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

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freedomrun

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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?
 
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1

1shortcircuit

  • Thread Starter Thread Starter
  • #2
How long is a shower on for?

How long is a cable tested for to prove rating of cable?

Go through your cable calcs applying the correct factors and PROVE wether a 45A mcb will be adequate:thumbsup


Hang on a minute, you're just wanting to cheat the system and get a quick answer aren't you ;)

Stop being lazy lol
 

kingeri

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Arms
The section of cable in the mini trunking should technically be de-rated, don't have my books handy so can't be more specific, sorry. And doesn't matter what he says about main bonding, if he won't let you do that then just walk away!
 

darkwood

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Measure the supply voltage, if its 230v use the 230v rating if its as i suspect 240v or their abouts use that rating. Yes agree we use on paper 230v now but rarely come across this in domestic.
 
K

Knobhead

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  • #6
Measure the supply voltage, if its 230v use the 230v rating if its as i suspect 240v or their abouts use that rating. Yes agree we use on paper 230v now but rarely come across this in domestic.
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
 

Richard Burns

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You have current of about 42A with either value of power as they are almost the same within a fraction.
If you have 6mm T&E in trunking this is a CCC of about 36A so this would not meet the regs. The clipped direct would meet the regs at 46A.
You could use 40A RCBO as protection since it is on for only a few minutes, but you would need to justify the CCC for trunking run; is it horizontal or vertical is it buried in insulation, is there clear air flow through the trunking or is it sealed at either end.
 
Very rarely is cable clipped direct along its whole length for a shower. Most of the time it will be run through conduit inside the wall, or pass through insulation in a loft.
I can't believe we are doing the whole shower thing again! :)
 
F

freedomrun

  • Thread Starter Thread Starter
  • #9
This install is in a GFF. Cable runs under the floorboards and then up a wall in mini trunking - is a value stated anywhere for the minimum legnth of cable permitted in trunking BEFORE de-rating has to be calclated?

I'm only asking for some advice here, not for calculations to be performed for me - I'm not lazy. Thanks.
 
D

drew35

  • Thread Starter Thread Starter
  • #10
If he doesn't want bonding ask him if he ever wants to be found lying dead and naked in the shower, after electrocution, and in a pool of his own pee n' poo? Ambulance staff pointing and laughing at his tiny pee pee!

Basically how I'm found after each and every Birthday party, apart from the dead part, and only occupationally electrocuted!
 
F

freedomrun

  • Thread Starter Thread Starter
  • #11
The bonding is not a problem - he has it installed or i'm not doing the work, that's it. My question is about the overcurrent protection at the top end of stated reg limit.
 

Richard Burns

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Under the floor boards is reference method B as well, not reference method C, so it looks like you are running a cable with too low a CCC for the applied load!
 
F

freedomrun

  • Thread Starter Thread Starter
  • #13
True. I have no access to the cable installed under the floor, so i should assume worst case install method for T&E within reason - would'nt of thought insulation would be present in a ground floor flat and it's an old Victorian property converted years ago.
 

telectrix

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Under the floor boards is reference method B as well, not reference method C, so it looks like you are running a cable with too low a CCC for the applied load!
why? under floor is either clipped direct od in free air.
 
F

freedomrun

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  • #15
Well, my original thoughts on this were that Ref C was the one to go for.
 
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