Volts drop too high...

[Brutal]

I have installed Ring Final Circuit in 2.5 LOR 120m protected by 32A breaker

Volt drop calcs - 18mv/a/m x 32A x 120m / 1000 = 69.

So the volts drop is 69 volts? which is way over the stated 3 or 5 %.

Am I diong this corectly and if so what can I do about the volts drop?

Cheers

 

[chrish_1977]

That does seem like a very big ring, you could try splitting it up into a smaller one(s)

 

[MDJ]

sure that is not 6.9?? I haven't got my brain on but that seems wrong

 

[Brutal]

volts drop of 6.9 you think? sounds better

 

[MDJ]

yep sounds better, 69 volts is a lot on 120 meters buddy.

 

[Brutal]

yep sounds better, 69 volts is a lot on 120 meters buddy.

It does seem a lot but the calcs give 69 volts.

If my calcs are wrong.... Where am I going wrong?

mv/a/m - 18

Ib - 32a

LOR 120

18 x 32 x 120 = 69120

divide by 1000 cos in milli volts gives me 69.1

The 120 meters is the whole circuit (as in ring)

surely I would just need to measure to the furthest point? say 60m

Also If it is a ring circuit and the CSA is doubled as two 2.5mm cables.... The total CSA is 5mm - does this have an impact?

I've been running my business for a while now so need to start getting this stuff right!

 

[Brutal]

That does seem like a very big ring, you could try splitting it up into a smaller one(s)

Splitting it would do no good... The rings are split into three with the longest being 120m. Its the distance travelled from the DB thats the problem

 

[MDJ]

You may be right, as I said I didn't have my head on, but that sounds a hell of a lot of volt drop for that circuit, however If your sure?? then fair enough

 

[Brutal]

You may be right, as I said I didn't have my head on, but that sounds a hell of a lot of volt drop for that circuit, however If your sure?? then fair enough

ok.... so what can I do about it? I have to provide calcs for the job, I can't show 69v volt drop

 

[MDJ]

One question fella, have you actually measured the results by instruments?

 

[Brutal]

One question fella, have you actually measured the results by instruments?

used a meter to measure the volts at the furthest point? I'm sure it was no less than 236 volts when I measured EFLI

 

[telectrix]

it's more like (18 x 20 x 120)/4000 = 10.8V.

20A because that's about the correct value of Ib. 4000 because it's a RFC. so you divide by 2 for the 2 cables, and 2 again because the cables are in parallel. same as R1+R2= ( r1+r20/4.

 

[Brutal]

it's more like (18 x 20 x 120)/4000 = 10.8V.

20A because that's about the correct value of Ib. 4000 because it's a RFC. so you divide by 2 for the 2 cables, and 2 again because the cables are in parallel. same as R1+R2= ( r1+r20/4.

so I need to put circuits on 20a breakers?

 

[telectrix]

no. it's a 32A RFC. you assume 1/2 the current goes down each leg, but allow a bit more ,20A instead of 16A, as at most points on the ring, 1 leg will be shorter than the other and so carry more of the load current (less resistance). it's only at the mid point that each leg will carry the same current.

 

[Brutal]

so I need to put circuits on 20a breakers?

ok, so if I put the calcs you quoted above I won't have any probs? the lighting circuits worked out fine anyway

 

[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.