Discuss Advice on MCB size please - domestic shower cct in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

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Best practice rules here , Dont take crap off customers who winge at the cost as its your --- next if all goes wrong . Hes employing you to do it right not DIY it as before , If he dosent like whats best then walk ! , Its suprising how good it feels & i bet he calls you back .
 
I'll use Ref B then - what would be the minimum permitted length of cable within trunking before factors need to be applied?
 
Let’s summarise here
All this is only considering the installation method, the regulations, my thoughts.
This does not consider volt drop or any other applicable derating factors.

Cable run 15m of 6mm[SUP]2[/SUP] T&E all installed to reference method B (in my opinion)
Shower 9.6kW @ 230V or 10.5kW @ 240V
Protection device not yet selected.

CCC for 6mm[SUP]2[/SUP] multicore 70[SUP]o[/SUP]C
Ref B 38A
Ref C 46A

Current demand of an instantaneous water heater (no diversity permitted)
9.6kW @ 230V I=P/V = 41.7A
10.5kW @ 240V I=P/V = 43.75A

So in order to carry a current of maximum 43.75A then a 6mm[SUP]2[/SUP] cable must be installed to reference method C and cannot be installed to reference method B.

Questions you have:

Can I use a 45A MCB?

For the shower as an appliance and if the cable were suitable – Yes
For 6mm[SUP]2[/SUP] T&E installed to Ref method B – No (also you cannot use a 40A MCB)
For 6mm[SUP]2[/SUP] T&E installed to Ref method C – Yes

What is the maximum length of trunking before derating is required?

The regulations do not state a minimum length before derating is applied.
You could possibly consider as an electrician that the regulations do give variable derating factors for different lengths of cable in insulation. If you were to apply the same considerations to trunking this would mean that full derating is applied if the length is greater than 50cm. As your length is 200cm full derating would apply.

Solutions available:
Use the 6mm[SUP]2[/SUP] cable as a pulling wire to draw 10mm[SUP]2[/SUP] cable in to replace it, use a 45A MCB, upgrade bonding.

Decide that under the floor is reference method C, remove the trunking, use the existing cable, use a 45A MCB, upgrade bonding.

Read the manufacturers instructions and decide if they specify that you can use any other method of safe installation as these would override the wiring regulations.
 
alternatively, change the shower unit for one with a sensible KW rating. e.g. 8KW.
 
Replacing the Shower would be my pref choice - customer not having it, but all i can do is state all the possible options I have available.
 
Richard Burns - Thankyou for taking the time out to give such a detailed asnswer. Much appreciated.
 

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