I am converting a barn which is about 20 meters long. The electricity supply enters the property at on end. The question I have concerns the length of the various circuits. As far as ring circuits are concerned the property fits within the 100 square meters restriction on any ring circuit but by the time we have gone round the walls and up/ down to sockets the length of a ring circuit could be 70 or 80 meters. As far as I know there is no specified max length for a ring circuit, but I am concerned about the size of the cable to use if we are running at these sort of length for a ring circuit. Should I use 4mm cable rather than 2.5mm?. The first floor circuit will only have a couple of TVs and the vacuum cleaner as the main demands on the circuit., so would 2.5mm be OK here? The kitchen does not currently have washing machine/dryer (they are on the garage circuit), but they could be fitted in future. Would this affect the decision?.

A similar issue arises about the cooker cable. It has a length of about 30 meters. 6mm cable is good for 12 KW cooker, but should I be think about 10mm cable in this situation?

Views please

Barney's right, this sounds like a manageable situation but you need an electrician who can actually do the sums and take responsibility (in a way that forums can't!). It has to be designed right. It's no good just hoping it will test OK when it's finished.

There isn't an inviolable max length for a standard 32A, 2.5mm2 ring circuit. You can't really go over 106 metres without hitting voltage-drop or other problems. But 70-80m doesn't sound all that hairy.

Even if you don't go for the kitchen fuseboard idea, it may still be a good idea to put heavy loads like dishwashers on separate radial circuits wired all the way from the main board.

For sure - for any given circuit cros sectional area then there is resistance - if you make the circuit longer then resistance increases. - With me so far ?

for volt drop, you want the kit to still keep working at the end of that long circuit so from Ohms law we have V = I x R (current x resistance). for a given load, increasing cicuit length increases resistance and hence causes a voltage drop - so from 240v at your meter, you should constrain the volts drop to about 5% of that ie about 12V, so at the end of the circuit, the voltage is still 240 - 12 = 228V

For operation of fuses/circuit breakers then you need high current to flow to operate them quickly. lets say you need 160A to oprate 32A circuit breaker on your cooker in 0.1 seconds under fault. Again from ohms law we have R = V/I = 240/160 = 1.5 ohms. take away say 0.5 ohms for the external system and that leaves you with a circuit that can't be more than 1 ohm - so that sets a limit on circuit length.

as I said, for earth faults you'll have an RCD so a limit here of about 1660 ohms (as it operates at 30mA) - so no issues with circuit length.

What I'm saying is that circuits have practicable limits based on the above - but as your building isn't huge, any sensible spark will be able to resolve the slightly longer circuits within conventional cable sizing and design

any help

Regards

barney

Barney Thanks for all that, I recon you ought to be in school teaching this stuff to the next generation. Thanks to all for the additional insights into the magic of electricity in the home

LoL - those who can - teach

No thanks, I did a bit of part time teaching to engineering students - I'd have had half the class out on it's collective ear if I'd had my way

Diversity - now there's a black art - you'll only need to worry about it from a volt drop perspective as it sets the design current for the circuit. It won't be a concern for fault conditions.

Any domstic installer will be familiar with IEE Guidance on domestic diversity criteria - it'll form the basis of selection of "standard" circuit arrangements in domestic dwellings.

Do tell him though if you plan to have welding parties in the lounge - he might need to chuck in another ring final circuit

regards

barney

Generally I don't - it was only a favour really - trying to inject a bit of real world design thinking into HNC/HND building services engineering courses.

The bit I do miss is the emerging grasp of what's going on by the quiet kid in the corner who generally gets his thoughts and ideas swamped by the know it all bugger being paid to be there, not interested in learning anything and generally enjoys being disruptive.

Teaching is an art for sure, if you are going to reach all the students - the only bit i can say i enjoyed was passing on knowledge rather than just hitting them with information


regards

Barney

LoL - well in my experience ther is no accounting for what crazy people do in houses. I went to one place where a guy was totally renovating and rebuilding a mark 3 Cortina 2000 GLX (Coke bottle style) in his dining room - it was a complete work of art including the airbrush work on the body - spoilt only by the fact that he couldn't get it back out again without removing completely his patio doors and a goodly part of the reveals (they had been taken out to get the shell inside). It was a complete workshop in there - and he looked perfectly normal !!

Regards

Barney