Grouping factors

[Bailey326]

2 x 2.5mm twin and earth (ring circuit) in 25mm pvc conduit buried (plaster/concrete).

Domestic.

Who here would calculate grouping factor to further derate the cables?

 

[GBDamo]

2 x 2.5mm twin and earth (ring circuit) in 25mm pvc conduit buried (plaster/concrete).

Domestic.

Who here would calculate grouping factor to further derate the cables?

This is what the 80% 'rule of thumb' figures in the OSG are there for, to save us the brain ache of calculations for every eventuality.

 

[buzzlightyear]

Who here would calculate grouping factor to further derate the cables

You, read the big blue book.

 

[Ian1981]

2 x 2.5mm twin and earth (ring circuit) in 25mm pvc conduit buried (plaster/concrete).

Domestic.

Who here would calculate grouping factor to further derate the cables?

Assuming you mean that they are 2 cables, They are the same circuit, why would you apply grouping?

 

[Bailey326]

Assuming you mean that they are 2 cables, They are the same circuit, why would you apply grouping?

This is exactly what I was thinking/remember being taught.
Yet some people I've spoken to in the past reckon they would put the ring on a 20amp breaker.

 

[Julie.]

Assuming you mean that they are 2 cables, They are the same circuit, why would you apply grouping?

Because they both produce heat.

One cable in conduit in a wall - say 23A rating will produce a certain heat per unit length, now put another cable also carrying 23A it will also produce the same heat per unit length - so you end up with twice the heating effect without changing the corresponding cooling effects of the wall etc.

2 cables in conduit has a derating factor of 0.8 (table 4c1) on methods A to F so if you install to method B then table 4d2a gives 23A for 2.5mm^2 which with the derating factor gives 18.4A

 

[Ian1981]

Because they both produce heat.

One cable in conduit in a wall - say 23A rating will produce a certain heat per unit length, now put another cable also carrying 23A it will also produce the same heat per unit length - so you end up with twice the heating effect without changing the corresponding cooling effects of the wall etc.

2 cables in conduit has a derating factor of 0.8 (table 4c1) on methods A to F so if you install to method B then table 4d2a gives 23A for 2.5mm^2 which with the derating factor gives 18.4A

Grouping factor needs to take into account number circuits, not the number of cables, this is a ring final , you can also take into account that if it’s not pulling more than 30% of its grouped rating, then it can be ignored.

 

[pc1966]

The RFC is a slightly odd case because the two cables are both generating heat, but they are limited to a total of 32A between them by the OCPD.

Best-case is an equal share where both 2.5mm will have to cope with 16A each and as Julie has computed that is in the acceptable rating.

Worst case of 32A in one leg results in double the heat (4 times 16A case in one leg, none in the other, from I2R). This should not occur, but there is a small chance if both L & N loops open at one last socket, but in this case you have no grouping factor to worry about!

 

[Julie.]

Grouping factor needs to take into account number circuits, not the number of cables, you can also take into account that if it’s not pulling more than 30% of its grouped rating, then it can be ignored.

Where does it say that the regulations?

So if two cables are taking rated current they will produce different amounts of heat if they happen to be the same circuit, or different circuits?

How will that work?

 

[Ian1981]

Where does it say that the regulations?

So if two cables are taking rated current they will produce different amounts of heat if they happen to be the same circuit, or different circuits?

How will that work?

It says number of circuits or multi core cables to be more accurate.
I have taken this as 2 cables as a ring Final is one circuit.

 

[Julie.]

The RFC is a slightly odd case because the two cables are both generating heat, but they are limited to a total of 32A between them by the OCPD.

Best-case is an equal share where both 2.5mm will have to cope with 16A each and as Julie has computed that is in the acceptable rating.

Worst case of 32A in one leg results in double the heat (4 times 16A case in one leg, none in the other, from I2R). This should not occur, but there is a small chance if both L & N loops open at one last socket, but in this case you have no grouping factor to worry about!

Not quite, if there is no load on the socket the two cables are feeding, then whatever is going down one must return via the other.

So if all the loads happen to be at one end of the ring, then you could have 20A down the short leg and 12A via the long leg.

Of course it is likely to be more balanced than this, and could be worse, but this is the worse condition considered in the design of rfc's

 

[pc1966]

Not quite, if there is no load on the socket the two cables are feeding, then whatever is going down one must return via the other.

The OP did not say it if feeding just one socket so worst-case it is both end legs coming from the DB in the conduit.

The 20/12 split as you say is the worst-case RFC design case but not fault case!
[automerge]1600013028[/automerge]
Though I guess it is not normal to consider that, otherwise we would always use 4mm cable instead of 2.5mm

 

[Julie.]

The OP did not say it if feeding just one socket so worst-case it is both end legs coming from the DB in the conduit.

The 20/12 split as you say is the worst-case RFC design case but not fault case!

The worse case isn't there, that could have 20/12 or 16/16 or whatever.

The worse section is where the 20A leg goes down the wall to an unused socket, then back up the same way to the next circuit etc.

In reality I think rfc's tend to be more balanced than 20/12 and even then unlikely to be at 32A total in any case

 

[pc1966]

The worse section is where the 20A leg goes down the wall to an unused socket, then back up the same way to the next circuit etc.

Doh, of course it is!

 

[Julie.]

It says number of circuits or multi core cables to be more accurate.
I have taken this as 2 cables as a ring Final is one circuit.

I take it in relation to that table:
single core cables - be it a line and neutral, or three lines (+n) is considered a single circuit.

Twin and earth would be a multicore cable

Not the eventual use of the cable.

 

[Ian1981]

I take it in relation to that table:
single core cables - be it a line and neutral, or three lines (+n) is considered a single circuit.

Twin and earth would be a multicore cable

Not the eventual use of the cable.

That is a Fair point