Cable Q

[empip]

Pvc, flat twin and earth.
Coarse excess-current protection.
Domestic 240V supply

Surface clipped single circuit ... Live and Neutral both comprise 3 stranded conductors, each strand 0.911 mm diam. CPC single strand 1.118 mm diam.
Length of cable 5.2 metres.

Cable resistances at 20°C and when running full rating load as defined by Regulation for that cable type and fixing, at 70°C ?

Volt drop under that load?

What resistances would (do) you expect to find by actual measurement at the wall socket ?

 

[Adam_151]

3/0.036 with 1/0.044 cpc?, volt drop - haven't a clue, in today's money it would be 2mm² if you really wanted you could work it out from the resistivity of copper, or esitimate it by the taking the mid point between 1.5 and 2.5. As for current ratings, 16A by the 14th edition I'm told, and it often got used for immersion heaters, etc

Note that 1/0.044 comes out as a CSA just less than 1mm² , and as we don't allow conductors smaller than 1mm² (there are exceptions... flexible cables, etc see table 52C for full details)

 

[empip]

3/0.036 with 1/0.044 cpc?, volt drop - haven't a clue, in today's money it would be 2mm² if you really wanted you could work it out from the resistivity of copper, or esitimate it by the taking the mid point between 1.5 and 2.5. As for current ratings, 16A by the 14th edition I'm told, and it often got used for immersion heaters, etc

Note that 1/0.044 comes out as a CSA just less than 1mm² , and as we don't allow conductors smaller than 1mm² (there are exceptions... flexible cables, etc see table 52C for full details)

The relevant page of the 14th is in the Wiki ! But it doesn't mention the CPC ...

Are the calcs something like? :-

Using 18.3 x10^-9 Ohms.M being the average figure used in the OSG for resistivity relevant to csa's up to & inc' 25mm² ( corrected to mm from metres through division by 10^-6 )

R1 + RN (18.3e-3 x 5.2 x 2) / 1.97 = 0.1 Ohms at 20°C

And at 70°C 0.097 x {1+0.004(70 - 20)} = 0.097 x 1.2 = 0.12 Ohms

Volt drop at 19Amp (14th max rating) 19 x 0.12 = 2.3 V

R1+R2 [(18.3e-3 x 5.2 x 1.2 ) / 0.98 ] = 0.117 + (0.116/2) = 0.175 Ohms

TNS (by enquiry) Ze = 0.6 Ohms : Zs = 0.78 :

EFC = 240 / 0.78 = 307 Amp 230 / 0.78 = 295 Amp
PFC = 240 / 0.12 = 2kA 230 / 0.12 = 1.916kA

16Amp MCB then, with max expected load of 13 Amps which can be reduced by diversity...

Do give me a prod if the calcs are wrong...

Do the regs actually outlaw a 1mm² cpc, if previously installed under the 14th?

If both ends of the 5.2 metre cable are bright and tight, with a new 'quality' socket, what can affect the cable actual resistance apart from loading?
How near to the theoretical values would we expect to physically measure?

 

[securespark]

1mm² is NTCS, but I would say if your mid-point or destination EFLI is hunky dory, then all is well.

NIC guys go hell for leather against 1mm² cpc's in their PIR's....

 

[RF Lighting]

I don't

If the R1 + R2, and the EFLI are ok then it only gets a code 4 from me.

 

[Adam_151]

Thats different though, thats 1mm cpcs on rings we are talking about there? not the fact that 1/0.044 is *less* than 1mm which isn't allowed by something in section 5!.

As for 1mm on rings (and same process applies here for this issue), as its not a standard arrangement (like the one that automatically allows 2.5/1.5), it has to be selected or calculated, fails by 54G, so only option is the adiabatic, where it fails on a 3036, passes on a 1361 as long as zs lower than 0.8, and and may go either way on a breaker depending on fault level and what I²t you get out the maker for that fault level (calcing it from fault level and 0.1 gives you a max allowable fault level of 0.36ka!)

 

[empip]

The circuit in question is a radial.

My ring (main!) has a cpc 3 x 0.036" csa being 1.97 mm² that is 30% greater than the 1.5mm² standard.

Would it be fair to say that given no cable damage, high impedance would be generally caused through termination or associated problems?

 

[Adam_151]

The circuit in question is a radial.

My ring (main!) has a cpc 3 x 0.036" csa being 1.97 mm² that is 30% greater than the 1.5mm² standard.

I though 7/0.029 had a 3/0.029 CPC, not a 3/0.036 one ?

 

[empip]

The circuit in question is a radial.

My ring (main!) has a cpc 3 x 0.036" csa being 1.97 mm² that is 30% greater than the 1.5mm² standard.

I though 7/0.029 had a 3/0.029 CPC, not a 3/0.036 one ?

I had measured the ring main ODs previously using a Vernier Caliper, I saw that 0.029" figure quoted elsewhere, so I opened the box and remeasured using a micrometer 0.036" x 3 is correct.

I have another radial in 7 x 0.036" with 7 x 0.029" cpc. (4.59 x 2.98mm²). some lumpy stuff... All with pvc insulation (marked as such).

Useful .. 'ennell' Re: old rubber cables? Apr 7, 2005 8:51 PM Click for info'
14th Ed. cable ratings Here

 

[denisthedolphin]

The previous imperial cable sizes were
1.044 ie normaly called Bell wire
3/ 029 now 1.0mm
3/036 now 1.5mm
7/029 " 2.5mm @ so on
7/036
7/044
7/052 etc then on to 19/ 044
Hope this is some help

 

[Jim2287]

are all rubber insulated cables imperial in csa ?

 

[denisthedolphin]

No, allthough when cables changed from imperal to metric sizes , insulation on domestic cables started to change to pvc, however rubber insulation is still in use today TRS also as butyl rubber insulated which is very high temperature cable, also some tri- rated panel wiring is butyl TCP. For example, PVC cable is not used in marine practice, even high temp. PVC because of the fumes given off, but generally in domestic and industry the vast ammount of cable is pvc.
The old domestic cable used was rubber with a cotton exterior cloth, this was called VIR,

There endeth the lesson, amen!

 

[empip]

I had asked about the difference in Rho, as used in OSG - to indicate cable resistance - From the Rho figures, apparently for cable, more likely for pure Copper, sprinkled with differing values about the web.
------------
I found the average Rho at p.158 current OSG (Up to 25 mm²) to be 18.3e-09 Ohm.M
------------
Following some rummaging around the net... HERE I find, hopefully, the answer !!
Copper conductor class 1 & 2 IEC 60 228 ... Max res' figure (manf' limit I hope), the mean being 18.307e-09 for csa's 0.5 to 16.0mm² at 20°C

Interestingly the figure from 16th Ed. Table3. - Wiki - http://electrical-contractor.net/PC/IEE1966_T3.jpg For 3 x 0.036" (1.97 mm²) at 19 amp, volt drop 12V / 100ft (30.48 m) at, I imagine, 70°C .
Rho used = 17.0e-09 Ohm.m

So, could there have been more pure copper per vol in that old cable standard ?