Shower Consumer Unit - Sizing of tails

[Hefferl]

New shower to be installed (10.5kw).

Current consumer unit is full / not RCD protected.

Intention is to split out tails and use seperate Shower CU (c/w RCD)

When sizing tails to new shower CU, as its a fixed load, do I just have to consider the shower load (approx 44A) or will they need to be able to carry the current that the main fuse will allow (80A)?

 

[Sidewinder]

Shower is fixed load but, for the difference just fit the bigger sized tails.

 

[Hefferl]

Thanks for help so far but I'm still not 100% clear on this.

When adding a secondary CU in this manner (Garage CU or Shower CU) do I have to size the new tails solely according to the main fuse rating to protect the tails themselves?

Or do I take account of what is being supplied by the new CU and the associated max current?

e.g. New garage CU with only one 20A MCB and one 6A MCB means the load on its tails is limited so even if connected via henley to 100A cutout fuse 16mm tails would be more than adequate. In this scenario what about short circuit protection in the unlikely event of damage to the tails somehow?

 

[ProDave]

e.g. New garage CU with only one 20A MCB and one 6A MCB means the load on its tails is limited so even if connected via henley to 100A cutout fuse 16mm tails would be more than adequate. In this scenario what about short circuit protection in the unlikely event of damage to the tails somehow?

Well, considering there are still millions of houses here the whole house is running on 16mm tails, and they don't catch fire very often, I think 16mm will be fine for your garage CU.

 

[Revved Up Sparky]

I would still fit 25mm tails. My way of thinking is that you cannot rule out a fault condition occuring which might create 80A of fault current in a cable which is only designed to carry up to 63A.

 

[Nicky Tesla]

10mm will be fine.............overcurrent protected by the shower MCB............adiabatic equation for the minimum CSA of the cable before it melts in a live/neutral short ..................live/cpc short will be minimum if after the RCD

 

[steptoe]

I'd use 6mm tails on a lot of occasions if only the wholesalers stocked them!

I use 16mm a lot though, just so much easier to terminate in a little 2way enclosure.

 

[andy_spark]

Well, considering there are still millions of houses here the whole house is running on 16mm tails, and they don't catch fire very often, I think 16mm will be fine for your garage CU.

Down where I am Western Power use 16mm tails to supply their isolator switches they install after the meter, in fact the supply to the main fuse is in 16mm. After all this I need to use 25mm to supply the CU. It doesn't make much sense but there it is.

 

[NozSpark]

Why's that then?

I bet in most cases that the main fuse isn't anything more than 80A

 

[Nicky Tesla]

mainly they are 100A, i know cos i often check. 80A is common also 60A, but i have seen as low as 35A for a mains fuse. This didnot blow even though there was an electric shower in the house, yes house not bungalow or flat,

The current ratings are for continued use at the rated amperage for an hour or so. Using seamingly underated cable will in most cases be fine due to the sporadic nature of current use.

 

[Manator]

I am just having a laugh, but, have any of you tried to follow the fault curves for any given fuse? Or breaker? If you design any circuit, you need to know how fuses and circuit

breakers work.

 

[andy_spark]

Agree with Nicky, lots of main fuses are 100A usually where there is an off-peak supply for storage heaters. I too check the fuse size, but as he says diversity etc is what the the supplier will use as a reason.

I have tried following fault curves and you can see some interesting results, although the charts could do with being a bit larger!