Advice on MCB size please - domestic shower cct

[freedomrun]

TN-S. Domestic 1.5 kW shower on a 6mm2 T&E clipped direct for the most part and in some plastic trunking for about 200 cm Total cct length is about 15M. The manufacturers label inside the shower itself states that it is a 9.6kW shower at 230V - should i take this as the power rating or the one also stated as 10.5kW at 240V (now we operate to 230V maybe i should take this as the power rating?) But I refer here to the 10.5kW rating.

Trips out a 32A MCB no problem (DIY installed shower and shower upgraded from previous). Told the customer i've got to include 30mA RCD and increase the MCB size. From what i can work out, the maximum i can go to will be at 46A. I would rather use a lower size cct protection at 40A, but am i likely to find this will trip out with the power rating here.

How much of an issue is the 200cm of cable enclosed in the plastc mini trunking, will this significantly decrease the current rating in this case, or at this length not an problem?

The customer is going to have the hump when i tell him i've got to put Main Bonding in as none present, not keen on swaping out the shower for a less powerful one. Changing the T&E from 6mm2 to 10mm2 is the obvious solution, but wanted to double check the MCB situation - would i be risking it using a 45A MCB on a cct when the regs say 46A is the maximum this cct design can take?

 

[SirKit Breaker]

The shower is irrelevant in this calculation. You need to spec your protective device for the rating of the cable, not the rating of the appliance. If the appliance needs more power than the cable can supply, then the cable needs to be upgraded to allow higher rated breaker to be fitted. This must of course be done with consideration to correction factors, volt drop and so on.

Cheers...........Howard

 

[Darkwood]

The resistance of 9.5KW @230V is 5.568 Ώ giving a current of 41.3A.
@240V the current through 5.568 Ώ becomes 43.1A giving an output of 10.34KW

Not sure why you given me this info the OP has already expressed the shower has a duel rating lable of both 230v and its equiv' at 240v, i wasn't in any doubt that the element resistance remains constant while the ampage increases with voltage increase or vice versa. I mearly asked him to measure the voltage to guide him to which one to work his calcs to.

 

[Richard Burns]

why? under floor is either clipped direct od in free air.

From BS7671 installation method numbers 40 or 47 seem to apply here, either in a building void or in a suspended floor, unless the space is deeper than about 200 or 600mm respectively, which could be the case but is probably unlikely. Both of these are method B.
Looking at the intent of the installation methods, if a cable is in an enclosed space without being in close contact with a conductive surface this is method B.
Alternatively if the cables were stretched across and did not reach the floor then you maybe able to call it free air, but an unlikely installation method (perhaps!)
I would tend to be erring on the side of B.

 

[Geoffsd]

Either way, it doesn't matter.

Both ratings are over 40A - so too high for 40A mcb and too high for 6mm² in conduit.

Smaller shower or lots of work.

 

[Guitarist]

Here we go again....
Don't forget manufacturers' instructions....

 

[valleybilly]

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 .

 

[somersetsparks]

in your average gaff, under the floor is reference method B, according to my tables, I seem to remember at college it has something to do with air movement or lack of it

 

[freedomrun]

I'll use Ref B then - what would be the minimum permitted length of cable within trunking before factors need to be applied?

 

[Richard Burns]

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.

 

[Alot Isaiah]

measure your supply voltage if it is 230v use it

 

[telectrix]

alternatively, change the shower unit for one with a sensible KW rating. e.g. 8KW.

 

[Guitarist]

Nicely put Richard.
Good idea tel.

Both very sensible and succinct answers

 

[freedomrun]

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.

 

[freedomrun]

Richard Burns - Thankyou for taking the time out to give such a detailed asnswer. Much appreciated.