Adding sub board to main consumer unit on tt

[davesparks]

Why do you want to protect submains with MCBs? This is very poor design practice.
Why do you want the submains on RCDs?

 

[davesparks]

If the existing submain is not suitable for the increased load then the answer would be to replace it, not to bodge around it with poor design choices

 

[buzzlightyear]

you took the words right out of my mouth dave sparks

 

[davesparks]

you took the words right out of my mouth dave sparks

There's a song in there somewhere!

 

[buzzlightyear]

this farm shop you saying ,calculate the existing load before the extra load .
you need to put some pics on here to so we can see the existing supply.
candy man !

 

[davesparks]

this farm shop you saying ,calculate the existing load before the extra load .
you need to put some pics on here to so we can see the existing supply.
candy man !

Calculate? What on earth would you want to do that for?
Just pick an MCB rating that sounds good and add a generous quantity of RCDs 'just in case'

 

[bigspark17]

Sounds if you pulled a 16mm cpc in with the sub then this would get you too your max zs value. Is this doable? Stay away from rcds & mcbs on large subs if possible for discrimination of ocpd in series during fault condition. Even if you end up TT the barn the sub would still need to meet TN disconnection times up to the isolator. Understand your in a pickle but dont get led by the customers "tight budge" as you will still need to cert your work.

 

[davesparks]

What size is the existing sub-main? Is it ok for the increase to 63A, what sort of volt-drop is there going to be on it?

 

[fatgreeny09]

What size is the existing sub-main? Is it ok for the increase to 63A, what sort of volt-drop is there going to be on it?

It's a 16mm 4 core armoured. Not much volt drop at all. It's about 30 meters but that's just from r1 and rn test and rough distance

 

[davesparks]

It's a 16mm 4 core armoured. Not much volt drop at all. It's about 30 meters but that's just from r1 and rn test and rough distance

Not much at all? That's a great answer to a technical question.

A quick smoke packet calculation based on the full ratings of the ocpds suggests that your looking at 4.5V drop on the 63A submain and 3V drop on the 50A submain after that, giving a total of 7.5V drop on the submains alone.
That leaves you very little left for your final power circuits and nothing at all for any final lighting circuits.

 

[marc8]

Make sure that RCD up front (100ma) is time delayed, if you install a 30ma rcd down stream the 100ma rcd could still trip unless time delayed.