Level 3 C&G cable sizing calculations

[richardindorset]

As well as that, as I pointed out, the regulation is written for a BS3036 30A fuse so the cable has to be derated by the 0.725 factor.
27 x 0.725 = 19.575.

Therefore, it is not really possible to match values to a 32A MCB with the mandatory things you must do.


Yes, but it does not allow you to use a 20A cable (except MICC); you must use a 27A 2.5mm² T&E.


Obviously it cannot. See above.


That is not the correct answer.

Using 4mm² cable it would not need to be a ring final; it could be a radial; therefore the question is invalid.

433.1.204 Accessories to BS 1363 may be supplied through a ring final circuit, with or without unfused spurs,
protected by a 30 A or 32 A protective device complying with BS 88 series, BS 3036, BS EN 60898, BS EN 60947-2
or BS EN 61009-1 (RCBO). The circuit shall be wired with copper conductors having line and neutral conductors
with a minimum cross-sectional area of 2.5 mm2 except for two-core mineral insulated cables complying with
BS EN 60702-1, for which the minimum cross-sectional area is 1.5 mm2. Such circuits are deemed to meet the
requirements of Regulation 433.1.1 if the current-carrying capacity (Iz) of the cable is not less than 20 A and if,
under the intended conditions of use, the load current in any part of the circuit is unlikely to exceed for long periods
the current-carrying capacity (Iz) of the cable.

No sums necessary.

Alas putting "No sums necessary" would not be the 'right' answer as far as the C&G exam marker is concerned...

This wouldn't be the first exam question I've read that has a 'correct' answer that is very dubious...

At least it's not me being thick. Somehow I just need to figure out how C&G come up with their answers...

 

[EFLImpudence]

At least it's not me being thick. Somehow I just need to figure out how C&G come up with their answers...

You could ask them; after reading 422.1.204 how they came up with the question.

 

[securespark]

I am not au fait with the 18th, but a heating eng I know said that a spark he knows told him that ring finals and 3036s were deleted in AMD3.

I guess they are both still very much part of 7671?

 

[securespark]

Re C&G questions, when I took my 17th update, I got one question wrong out of 60 and was disappointed. I knew what question it was, and I considered it very confusingly worded, but they insisted it was good.

 

[flameport]

I guess they are both still very much part of 7671?

They are. Nothing has changed for either of those things for decades.

 

[securespark]

Thought so. Wonder where the spark got those ideas from?

 

[EFLImpudence]

that ring finals and 3036s were deleted in AMD3.

Indeed - and what does that even mean?

 

[richardindorset]

So, C&G's answer to this question is 4mm2 and how did they come to this answer?

Well, it appears that for radial and lighting circuits, you use the breaker rating plus any factors to get It and then look up the relevant installation method.

For a Ring Final, you take the Ib, the design current and ignore the breaker rating. Apply factors to that and then look it up.

I don't know why they do this, as I cannot find any regs that say to to this....

 

[plugwash]

I thought for rings the regs specified a cable rating of 20A explicitly.

 

[richardindorset]

I thought for rings the regs specified a cable rating of 20A explicitly.

Yes, they do, but not as far as C&G is concerned! This is what I'm trying to get to the bottom of. We're being taught the process of cable calculations which is great for radials and lighting etc but doesn't work for Ring Finals for obvious reasons, however there is nothing in the course notes to say how C&G come up with their answer to the question of what cable to use in a Ring with a 32A breaker. It appears they use Design Current instead of Tabulated Current to find the appropriate cable size in table F6 in the OSG.

I've googled this a lot and it is a very common question, where people are trying to find calcs to get Ring Final cable sizes, when there are none, just regs to cover Ring Final saying to use 2.5mm2 as long as there's a 32A breaker, plus table H2.1 in the OSG showing circuit type A1, Ring, 2.5mm2 and floor area of up to 100m2.

 

[EFLImpudence]

I thought for rings the regs specified a cable rating of 20A explicitly.

They do - but then state you cannot use 1.5mm² which has a CCC of 20A but you must use 2.5mm².

This is because the regulation is still written for a BS3036 30A fuse and presumably imperial cable.

This is why modern calculations do not work for the ring final.

 

[securespark]

If you can post the whole question here, I shall query it with C&G themselves.

 

[richardindorset]

If you can post the whole question here, I shall query it with C&G themselves.

I think I have above, but I have a few instances of this issue.

Basically we are given a design brief, such as: Fill out a design grid showing the correct cable sizes for all the circuits required in the new student accomodation block, shown in the attached plan.

The design grid has each circuit along the top and rows down for each stage of the calculations. Circuit 3 is the ring circuit:

Circuit: 3
Description: Power ring final for kitchen
No. of points: 7
Type of Wiring: 70 deg C thermoplastic flat multicore
Design Current Ib: 20A
Type and nominal rating In (of protection device): 32A type B breaker
Length: 37M
Installation Method: 100 *
Ambient Temp: 35 deg C
Rating Factor Ca: 0.94 *
Other factors (ignoring grouping): 1.0 *
Minimum Current capacity <It: ?
mV/A/M: ?
Volt drop: ?
Minimum conductor Area: ?

The numbers marked * are filled in by us using comments on the plan. All other data is provided for us already on the grid.

Following standard proccedure for radials (in the abscence of any other proceedure) we take In as 32A and apply facotrs to get It at 32/0.94 = 34.04A. We'd look at table F6 in the OSG, ref method 100 and find that 6mm is just, only just, too small, at 34A, so we 'should' go to 10mm2 cable. Obviously the voltage drop would be minimal, but of course no one in their right mind would use it!

So what C&G do in this case (which is an undocumented proceedure) is use Ib, the design current and not In, the breaker rating, to get 20A, then apply factors to get 21.23A and then look up table F6, ref method 100, 4mm2 cable which has a capacity of 27A. Volt drop is within limits.

What the regs say, is to use 2.5mm2 cable and don't do any calcs. If you work out the volt drop for this you'd get 13.32v which is above the limit, but I guess that being a ring the drop could be half that?

Anyway, the task is presented in a way that you are expected to follow the proceedure and show calcs, not just refer to Reg 433.1.204

 

[EFLImpudence]

Until something definite comes along:

Minimum Current capacity <It: ?
mV/A/M: ?
Volt drop: ?

How can you work out those values until you know the conductor CSA?

Minimum conductor Area:

What the regs say, is to use 2.5mm2 cable and don't do any calcs. If you work out the volt drop for this you'd get 13.32v which is above the limit, but I guess that being a ring the drop could be half that?

Actually a quarter - like the resistance.

Anyway, the task is presented in a way that you are expected to follow the proceedure and show calcs, not just refer to Reg 433.1.204

Ok, but that is what a ring circuit is.

 

[richardindorset]

Until something definite comes along:


How can you work out those values until you know the conductor CSA?




Actually a quarter - like the resistance.


Ok, but that is what a ring circuit is.

So what you do is, get a figure for It, look up a cable size that has capacity of equal to or greater than that, find the voltage drop, calculate it, check against limits (<11.5V) and if bigger, go to next cable up. That's how you get the numbers.

Regarding resistance, when you say quarter, you mean because it's Line and Neutral lengths in parallel with Line and Neutral again to form the ring?

I'm not sure thats right as the length given is the length of the whole cable, not the middle of the ring. Table F6 gives drop for a 2.5mm2 cable of 18mV, so this is for the line and neutral, so 9mV drop on each per meter, per amp. 9mV drop on 2.5mm2 wire gives a resistance of 0.009 ohms. A resistivity of 2.25x10-8, pretty close to pure copper of 1.77x10-8