Minor Predicament over RFC Length

[uksparks]

Hi,

I have just started a new build today, its a large U shaped bungalow with vaulted ceilings etc.

I have split the property down into a lot of circuits to help with things due to cable runs etc.

I have just wired a section of it today, I was going to do it all in 4mm radials, but by the time I had got to the end of the section (bedroom, Wardrobe, Study & Entrance Hall) I had used 96 meters, this does not include the leg back to the board which is a further 13 meters....

I had given it though on paper, but then when I started cable routes were not quite so straight forward etc.

Anyway, my problem I have now is that the R1+R2 for that circuit at say 110 meters, will be 1.83 Ohms, which is way too much as was going to put it on a 32A, jees I would have to drop it to a 16A to comply.

So then thought, its ok, illt turn it into a ring, total length will then be 125 meters give or take a meter.

But then thats introduced another issue...

So I am thinking about VD,

Assuming 20A at the furthest point plus an additional 12A evenly distributed, the Ib of 32A then becomes (32 + 20)/2 = 26A, which I would think its fine.

4 x 11.5 x 1000 / 26 / 16 = 110 meters....

So i have got 125 meters, and to stay inside 11.5v I need it to be 110 meters.

However, the predicted readings are:

r1 = 0.57
rn = 0.57
r2 = 1.51

R1+R2 = 0.59

Should I be concerned or even bothered about the 110 meters issue or do I drop it to a 25A or 20A RCBO? but thats not going to overcome the VD.

The Zs will be well within the 1.1 Ohms for 32A

The problem is, there's no way I am going to run 6mm radials, thats just silly.

The other half of the house is even further away from the board, I have intentionally split the circuits down to minimize on length but the further circuit the other side, will be over 100m for a ring and about 80m as a radial...

 

[Pat H]

Never had to deal with such long runs as that. Wouldn't one option be to run say a 10mm tail from the broad to closer to the ruing and then run the ring from the end of the 10mm tail?
You'll lose less VD over the 10mm than you would over 2 X 2.5 (you'll lose half as much in fact)

 

[Octopus]

I think you are assuming the load is far too high given the room names you have mentioned...

 

[uksparks]

Well i was only planning on 20A radials to be honest, but then its too long,

Ill work it out, ill just ring them.

 

[Dave OCD]

With the size of this place would it be worth considering a sub-main with 2nd CU for the furthest 'half' from the mains position ?

 

[telectrix]

in your first scenario, as murdoch said, your assumed Ib is way over. i'd be inclined to assume more like 10 - 12A.

as for the second, you could run a 10mm feeder and then create a ring from that, forming a lollipop circuit.

 

[Octopus]

in your first scenario, as murdoch said, your assumed Ib is way over. i'd be inclined to assume more like 10 - 12A.

as for the second, you could run a 10mm feeder and then create a ring from that, forming a lollipop circuit.

Even 10A seems high for those rooms..

 

[uksparks]

I take on board what you are saying about the loadings, however, how does it work in the event that someone does pull say 2o amps? it then doesn't comply.

 

[Wilko]

Just to confirm - it's a 4mm rfc of 125m total length with a 32A mcb?

 

[sparksburnout]

Was the design of the electrical installation not part of the original project? Sounds like quite a substantial undertaking.

 

[bigspark17]

Is the consumer unit in one side of the build, could you not have put the consumer unit in a central location or run a sub board.?

 

[Leesparkykent]

4mm RFC would be ideal.

View attachment 35561

 

[Wilko]

Spoiled all my fun

 

[telectrix]

4mm RFC would be ideal.

View attachment 35561

looks like that's a sauna. lol. 30 degrees ambient? our boiler would have a heart attack if i called for that much.

 

[Leesparkykent]

What you have to remember is the mV/A/m values are based on maximum CCC/70 degree conductor operating temp...The cable is not going to be maxed out so in reality the volt drop is going to be less than calculated as it proportional.

 

[Dave OCD]

Can you imagine a 70 degree conductor temperature in reality ? It would feel hot enough to the touch on the outer sheath to worry me !

 

[uksparks]

I

Was the design of the electrical installation not part of the original project? Sounds like quite a substantial undertaking.

It wasn't quite as simple as that, you can only work things out on the information you have been provided, and then when it comes to it, where you thought you could run a cable is now not possible.