formula for working out ring main cable sizes

[electro]

[FONT=Times New Roman, serif]Hi [/FONT]


[FONT=Times New Roman, serif]Can anybody remember the formula for working out ring main cable sizes (Cable Capacities). I know we use 32amp MCB with 2 x 2.5mm² but its been so long I cant remember the formula that proves 2.5mm² is capable of carrying a ring main load.[/FONT]


[FONT=Times New Roman, serif]Thank you in advance[/FONT]


[FONT=Times New Roman, serif]Sorry if this has been discussed before but I have done a search with no results.[/FONT]

 

[Guest123]

It >

___20____
Ca Cg Ci Cc


We are allowed to use this formula with the Ib of 20A for ring circuit cables as it is unlikely that the current in any part of the ring circuit will exceed 20A.

Another way is to assume a load of 20A at the furthest point of the ring circuit with an additional 12A evenly distributed so........(32+20)/2 = 26A.


HTH.

 

[electro]

Thanks Lenny

My mind went blank on the matter, I've got the old books out and read the reg 433.1.4 for ring mains, thanks for your answer, with 60 people looking at the post with no answer i dug out the ECA book.

thanks Again Lenny

 

[pushrod]

I thought it was always just worked out from table 7.1 of the OSG and the installation method, protective device and max run length (mind i suppose it had to be calculated somehow in the first place )