Help with Ring Tabulated Current Equation

[smokeyc]

Hi there

I am struggling with the following calculation.

4mm Ring Final Circuit,

In = 32A
Cg - group of 6 with a correction factor of 0.57
Ci - 50mm insulation of 0.78

It = In/CgCi
It = 32/0.57x0.78
It = 71.97A

Then I get stuck here:
Ib < In < It < Iz
4mm Cable from Table 4E2A gives Iz of 36A

As this is a ring circuit - is the current It halved for each leg? E.g 71.97A e.g. 35.99A

Thank you

 

[Aaron b]

Each leg needs to be able to carry 20amp. Isn't the insulation derating already accounted for by using the correct column?

That table is for thermosetting cables, does that match your selection? Also see the note regarding operating temps. Sockets are unlikely to be rated 90'c

 

[smokeyc]

Each leg needs to be able to carry 20amp. Isn't the insulation derating already accounted for by using the correct column?

That table is for thermosetting cables, does that match your selection? Also see the note regarding operating temps. Sockets are unlikely to be rated 90'c

Sorry its twin and earth, thermoplastic PVC at 70C, ambient 30C
With clipped direct giving 37A - table 4D5.

This is where I get so confused.

So the bulk of the run - the reference method probably most suited is 103 (I think).
But that does not feel entirely accurate.
Its in the ceiling with 200mm traditional joists, holes in the center, due to the size of the joist - I have had to group circuits.
Hence I have six twin and earth cables going through each joist hole; they go across a lot of joists. Including this ring.
The void is 200mm depth with 50mm rockwool.

Using 103 gives 17.5A
From what I understand from your comment means that a 32A RCBO would not work with this circuit, given 20A per leg, as each cable can only achieve 17.5A, not 20A.

I wanted to try a different approach using the tabulated current (It) with the correction factors Cg and Ci

Any help would be much appreciated, I am fairly confused, and desperately trying to find something to help online with little joy.

 

[smokeyc]

What's even more confusing, when reading the OSG in 7.2.1
"grouping is not necessary..."
"cables above ceilings are clipped to joists as per installation methods 100 to 103 of Table 4A2"
"otherwise refer to Appendix F"

103 isn't clipped.

 

[Aaron b]

I wouldn't consider the grouping factor an issue in most domestic dwellings. EV is probably the only circuit likely to draw current anywhere near its capacity and for a significant amount of time. But the tabulated values don't take that into account.

If you look at table 4A2 on pg 437 you'll see 102 and 103 is for cables covered in insulation in a void and 100 and 101 are open to air.

With free space around the cable (as in laying on top in a roof space) then it could be considered method B. Personally I have used 2.5 and downrated to 25amp for 101.

 

[smokeyc]

Table 7.1(ii) says a ring 32A with 4.0/1.5 can be installed in 103 with a max length of 171vd.

However as mentioned the Iz CCC for 103 is 17.5A.

 

[smokeyc]

I wouldn't consider the grouping factor an issue in most domestic dwellings. EV is probably the only circuit likely to draw current anywhere near its capacity and for a significant amount of time. But the tabulated values don't take that into account.

If you look at table 4A2 on pg 437 you'll see 102 and 103 is for cables covered in insulation in a void and 100 and 101 are open to air.

With free space around the cable (as in laying on top in a roof space) then it could be considered method B. Personally I have used 2.5 and downrated to 25amp for 101.

This is between floors, so a void.

 

[Aaron b]

Table 7.1(ii) says a ring 32A with 4.0/1.5 can be installed in 103 with a max length of 171vd.

However as mentioned the Iz CCC for 103 is 17.5A.

That doesn't seem right. Perhaps its a throw back from the changes in installation methods from the 16th.

 

[timhoward]

I'm understanding that you have a ring final circuit, run in 4 sq mm T+E and it's grouped with 5 other cables, and it is all under 50 mm of insulation.
It's fairly simple until you get to the grouping.

Ci - 50mm insulation of 0.78

No. Table F2 is length in insulation, not depth of insulation.
However, read the Note on first page of appendix F, Ci is 1 if you use the tables in that chapter to get the CCC.

Then I get stuck here:
Ib < In < It < Iz

For a ring final circuit, reg 433.1.204 is a special case that overrides 433.1.1.

From 433.1.204
Minimum cross sectional area of 2.5 sq mm (tick)
Such circuits are deemed to meet the requirements of 433.1.1 if the current carrying capacity of the cable is not less than 20A and (paraphrasing) no bit of the circuit will pull > 20A for a long time.

So you need the tabulated current carrying capacity of 4 sq mm PVC (I think you had the wrong table above). It sounds more like reference method 100, which gives you 27 amps. This exceeds 20 amps and is deemed to meet 433.1.1 if no further derating considerations apply.
So far so good. (Why are you saying 103 as that is for inside a stud wall?)

The complication is the grouping as you have 6 cables together for a long distance.
I agree that you are interpreting section 7 of the OSG correctly, the three exemptions don't apply and it refers you to Appendix F. (Personally I make flipping sure I don't end up in this situation! Couldn't you have used more than one hole?!)

This is where it gets complicated. The lb for a RFC is 32 amps but each cable doesn't have to carry 32 amps because of 433.1.204.
If you proceed anyway and correct Lb to 56 amps, then half it (28 amps) then I think that is a reasonable attempt.
In reality it won't be a perfect 50/50 split.
28 amps is slightly over 27 amps of course. Maybe an ambient temperature correction could save you?!

I think the real solution to be honest is re-route a couple of them in a 2nd hole.
I'd be interested in the views of some of the forum legends on this one. @davesparks @pc1966

Out of interest, if you do the grouping derating for the other circuits passing through these joists do they stack up?

 

[Aaron b]

It doesn't sound like your cable is surrounded by insulation and will have air space on one side so it would be method B.

 

[pc1966]

Is the supply capable of 5*32 = 175A? If 60-100A SP domestic you could never run all at max for any length of time anyway!

As above, generally your are looking to have as-installed CCC of 20A and it seems the issue is the grouping factor if it applies or not. As @timhoward says you would be best to try and avoid having the cables too tightly bundled for various reasons, the potential derating due to group factor being a significant one!