Electric Shower Cable

[Dunton]

Just had an electrical installation test done on a house I am moving to. Mostly fine except that the electric shower (Triton "Pearl", which I believe is 10.5kw) is wired with a 6mm cable. I have the same shower in my current house and I used 10mm.

The report doesn't say that it the wrong size cable, just that the 40A MCB protecting it should be 32A (failed inspection because of that one thing).

IS it ok to run this on 6mm? what are the risks?

 

[ChrisR]

NO.
Fire, death and stuff.
Try the electrical forum!

 

[gas4you]

I would expect a 10.5Kw shower to be wired with 10mm T&E AND be protected by a 40amp mcb, not a 32amp and have a 10mm earth bond on the cold mains pipe if it is copper

Are you sure it is not 8kw or less? That would fit in with your report results

 

[baxpoti]

If your 6mm is standard p.v.c/p.v.c.

Refer to BS 7671
Table 4D2A

Current - carrying capacity of multi-core p.v.c.-insulated cables, non armoured (copper conductors)

What Method of Installation of the cable is used on your shower and how long is the cable?

 

[Dunton]

I can't remember the wattage of the "Pearl" but think it was 10.5Kw - may have been less.

The cable is std pvc and is about 4 or 5 metres length (judging by the CU position and floor plan). I suppose some of it is buried in plaster and the majority in the ceiling void.

The report didn't say the cable was wrong (but then he probably wouldn't have known the wattage of the shower I suppose).

Is 8Kw the maximum for 6mm cable then?

(by the way, the current owner is an electrician so I assumed all would be well)

 

[Dunton]

Ooops! must be my age - it's aTriton "Opal" not pearl Iknew it was a gem.

And it is 10.5Kw

Looking at that BS table, the 6mm can take 51A in "Method 13", depending on what 'free air' is. Maybe the electrician who fitted it decided it was mostly that.

 

[Spark123]

10.5kw equates to approximately 44A meaning a 45A MCB is required. According to table 4D5A 6mm is rated at 47A clipped direct before any relevant correction factors are applied i.e. it is only rated at 47A as long as it is clipped direct, not subjected to an ambient temp over 30degC, not grouped with any other cables and kept away from any thermal insulation (also not using rewirable fuses). There are also other factors such as cable length (volt drop and efli) which also need consideration. Any of these could be why it was failed.
The remedial work will almost certainly be notifiable to your LABC under part p of the building regs too I'm afraid.

 

[Dunton]

So, might be easier to change it for a 8Kw model then?

 

[Dunton]

I see there are a number of 8KW or less electric showers.

6mm cable and 32A MCB are ok for 8KW, yes?

Does anyone know of one that EXACTLY matches the water and electricity connections of the Triton "Opal", so it can be changed relatively simply?

 

[Spark123]

7.5kw is about the maximum for a 32A protective device. The current carrying capacity of a cable is given by how it is installed and how long it is, a 6mm does not automatically carry 32A i.e. if it is laid in insulation for 500mm it halves its current carrying capacity.

 

[Dunton]

Ok thanks.

I'll ask about a possible replacement in a separate topic and close this one.

 

[dingbat]

If your 6mm is standard p.v.c/p.v.c.

Refer to BS 7671
Table 4D2A

The figures in table 4D2A do not apply to twin & earth.

Table 4D5A is specifically for flat T & E cable. It was included as an addendum at the back of the blue book and included in Appendix 4 when reprinted with a brown cover in 2004.

What colour is your regs book?

 

[obmitty]

I would expect a 10.5Kw shower to be wired with 10mm T&E AND be protected by a 40amp mcb, not a 32amp and have a 10mm earth bond on the cold mains pipe if it is copper

Are you sure it is not 8kw or less? That would fit in with your report results

I'm a bit confused...

The current flowing is given by

Power W = current (I) x voltage (V)

Therefore, for a 10.5 kw shower,

I= 10500W/230V

I= 45.7 Amps

Surely the MCB should be atleast this value or it will trip more often?


I'm not trying to be a smarty pants, I'm at the start of the learning curve and I assume you know something I don't (yet!).

 

[davy_owen_88]

The highest rating on a shower is for 240V. If you read the manual it will usually say something like:

power: 10.5/9.6kW
voltage: 240/230v

Because a shower has a fixed resistance a drop of voltage gives a drop in current.

At 240v:

10500w / 240v = 43.75A

At 230v

9600w / 230v = 41.74A

Although the voltages are now taken as 230v almost everywhere the voltage is still closer to 240v. Using 230 in the equation just gives a higher current than what you actually have.