Lighting Circuits......

[Toneyz]

Without going into calculations, at what length of the main run from the D.B. on a lighting circuit do you get a gut feeling that it needs to be upgraded from 1.5 to 2.5 cable more referring to singles than T & E. 6/10A type C mcb LED lighting low design current so more of a V.D max loop etc.

 

[Toneyz]

If my gut feeling was that the first light was a bit of a stretch then I’d install say 2.5mm singles to the first point then perhaps 1.5mm to the other points however I’d be estimating the run and writing down some design calcs as well , as quite often it’s the max zs you need to watch when installing long runs especially if it’s a type c mcb.

And at what point mtrs would that gut feeling kick in ? for you .

 

[Andy78]

I think the OSG has a ready reckoner table giving maximum lengths of standard circuits using the method Strima mentioned.

I have installed 4mm in a large lighting circuit before after doing the VD calcs. Was a bit of a fiddle to terminate but with deep boxes was OK in the back of a Klik plate.

 

[westward10]

Must admit I have done so many Zs readings over the years I am forever guessing the reading before I take it. If I was putting in some lighting runs across a warehouse I would look and think yeah that needs 2.5 and maybe 4.0 over there, couldn't give you a distance though. Went to a huge logistics warehouse last year and someone had run a new emergency lighting circuit snaking around six mez levels, all one circuit in 1.0 T&E can't remember the exact Zs but it was over 12 ohms!

 

[Toneyz]

If my gut feeling was that the first light was a bit of a stretch then I’d install say 2.5mm singles to the first point then perhaps 1.5mm to the other points however I’d be estimating the run and writing down some design calcs as well , as quite often it’s the max zs you need to watch when installing long runs especially if it’s a type c mcb.

If you take what the iet design guide says then if the load is evenly distributed along the circuit the voltage drop is reduced.
A lighting circuit with evenly distributed luminaries the average current is half the total load current
Max volt drop
6.9x1000/(Ib/2) x (mV/A/m)

At last, to what I am getting at there is a point when it kicks in followed up with some rough calcs.

 

[Pete999]

Not relevant to the question that I am asking sorry Pete to be blunt to you Pete ( with respect). We are not all robots to what something is calculated off site getting a cable A to B is not always a straight line as it is on paper we must all get to have a experienced view when looking at a practical run.

You haven't answered my question Ant, which

At last, to what I am getting at there is a point when it kicks in followed up with some rough calcs.

A bit condescending there Ant, if I may be so bold.

 

[Toneyz]

Must admit I have done so many Zs readings over the years I am forever guessing the reading before I take it. If I was putting in some lighting runs across a warehouse I would look and think yeah that needs 2.5 and maybe 4.0 over there, couldn't give you a distance though. Went to a huge logistics warehouse last year and someone had run a new emergency lighting circuit snaking around six mez levels, all one circuit in 1.0 T&E can't remember the exact Zs but it was over 12 ohms!

Thanks for answer Westy could of looked on paper for calc ok for 1.0mm at some stage an electrician pulling that in alarm bells/experience should of kicked in.Your point regarding warehouse lighting was the exact answer that I was trying to gain you looked at length and from experience what was thought to be 1.5 should be 2.5 .

 

[Toneyz]

You haven't answered my question Ant, which

A bit condescending there Ant, if I may be so bold.

It wasn't intended to be the view that I was trying to gain was what ever has been designed or could be recalculated when installing a circuit things such as this is a long run for this cable start to go through your mind, if you have installed that length of run a few times in the past and after testing it is well within next time you that experience helps you, for example a 40mtr run may have been coming on the high side for Zs when you come to install another circuit at a different location you start to think this needs to be 2.5 when looking at the run.

 

[Darkwood]

Anthony - I can see what your original question was trying to aim at but you barred us from using any calcs and only rule of thumb, considering you have stated singles then surely you must realise how this singles is contained and grouped has a very big bearing on the answer even for an approx' rule of thumb or experienced guess.

Example - if the containment was metal with a good surface area and provided a designed earth path then compliance with ELI would be less of a worry, volts drop also has some flexibility at the installers discretion.

This is why I believe you asked a question that was relatively difficult to give a straight answer to given the lack of info, and why you got probed for more info' - if I was to browse using my experience and judge such an install in order to make a guess (no calcs) then I would still take into account installation methods in order to weigh up a reasonable guess, it can make a big difference to the guess.

 

[ipf]

Anthony - I can see what your original question was trying to aim at but you barred us from using any calcs and only rule of thumb, considering you have stated singles then surely you must realise how this singles is contained and grouped has a very big bearing on the answer even for an approx' rule of thumb or experienced guess.

Example - if the containment was metal with a good surface area and provided a designed earth path then compliance with ELI would be less of a worry, volts drop also has some flexibility at the installers discretion.

This is why I believe you asked a question that was relatively difficult to give a straight answer to given the lack of info, and why you got probed for more info' - if I was to browse using my experience and judge such an install in order to make a guess (no calcs) then I would still take into account installation methods in order to weigh up a reasonable guess, it can make a big difference to the guess.

In other words.......it's guesswork without involving a bit of calculation and info.

 

[Toneyz]

Anthony - I can see what your original question was trying to aim at but you barred us from using any calcs and only rule of thumb, considering you have stated singles then surely you must realise how this singles is contained and grouped has a very big bearing on the answer even for an approx' rule of thumb or experienced guess.

Example - if the containment was metal with a good surface area and provided a designed earth path then compliance with ELI would be less of a worry, volts drop also has some flexibility at the installers discretion.
Yes of course you would follow up with calculations and I was referring to one type of circuit as many electricians on here must of installed at some point. If on a proposed circuit chart it is shown 1.5 and you are looking at the run and you are going to be more than half of that drum to the first light in that room past experience/thoughts go through your mind to rethink/check.
This is why I believe you asked a question that was relatively difficult to give a straight answer to given the lack of info, and why you got probed for more info' - if I was to browse using my experience and judge such an install in order to make a guess (no calcs) then I would still take into account installation methods in order to weigh up a reasonable guess, it can make a big difference to the guess.

Post edited to highlight response - re' Darkwood.

 

[Toneyz]

In other words.......it's guesswork without involving a bit of calculation and info.

Experience is different to guesswork and experience would be followed by calculation.

 

[ipf]

Experience is different to guesswork and experience would be followed by calculation.

No....experience is a great string to your bow, but in this simple case, calculation would replace guesswork.

 

[Lucien Nunes]

There are situations where gut feeling is helpful, maybe essential. Complicated tray / trunking / conduit bends etc, where calculation is either impossible or unhelpful as far as producing a good end result. Assessing suitability of a cable gauge is not one of them, since it takes all of 10 seconds to work out the correct answer to a much higher level of certainty than any gut feeling can provide. You do not rely on gut feeling to work out 6.7 x 15.2 ... you work it out using either long multiplication (if you need the full accuracy in the answer) or a numerical method of approximation (if you don't). And so it is with long cable runs. I carry around one figure in my head, which is the resistance of 1m of 1.0mm², and do the necessary multiplication and division to get a first-order accuracy on any VD while pacing out the run. If more accuracy is needed then I go to the specific VD in V/mA/m for the cable in question. If someone proposed to run 250m of 1.0 for a 1kW load, yes gut feeling would take over. But 50m for a bunch of lights, I would do the maths.