16mm twin &earth sub mains separate earth

[buzzlightyear]

been to look at a job , customer needs sub mains and meter moving to the under stairs from main cut out from hallway a new 100 amp cu . 16mm twin earth do I need separate cpc for that according to table
54.7 16 mm cpc any thoughts on this .

 

[Andy78]

Table 54.7 is an alternative to using the adiabatic equation. Both are acceptable for determining cpc sizes.
What are these distribution circuits supplying ?

 

[Leesparkykent]

Where's the bonding connected?

 

[buzzlightyear]

andy 78 free standing cooker ,5lights approx 6xd/sockets a tn-s system

 

[Andy78]

andy 78 free standing cooker ,5lights approx 6xd/sockets a tn-s system

I'm assuming these are the final circuits not distribution circuits ?
I'm confused now.

 

[buzzlightyear]

sorry andy 78 ,yes final circuits. domestic .

 

[Andy78]

Right, I've re read the first post a few times and I've now understood that it's a domestic CU relocation. (sorry for being slow)

My post #2 stands for the size of the cpc for the circuit supplying the new CU. Lee's post #3 needs answering to help further.

 

[buzzlightyear]

sorry andy 78, 16mm t/e flat. sub mains cpc for that is 6mm sq.
the question, would you run a separate 10mm cpc with that .

 

[davesparks]

sorry andy 78, 16mm t/e flat. sub mains cpc for that is 6mm sq.
the question, would you run a separate 10mm cpc with that .

You can only answer this by doing the calculation for this particular installation.
You also need to consider the bonding, will the cpc of the submain also be acting as a main bond? If so it may affect the sizing of it. Although with a TNS system you will likely only need 6mm main bonds.

 

[Wilko]

Hi - not that I think you need 100A, but from OP if it's planned to have 100A fuse then 16mm tails are too small I think. But I'm probably being too literal, sorry. As DaveSparks says, you need the calc if you want to do protective conductor less than 16mm.

 

[buzzlightyear]

the plan is, new one way cu with 63 amp mcb/100 isolator with 25mm tails from dno to new cu with 16mm sub main cable on the end of that 100 amp fuse from dno 16mm cpc,to ads 10mm gas /water
would I need a separate earth has well

 

[bigspark17]

Where is bonding ran too??? U seem to be avoiding the question. It has a lot of relevance or people wouldnt ask. Id recommend using switch fuse at DNO side of things for the sub to provide discrimination between OCPD in series.

 

[westward10]

If your 10 bonding is going to new CU then yes you need a separate conductor. If not probably no but use the adiabatic equation to see whether the cable cpc is adequate which it probably is.

 

[buzzlightyear]

thanks westward .

 

[davesparks]

the plan is, new one way cu with 63 amp mcb/100 isolator with 25mm tails from dno to new cu with 16mm sub main cable on the end of that 100 amp fuse from dno 16mm cpc,to ads 10mm gas /water
would I need a separate earth has well

Submain Fed via an mcb is rarely a good idea, have you considered using a switchfuse?

 

[buzzlightyear]

daves parks I'm still trying to get my head round sizing protective conductors table 54.7
and you write switch fuse would be better with switch fuse 60 amp
but the size of the 16mm twin not much room to play with in unity area

 

[davesparks]

daves parks I'm still trying to get my head round sizing protective conductors table 54.7
and you write switch fuse would be better with switch fuse 60 amp
but the size of the 16mm twin not much room to play with in unity area

Why are you 'trying to get your head around sizing protective conductors' if you are designing and installing things on your own? You should have learned this during your apprenticeship and not be learning as you go and practicing on your paying customers.

What is your concern with space of? The physical size of the switchfuse or the space inside it to work?
A KMF switchfuse is smaller than any small CU you are proposing to use!

 

[buzzlightyear]

I could not remember the formula fault current if = v/s =I2t/k.
has for the kmf switch fuse a bit small for the tails I think.
plus it is the size of the space to work in, old utility box.

 

[westward10]

Those switches are relatively small and they are designed to take 16.0 conductors, it is a better option than an mcb.

 

[buzzlightyear]

westward do you know what the characteristic for 60 amp bs 1361, could not find it in the yell regs