Hi folks,

I recently came across some ratings for a single core cable with a 300 sq mm conductor. It is claimed that the current carrying capacity is 1200A. Compared to standard cable types this seems to be very high. Please can someone advise, is this a common rating for a conductor of this size?

I've looked at the tables in BS7671 and the ratings are much lower. Also tried some calculations to IEC60287 but they also give much lower values.

I'm particularly concerned about this particular cable as it is neutrally buoyant and the core and inner sheath will be surrounded by a thick layer of buoyant (and thermally insulating) foam! The current carrying capacity should therefore be considerably lower than standard cables that don't have the foam.

Thanks,

Steve

What's the stated operating temperature of this cable, is it at -273k?

If it's buried it might, if you're lucky, be able to do this for 1 hour every day with no load the rest of the time!

Cheers,
Ian.

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irevans

Hi Backintime and Ian,

Do you mean the installation ambient? Unfortunately no, the maximum its supposed to be able to operate at 1200A in is 30°C. No specific duty provided so surely this is a continuous rating and the application its for wouldn't be running any less than 24 hours at a time, probably a lot more. Scary!

It is actually a subsea cable which would be operating in relatively warm seas and, as mentioned previously, it has an extra layer of insulation around it in the form of bouyant foam. Thermal resistance of this stuff is very high so I imagine it would reduce the current carrying capacity significantly.

Thanks for your contributions. Glad to hear I'm not the only one that thinks this must be magic cable... Would be interested to see if anyone else can explain how it could be achieved.

Cheers,

Steve

If you read the current rating tables in BS7671, they all start at a base ambient temperature (30 degree C) which is the assumed maximum ambient temperature. The designed working temperature for thermosplastic cables is 70 degree C and thermosetting cables 90 degree C.

For your subsea cable, let's assume that the cable designer had stated the based temperature (possibly -5 degree C) and a designed operating temperature of 155 degree C (because it has a super-thermal capability jacket and there is no fire risk to adjacent materials!). Considering for normal house wiring cables, the allowable temperature rise is only (70-30)=40 degree C, your subsea cable can allow a temperature rise of (155 - - 5)=160 degree C. Therefore, it is possible to carry around 1,200A under the circumstances.

Regards

marty