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?

 

[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

 

[pushrod]

THe OSG quotes 176M for a type B 32A mcb in a 4mm ring. Doesn't give a figure for a 20 A breaker. Could be even slightly larger for the 32A as the Ct factor might be reasonable as the loading would be relatively small for it. Might have a go at calculating it tomorrow - don't hold your breath mind - JUD is probs one of the best for giving you an answer if he reads this.

 

[vitoboy]

for what method of installation and at what ambeint temp...

 

[ted26]

for what method of installation and at what ambeint temp...

as far as i can gather the route will be above a false ceiling in an office block and guessing 22 degrees! and attached/hung from beams (in free air)

 

[pushrod]

Just wondering are the 4 loads close together? If they were would it be worth considering putting a sub-main close to them?

 

[ted26]

Anyway thanks to everyone for help i have enough info now to meet the regs requirments.

 

[JUD]

THe OSG quotes 176M for a type B 32A mcb in a 4mm ring. Doesn't give a figure for a 20 A breaker. Could be even slightly larger for the 32A as the Ct factor might be reasonable as the loading would be relatively small for it. Might have a go at calculating it tomorrow - don't hold your breath mind - JUD is probs one of the best for giving you an answer if he reads this.

The calculations for working out max ring circuit lengths for Zs and Voltage Drop are:

Zs = 4 x (Z41 - Ze) / ((R"1 + R"2) x Cr)

where:

Z41 = Max earth fault loop impedance from tables 41.2, 41.3 or 41.4 of BS7671
Ze = External earth fault loop impedance
R"1 + R"2 = Resistance per meter of line and cpc conductors from table 9A of the OSG.
Cr = Rating factor for conductor operating temperature from table 9C of the OSG



VD = 4 x (11.5 x 1000) / Ib / (mV/A/m) / Ct(where applicable)

where:

Ib = Design Current
(mV/A/m) = Voltage Drop per Amp per meter
Ct = Correction factor for the reduced operating temperature of a cable if it's current-carrying-capacity is significantly higher than the design current (only applicable at an ambient temperature >30°C)

As Lenny said, voltage drop is usually always the limiting factor so for a 2.5mm² ring circuit on a 32A type B MCB with a Ze of let's say 0.8 the calculation would be


Zs = 4 x (1.44 - 0.8) / ((19.51 / 1000) x 1.2) = 109m

and for Voltage Drop...

VD(without Ct applied) = 4 x (11.5 x 1000) / 26 / 18 = 98m
VD(with Ct applied) = 4 x (11.5 x 1000) / 26 / 18 / 0.923 = 106m