Ring main design calc

[SafetyEICR]

Going through a ring main design calc for a course I’m on to determine c.s.a of ring final conductors.
As we don’t know the load current for circuit we assume this to be the rating of the overcurrent protection device. I.e 32a.
Only correction factor I need is grouping factor which is 0.45.
32 / 0.45 = 71a
Meaning I should use 6mm ring? Speaking from experience this is never the case. Am I missing something?

 

[Sintra]

Where did you get the 0.45 figure from?

 

[kingeri]

A hefty derating factor......and revisit the calc.

 

[telectrix]

first, as it's a RFC, you assume a max. current of 26A in each leg as a worst case. and can't see 0.45 being the correct factor.

 

[SafetyEICR]

The circuit is grouped with 12 other circuits giving 0.45 grouping factor. it = in / cg

- - - Updated - - -

it = 32 / 0.45 = 71a

 

[Guest55]

all sounds familiar .........
is this the 2396 project ?
;-D

 

[telectrix]

not enough info. for example, if any cables are loaded no more than 30% of their rating ( grouping allowed for), they can be ignored when calculating the grouping factor for your circuit/s.

 

[kingeri]

Basic maths, let's assume your derating factor is 0.5 for easy reckoning, if you divide 32 by a half, you are effectively doubling it! If, however, you multiply 32 by half, you'll get 16......which may well fit in more with your line of thought. However, it's an outrageous derating factor!

 

[SafetyEICR]

Telectrix I did read that somewhere. The circuit in question is grouped with 12 other circuits. All which are socket circuits apart from 2. As we cannot fully determine the design current for these circuits and how they will be loaded, can we just use grouping factor for the two circuits. it 32 / 0.7 = 45a, which seems more reasonable.

 

[Badged01]

The way I would think of it is whatever current capacity you require after applying factors etc .... this is for a radial circuit. Assume in your case a 6mm cable is required (i.e. 1 cable going into the socket). For a ring circuit you have two cables going into the socket so each needs to be 3mm to give you the effective 6mm. Cant get 3mm so would do a ring in 4mm --- whether you'd get them into the back of the socket though is another question!!

 

[telectrix]

buy decent sockets and it's easy to get 2 x 4mm into them.