Rewiring for a 10.8kW shower

[BrokenWing]

My fiancee and I have naively bought a 10.8kW shower thinking "biggest is best" only to find that the old shower was only wired with 6mm cable and a 30amp fuse on the main circuit board.

I've spoken to 2 electricians who claim that to rewire for 10mm cable will require tearing up floors. OK I'm naive but I'd have thought you could attempt pulling the new cable through along the track that the 6mm currently exists.

I'd imagined attaching the 2 cables securely at the bathroom end with some overlap, using a hefty wrapping of electrical tape and a bit of manly tugging at the fusebox end.

It's a 3 floor townhouse, therefore tall and narrow, with the bathroom on the middle floor. The fusebox is downstairs in the back of the garage, above the connecting internal door so horizontal distance from the shower is no more than 10 feet.

Because the fusebox is high, the wire has less than a foot to reach the under floor space, then that 10 feet of horizontal travel and finally about 4 feet to the height of the back of the shower.

Am I naive? Surely we could attempt my method before resorting to tearing up the floor (which will involve new carpet across 3 floors of stairs and 2 small landings)?

 

[holmslaw]

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[BrokenWing]

In that case what's the biggest shower you can have on 6mm wiring with a 30amp fuse? 8.5kW? 9.5kW?

Would the MCB take a 40amp fuse on this wiring for the 9.5kW shower?

 

[ban-all-sheds]

What is the spark supposed to do when the joint in the cables snaps? Or the cables get gets jammed.

Charge for his time so far, and walk away leaving BrokenWing with no shower circuit at all.

 

[ban-all-sheds]

In that case what's the biggest shower you can have on 6mm wiring with a 30amp fuse? 8.5kW? 9.5kW?

http://www.angelfire.com/pa/baconbacon/page2.html

Would the MCB take a 40amp fuse on this wiring for the 9.5kW shower?

• 
  
  
  
• //www.diynot.com/wiki/electrics:mcbtocable
  
  
• http://www.tlc-direct.co.uk/Book/4.3.1.htm
  
  
• http://web.archive.org/web/20080213151445/http://www.kevinboone.com/cableselection_web.pdf
  
  
• http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html
  
  
• http://www.tlc-direct.co.uk/Book/8.1.1.htm

 

[holmslaw]

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[holmslaw]

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[Taylortwocities]

I'd have thought you could attempt pulling the new cable through along the track that the 6mm currently exists.

I'd imagined attaching the 2 cables securely at the bathroom end with some overlap, using a hefty wrapping of electrical tape and a bit of manly tugging at the fusebox end.

 

[ovoloknight]

Trying to pull a cable the way you described is nearly (if not) impossible. it might not sound far but trying to pull a 10mm cable vertically up while it is running through some joists isnt advised.
Maybe you should try it first to see if it can be done?
You say you would have to have 3 new carpets? Why couldnt you just lift the ones you have and put them back down afterwards?
Have you considered an alternative installation method such as trunking?

Personally, i would tie the new cable on the old one and give it a quick pull, you will know within 5 seconds if its gonna go. If not then i would start lifting floors.

 

[EFLImpudence]

I'd have thought you could attempt pulling the new cable through along the track that the 6mm currently exists.

I'd imagined attaching the 2 cables securely at the bathroom end with some overlap, using a hefty wrapping of electrical tape and a bit of manly tugging at the fusebox end.

Drill a hole in a piece of wood and try pulling through a cable at a slight angle.

 

[bernardgreen]

Pulling a cable through a joist can damage the sheathing and this happens unseen. Pull really hard and the sheath and core insulation can be damaged to the point of exposing the copper.

Also cable subjected to strong pulling can stretch reducing the cross sectional area of the copper which will result in warm ( possibly very hot ) spots in the cable.

 

[Taylortwocities]

Also, holes in joists often have several cables in them.
Dragging several metres of cable through the hole, without being very careful, can easily wear the sheath of the other cables, exposing live copper conductors.

 

[bernardgreen]

I just remember the other hazard, water pipes tangled up with the cable used as the draw cord for a new cable. New cable installed ( after floor boards were finally lifted ) and damp patch on hall ceiling. Floor boards up again to repair weaping joint.

 

[JohnW2]

I just remember the other hazard, water pipes tangled up with the cable used as the draw cord for a new cable. New cable installed ( after floor boards were finally lifted ) and damp patch on hall ceiling. Floor boards up again to repair weaping joint.

Probably much more common than that (since pulling pipework apart with a cable is not that easy) is the situation in which the cable being used as a draw cord is tangled up with other cables. If one 'gets heavy', it's then very easy to rip other cables out of (concelaed!) JBs or accessories or, even (particularly if one is pulling on a 6mm² cable, per this thread) ripping apart a small (e.g. 1mm²) cable. ... not to mention corresponding damage that may be done to the new cable being pulled in.

Kind Regards, John

 

[skenk]

Because of the installation method, the 6mm cable will have a maximum rating of about 35A, so you are stuck with the 30A fuse.

It will? Is there something about the installation method I missed?

To the OP, if the shower is 14' away then I can't see how you will need 'new carpet' on 3 flights of stairs and two landings however you will need an isolation switch so the length might be a bit more than that?

Depending on installation method and length, 6mm may be able to carry 40A which is a 9.6kw shower due to shower power ratings being quoted at 240v. However your fusebox won't be up to the job so you will need a new CU or additional small CU with MCB and RCD for the shower (you will need an RCD no-matter-what if you alter the circuit).