Volts drop too high...

[telectrix]

well that's the way i've always calculated VD for a ring. chances are that most of the time the load will be well less anyway. most of the time, except for kitchens, the current will be < 5A.

 

[malcolmsanford]

Tel is just about there

You Divide the length by 4 so your calc will be 18 x 26 x 30/1000 = 14.04v then corrections for operating temperature and that would be 14.04 x 0.923 = 12.95 volts

This assumes 20 amp at the furthest point and 12 amp distributed evenly around the ring so, average = (32 + 20)/2 = 26

So I'm afraid your circuit fails. And you could check this as the OSG guide Table 7.1 (i) for a 2.5mm RFC is 106mts on RCD/MCB. It is slightly longer on a BS 3036 fuse becasue of the 0.75 rating factor

 

[telectrix]

i was a bit more generous with current demand. apart from a kitchen RFC, it's extremely rare to see anywhere near 20+A on a RFC.

 

[malcolmsanford]

Agree 100% mate, and in the real world a 120m ring is not going to fail, unless it's going to be loaded up, but the OP said that he needed to show his calcs, and if he does it fails.

If he needs to show his clacs somewhere along the line there is going to be an ole f##t like me telling him sorry son that is not acceptable

 

[telectrix]

malcolm. you're allowed to say "fart" as it's not in the banned list. it's also in the OED.

 

[Brutal]

Morning Chaps,

Thanks for your continued assistance. It is looking rather like the calcs will fail.

Does it matter that the building is empty and unlikely to be used for anything other than visiter centre in the summer?

What is the maximum LOR for RFC? 120 Meters is roughly worked out, lets say it would be the worst case. It is more likely to be about 100m if measured with a tape measure

If we calc 18 x 20 x 100 / 4000 = 9

Would it be acceptable to show these calcs?

ALso, should a job like this come up in the future how would I get round the VD?

Surely you wouldn't wire a RFC in 4mm? And there is nothing I could do about the LOR. It is purely down to distance from the DB

And my final question would be.... Am I correct in stating LOR as beggining at DB going to every socket and include the return leg to DB?

 

[malcolmsanford]

Morning B

As Tel and I were saying in the real world your 120m RFC will most likely never cause you a problem and would chug away happily but you said you needed to show calcs, and on paper it is non compliant with VD

If you use the OSG guide of 106m for a normal 2.5 RFC it would calculate @ 26.5 x 18 x 26/ 1000 = 12.402 v then you have your correction factor of 0.923 that would bring it to 11.45volts & your 5% is 11.5volts

So rule of thumb is the 100 sq mt one as maximum

I have seen RFC wired in 4mm, in fact for IT solutions if I decide to use a ring final I would do it is 4mm. This though is not for VD but to adhere to the regs with regards to protective conductor currents. So a 4mm ring would be fine, in fact a 4mm ring would give you an approx sq area of 175mt

 

[Steve_P]

It's a longshot but can you re-route some of the cables and make it 14m shorter? You wouldn't have to show calcs just quote the OSG if you could.

 

[Farmelectrics]

just use a shorter lenght in your calcs so it works out ok lol joking of course

 

[ROADESPARKY]

Just a bit of food did thought. I don't do much domestic more industrial but I am currently working with sparks who do quite a bit. The subject of ring mains and NIC inspections come up. At his last inspection the NIC inspector told him that a ring should be no more than 100M. Based on this a ring of 120 M is incorrect anyway.

 

[Farmelectrics]

if thats correct your stuffed its not what the books say though i havnt got them with me at mo so cant qoute

 

[MDJ]

I have heard 100 square meters from them not 100 mtrs, not sure what the difference is or even if it is true, I always have at least 3 rings in a house anyway and it is never anywhere that amount of length so I don't take any notice about that rule, even if there is something in it, and ahem there may not be, quick check the regs.

 

[durhamspark]

As Malcomsandford said,OSG table 7.1 gives you max length of 106m for 2.5mm , table H2.1 gives the 100m2 floor area.

Using 4mm it's 176m , not as uncommon as you think. Plenty of designers now are covering themselves by upping the cable size if the calcs are close.

 

[MDJ]

yeah thats very true durham, a few consultancy's I do work for specify 4mm2 as standard now and also it has to be LSF too.

 

[tony mc]

And when you try and spur from a S/O wired in 4 mm !

Remember this calc re max lenth of a ring.

Max VD x 4 x 1000 / Ib x mv/A/m.