Grouping factor for JB

[Jonspark]

Opened an IP65 JB surface mount box today (internal conservator) 2 lighting circuits including Switch cables and radial feed from last light all coming into the box. All fitted ok and all secure with Wago connectors no strain on connections.

My question is would you need to apply grouping factor for JB is multiple cables for same circuits?

 

[telectrix]

are they all 1 circuit. i.e. all off the same MCB? if so no grouping applied.

 

[Jonspark]

are they all 1 circuit. i.e. all off the same MCB? if so no grouping applied.

Yes all on same MCB. Also noticed no RCD on circuit.
[automerge]1590700790[/automerge]

Yes all on same MCB. Also noticed no RCD on circuit.

I guess that makes sense of course given its all one circuit thanks

 

[Lucien Nunes]

Well, yes and no. Grouping factors are determined by how much current flows in each cable, regardless of which circuit it belongs to. So, if for argument's sake you have the full current of a circuit zig-zagging up and down a bundle of cables, then the factor applies for the full number of cables even though only one circuit is involved. E.g., a 20A load on a DP switch where the cables on the supply and load side of the switch are bunched, there are two cables carrying 20A and giving off the same amount of heat as two separate 20A circuits, so require a Cg of 0.8.

OTOH, bunched cables each carrying part of the total current in one direction only, cannot carry more than the circuit rating. E.g. a 6A lighting circuit that splits into three cables each serving one area, the total on all three cables cannot exceed 6A. No matter how the current is divided between the cables, e.g. 3 x 2A, the heat dissipation will be less than when one cable carries the full 6A, therefore no factor needs to be applied.

 

[Spoon]

Well, yes and no. Grouping factors are determined by how much current flows in each cable, regardless of which circuit it belongs to. So, if for argument's sake you have the full current of a circuit zig-zagging up and down a bundle of cables, then the factor applies for the full number of cables even though only one circuit is involved. E.g., a 20A load on a DP switch where the cables on the supply and load side of the switch are bunched, there are two cables carrying 20A and giving off the same amount of heat as two separate 20A circuits, so require a Cg of 0.8.

Now this is interesting as I thought that you went by Circuit.... Yes Table 4C1 does state "Number of circuits or multicore cables"
So you are saying that RFC's, where you have 2 cables going from the floor boards to the socket should be de-rated by 0.8 because of this?

@Jonspark Lighting circuits are a bit different to above as it depends on how much current the cable will carry. Most light in houses are LEDs so don't us a lot. As Note 9 states for Table 4C1 (page 396 BBB) "If, due to known operating conditions, a cable is expected to carry not more than 30 % of its grouped rating, it may be ignored for the purpose of obtaining the rating factor for the rest of the group."
As most houses use LED's (about 4W) then this might apply...

 

[Jonspark]

Now this is interesting as I thought that you went by Circuit.... Yes Table 4C1 does state "Number of circuits or multicore cables"
So you are saying that RFC's, where you have 2 cables going from the floor boards to the socket should be de-rated by 0.8 because of this?

@Jonspark Lighting circuits are a bit different to above as it depends on how much current the cable will carry. Most light in houses are LEDs so don't us a lot. As Note 9 states for Table 4C1 (page 396 BBB) "If, due to known operating conditions, a cable is expected to carry not more than 30 % of its grouped rating, it may be ignored for the purpose of obtaining the rating factor for the rest of the group."
As most houses use LED's (about 4W) then this might apply...

Great ?? did have a read of this section but it was late on and often you get a better explanation from someone who can speak plain English as you have sir thank you. The phrase “30% of its grouped rating” confused me. ?

 

[Spoon]

Great ?? did have a read of this section but it was late on and often you get a better explanation from someone who can speak plain English as you have sir thank you. The phrase “30% of its grouped rating” confused me. ?

I have to use Google to find out how to spell stuff... I couldn't count on my English mate.

 

[Jonspark]

I have to use Google to find out how to spell stuff... I couldn't count on my English mate.

?? what did we do before spell check and google!

 

[Baddegg]

He can spell cheese cake just fine though!

 

[telectrix]

but "just fine" does not spell cheesecake.

 

[Lucien Nunes]

So you are saying that RFC's, where you have 2 cables going from the floor boards to the socket should be de-rated by 0.8 because of this?

RFCs are a special situation because the normal principle of Ib<In<Iz is not applied. It is accepted that two legs of a ring can run together and be counted as one circuit. In theory it is possible to overload the cable but empirically it is cheesecake.

 

[telectrix]

RFCs are a special situation because the normal principle of Ib<In<Iz is not applied. It is accepted that two legs of a ring can run together and be counted as one circuit. In theory it is possible to overload the cable but empirically it is cheesecake.

stop it. you'll get spoon droolong like Hooch.

 

[Spoon]

stop it. you'll get spoon droolong like Hooch.

View attachment 58516

I do that anyway mate.

 

[telectrix]

?? what did we do before spell check and google!

i lernt to spell at skewl. musth ave bin rate coz. it were approved.