2.5mm swa from 4mm radial?

[JohnW2]

I thought he was trying to point out that if the voltage drops because of current drawn by a large resistive load, e.g. a 2.5kW kettle, the increased draw from a few hundred watts of SMPSU would not cancel out the decreased draw from the kettle.

Maybe - and, if so, he would obviously be right, but I don't think that anyone has suggested otherwise.

As you are aware, this started simply because I pointed out, as and educational exercise for someone who felt that he was 'totally stupid', that it is actually not true for all types of load that decreased voltage means decreased current - I wasn't implying any 'practical relevance'.

Kind Regards, John

 

[ericmark]

If one looks at the formula for volt it has a lot of items which really do very little.

I have also tried to work it out and for years used an Excel program. New phone needs for me to pay for Excel so Java Script seemed a better idea.

I was helped with this calculator by people on here and it seems I had made a mistake with original which only came to light as I moved it to Java Script.

I am sure there are better calculators out there I used method 100 with my calculator and clearly with other installation methods there will be errors.

However to design an installation to run right on the limits would be rather fool hardy and it does do a near enough job for me.

Some of the variables do seem questionable "Max permitted operating temp tp" will alter the "Correction factor Ct" which in turn will alter volt drop. I know this is the formula given in BS7671 but personally I just can't see how using different insulation on a cable can mean the volt drop changes?

Since building the calculator I have not done enough to really test it the point is of course we want to ensure once complete and we test and inspect that it will fall into the laid out limits.

However if on the edge then the next question is how accurate are our testers. Take any tester and measure PSCC three times and each result will differ. 10 amp on the PSCC can mean 1 volt extra or less on volt drop and that is both for the source and destination so best will in the world if the volt drop is calculated at being 2 volts over it would be hard to say if that was due to measurement error or design error I would not want to fail an installation if just 2 volts over the limit.

As to loading pick up a laptop power supply and it will say something like 2.4A yet I have seen school laptop charging trolleys with 20 laptops being charged from a single 13A lead without blowing the fuse. As you say switch mode power supplies cause some odd results.

 

[ban-all-sheds]

Some of the variables do seem questionable "Max permitted operating temp tp" will alter the "Correction factor Ct" which in turn will alter volt drop. I know this is the formula given in BS7671 but personally I just can't see how using different insulation on a cable can mean the volt drop changes?

If different insulation, e.g. XLPE, allows a higher conductor temperature then the resistance of the conductors will be higher and the volt drop will be higher.

If you look at the thermal coefficient of resistance of copper and see by how much resistance increases at 90° vs 70° you'll see it's in line with the different volt drop figures in the tables for 90° & 70° cables.

 

[JohnW2]

Some of the variables do seem questionable "Max permitted operating temp tp" will alter the "Correction factor Ct" which in turn will alter volt drop. I know this is the formula given in BS7671 but personally I just can't see how using different insulation on a cable can mean the volt drop changes?

If different insulation, e.g. XLPE, allows a higher conductor temperature then the resistance of the conductors will be higher and the volt drop will be higher.

Indeed - IF running at the maximum permitted temperature. However, as often discussed, what none of us seem have a clue about is whether the flow of a (BS7671) 'maximum current' (for the cable type and installation method) will, in truth, actually get the conductor temp anywhere near the 'maximum rated' figure.

For the calculation of 'worst possible case' VD, I suppose it's fair enough that the regs assume that the conductor is at maximum permitted temp - but whether or not that can/does ever happen with BS7671-permitted currents is something I just don't know. Do you have any knowledge/information about this?

Kind Regards, John

 

[ericmark]

My thanks you have highlighted another error in my calculations. It was if a = 2 and b = 4 then a+b = 24 where what I wanted was value of a + value of b which = 6 as a result my correction factor Ct was working wrong way around.

Now corrected. This also means all the values given are also out.