CPC calculation

[ETradesmen]

Just got back from looking at a potential job. What a bodge up!

I was called around to be asked to install an isolator for a sub main feeding a consumer unit which a car charger company will come off feeding their charger.

New mains upgrade recently done and the previous electrician has bodged a bit of 16mm t and e straight into a henly block from the meter to feed secondary consumer unit.

I’m thinking the cpc within the 16mm may be undersized but just wanted to clarify if my calculation is correct (it’s been a while)...

The size of fuse I’m thinking of installing is a 50A type b (either RCBO or mcb with rcd main switch).

S = 250A x 11 (squared) / 115 = 7.2mm??

CPC is 6mm.

The sub main in question has no bonding attached to it and is approximately 15mtrs away from main board running underneath the floor.

Please correct me if I’m wrong, it’s been a while!

thanks

 

[davesparks]

I don’t need to know the load at the consumer unit end, I just want to make the sub main safe which is why I’m going to add a fuse making sure it’s the biggest / safest one.

My thoughts were to add a 63a double pole RCD controlling a 50A MCB, the part I’m stuck on is the CPC calculation,

You say you want to fit the best fuse for the job, then say you are going to fit an MCB, this makes no sense? An MCB is not a fuse, and a fuse is better than an MCB for protecting distribution circuits.

If you don't know the load at the consumer unit how can you know whether the ocpd you install is correct?

 

[buzzlightyear]

If you don't know the load at the consumer unit how can you know whether the ocpd you install is correct?

I asked that in post #8.

 

[davesparks]

I asked that in post #6.

Eh? Post 6 asks what size the tails are.

 

[buzzlightyear]

Eh? Post 6 asks what size the tails are.

What I meant was the loading put wrong number.