fiftyhertz:

 

OMS:

If you think about a ring circuit, if there is 20A in one leg, then there will only typically be 10A in the other leg (otherwise we'll be operating the CPD)


I believe amtech uses standardised grouping factors for RFC's based on:


1 ring - Factor 1.5

2 Rings - Factor 1.19

3 Rings - Factor 1.03

4 Rings - Factor 0.94

5 Rings - Factor 0.87

6 Rings - Factor 0.82


If we call the factor Cr then Itabulated > In x I/Cr (Amperes) and thus we need a cable rating of 21.33A for a single RFC (now BS 7671 mandates 20A and 2.5mm2 (which will carry 27A) - the 20A assumes the worst probable division of current around the ring. We tend to use the rating of the CPD rather than a RFC design current as it's quite probable the rings may well be subject to simultaneous overload (for design purposes)


You may need other factors as usual for ambient temperature exceeding 30C and for cables in insulation


Does that help


Regards


OMS

OMS,


I have saved that list down for future grouping as it very helpful. 


In this case i dont think it fits as i am using Ib, and the correction factor is constant at 0.8. This can be seen from the last line of the table above. 32A design current @ 0.8 Cg. 20A/0.8 = 25A as calculated by Amtech.

You can't use Ib as you have no real control over the usage - it could easily be fully loaded with adjacent fully loaded (or overloaded) circuits


The bit i dont understand is as per the table, if the design current is 21A @ 0.8 Grouping (2 circuits), how the tabulated current Amtech spits out can be 21.1A.

Because I'm using a design current of 32A and approximating a 0.66 and 0.33 split of current in the legs of the ring (21.3 A and 10.7A) - and thus one leg of the circuit is less than 33% loaded and can be effectively ignored for grouping purposes. What you should be seeing is that you can sensibly group 3 No fully loaded RFC's when using a 27A cable with no derating (ie Cr in each case is greater than 1) due to the headroom difference between 20A and 27A



Hope this makes sense

 

fiftyhertz:

 

OMS:

If you think about a ring circuit, if there is 20A in one leg, then there will only typically be 10A in the other leg (otherwise we'll be operating the CPD)


I believe amtech uses standardised grouping factors for RFC's based on:


1 ring - Factor 1.5

2 Rings - Factor 1.19

3 Rings - Factor 1.03

4 Rings - Factor 0.94

5 Rings - Factor 0.87

6 Rings - Factor 0.82


If we call the factor Cr then Itabulated > In x I/Cr (Amperes) and thus we need a cable rating of 21.33A for a single RFC (now BS 7671 mandates 20A and 2.5mm2 (which will carry 27A) - the 20A assumes the worst probable division of current around the ring. We tend to use the rating of the CPD rather than a RFC design current as it's quite probable the rings may well be subject to simultaneous overload (for design purposes)


You may need other factors as usual for ambient temperature exceeding 30C and for cables in insulation


Does that help


Regards


OMS

OMS,


I have saved that list down for future grouping as it very helpful. 


In this case i dont think it fits as i am using Ib, and the correction factor is constant at 0.8. This can be seen from the last line of the table above. 32A design current @ 0.8 Cg. 20A/0.8 = 25A as calculated by Amtech.

You can't use Ib as you have no real control over the usage - it could easily be fully loaded with adjacent fully loaded (or overloaded) circuits


The bit i dont understand is as per the table, if the design current is 21A @ 0.8 Grouping (2 circuits), how the tabulated current Amtech spits out can be 21.1A.

Because I'm using a design current of 32A and approximating a 0.66 and 0.33 split of current in the legs of the ring (21.3 A and 10.7A) - and thus one leg of the circuit is less than 33% loaded and can be effectively ignored for grouping purposes. What you should be seeing is that you can sensibly group 3 No fully loaded RFC's when using a 27A cable with no derating (ie Cr in each case is greater than 1) due to the headroom difference between 20A and 27A



Hope this makes sense