Calculating It

[TommyB123]

Hi Again,

I'm trying to work out the It of a ring final, and can't get a logical answer that I understand.

It must greater than or equal to In/(Ca*Cg*Ci*Cf)

32/(1*0.57*0.5*1) = 112.2A ?!

I've never designed a ring final before so my knowledge is poor, however I do know 25mm2 cable is abit big for a domestic ring circuit

 

[PrenticeBoyofDerry]

Firstly from were/how did you get your figures?

 

[TommyB123]

32A Ring circuit

Ca - Ambient temp - 30c - 1
Cg - Cable grouping - (6 cables bunched in air or embedded in a surface) - 0.57
Ci - Thermal insulation factor (run in a length greater than 500mm) - 0.5
Cf - CPD type - Using a value of 1 - 1

 

[ricicle]

You only need to apply the worst case factor, unless all conditions occur simultaneously.

 

[TommyB123]

You only need to apply the worst case factor, unless all conditions occur simultaneously.

Sorry I don't follow?!

 

[PrenticeBoyofDerry]

Is your question related to a standard ring final circuit within a domestic property?
If so, calculating the design of the circuit and rating of the current/devices, using this method would not be the practical way.

 

[TommyB123]

Is your question related to a standard ring final circuit within a domestic property?
If so, calculating the design of the circuit and rating of the current/devices, using this method would not be the practical way.

It is Yes. What method would be used instead?

 

[PrenticeBoyofDerry]

https://www.tlc-direct.co.uk/Book/6.3.2.htm

Where possible, I would avoid cable being totally surrounded by thermal insulation. If you can clip to joist or rest on plasterboard surfaces, of above the insulation, this would benefit the dissipation of heat, thus improving the current carrying capacity of the cable.

 

[plugwash]

In your proposed design does the grouping and the thermal insulation occour at the same time?

 

[TommyB123]

My issue is that all circuit cables will be run through the cavity of a wall (insulated) and bunched together, untill they reach the void between upstairs and downstairs floor/ceiling.

How would you categorize the insulation in a cavity wall?

Am I right in taking this into account with the grouping and insulation factors?

The website you suggested quotes;

"These sizes assume that sheathed cables are clipped direct, are embedded in plaster, or have one side in contact with thermally insulating material. Single core cables are assumed to be enclosed in conduit or trunking. No allowance has been made for circuits which are bunched, and the ambient temperature is assumed not to exceed 30°C."

This is where I am getting confused!

Also - Appendix H in the OSG makes no reference to I/CCCC when using 'Standard circuits' and states instead, that a Ring circuit protected by a 32A CPD must have a cables with a minimum 2.5mm csa and the as installed current carrying capacity must not be less than 20A for a ring circuit. I make mine 27A with installation method C - So am I taking this way out of context?

 

[TommyB123]

In your proposed design does the grouping and the thermal insulation occour at the same time?

Yes, then the insulation factor becomes minimum and the cables take different routes and are no longer 'grouped'.

 

[ban-all-sheds]

Cg - Cable grouping - (6 cables bunched in air or embedded in a surface) - 0.57

Is that 6 ring finals, or 3?

You should be using the number of circuits, not cables, as the group size.

Also take note of Note 9 to Table 4C1.

Finally, if you can get the cables, or each circuit pair, 20mm apart then they aren't actually grouped at all.

Ci - Thermal insulation factor (run in a length greater than 500mm) - 0.5

So that's Method 103. Can you install them so they are touching the plasterboard, i.e. Method 102, i.e. Ci = 0.78?

 

[TommyB123]

Is that 6 ring finals, or 3?

You should be using the number of circuits, not cables, as the group size.

Also take note of Note 9 to Table 4C1.

Finally, if you can get the cables, or each circuit pair, 20mm apart then they aren't actually grouped at all.

Ci - Thermal insulation factor (run in a length greater than 500mm) - 0.5

So that's Method 103. Can you install them so they are touching the plasterboard, i.e. Method 102, i.e. Ci = 0.78?

It is a mixture of circuits. 2 Ring finals, a Kitchen radial, 2 lighting circuits and a circuit for the boiler. So using Note 9 on 4C1 i could probably discount the two lighting cables as they are likely to less than 30% of the others? The grouping as i say is only to get them from downstairs, up into the roof space between the two floors. I suppose i could then separate them so they are not 'Grouped' would this value then be ignored in the It calculation? or substituted with another value?

Having a re think on the cable route now, i could ignore the initial route and go from the CU straight into the void, then using method 102 as you said!

 

[JohnW2]

Finally, if you can get the cables, or each circuit pair, 20mm apart then they aren't actually grouped at all.

I get a bit confused by this, and the intended interpretation.

If, for example, one has 6 cables (3 ring final pairs) neatly arranged side-by-side, the first and third cable pairs will be >20mm apart - so does that count as two pairs or three pairs as far as grouping factors are concerned?

Kind Regards, John

 

[TommyB123]

I've got lost here guys, can anyone put be back on the right track?

I've now got;

It - 32/(1*1*0.78*1)

It - 41A

So It > In - All Good?

But then using table 4D1A And Reference Method B - It has to be 6/10mm cable - Obviously not right.

Is there a rule for sizing cables in a RFC? Two legs, reduction in CSA for instance? (I'm really not sure and struggling with this! )