Maximum cable length

[ted26]

Does anyone know the maximum cable length that can be used for a ring circuit using 2.5 t+e with a 32amp breaker?

 

[ukpablo]

page 362 of brg

100m squared as a rule of thumb.

 

[ted26]

I thought that was floor area 100m squared!

I was concerned with atual cable length of total ring circuit

 

[telectrix]

ithink its about 50m end to end but that's just a guesstimate.

 

[malcolmsanford]

the OSG has a table in there giving maxium cable lengths for cable size but as I haven't got mine with me can tell you which one, sure if one of the other lads are following this thread they can give you the table reference

 

[Hawk81]

Table 8a in the OSG gives you the maximum length. Which is as someone has already said is 100m2

 

[vitoboy]

page 44 table 7.1
it depends if you got a fuse or mcb what type of mcb or rcbo ect ect..
also you need to get past your max zs and thats the real limit..

 

[J@mes]

if the area covered is 100m2 then does that mean that the cable can be 40 meters long end to end (ie going around a square room that is 10x10m meaning it would cover 40m end to end?

subject to all other criteria being met

 

[vitoboy]

100m squared is not a cable length!
its an area to be served.
you could still have a zs thats to high even within that..
and then it wont comply..
honest.

 

[vitoboy]

in a 10X10 meter square what about the drops or rises.....if the rooms got a 5 meter celing hight and your drop is from there then how big would your ring main be...
its all about cable resistance and what protection you have and the temperature and the insulation around the cable.....
if i were you id look into some cable calcs....they are a wonder to behold..

 

[ted26]

I atually searched and found a reference earlier which said something like 98m for 2.5 and 178m for 4.0

this i think must be the maximum for each and could be lower depending on other influences

 

[telectrix]

what about the drops/rises to sockets. as vito says, have to meet Zs less than 1.2 for a 32A type MCB, so knowing Ze and the resistance of R1 and R2 per 100m each, you can work it out

 

[ted26]

in a 10X10 meter square what about the drops or rises.....if the rooms got a 5 meter celing hight and your drop is from there then how big would your ring main be...
its all about cable resistance and what protection you have and the temperature and the insulation around the cable.....
if i were you id look into some cable calcs....they are a wonder to behold..

You are right but have forgot more than i now know!! been working easy for 10 years and need to refresh my electrical installation theory.

anyway thanks for any help from all

 

[vitoboy]

and table 8a...is for minimum live size in mm for final circuits...

where are lenny and the big boys on this...

 

[pushrod]

type b circuit breaker, tn earth,installation method a, c, 100 or 102 length = 106 metres.

 

[Guest123]

Volt drop is the over-riding factor here.....I'll see if I can dig up the other recent thread on it.


http://www.electriciansforums.net/e...in-100-square-metres.html?highlight=volt+drop

 

[Des 56]

I think its a bit of no can answer question
As Lenny said,voltage drop is the main factor because of the limits now in force
Voltage drop is dependant on not only the length but the load and the cable resistivity
and the cable size

If any of these are changed ,the answer will also change
We can follow the general guidance for 100meters square and it would probably be within limits.but as pointed out by others even then it could be outside that limit
Also the type of load that affects voltage drop limits,if there are lights in circuit,there is less to play with

Tricky question I reckon

 

[ukpablo]

table 9a osg page 166

2.5 cable with cpc 1.5 has resistance of 0.01951 ohms per metre

x by 100 metres = 1.951 ohms

now max zs for 32 A mcb type b is 1.16 ohms.

a ring has double the csa therefore if u divide it by 2 you get 0.9755 ohms

ie 50 metres out on a ring cct and 50 metres back.

this does not take into account any other losses, so answering your question, the rule of thumb is 100m squared, give or take a few metres!? as long as the max zs is within limits, it should not matter.

hope this helps and that im not talking poop?!

 

[ted26]

Its a funny job which i have been left with and can only give vague details as i have only vague details thus far!

I know this maybe a tuff question but i will tell you all i know and lets see where this goes

the circuit i have to install is on an industrial site and compromises of 4 switched fuse spurs taking 5 amp each and the ring will be about 130m!!

so do you think 4.0mm with a 20amp breaker is my going to be the only option that will comply!

and again thx for any help

 

[telectrix]

i.d go with a 32A MCB as a 20A is on its limit at 4 x 5A. Volt drop and cable resistance are the 2 factors you have to watch for. Find these values per 100m and per amp and you've answerede your query