Grouping Factor is giving me a headache.

[LeeH]

I'm in the process of completing my 2396 project (A college) but the due to the grouping factor I'm getting some unrealistic and impracticable cable sizes.

I have got around some larger cables sizes by changing from thermoplastic to thermosetting and that has sometimes been just enough to drop a cable size.

But for example, due to a Cg factor of 0.6 on a 20A radial with a thermoplastic single it works out at a 6mm csa. MK states the there 13A sockets will take 6mm but in reality I doubt many of you have ever done this have you?

I cannot use thermosetting singles in this case as I'm using PVC conduit in some of the circuit so I cannot use that to reduce the CSA.

I have already separated some of the circuits to reduce the grouping as much as I can too.

I know as a designer I have some movement but for the purpose the of the assignment I have to cross every t and dot every i.

Thanks in advance. :annoyed:

 

[wozz]

redesign your containment to limit the number in each section, also are you using the paragraph below the Cg table?

 

[Marvo]

6mm for a 20A radial is a massive cable. Are the cables all the same load ans size? Are any of the cables at < 30% of their grouped rating? What table are you using for your derating figures?

 

[LeeH]

redesign your containment to limit the number in each section, also are you using the paragraph below the Cg table?

I have already split the lighting circuits away from the socs etc.
Are you referring to Note 9?
If a cable carries no more then 30% of its grouped rating then it may be ignored or the purpose of obtaining the rating factor.

So, a 20A circuit with no factor is 2.5mm. With the grouping its 6mm which has a tabulated value of 41.

30% of 41A is 12.3A which is less then the Ib of 20.

Is that what it is saying? Then do I take that 1 circuit away from the total grouped circuits?

Cheers.

 

[LeeH]

6mm for a 20A radial is a massive cable. Are the cables all the same load ans size? Are any of the cables at < 30% of their grouped rating? What table are you using for your derating figures?

I know marvo, hence the question and my comment about them being unpractical.

No, in this instance which is the office block its 20A radials and 6A lighting circuits.

Cable sizes got extremely mad in the workshop area with 20+ circuits......

 

[LeeH]

Don't know why this wasn't covered in class.

But then again I do because there is far to much ground to cover in 12 weeks!!

 

[Paul651975]

When I did the Johnstone catalogue shop I managed to group approx 12 circuits in one trunking run. You have to look at using the <30% of the grouped rating for the cable i.e. 1.5mm cable but circuit only loaded with a couple of amps. I ended up having to run some lighting circuits in 2.5mm to negate them from grouping calc for the RFC circuits. Even then couldn't group more than 4 power circuits.

 

[LeeH]

Paul, on post 4 I have mentioned the 30% rule.

Is the way I have explained correct?

Thanks.

 

[Richard Burns]

I have already split the lighting circuits away from the socs etc.
Are you referring to Note 9?
If a cable carries no more then 30% of its grouped rating then it may be ignored or the purpose of obtaining the rating factor.

So, a 20A circuit with no factor is 2.5mm. With the grouping its 6mm which has a tabulated value of 41.

30% of 41A is 12.3A which is less then the Ib of 20.

Is that what it is saying? Then do I take that 1 circuit away from the total grouped circuits?

Cheers.

Paul, on post 4 I have mentioned the 30% rule.

Is the way I have explained correct?

Thanks.

You have described a circuit planned to be run in 2.5mm² cable, this is grouped with four other circuits so the group rating factor is 0.6.
Using thermoplastic multicore clipped direct the current carrying capacity of 2.5mm² cable is 18.5 A on single phase.
So therefore the corrected CCC is 11.1 A.
If the design of the circuit is such that it will carry less than 30% of 11.1 A i.e. 3.33 A (so less than 766 W) then this circuit can be discounted from the grouping calculation.
So then the calculation would be done for the other circuits as if there were only 4 circuits in total so a grouping factor of 0.65.
If all the circuits bar one were carrying less than 30% of the grouped CCC then the heavily loaded circuit could be considered on its own!
Similarly for lighting circuits in a domestic situation they are often carrying less than 2 A in reality and so a 1.5mm² cable can be disregarded.

 

[telectrix]

and have you used the design current for calculations, or the MCB rating?

 

[LeeH]

Hi Richard and thanks for that little gem of info.

The circuits in question are 13A socket outlets so they have a varying load. I cannot apply diversity obviously so the load is 20A.

So the 30% rule does not help and I'm still stuck with 6mm radials FFS.

 

[LeeH]

and have you used the design current for calculations, or the MCB rating?

Hi tel,

MCB rating as it's a A3 radial in this case.

 

[Richard Burns]

13A socket outlets may be used for particular purpose, such as in an office they may be only used for computers and IT equipment so in your design you can say that any particular socket may be using about 300 W, if you have six sockets on the radial then that would be 1800 W so a design current of 7.8 A.
In the design you specify that only IT equipment is to be used on this radial.

This is an extreme case situation so not a general solution, but you should be looking at the design of the circuit and its intended use instead of assuming maximum current draw at all times.
Occasional other uses should not seriously detract from this design and if the users deviate from your design then that is not your concern, though some level of safety from normal use should be envisaged.

Also consider changing your containment method if possible, for instance if the cables are on basket then the CC is larger and may allow greater freedom in grouping.

 

[D Skelton]

Many people seem to forget that the appendices after 1 are only informative. Also, when it comes to the grouping factors shown in appendix 4, these only apply to cables under prolonged and steady-state operation at 100% of load factor. For other situations the grouping factors will be considerably different.

The fact is, you, as the designer, can do whatever you like so long as you can justify that your decision is safe.

If you decide that only 4 out of 20 circuits in a length of steel trunking will be under prolonged and steady state operation at 100% of load factor, then apply a suitable rating factor taking into accout the likelyhood of the other circuits being fully loaded, even operational at that exact time.

I said this in another post a day or so ago, how many times do we come across steel trunking stuffed to the brim with ten, twenty, even thirty circuits, yet it isn't hot to the touch even in the slightest? I don't think I've ever touched a piece of trunking sat there at 70 degrees temp?! Which according to the informative guidance given in appendix 4 it would be if the factors stated weren't applied.

Think of it another way, say you have 10 x 20A radials run in a single length of steel trunking, what is the likelyhood of each of those 20A radial circuits being loaded to 20A all at the same time???

Another way to approach it in order to follow the guidance of appendix 4 to the letter, is to install three times the amount of circuits. Instead of one 6mm radial to supply 3 motors, run three 2.5mm radials, therfore keeping the load factor for each circuit below 30%. This isn't the way I'd approach it, but is worth looking into if you are concerned about following things to the letter, guidance or not.

 

[Chr!s]

I'm in the process of completing my 2396 project (A college) but the due to the grouping factor I'm getting some unrealistic and impracticable cable sizes.

I have got around some larger cables sizes by changing from thermoplastic to thermosetting and that has sometimes been just enough to drop a cable size.

But for example, due to a Cg factor of 0.6 on a 20A radial with a thermoplastic single it works out at a 6mm csa. MK states the there 13A sockets will take 6mm but in reality I doubt many of you have ever done this have you?

I cannot use thermosetting singles in this case as I'm using PVC conduit in some of the circuit so I cannot use that to reduce the CSA.

I have already separated some of the circuits to reduce the grouping as much as I can too.

I know as a designer I have some movement but for the purpose the of the assignment I have to cross every t and dot every i.

Thanks in advance. :annoyed:

I think you need to read through appenix 4 from the beginning, try and ensure there are no more than 5 derateable circuits in your containment.

cheers
chris