Cable limitations

[ph]

What is the maximum permissable length of a 2.5mm2 cable on a ring main with a 32a mcb and does it matter how many sockets are fitted as long as you are within the limits?

 

[andy]

sockets are unlimited, but the floor space must not exceed 100m^2

 

[ph]

do you mean the cable length from consumer unit and back should be 100m max in length? also how do you work out the volt drop across the ring main?

 

[felix]

No. What Andrew2022 wrote, using the limited symbols available on a standard keyboard, was 100 SQUARE metres. That's the maximum floor area that the ring can serve. Add up the floor areas of all rooms that have sockets or anything permanently connected on the ring.

Working out voltage drop is complicated by the fact that you have two paths to each socket and multiple sockets. If there's a standard calculation for this then I don't know it, though I'm sure somebody else out there will. To get a ball park figure I would do this.

Find the total length of wire right round the ring. Half it to find the distance to the furthest point. Look up the resistance of this length of wire. I would guess at about 0.015 ohms per metre but that's only a guess based on the resistivity of copper. Half it because you have two cables in parallel. Multiply by 32 amps. This is a worst case calculation and I would expect actual voltage drops to be less than this.

 

[ph]

Too make things easier how do i calculate a voltage drop from the consumer unit to the first socket on the ring or for example 20m of cable on a radial circuit using 2.5mm2 cable ?

 

[Damocles]

On site guide quotes 84 metres as max permitted length for a 32A ring protected by type B mcb. This is probably a worse-case limit, but good to follow the recommendations.

Voltage drop is not the most important thing to think about. Maybe your lights might dim, but that is generally not dangerous. The important limit is that resistance must be low enough so that a BIG enough current will flow to trip the breaker if their is a short.

Having said that, if every circuit in the country was designed to save 2V final volt drop, then that would save 1% of the nations electricity bill. Which is quite a bit.

Worst case resistance to a point on a ring is 1/4 cable total resistance. Half because there are two cables and halved again because the furthest distance from the origin is half way round. Dont forget resistance to be counted is that of earth conductor, which is thinner than the others.

 

[Scoby_Beasley]

Too make things easier how do i calculate a voltage drop from the consumer unit to the first socket on the ring or for example 20m of cable on a radial circuit using 2.5mm2 cable ?

See here, it might help you if you know what you're going to plug in.

 

[ban-all-sheds]

Having said that, if every circuit in the country was designed to save 2V final volt drop, then that would save 1% of the nations electricity bill. Which is quite a bit.

Is that your final answer?

Would you like to phone a friend?

Or how about this - "Computer, take away 2 wrong answers"

a) if every circuit in the country was designed to have 2V final volt drop, then that would save 1% of the nations electricity bill.
b) if every circuit in the country was designed to have 2V final volt drop, then that would save 1% of the nations electricity bill.
c) if every circuit in the country was designed to have 2V final volt drop, then that would save 1% of the nations electricity bill.
d) if every circuit in the country was designed to have 2V final volt drop, then that would make no difference electricity bills, because the voltage drop is caused by current flowing through the cables, and counts as consumption just like the electricity actually used by appliances.

 

[Damocles]

Did I use the word 'have'?

I know it depends a bit on the nature of the load, but pretty broadly if you eliminate waste amounting to 2V *1,000,000,000 A then you save quite a bit of power.

 

[Adam_151]

Actually ban, becuase the total resistance in the circuit had increased, the current drawn would drop, but the efficenty would have decreased as the resistance in the heating element would be the same but the resistance of the cable increased, if its something like a kettle where you need to transfer x amount of energy before its boiled, then you are going to be wasteing energy, because while you are using less total energy, you are putting more into the cable than before, which is wasteful, you would need the kettle on for longer, using more total energy. If however it was a shower, haveing a higher volt drop would still be inefficent, but this is likely to show its self in a less powerful shower, as opposed to a higher amount of total energy used, either way you loose when you take into account the full picture

 

[ban-all-sheds]

I know - many apologies to Damocles - something just went kerching! - I did a couple of example caculations, and thought "Oh s**t". Came back to the PC to erase evidence of my stupidity, but too late.....

I know we were actually both wrong with our maths, but Damocles was out by a lot less than I was. It wouldn't have been so bad if I'd given a straight reply, but no - I just couldn't resist, could I, so it's oef sur la visage pour moi.

Very sorry Damocles.

 

[felix]

If you have all electric heating then power losses in your cables are not losses at all, as long as those cables are within the heat containment envelope. (I hope you like that phrase).

Of course you shouldn't really be using electricity for heating in the first place. Given that the universe as we know it is slowly but surely running down (second law of thermodynamics), converting high grade electrical energy into low grade heat without even bothering to run a heat engine along the way is a blatant waste of a non-renewable resource. Remember that next time you put your foot on the brake pedal.

Time I wasn't here. Beam me up somebody!

 

[ban-all-sheds]

I read a fascinating article once about downhill vehicle racing - basically adult soap-boxcarts, although anybody whoever built one of these as a kid would scarcely recognise these machines. There are restrictions on how much you can spend, but stick to the budget and AFAIK you can do what you like, technology wise. They use flywheel-based regenerative braking systems......