# cable calculations for ring mains

Discussion in 'Electricians' Talk' started by home automator, Mar 28, 2004.

1. ### home automatorNew Member

I am doing some cable calculations for a large ring main
I can do the maths for radial circuits fone but cant find a reference to ring main calculations
I have to install a ring main in a large house the lenght is about 70metres of cable in all
i am guessing 4mm cable protected with a 20 amp breaker should keep it within regs. The circuit loading will be nominal 2 tvs and a vaccuum (no other electrical equipment is to go in the rooms as it is an automated home and video is transmitted from elsewhere)
do i have to half the bale length as it is a ring main ???

2. ### ban-all-shedsNew Member

What calculations are you trying to do?

Voltage drop?

Disconnect times? Do you know what type of breaker it is, and what the value of your Ze is?

Current carrying capacity? What installation method is being used? What grouping or ambient temperature factors are there?

Why are you guessing at 4mm cable for a load that is almost certainly well under 10A?

Or you could just buy a copy of the OSG?

3. ### plugwashNew Member

from the point of view of voltage drop and disconnect time we have to consider which point on the ring has the highest earth loop resistance i'm pretty sure this is the centre of the ring though i haven't checked out the maths in full (bascially you would need to work out a formulaue fot the resistance at any point on your ring then differentiate it then set the derivative to 0 and solve)

this means the r1 and r2 would be half the value they would be in a radial of the same wire so you can use twice the cable before meeting disconnect time and voltage drop issues as on a radial of the same cable with the same breaker also this is the distance to the furthest point on the ring so you can double the length again to get your total max cable

there is also the issue of unbalanced current if the ring is heavilly loaded near one end this is a particular issue with rings that have sockets near the cu or rings that have one leg much longer than the other this is one of the reasons kitchens go on thier own rings

4mm on a 32A breaker is probablly advisable for a ring this size just because you don't plug in fan heaters doesn't mean future occupants won't

4. ### Rabbit RabbitNew Member

4.00 mm on a ring! ya like throwing money away?

4.00mm with 1.5mm cpc = 16.71 milliOhm / metre
4.00mm with 2.5mm cpc = 12.02 milliOhm / metre
2.50mm with 1.5mm cpc = 19.51 milliOhm / metre

Buy the OSG book its al in there.

As someone here says more info needed what is the use/mcb and what type if an MCB. Also the fact that you are not going to plug anything taking power into it does not mean that someone else will not, if it has a 13 amp socket they will one day. I have to be careful saying things like people CAN or WILL coz G20 tells me off!

5. ### sparkedoutNew Member

Ring circuits are usually done by the amount of floor area they serve a 32a 2.5 cct is standard unless you have to de rate the cable for some reason ie thermal insulation

6. ### home automatorNew Member

the problem i have with usin 2.5mm cable is that the runs are so long that the volt drop will be too high so i was going to suggest using 4mm to compensate for the volt drop.
i appreciate that the system could have future loading added to it but as this home is 100% automated the loading will be minimal as there will be no things like dvd or video to add, tv's will be wired on spurs as they are going to be wall mounted plasmas. The kitchen will be on a separate ring to so the loading will be tiny on my ring and future loading wont be that much of an issue in the areas i am concerned with.
Im going withj MCB's for protection

7. ### ban-all-shedsNew Member

I can't be a*d to do the sums myself, but the OSG says for a 2.5mm ring on a 30A Type B, Installation Method 6, Ze<=0.8 ohms for TN-S or 0.35 ohms for TN-C-S, max length 88m.

For ditto but 32A Type B it's 84m.