8.5KW shower on 6mm cable??

[la poste]

Howdy.

We have an old Mira pumped electric shower, it's not mains fed but gravity fed and it's kaput. The only available replacement is an 8.5KW model.

The supply cable is 6mm multi-stranded and about 15m long clipped direct up to the loft and then dropped down in pvc trunking to the shower. It is fused at 32 Amps.

My question is would an 8.5 KW shower need a 10mm cable or would we be able to use the existing 6mm?

I have read the OSG and it says that 6mm cable can carry up to 40 Amps depending on method of installation.

Also bearing in mind our supply is 240 Volts and not 230 volts as the shower manufacturer assumes.

Any advice would be appreciated.

Thanks.

 

[NozSpark]

Showers are usually rated firstly at 240V and then at 230V

so an 8.5kW (at 240V) shower is something link 7.8kW at 230V

that will give you ITRO 37A @ 8.5kW and 34A @7.8kW..

TBH a 32A MCB will stay in a lot longer than you'll need for a shower at those values

 

[la poste]

Thanks.

I actually think it will be fine.

The shower will be used for short lengths of time and I will make sure there is no insulation covering the cable in the loft.

I will probably fuse it at 32 Amps IE leave it as it is and just to see what happens.

If I have a fusing problem I will decide what to do when it happens.

I think the maximum the shower will be used is five minute bursts three times a week.

 

[NozSpark]

I've not got my regs book to hand, but you could look up the tripping times in the graphs at the back.. TBH it's probably off the graph

 

[Larnacaman]

TBH a 32A MCB will stay in a lot longer than you'll need for a shower at those values

Well you obviously don't know much about WOMEN then!!!! ROTFWL

 

[la poste]

I work on the assumption an MCB will trip at roughly 1.45 the fuse rating EG 1.45 times 32 Amps.

Is there anything wrong with this logic?

 

[revjames]

TBH a 32A MCB will stay in a lot longer than you'll need for a shower at those values Well you obviously don't know much about WOMEN then!!!! ROTFWL

I have a 10.5kw shower and would love it to trip after 5 mins

 

[binky]

you can always upgrade to 40A on 6mm cable given only very short section is run in conduit.

 

[PC Electrics]

I'd agree with Binky. Clipped direct 6mm is good for 47A and strictly speaking the section in the trunking is 38A. What's 2A between friends? It's supplying an occasional load (if you ran an electric shower continuously I'd bet the shower would give out before the cable) in an ambient rarely exceeding 20C and it's only a short section.

Most manufacturers state an 80% rating factor for MCBs run close to their rated capacity if they have other devices each side 'cos there's less cooling available. So a 40 would be better in that situation.

 

[sparkytim]

As this is a fixed load then NO OVERLOAD protection required

see BRB Pg 74 433.3.1 (ii)

So 40 A MCB OK

 

[kme]

Concur with Tim, Binky & PC here - fit a 40A.

KME