4mm wired 9.5kw shower - cable hasn't blown

[danrich123]

I started a rewire today, ripping out the old wiring etc. Anyway, i noticed what looked like a small cable going to a quite new shower. Upon inspection i found a newish 9.5kw shower run in 4mmt/e.
I asked the owner if it had been working etc. and she says its been fine, she had the shower upgraded to 9.5kw a few years ago from a 7.5 and has never had a problem.
Why would this cable never of gone 'bang'. It had been run in trunking grouped with the other cables in the installation. so wasn't like it was in free air, or had minimal insulation around it.
Any possible explanations?

 

[ban-all-sheds]

533.2.1

 

[1john]

Why would there be a problem, we dont install minimum sized cables, we take worse case scenarios into consideration, cables are calculated to run safely at capacity, therefore there is every likelyhood that a 4.0mm2 cable could be capable to carry 40A for short periods of times, I have witnessed whole domestic installations with a 60A DNO service fuse using 6.0mm2 for the tails. Did you really think the cable would have blown?????


(Incase any DIYers are reading this and thinking about using an undersized cable, DONT, there is every chance something may go wrong. It is possible for an undersized cable to over heat and catch fire or set something else on fire)

 

[spalding1sparks2]

re shower 4mm was to small for the 7.5 shower what circuit protection was there ie 3036 rewireable fuse or MCB and what size was it. The shower would quite possible be used for very short periods and if anything the connections are usually the first point to suffer and burn did you look in the DB and the Isolating switch for heat damage, recently found 1 on a 2.5mm from the pull to the shower that burnt out at the connections DIY job tut tut.
where was part P
kelve

 

[danrich123]

I thought it would be ok in when used for a very short time but not 3 years...(unless shes abit dirty and doesn't use it much)
I'll have a good read of 533.2.1 tomorrow, I probably need a refresher.

 

[Taylortwocities]

re shower 4mm was to small for the 7.5 shower what circuit protection was there ie 3036 rewireable fuse or MCB and what size was it.

7.5KW is less than 32amp at 240v (and that was the standard 'back then').
9.6kW is about 42A at 230v and a 32A cpd isn't going to blow at that current. Check the time/current graphs in the good book.

Nominal rating of 4mm² is about 32A, so it will get a little warm and the shower may not get quite so hot (cos of vd). But it will live through the experience of a 10 minute shower now and again.

Note: this is not a design criterea to be followed!

 

[radioredcat]

I saw a shower that was installed in 1975 by the council that was connected to the sockets ring main.

The cable to the shower was the only piece of cable in PVC the rest of the house was rubber cable.

I was called to have a look at a light that was flickering when it was switched off when i took the light switch off there were bare wires and a pile of rubber in the back box.

Needless to say i told them they need to have the house rewired which they did.


Andy

 

[EngStudent]

Nominal rating of 4mm² is about 32A, so it will get a little warm and the shower may not get quite so hot (cos of vd). But it will live through the experience of a 10 minute shower now and again.

Note: this is not a design criterea to be followed!

Shower in the last flat I rented was 8.5kW on a 4mm² cable. I'd not noticed until I felt the trunking running alongside the doorframe one day and it felt quite warm. Upon further inspection, they'd run out of ways in the CU (old 4-way Wylex) so had run it from an MCB next to the service head, a run some ~16m in length. Obviously the first attempt didn't go too well, because I found an enclosure lying in the bottom of the cupboard, complete with partially melted MCB.

 

[ban-all-sheds]

had run it from an MCB next to the service head

Connected before the meter?

 

[EngStudent]

had run it from an MCB next to the service head

Connected before the meter?

Nah but they had mounted it in very close proximity.

That place was a deathtrap.

 

[1john]

When I bought my house there was a 8.5kW shower wired in 2,5mm from the pull switch! These botches are around, I had photos somewhere, it was new colours so quite likely a Part P job that was done illegally.

 

[ban-all-sheds]

so quite likely a Part P job that was done illegally.

How can it be a "Part P job" when it doesn't comply with P1?

 

[xr4x4]

flat i used to live in, boss owned it and we worked in the shop below.

he fitted a 8.5kw shower before i moved in, he tapped into the 6mm feed to the hob in the loft with a junction box...or choc blocks.

this was fed from a 30a rewireable in a old 4 way wylex CU.

prob still like it today.

 

[1john]

so quite likely a Part P job that was done illegally.

How can it be a "Part P job" when it doesn't comply with P1?

I meant to mean that it could have been a 'part p' job, but clearly wasn't.