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

 

[ianphilip007]

so you can feed a (63a/30ma rcd/ 45a MCB shower cu) from a 80amp fused tails?

 

[sedgy34]

Read the question snapester

 

[ianphilip007]

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.

i agree reg not very clear

 

[D80]

Yip! Stick in 10mm tails if you have them.

Seen on a few sites the fire alarm on the mains panels fed directly off the busbar in 1.5/2.5MM cable. 1000Amp supplies to the panel and the like.

 

[Snapester]

I think what he is saying is if the feed ie 16mm tails are over 3m from the origin then you will need a KMF switch at the origin of supply.
So if your shower CU is in the airing cupboard or something and over 3m from the origin of supply you will need a isolator/KMF switch.

 

[ianphilip007]

yes i agree snapster, if the shower cu is within 3 meters no need for a switchfuse to protect the 16mm tails. the way i see it they would be protected via the mcb/rcd in the new shower cu. if over 3 m the 16mm tails would have to be protected and fused down via a switchfuse at origin before beeing routed to shower cu.

so i think that the 16mm tails can feed the 63a/30ma rcd mainswitch of the shower cu directly from the 80amp/100amp main fuse via henly block.

 

[Guitarist]

As long as your reduced fuse or breaker is within 3M of the supply point, then the cables only need rating at the new fuse/breaker. Hence, if your new shower CU had a 40A MCB installed and that is therefore the maximum current which can flow through that system, then 10mm tails would suffice (or 63a/16mm). As said previously tho, the RCD rating is NOT your circuit protection. You will need an additional MCB or RCBO

 

[ianphilip007]

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

 

[Snapester]

The RCD is just a switch it will not operate as an overload device, you will need MCB to proved overload protection.

 

[telectrix]

but a 63A/30mA RCD is fine.

 

[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