Maximum length

[studentspark]

Just trying to understand the maths behind table 7.1 in the OSG ... Maximum length

For a 20A RCBO (Type B) cable 2.5 - 1.5 (T&E)

I'm presuming clipped direct as this will give you the longest run

The Maximum Length is 42m.

Now the volt drop calculation. Using Rated current of protective device
18 x 20 x 42 /1000 = 15.12 volts

Which would suggest 42 M is too long

So I must be missing something from my calculation.

And...

Note one says VD is the limiting factor (unless marked ad-ol-zs-sc ) There is no mark.

Max zs is 1.75 for a 20 RCBO/CB, and if Ze was 0.8Ω - would that suggest a max length in relation to Zs as

1.75 - 0.8 = 0.95

0.95 / 0.0195 = 48.7m

No a mile off, but just wondering how the calculations are done.


many thanks

 

[EFLImpudence]

If you look on Page 64, it says the assumed load for 20A circuits is 16A.

That still doesn't work out exactly so maybe they are using a more accurate figure than 18.


Another example of the pointlessness of the OSG. Throw it away.

 

[ericmark]

At home I have a Java Script program to work it out, started when told 106 meters now allowed for ring final and I could not get same figure, two reasons one calculated at 26 amp average not 32, and two used the correction for volt drop when current not at max. The latter uses Sq root etc and not easy to transpose. And easy to make errors.

 

[JohnW2]

If you look on Page 64, it says the assumed load for 20A circuits is 16A.
That still doesn't work out exactly so maybe they are using a more accurate figure than 18.

Taken together, those above two statements 'ought' to provide an explanation, but there appears to be, at the least, some 'over-simplifications' in the OSG, which will probably render their figures 'wrong' in many situations.

Tables of current-carrying-capacity [4D2A in Bs7671 = Table F5(i) in OSG - or 4D5/Table 6 ] give the CCC for a particular cable with a particular installation method for a conductor temperature of 70°C - for example, 27A for Method C 2.5mm² T+E. That presumably implies that the expectation is that with a current of 27A, Method C 2.5mm² would reach a conductor temp of 70°C. The VD table [4D2B = Table F5(ii)] then gives a VD of 18 mV/A/m (aka resistance/impedance of 18 mΩ/m), again at a conductor temp of 70°C.

As you say, the 'maximum cable lengths' in Table 7.1 assume a current of 16A for a 20A radial circuit. For several of the installation methods mentioned in Table 7.1, the CCC will be appreciably greater than 16A, hence the conductor temp will be appreciably less than 70°C when 16A is flowing. The resistance/impedance of the cable will therefore be appreciably less than 18 mΩ/m, so that the maximum cable length (for a 5% VD) would be appreciably greater than one would calculate if one assumed 18 mΩ/m.

A point to note is that, for reasons I've described, the maximum length (for a given VD) will depend upon installation method. However, the OSG gives single figures for a fairly wide range of installation methods (100-102, A, C).

One would have to do the sums to determine which installation method gave the closest to 'correct' figures for 'maximum length', but they clearly cannot be correct for more than one installation method. They may have tabulated 'worst case' figures (i.e. Method A, with 101 and 102 being very close to that) - but, if they have, the 'maximum length' figures tabulated will be appreciably lower than would be the 'true' figure with Method C.

I don't really understand why they have included installation method 101 (for a 20A 2.5mm² RCBO radial) since, with a CCC of 17A (4D5), that would not be acceptable for a 20A circuit.

Another example of the pointlessness of the OSG. Throw it away.

Quite - see above

Kind Regards, John

 

[EFLImpudence]

Yes, I didn't bother to mention the installation methods as clearly that alters everything.

With Method 103 and 2.5mm² and a CCC of 13.5A the maximum length figure cannot really be stated without knowing the actual resistance of the cable depending on how much of the cable is surrounded by thermal insulation and likely to be at 70° and how much is not and barely over ambient temperature.

 

[JohnW2]

Yes, I didn't bother to mention the installation methods as clearly that alters everything. With Method 103 and 2.5mm² and a CCC of 13.5A the maximum length figure cannot really be stated without knowing the actual resistance of the cable depending on how much of the cable is surrounded by thermal insulation and likely to be at 70° and how much is not and barely over ambient temperature.

Indeed - and the fact that they give just single 'max length' figures for a range of installation methods (when the truth is that they will be different for different installation methods) means that the OSG is less than useful in this respect.

To be fair to them, they didn't include Method 103 in the list for 20A circuits, but, as I said, they did include 101 (only 17A for 2.5mm).

.... without knowing the actual resistance of the cable depending on how much of the cable is surrounded by thermal insulation and likely to be at 70° and how much is not and barely over ambient temperature.

Yes, that's another complication which I didn't go into. In practice, accurate calculations are rarely going to be possible. It is rare for a cable to have the same installation method throughout its length, but when it comes to cable sizing, we have to assume that it is 'the worst case' throughout it's length. However, as you say, to actually work out resistance of a cable (hence VD) one needs to consider the cable on an 'inch-by-inch' basis in order to estimate what the temperature (hence resistance) will be in each of those 'inches'.

I suppose if one assumes that installation method is the same throughout the cable (at the level of the 'worst' part) one will at least end up with a 'conservative' (high) figure for max length.

Kind Regards, john

 

[EFLImpudence]

Agreed but if the worst installation method is in use then the MCB should be 10A, so all moot anyway.

 

[JohnW2]

Agreed but if the worst installation method is in use then the MCB should be 10A, so all moot anyway.

Well, yes, the assumption has to be (before one even thinks about VD), that the cable is adequately sized (given its installation method) for the OPD rating - that's why I questioned why the OSG had included Method 101 in relation to 2.5mm² 20A radials.

Kind Regards, John

 

[JohnW2]

.... As you say, the 'maximum cable lengths' in Table 7.1 assume a current of 16A for a 20A radial circuit. For several of the installation methods mentioned in Table 7.1, the CCC will be appreciably greater than 16A, hence the conductor temp will be appreciably less than 70°C when 16A is flowing. The resistance/impedance of the cable will therefore be appreciably less than 18 mΩ/m, so that the maximum cable length (for a 5% VD) would be appreciably greater than one would calculate if one assumed 18 mΩ/m.

Some actual (rough) figures, for illustration ...

Table I1 of the OSG gives the L+N resistance for 2.5mm² T+E as 14.82 mΩ/m (7.42 mΩ/m x 2) at 20 °C. If (as the OSG does) one assumes a resistance temperature coefficient of 0.004 per °C, then at 70 °C the resistance will increase by a factor of 1.20 [ 1- (0.004 x {70-20}) ], which is the figure for 70 °C given in the OSG's Table I3. Hence, taking all that together, one would expect the L+N resistance or 2.5mm² T+E to be 17.78 mΩ/m (14.82 mΩ/m x 1.20) at 70 °C - which, when 'rounded', becomes the 18 mΩ/m (mv/A/m) figure quoted for VD in BS7671.

As previously said, the CCC tables imply that the conductor temperature of Method C 2.5mm² T+E will rise to 70 °C when 27A is flowing, with an ambient temperature of 30 °C - hence a temperature rise of 40 °C (70-30) at that current.

The amount of power dissipated in the conductors (hence approximately the amount of temperature rise) will be proportional to the square of the current (W=I²R). Hence, when 20A flows through Method C 2.5mm² T+E, one would expect the temperature rise to be about 0.55 [ 20²/27² ] times the temperature rise with 27A flowing. If, as above, 27A results in a 40 °C rise, then 20A would be expected to result in roughly a 22 °C (40 x 0.55) temperature rise - i.e. a rise from the (BS7671) ambient of 30 C to about 52 C.

Going back to the resistance/temp calculations mentioned above, if the L+N resistance of 2.5mm² cable is taken (per OSG) as 14.82 mΩ/m at 20 °C, then it should be about 16.12 mΩ/m (mv/A/m) [14.82 x (1 + {0.004 x 22}) ] at 52 °C.

Hence, those rough calculations would suggest that (when 'fully loaded', to 20A), a 20A 2.5mm² Method C radial will have voltage drop of about 16.12 mΩ/m (mv/A/m), which is appreciably less than BS7671's 18 mΩ/m (mv/A/m) figure.

Kind Regards, John

 

[JohnW2]

Grrrrr - this again! ....

 

[JohnW2]

... I should perhaps have added ...

... whoever created the OSG's "maximum length" tables could have undertaken (but did not) the above calculations just as easily as I did, although they would then have to have given separate figures for each installation method - and that would perhaps have been more useful than what they have actually done.

Kind Regards, John