length=15&submit=Calculate+Min+Cable+Size

 

[riveralt]

My concern is, he doesn't have to live here and I don't want another fire!

 

[thedigger]

My concern is, he doesn't have to live here and I don't want another fire!

Yeah, we had one just after we moved in.... dodgy electrics, would you believe...

Edit... before we moved in, a few days after we picked up keys