cable size?

[gesl]

Hi there, I hope you can shed some light on this for me. I have an installation with two 32A frc's both in t&e what I measure at 2.5/1.5. Measured end to end resistance on L N and cpc, in both circuits the cpc is showing double the value of the L and N conductors to within .o5 ohms. I have tried my best to measure the sizes by using a vernier to find the diameter of one core then doing the maths and times by seven (cores), but there is too much error in this method for it to be accurate. It looks like the cpc is half the size of the L and N but I can't find any references to cables that exist like this?

What do you reckon?

Thanks

Stu

 

[Bradzz]

2.5/1.5 gives a reading of 1.67 times that of L.N so cpc is 1.67 times greater, make any sense.
When i say its 1.67 times greater, i mean 1.67 times greater than L end to end.
say 2.5mm phase comes in at 2ohms then the cpc might come in at 3.34ohms thus being 1.67 times greater than the Phase. I hope this helps.

 

[gesl]

Yes total sense, thats why I'm conufused that I'm getting double the value of the L and N on both circuits. The values are within 0.05 ohms on both circuits so its unlikely to be bad connections giving me a high reading.

 

[telectrix]

sounds like its imperial cable. (stranded). i that case the live conductors are nearer 3mm, so your readings appear to be fine.

 

[gesl]

How old would that make the cable approximately? all the IR tests are good.

 

[telectrix]

1960's /70's. as long as IR tests are OK< it's better cable than the solid crap we're forced to use these days. i'm assuming it's pvc/pvc and no green goo leaching out at accessories.

 

[gesl]

Yes, its PVC/PVC. what do you put down for the cable sizes and max zs on the eic in this case then? I've no idea what the old imperial sizes would be equivolent to?

 

[telectrix]

Max. Zs is determined by the MCB/fuse, not the cable. as for size, put 2.5/1.5mm otherwise you'll confuse the suits that have never heard of 7/029.

 

[gesl]

Cheers for your help.

 

[Deleted member 26818]

It is likely that you have imperial cables.
With modern T&E, 2.5mm² has a 1.5mm² CPC.
With older cables, 2.5mm² could have either a 1.00mm² or 1.5mm² CPC.
The ratio between the resistances would be just under (2.5 : 1) with the smaller CPC, and just over (1.5 : 1) for the larger CPC.
With imperial cables you had a ratio of (2 : 1) for the smaller CPC (0.0015") and a ratio of (1.5 : 1) for the larger CPC (0.002").
As you have a ratio of (2 : 1) between the resistances of the live conductors and the CPC, it would indicate that you have imperial 0.003" T&E with a 0.0015" CPC.
I have a feeling, that with imperial sizes the minimum CSA of the CPC for 0.003" T&E was in fact 0.001" which would of had a ratio 0f (3 : 1).