from an 8kw shower to a 10.5.

[Phenz]

Hi, i've taken the old shower off the wall and i'm looking into getting a 10.5kW shower. I've heard that you need better wiring for this kind of upgrade.

I've heard talk about 6mm and 10mm wiring and the like, but i'm kinda new to diy and was wondering...

the grey cable measures 15mm accross (including insulation) and has 3 wires inside, the red and the black measuring 5mm accross (including insulation)

i'm not sure whether this is 10mm wiring or not?

The wire comes off a dedicated circuit breaker which has B45 written on it, does this mean i have to upgrade it.

from what i've read, you need to have a separate circuit for the shower... a few years ago my dad and i put some lights and sockets in the loft taking a spur from the shower power supply - was this wrong?

there are two lights, and two sockets, (powering a tv signal booster) will this affect the breaker i need to use - what should it be?

i'm pretty good at diy but i'm a mechanical engineer and not too clear on electrical spec - just want to run this by people who know/done it before.

thanks.

 

[davy_owen_88]

the grey cable measures 15mm accross (including insulation) and has 3 wires inside, the red and the black measuring 5mm accross (including insulation)

i'm not sure whether this is 10mm wiring or not?

The best way to check it to get a small length of 6mm² and 10mm² cable from your local shed and compare the conductor sizes (not the insulation).

The wire comes off a dedicated circuit breaker which has B45 written on it, does this mean i have to upgrade it.

A 45A MCB is fine for 10.5kW.

from what i've read, you need to have a separate circuit for the shower... a few years ago my dad and i put some lights and sockets in the loft taking a spur from the shower power supply - was this wrong?

Yes! Very very wrong! I just hope you found sockets and lights that accept 6mm² cable or you fused it down appropriately? Regardless of those issues, you shouldn't spur anything off a shower circuit, ever.

there are two lights, and two sockets, (powering a tv signal booster) will this affect the breaker i need to use - what should it be?

No it won't affect the breaker because you should disconnect them and do the job properly. You were in your loft probably only a few feet away from a suitable supply and you chose the worst possible option. Fit the lights to the lighting circuit and as the TV signal booster uses minimal amounts of 'leccy fit a 3A fused spur and feed the socket from there too.

 

[Phenz]

thanks for the reply, i can now fit my shower with confidence and i'll find a lighting circuit and take the spur off there for the loft lights.

i'm pretty sure its 10mm wiring that feeds the shower and i've got 6mm wiring as the current spur taken off it - but i'll change it.

cheers

 

[Phenz]

ARG! i just found out that my cable is only 6mm, checked it with the local electrical shop - like suggested - good call.

is there a straight forward way to run 10mm cable around the house to the circuit breaker? its a small two story house and the shower couldn't be further from the fuse box if i tried. the guy said it was £1.50 a metre for 10mm cable? that sound about right?

 

[davy_owen_88]

Just surface mount it if you don't feel like the hassle of lift floorboards etc.

At £1.50 per metre I would be interested to know where you are getting your cables. TLC and screwfix are closer to £2.50 per/m for 10mm² twin & earth.

 

[obmitty]

Alert Electrical sells 10 meters for £18. Unless you spend over £50, postage is free.

Wilkinson's is the cheapest place if you only need a few meters... its about £1.95/meter if I remember correctly. Cheaper per meter than buying a whole roll at B+Q, and you can just buy 1 meter if thats all you need.

 

[obmitty]

BTW, when you do the sums, power through the cable comes out at more like 46A. How come a 45A mcb is fine, will it not be more likely to trip?

 

[Phenz]

surface mounting it might be a possibilty might have a look at taking up the floorboards in my cupboard... and feeding it to downstairs thought there.

the 'fuse' box looks like it has had the shower mcb added to it at a later stage cause it goes off in a different direction to all the other electrical wiring which takes off into the wall, so should be easier to wire.

if i flip the main breaker, will that allow me to safely rewire the shower breaker?

i get it from Pearsons in the barras, glasgow (live right next door) - he siad it was £1.50 and i asked if i got a discount for buying a lot of it... apparently 30m isn't very much. still going to set me back £45 + the shower unit. It better bloddy work!

 

[Spark123]

BTW, when you do the sums, power through the cable comes out at more like 46A. How come a 45A mcb is fine, will it not be more likely to trip?

Power ratings are normally given for 240v, this equates to somewhere around 44A for a 10.5kw shower. If the supply voltage is dropped to 230v the rating of the Kw rating of the shower also drops to somewhere around 9.6kw which requires about 42A.

 

[Phenz]

i don't think i'm going to replace the 6mm cable with 10mm, just too much hastle just for a slighly better shower.

i've been looking at the spec for 6mm and it says that i could have it supplying 45A but according to people on here - it just isn't done!.

The cable runs from the shower up into the loft, there is loft insulation, but its not a problem to clear the 50cm run that it has of all insulation. it goes to a pull cord and then back into the loft, and down inside the wall to the ground floor where there is a switch in the kitchen. as far as i'm aware, it then goes underneath the house, underneath the kitchen, stairway, lounge to the circuit box in the porch where its got a dedicated 45A mcb.

what is the most powerful shower i could connect to this circuit?

 

[Softus]

What's the total length of the 6mm² cable run, including the segment from pull switch to shower unit?

 

[Phenz]

using very very conservative measurements:

shower to pull cord : 1.5m

pull coard to kitchen switch : 6m

kitchen switch to mcb : 12m

In total about 20m

 

[Softus]

using very very conservative measurements

Sorry - which way are you erring?

 

[Phenz]

when measuring the heights of my walls, the lengths of my floors etc to give an ida, i added about half a metre to each measurement,

length from shower to celing
+50
length to pull cord

length of cable till it dissapears into the wall
+ height of upstairs wall
+ 50cm
+height of downstairs wall
+ 50cm

kitchen socket

+length to floor
+50cm
+ length of kitchen
+50cm
+width of house
+50cm

mcb

comes out at about 20m

does it make a significant difference if the cable length is 20m as opposed to 25 or so?

 

[Stulz]

does it make a significant difference if the cable length is 20m as opposed to 25 or so?

It can do. The cable loses energy along it's length, this is called VOLT DROP. It has an inherent relationship to the length of the cable, the size of the cable, the material the cable is made from and the size of load that is attached to it. If you inspect the appendices of the Wiring Regulations (BS 7671) you will find that for general use copper and aluminium cables the data required to calculate their respective volt drops have been put into tables so that engineers and electricians etc can calculate the voltage lost in a circuit over a prescribed distance. The voltage drop is important as it dictates many features of how the electrical circuit actually operates in use, and importantly, how any circuit protective equipment, such as RCD's, MCB's and Fuses, actually operate. They must operate within 0.4s or 5s depending on the circuit. As this is a shower and the lower body resistance that would be intorduced by being immersed in a water stream, any protective device must operate to disconnect the circuit in the event of a fault within 0.4s.

A quick calculation shows that to run your 10.5kW shower in compliance with the Wiring Regulations, which it would be prudent and common sense to follow, you will need to either install 10mm² cable, or downgrade your shower aspirations.