4.0mm shower cable

[nc12345]

Hi Guys

I have a small job lined lined up , basically theres a 4.0 SWA from a RCD shower unit on a 40A breaker run 11m to a 45A switch then 1/2m in to 6.0 T&E to a 8.5Kw shower . The 4.0mm SWA is clipped surface for 2m then underground for 2m in its own pipe then 6m through a stud wall with insulation no greater than 100mm. Roughly the draw will be 37A and i know the 4.0mm SWA is rated at 43/45A at 90 degrees and it on a 40A breaker so alls good there but the problem i have is i think domestic switch gear is only rated at 70 degrees ? Am i worrying too much and i can tell the client its safe ? Im pretty sure he wont have the cable re run so the only thing i can think of is to ruduce the shower to a 7.5Kw and then the breaker to a 32A ?
Any ideas or feedback would be much appreciated .
Thanks to any replies in advance .

 

[telectrix]

if he won't have the cable uprated, stick in a 32A MCB to protect the 4mm. then if he does get tripping problems, you can say " i told you the cable neede replacing"

 

[Deleted member 9648]

Thing is with this 70 and 90 degrees business is that they never ever run that hot despite what it says on the tin.....90 degrees?....you could boil a kettle on that.We panic if it feels a bit warm never mind 90 degrees. Crack on I say mate....If it's a shower it'll only be on load for short periods anyway. Just make sure your terminations are good.....double over or wrap the wires so big terminals are filled with copper and good and tight.

 

[telectrix]

reminds me of a job i had at a regular client's pub. 16mm running 110A. after 2 hours at steady load, it managed to attain the fantastic high termperature of 39deg.C.

 

[Geoffsd]

The shower is 7.8kW @ 230V = 33.9A

Also, is it 90° 3c swa?

 

[nc12345]

Hi Geoffsd
yes its 3 core .
Sorry what do you mean the shower is 7.8 Kw , are you saying thats what a 8.5 is ?

Thanks to all the other replies

 

[Geoffsd]

Yes 8.5kW @ 240V which is what will be stated on the box is equivalent to 7.8kW @ 230V.

7800 / 230 = 33.9A which are the values we should use for calculations.

 

[nc12345]

Understood . My gut feeling is that it will be okay but i just wanted to make sure as safety is the main concern. Would you reccomend i drop the breaker to 32A to safe guard the installation as tripping shouldnt be a problem as it would only be on for short periods of time ?
Thanks

 

[dim_bulb]

Yes 8.5kW @ 240V which is what will be stated on the box is equivalent to 7.8kW @ 230V.

7800 / 230 = 33.9A which are the values we should use for calculations.

Where did you get the 7.8 from?

 

[Farmelectrics]

Yes drop breaker size cables protected then

 

[Geoffsd]

Where did you get the 7.8 from?

P/V=A 8500 / 240 = 35.4A
V/A=R 240 / 35.4 = 6.78Ω R being the only constant (apart from heating which is the same)

P=V²/R 230² / 6.78 = 7802W
P=V²/R 240² / 6.78 = 8495W