adding shower CU

[ianphilip007]

hi

just wanted to check something,
when adding a seperate shower CU (split tails) from a 80amp or even 100amp main supply the tails feeding the shower CU do they have to now be 25mm? or will 16mm be adequate due to the fixed load of the shower

The seperate shower cu (63amp 30ma RCD) is this again due to the CU supplying one fixed load so no need to be rated at the 100 or 80amp supply fuse

many thanks

 

[Guitarist]

thankyou guitarist thats what i was thinking, so would you agree that as the shower cu can only draw 40amps the shower CU 63a/30ma rcd switch is adequate ?? no need to replace with a 80a/30ma

As long as that is all the CU is able to do, ie. 1 MCB, then yes. Just to reiterate, the 63A main RCD switch will not give you protection against overload and/or short circuit unless it is an RCBO. That said, even if it were, you would still need an appropriate MCB (40A depending on circuit characteristics) to protect your shower cable if it's only 10mm.

 

[ianphilip007]

yes but in the shower cu's its the main switch for the cu so i was concerned at it beeing rated (63a)below the main fuse which might be 80 or 100 amps. but as a shower is a fixed load i guess 63amps will be fine on a 40a or 50a MCB

 

[Guest123]

by backfusing Lenny are you saying that the cable feeding the shower CU (if under 3 meters) is protected by the MCB in the shower CU lets say 40amps for a 8.5KW shower.

Essentially, yes.

Its the same principal as taking a feed off a busbar chamber for example.

It may be a 400A chamber, doesn't mean every cable taken from it needs to be rated at 400A, as long as there is a fuse within 3M, the supply cables can then be rated to that fuse size.



Sent from my BlackBerry 9800 using Tapatalk

 

[Snapester]

Basically forget the 63amps it is only what the switch will work safley at and what max current can pass through it, the 40amps will protect the shower cable not the main incoming feed!

 

[i=p/u]

so 63A main switch is just an rcd

 

[Snapester]

Yes i won't give overload protection hence the need for MCB along side the RCD.

 

[ianphilip007]

yes the mcb will protect the shower cable but i was mistakenly thinking that the rcd switch/current rating would have to be 80 amps for a 80amp supply fuse but if it is a fixed load of only 45amps then thats the maximum the rcd would need to be rated at.

 

[Snapester]

Exactly so if the shower is only ever going to have a max load of say 50amps then 63amps main switch is fine as your not exceeding what can safely pass through the switch.

 

[ianphilip007]

lol exactly thanks snapster.

 

[Guitarist]

Glad we cleared that up for you :star:

 

[ianphilip007]

many thanks for all your help