ring circuit length concerns

[mrloy99]

I have just recently started a new job-its a nursing home and I have been giving the work of 1st & 2nd fix to the 60 odd ensuite rooms.The job is on 2 floors and there is only one dist board on each floor.I have been instructed by the foreman to wire one ring(and one lighting circuit) to do a clump of 4 bedrooms.The first two we wired used 45m of 2.5 not including the leg back to the board which is an additional 43m.So by the time this first clump of 4 is finished we will have used around 200m for the ring.Some of the rooms are further away and will take around 250m.The circuit is to be protected by an 30ma rcbo.
It seems to me an awefull waste of cable and I would have thought submains were the way to go.The foreman says that there is to be very little load as the residents are to be old folks.I dont think the on site guide covers this installation as its not domestic and was wondering if any of you learn'd bretheren out there have any views on whether this installation would comply with regulations.The cables are to be installed on basket in the corridors and clipped to bison units in the rooms and I would imagine there will be sound proofing insulation in the studs between rooms.

 

[telectrix]

suggest you run the rings in 4mm. better to overkill than suffer volt drop

 

[mrloy99]

suggest you run the rings in 4mm. better to overkill than suffer volt drop

What would the volt drop be for this length of circuit?(I have suggested 4mm to foreman but he said no, 2.5mm)
I take it that earth loop/disconnection times are not an issue? as its to be on an rcbo.I would normally keep a ring to under 100m

 

[telectrix]

how long is a piece of string? volt drop is dependent of current, the higher the load, the larger the volt drop. ohms law, V=IR. or, volt drop =currentxcable resistance .

 

[pennywise]

Most of the nursing homes I used to work in had two boards a floor one at either end of the wing, and never more than 30m from the furthest point of supply it seemed to be a standard design from one new home to another. Every job is different though.

 

[Guest123]

A good thread here on ring circuits.......

http://www.electriciansforums.net/e...-forum/22127-ring-main-100-square-metres.html

 

[mrloy99]

how long is a piece of string? volt drop is dependent of current, the higher the load, the larger the volt drop. ohms law, V=IR. or, volt drop =currentxcable resistance .

ok got the osg out.....
mV/A/m
voltage drop -----------
1000

tabulated value of 2.5mm(table 6D2) is 18
from page 126 the max permitted volt drop for sockets is 5% ie 11.5 volts
so transposing the formula and assuming a design current of 32amps

11.5
m ------
18 x 32 x1000 is 19metres

or with a design current of 15amp
11.5
m -------
18 x 15 x1000 is 42 metres

surely this can't be right?or am I missing something

 

[Guest123]

I fear you are missing something......

Ring circuit VD = 4 x 11.5 x 1000 / Ib x (mV/A/m) x Ct = 4 x 11.5 x 1000 / 26 x 18 x 1 = 98M

I always use an Ib of 26A for ring mains as this is the average current drawn throughout the ring, 20A at the far end + 12A evenly distributed = 32 + 20 / 2 = 26A.

 

[telectrix]

11.5 / (18 x 32 x1000) =19.96m sounds right to me.

 

[Guest123]

11.5 / (18 x 32 x1000) =19.96m sounds right to me.

So all your ring circuits are 19M long from start to finish???

 

[telectrix]

oops. i really need to do some theory revision

 

[mrloy99]

I fear you are missing something......

Ring circuit VD = 4 x 11.5 x 1000 / Ib x (mV/A/m) x Ct = 4 x 11.5 x 1000 / 26 x 18 x 1 = 98M

I always use an Ib of 26A for ring mains as this is the average current drawn throughout the ring, 20A at the far end + 12A evenly distributed = 32 + 20 / 2 = 26A.

Thanks for that lenny......just wondering why we need to multiply result by a factor of 4?(I take it for a radial that there is no 'multiplier?--sorry about the daft questions--brain is getting a bit rusty)

 

[Guest123]

Thanks for that lenny......just wondering why we need to multiply result by a factor of 4?(I take it for a radial that there is no 'multiplier?--sorry about the daft questions--brain is getting a bit rusty)

Correct, there is no multiplier for a radial circuit.

A ring final in effect has 2 cables for L, N, & E run to the furthest point so resistance is halved, whilst the length of 'doubled cable' to the furthest point is also half the length of the cable in the ring final which gives the multiplier of 4.

 

[telectrix]

i missed something there, i was checking his arithmetic.

 

[nickblake]

When you think about it this installation should have been designed , 250 meters on a ring , way to big , ive had houses that have been large long cable runs and the calcs were to close to the mark for my liking so i installed the rings in 4mm just to get the readings to an exceptable level so realy the person doing the design of this one need to get back to the drawing board

 

[mrloy99]

When you think about it this installation should have been designed , 250 metres on a ring , way to big , ive had houses that have been large long cable runs and the calcs were to close to the mark for my liking so i installed the rings in 4mm just to get the readings to an exceptable level so realy the person doing the design of this one need to get back to the drawing board

I totally agree,But I think even with 4mm and Lennys recommended 26amps we end up with a maximum of 160metres to comply with the permitted voltage drop.I've voiced my concerns that we could be ending up with a voltage drop of 30volts.but I keep getting" there will be a very light load"I don't know if that means their design or rcbo will only be 15 amps(if that makes any difference or is permitted)
I know there is to be a wall mounted lcd tv in each of the 4 rooms on the ring...and also nothing stopping the residents plugging in their own personal kettle or suchlike...A sub dist board in a store would be the obvious way to go and also save them on cabling etc....but I'm only the numpty doing the work....All I can think of is to get their requirements in writing to protect my self from future comebacks

 

[nickblake]

Do you know what id do ,i would write down all your concerns on a piece of paper stick it in an envelope and send it to yourself dont open it ,just leave it , because if a problem arises from this ,and anything comes back to haunt you you are then covered by a your letter not saying anything will but you never know , but judging by what you have said i think a sub main would have been benificial in this case

 

[Steve Richards]

Are you signing the completion certificate as the designer, if no, where is your concern. the designer would take into consideration likely load and diversity.

I cannot understand Lennys theory where you multiply by 4, surely you multiply by 2 (2 x 2.5mm cables to the far end) and as I totally agree with your load of 26amps as an average gives you a circuit length of 49 metres.

A ring circuit 100 metres squared = 10 x 10 or 10 + 10 + 10 + 10 which is 40 metres of cable in total. Based upon the circuit being fully loaded at 32amps

2 x 11.5 x 1000/32 x 18 = 39.93 metres - am I missing something?

 

[Sintra]

Think about the formula for working out R1 + R2 on a ring

r1 + r2 / 4 = R1 + R2

Same principle applies.

 

[Steve Richards]

Of course.........Doh! thanks