calculating main tail size?

mark7600 · Mar 11, 2010

My question is on calculating the total load allowed on different main tail sizes. I have asked my colleagues and fellow students the same question, and got different answers. What is your opinon, do you follow the regs to the book by calculating the total current demand and apply diversity? Or just put the new circuits in?
I cant get my head round it as we are only starting to cover design in l3 2330.

The question is say you have a 60a cut out, 16 mm tails. Current board has 2 x 32a shower, ring, 1x16 Boiler,2x6 amp. If you wanted to add 3 x 32a , ring,Hob, double cooker and 1x6amp lighting.

Some one at college said just install it all on a big board the hrc fuse could take it applying diversity for a long time if it ever was all on at the same time?
Some one at work disagreed and said upgrade the supply?

What is your opinon from your experience?
Looking at table 6d1 in the osg the total demand in 87amps for 16mm clipped direct.

If you did upgrade what is the usual price to upgrade to a 25mm supply?
Thanks any help would be great as I cant get my head round the theory of diversity and the realistic day to day use of it. Cheersss!!!

ezzzekiel · Mar 12, 2010

your only concern is tail size which most sparks would upgrade to 25mm regardless, if you think supply needs upgrading inform dno.

anything 80a or under 16mm is fine, upwards of 80 then 25mm

KNIPEX · Mar 12, 2010

i believe if the tails are over 2m they will also need an isolating switch (anyone know the correct name for it?) so they can be isolated.

steamboatwilly · Mar 13, 2010

KNIPEX said:
i believe if the tails are over 2m they will also need an isolating switch (anyone know the correct name for it?) so they can be isolated.

Fuse switch KNIP....and the length of the tails differs according to your supplier's advice/policy. 2-3 metres is usual....over that add a fuse switch.

The problem you're having here Mark is that we live in a complex world and you'll *always* get conflicting advice from colleagues. The responses here are the right course of action for you....check in the Regs, if in doubt fit 25mm tails anyway, fit a fuse switch if >3m.

Diversity is one thing which everyone will disagree about. There is advice in the books but there's always personal judgement too and therefore you'll get 100 answers from 100 people. To get around this I wrote a quick Excel sheet into which you type the number of lights, rings/sockets, immersions, showers, etc and it calculates maximum demand for you. I print it out and keep it with the certificates for each job. 5 minutes work and it'll always be there as evidence of how you calculated MD. You're welcome to a copy if it'd help.