Main intake

[ezekiel]

how would you start to determine sizing of equipment at intake ie:

size of main fuse,
meter tails
main earth

never had opportunity to look into this and im puzzled

 

[nozspark]

how would you start to determine sizing of equipment at intake ie:

size of main fuse, Legally you can't as you would need to see it to be sure. Most cutouts that I see, although labeled as having a 100A 1361 II, have a 60A or 80A - if you look at the meter it usually has a current rating on it, the main fuse is likely to be the same or smaller ----- if you ring the DNO, they won't know and they are unlikel;y to send someone out to confirm it's rating

meter tails If you are having a problem with cable sizes then try using offcuts to compare - otherwise most of us can tell a cables size just by looking (experience)
main earth Same as above

never had opportunity to look into this and im puzzled

 

[ezekiel]

thanks for reply but what i mean is when would you consider upgrading main earth and tails, and what would be the reason for upgrading them?

 

[RF Lighting]

If the tails are too small for the rating of the cutout fuse, then they should be upgraded appropriately.

It is very common to find undersized tails on 100A cutouts.

They should be 25mm²

Normally the main earth should be 16mm² for a TN supply. If it is not it should be upgraded before any work is carried out.

 

[ezekiel]

ah so 16mm on a 80a cutout is acceptable?

 

[RF Lighting]

It is just ok, and would not be a concern if I was doing some additions to that installation, but I would tend to install 25mm² as a force of habit if I was replacing the CU.

 

[securespark]

You could always do the adiabatic to calc the csa for the earthing conductor.

 

[ezekiel]

if using the adiabitic to calculate the main earth would T (time) be o.2 for tn or 0.07 tt taken from table 41.1

and K taken from 54.2 143/133 (if so what is the difference between 143 and 133?

 

[Spark123]

Time is whatever time it takes for the protective device to disconnect. If you have 30mA RCDs on a TT system then this will be the time taken for the RCD to disconnect. For a TN system use the time for the main fuse to disconnect for the measured fault value.

I haven't got the regs book to hand, is one of those K values for a conductor over a certain size?

It's a lot easier to go with the norm and select the earthing conductor from tables taking into account the tail sizes tho

 

[Click-Sure]

the adiabatic is all very well and good, but I find you just can go wrong with rule of thumb sizings! (on the conventional stuff anyways)