Volt drop maths

[Dan Carroll]

Is the silence a good thing or a bad thing

 

[James]

What does your friend who is signing the work off think of it?

There are many knowledgeable people here who will gladly give there time up to assist in your learning.

This may not be the case in this instance but,
Quite a few diy, trainee, or chancers of some sort post things like this with the hope that they will get an endorsement from some qualified sparks.
Most have a “friend” or other contact who will be signing it off.

The reality is that this imaginary signature giver should be involved from the beginning to design, specify and oversee the work from start to finish.

It is not possible to have someone come in after it is installed to “check it over and write the certificate “
When you are signing an installation certificate, you are confirming that you are responsible for the design, installation and testing of it.

That, I am afraid is why the thread has gone a bit quiet.

 

[Dan Carroll]

OK well I appreciate what you are saying.
Allow me to say.
I completed my 2365 level 3, 18th and part p course last year. I am qualified enough now to actually become a certified domestic installer. I haven't because I want to get the NVQ and thought that I would be working for someone by now and therfore it would be wasted money for something in not using. I might revisit that option.
I come from a background of 16 years as a mechanical helicopter engineer, where safety is paramount. We were trained to never be afraid to ask a question. Even when we were trained to do all the jobs, there is a vast difference between a book and real world answers.
One person doesn't know everything and never be afraid to challenge those above you. They may have been making the same mistake with out realising it for years.

I do fully understand what you are saying, and I imagine there are people out there who would use this as authorisation to do the work.

My friend is happy with the design.
With the training I've had, I'd be happy to sign it off.

Please don't take this message as a rant or moan.
All I was after was some guidance that I'm going down the right track and my friend is correct as well. I'm not after someone to do the work for me.

 

[EricMark]

I did a quick calculation and get around 44.85 meters, correction factor Ct of 0.8854166666666666 yes I used java script which is why such a long number, but without the correction factor you will not get a true reading.

The formula for the correction factor is not simple, and I realised too easy to make an error, so by writing a program using java script it allows one to enter know figures, we know a ring final allows 106 meters and uses 26 amp as the design current Ib. So since the program gets that right, there is a good chance I have got the maths correct.

 

[Mark Wright]

I did a quick calculation and get around 44.85 meters, correction factor Ct of 0.8854166666666666 yes I used java script which is why such a long number, but without the correction factor you will not get a true reading.

The formula for the correction factor is not simple, and I realised too easy to make an error, so by writing a program using java script it allows one to enter know figures, we know a ring final allows 106 meters and uses 26 amp as the design current Ib. So since the program gets that right, there is a good chance I have got the maths correct.

Can you upload the source?

 

[EricMark]

The formula is in BS7671:2008 for working out the correction factor, I seem to remember involves square routes, as far as the 26 amp for ring final never found it written down, I was attending IET lectures when BS7671:2008 came in and in the lecture it was stated moving from the 4% for all to 3% lighting and 5% power resulted in the length of a ring final going from 80 something meters to 106 meters, however try as I could I could not get this 106 meters, so I asked and was told we consider 20 amp draw at centre and 12 amp even spaced so use 26 amp as design current. I have never found this written down, but it does give the 106 meters of 2.5 mm cable being just within volt drop limit.

With lights of course there is nothing to say 6 amp is the design current, it could well be considered as 3 amp if all lamps even spread, so 3 amp at 3% volt drop would allow 91 meters of 1.5 mm cable. If the DNO loop impedance is 0.35 ohms the end of run loop impedance should be 3.25 ohms.

However in real terms since only line goes to switch, not that easy to work out the limit.

My worry was if I do an EICR and say all is OK and there is excessive volt drop, could some one at a latter date say I got it wrong and try to claim I had not done the EICR correctly, or PIR as it was called then, however one there are laid out forms to show an EICR results and also set criteria as to issuing C1, C2, C3 etc. And unless volt drop was massively over limits it is unlikely to be picked up on.

My original idea was to include the calculations in the test results to auto flag up if non compliant, but a loop impedance meter is not that accurate, test three times and you will normally find 0.02 ohm variation, this is doubled up when also measuring the DNO at head loop impedance, 0.04 ohms makes a big difference to volt drop calculations, and if for example it was found you had 130 meters instead of 106 meters, all you need to say is worked at at 13 amp at centre and 19 even spread so using 22.5 amp as design current, and no one can say that's wrong.