Appendix 4, section 6.1 equation 6

Andrew the comment by Sparkingchip is not very complete above about cable temperature ratings.

A cable that runs through a “hot” area but is terminated in cooler ones can run at 90C in the “hot” area, as long as the terminations to most accessories remain below 70C, their normal rating. Special items can be obtained to run at higher temperatures but they are generally expensive.

You need to be careful too about the temperature rating of rubber cables, many are rated at 60C, because rubber deteriorates at higher temperatures much faster than PVC or XLPE. Refer to the manufacturer's data. Your description suggests all of these details can be ignored, but at 300m length you will find the voltage drop may be a problem, it all depends on the nature of the load. If a few lights no problem, if motors or appliances a bit of thought may be needed.

Regards

David CEng etc.

Thanks David, the cable for the application is actually a EN 50306 cable - 120degC, I just picked a 90c as an equivalent so I could work though the sum using everyday values from the tables for my understanding before adapting it. The load is electronic, pulling about 1.6A 

I believe mV/A/M can be calculated (for single phase) from 2 x ohms/KM x K, where K for 70 degrees (for example) is (234.5+70)/(234.5+20)=1.197   so would be 46 for my 20 ohm/KM cable.

So is this 300 metres through a railway tunnel or similar that already has cable supports installed?

In my head I was picturing a couple of guys trying to pull 300 metres of cable in one length through the roof of a commercial or industrial building, if it’s a tunnel then presumably the cable can be installed without stressing during handling, so does not need to be so robust.

Thanks Sparkingchip, it's laying on the ground and stored ona cable drum, so more like a long extension cord.  HO7esk resilience is still required though as it's a temporary install .

Hm, the resistance figures in BS7671 are given for “circuit meters” so your number of 2X is rather suspect. It is the normal way to specify multicore electrical cables, although obviously not for singles used for other purposes! Another confusion. This temporary cable thing being discussed is also somewhat suspect too, at no point in BS7671 does it say much about “temporary” except in areas covered by section 711, and that is not what you are doing. It seems to me that a few have tried to take BS7909 and apply it to your situation which is not necessary. Just use BS7671.

It will all be fine unless the sky falls in!

 

You need to be careful too about the temperature rating of rubber cables, many are rated at 60C, because rubber deteriorates at higher temperatures much faster than PVC or XLPE. Refer to the manufacturer's data. 

Butyl rubber cable can run up to 85 degrees C, but like all rubber and P.V.C. insulation, if run on the ground rodents just love to chew it.

Heat Resistant Butyl Flex 318*TQ at PEW Electrical

Then there is:

CENELEC - EN 50306-1 - Railway applications - Railway rolling stock cables having special fire performance - Thin wall - Part 1: General requirements | Engineering360 (globalspec.com)

Couldn't the supply Voltage be increased via a suitable transformer and reduced at the load end to reduce current flow?

Z.

I'm making the assumptionthat whatever the cable is it will be cut to length in the factory and delivered as one continuous 300 metre cable and installed without any plugs sockets or connectors.

So if or can be unrolled off the drum with minimal mishandling, possibly from the back of a pickup or from a cable trailer then positioned in a relatively safe place it should not be too stressed. 

When I was at college the lecturer said “Electricians don't do temporary ” in the context that we were training as installation electricians working to BS7671. 

Cable and drum around 100 kg.

Sparkingchip: 
 

Cable and drum around 100 kg.

That's only the weight of a big bloke.

Z.

Zoomup: 
 

Sparkingchip: 
 

Cable and drum around 100 kg.

That's only the weight of a big bloke.

 

Z.

And if that bloke went down on the floor it would  probably take an ambulance crew of two with an inflatable cushion to get him back up again, I would not be able to lift him as a deadweight.