That is always an option to consider - and the same for the 5% for non-lighting loads. Don't forget that the 3% and 5% figures only exist in an 'informative' Appendix of the regs.
Kind Regards, John
SWA earthing
- Thread starter RF Lighting
- Start date
Kind Regards, John
The software uses BS7671 values. With volt drop, over 100m, you can apply the 0.005% per meter factor. You can also apply a correction factor for operating temperature when you increase the cable in size to combat voltage drop - A 70mm can carry far FAR greater than 60amp. At 60amp it will NEVER approach the 70deg 'operating' temperature, Appx 4, 6.1.
If you did the calculations taking this into account, it should match. Theres something for you to do John
Wotz dat?
Agreed.
I could if I knew what the operating temperature would be at 60A, and if I knew what temperature your software has assumed. Is your software so clever that it has actually estimated the operating temp at 60A and then used that for its VD calculation?If you did the calculations taking this into account, it should match. Theres something for you to do John
As discussed above, it rather looks as if RF used the BS7671 70° figure.
Kind Regards, John
There is a set equation to use to correct for operating temperature. Don't think it needs the actual operating temperature. Can't remember the cable size where this equation cuts off though.
The 0.005% is in the regs - Any length of 100m or more, each additional meter over 100m can have the volt drop increased by 0.005%.
Not got the regs to hand to confirm fully, but it's something like that.
The 0.005% is in the regs - Any length of 100m or more, each additional meter over 100m can have the volt drop increased by 0.005%.
Not got the regs to hand to confirm fully, but it's something like that.
Right. That seems to be Equation 6 in 6.1. of (informative) Appendix 4. The initial pre-amble text seems to indicate that the equation only applies up to 16mm², but the subsequent text seems to imply that it can be used for any cable size although (obviously!) for sizes >16mm² the correction should only be applied to the resistive component of the cable impedance.
Found it - it's in a note to Table 4Ab, again in (informative) Appendix 4. I hadn't noticed that before, but that's probably because I have probably never used cables >100m! It actually seems a bit daft - if they feel that VD has to be limited to a certain level for the sake of the load, why should that get relaxed just because the cable is long?! Whatever, yes, this means that with 135m, one is allowed a lighting VD of 3.175% (which, incidentally, is still less than the 3.28% I calculated using 70° figures). In any event, this obviously does not explain the 'discrepancy' in the calculated VD figures - which presumably must be due to the temperature issue.
I'll see if I have enough information to do the temperature calculation!
Kind Regards, John
OK. If I assume that the factors Cg (grouping), Cs (soil thermal resistivity) and Cd (depth) are all 1, that the soil ambient temp is 20° (hence Ca=1), and that the CCC of buried (method D) 50mm² SWA at 70° is 140A, then for 60A I get the 'VD correction factor' (C1) as 0.9755. If that's right, it means that (based on the 70° VD of 7.53V), the VD at 60A should be 7.34V - still way above your software's 6.65V. So, either I've done my sums wrong or else we need to find some other explanation.
Kind Regards, John
I think RF is in Leeds, West Yorkshire, not Leeds, Guyana, so I doubt that the soil is ever going to be at 20° half a meter down...
Sorry, folks (particularly Lectrician) - I got my arithmetic wrong somewhere
Using above assumptions, the 'VD correction factor' (C1) should have been 0.891, which turns the VD (per BS7671 70° figures) of 7.53V into about 6.7V, therefore very close to the answer produced by Lectrician's software - which therefore is pretty clever. Apologies for the slight hiccup with the maths!Kind Regards, John
Probably true, although that's the default ground temp used by BS7671. The lowest the 'factor table' goes is 10°. If one used that, the 'VD correction factor' would become 0.865, leading to a VD of 6.50V, somewhat lower than Lectrician's software (which therefore was probably assuming about 20°).
Kind Regards, John
I selected 70mm², as 50mm² seemed far too close to the limit for my liking, and I still need capacity for the final circuits, which could be upto 50M.
60A is slightly high after diversity, but not a huge amount, and whilst there are no immediate plans to expand the installation, it's not totally implausible.
The price difference between 50mm² and 70mm² is only ~£400, which is nothing compared to the the overall project cost.
I have spoken to the client, and he was 'happy' to spend more on putting in a cable that will never give him any greif, rather than trying to save a few quid now and end up having to replace the cable a few years down the line.
Interesting stuff in this thread. I think I'm in need of a few more calculations, and a verification of Ze to make certain that a 2 core is adequate without a seperate copper earth.
60A is slightly high after diversity, but not a huge amount, and whilst there are no immediate plans to expand the installation, it's not totally implausible.
The price difference between 50mm² and 70mm² is only ~£400, which is nothing compared to the the overall project cost.
I have spoken to the client, and he was 'happy' to spend more on putting in a cable that will never give him any greif, rather than trying to save a few quid now and end up having to replace the cable a few years down the line.
Interesting stuff in this thread. I think I'm in need of a few more calculations, and a verification of Ze to make certain that a 2 core is adequate without a seperate copper earth.
Sorry, folks (particularly Lectrician) - I got my arithmetic wrong somewhere
Kind Regards, John
The software actually produced 6.7 when I set the install method to "user defined" and selected an install method correction factor of 1 just know.
Good to know it works, I like to manually check it every so often!
Yes, that's always reassuring. Mind you, that figure seems to assume a soil ambient temp of 20°C which, as BAS has pointed out,is probably a bit high for the UK.
Kind Regards, John
Just looked again, it was 6.63.
To clarify, the defaults I accepted for my calculation were a cable depth of 0.7m, a soil thermal resistivity of 2.5 K.m/W and ambient ground temp of 20° and a cable with tabulated VD given for 70° (at 140A, Method D). Do they all correspond to the parameters being used by your software?
Kind Regards, John
Not really looked. Tried to message you, but can't?
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