# ho7-rnf various ratings by manufacturer and bs7671- whats the correct safe answer



## sparkie (Apr 2, 2008)

Looking at an installation there are tri rated rubber cables as tails in the switch room. 
On the face of it there was a 400 amp supply per phase, label on supply

MCCB main switch is now set lowest setting at 504 constant 5 times overload.

DB1 with mccb main switch set to 320amp constant and 5 times overload, Also another board with main switch DB2

metering altered by the supply company and we now come to find that the supply fuses are 500 amp. tailed with 240mm ho7-rnf

There are several makes of HO7-rnf for instance eland cables quote on their site conductor operating temperature 85 degrees c ambient temperature 30 degree's c max amps capacity 467 amps.

Draka quote for over 4 mm to use 4h1a page 302 as a reference which gives you 392 amps.

If you look at 4F1A then 240mm is rated at 343 amps 30 degrees c ambient and 60 degrees operating temperature.

Or if you go with eland 4f2A and knock a bit off then with a 20 degree ambient temperature and 85 degree operating temperature then we are good for a 500 amp supply

Due to the size of the switch room and existing switch gear it would be a nightmare to alter the cable size. what confuses me is that depending who you choose for your information- you have a cable rated anywhere from 343 to 500 amp.


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## Trimix-leccy (Dec 4, 2007)

Didn't think you could use HO7-RNF for tails, I thought it was classed as a heavy duty flex [ but I am nearly always wrong...see wife for details ]

Can you find out the exact make of cable [prob not as you would not be posting here]?

We only use Tri-rated singles and install in a trunk and guess what? we have exactly the same dilema! 400mm can vary by upto 100A ffs!!! so we specify an actual make and use the makers tech dept to send us a 'ratings sheet' to attach to our paperwork. Sorry to have been of no help whatsoever:no:


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## sparkie (Apr 2, 2008)

it's double insulated singles and only tepid temperature with 280 amp load, what concerns me is the wildly differing load capacity. I would guess monitoring temperature the only way to reliably tell.


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