Calculating Voltage Drop On Lighting Circuits

[Marcus Vaughan]

Hello All,

I should know this and have been avoiding asking but just want to be sure.

So I am designing a downstairs lighting circuit for a domestic refurb/rewire.

I reckon max demand will never hit 3 amps.

So using the basic formula for voltage drop - (tabulated value x design current x circuit length) please put my mind at ease;

1) When imputing the design current, I can apply diversity to maximum demand first - right?
2) And circuit length - I take this to mean the total amount of cable used, not just the furthest point - right?

Could make a difference as to whether to use 1mm or 1.5mm.

Thanks in advance!

(please don't judge me for asking - just feeling a little this morning)

 

[Richard Burns]

For a domestic lighting circuit I would usually expect that there would not be a problem with volt drop.
The table 7.1 of standard circuits in the OSG with a B6 on 1mm² shows that the limiting volt drop is reached at 59m of circuit length, which is excessive for most domestic properties.

The design current is the expected current that will be drawn so diversity is applied. In your case you say you expect max demand at 3A so perhaps 10x70W lamps, quite high for a modern install but OK, say half the lights are on at any one time (all the lights if they have children!) so a design current of 1.5A.
If the circuit is just a small domestic circuit then approximately calculate the maximum length of cable (L) from CB to most distant light, including switch drops, and calculate your volt drop from Vd = Vd * A * L /1000. Total length of cable would only apply if the circuit is a straight line point to point to point radial with no branches.

If you have a more complex installation, as it sounds like you are describing, then you could, if necessary, break down the circuit into sections carrying the same current and calculate the volt drop for each section and add them together. Such as in the below diagram.

 

[buzzlightyear]

Shirley ,with all those awards from C/G .and you cant diced weather
1.00 or 1.5 cable to put in .pleases take me in a field and shoot me.

 

[Ian1981]

 

[Marcus Vaughan]

Thank you Richard - very helpful and definitely appreciated. Top drawer.

That all makes sense. I did make up a spreadsheet a few months ago for cable calculations - and I have attached it for this circuit. Actually gives volt drop at 2.2 volts so I am procrastinating for nothing. It's an early draft but does help me.

It was the circuit length that threw me - as every example in my training just showed a dedicated circuit for a shower or some such like. So I thought I'd better check to make sure I knew what I was doing, a bit embarrassing but I didn't want to let pride stopping me asking.

The circuit I am looking at isn't massively complicated - and the loads are teeny of course - so probably no need to break it down, but a great example for the future.

Excellent.

 

[Marcus Vaughan]

Shirley ,with all those awards from C/G .and you cant diced weather
1.00 or 1.5 cable to put in .pleases take me in a field and shoot me.

Do you know what Buzz, I am going to have to take it on the chin, and a wry smile has crossed my face as I suspected someone would make comments such as yours. I was tempted to kick the cat in embarrassment/frustration but then remembered I am a good egg really, and its not his fault, and actually only horrible people kick cats.

Oh - and it's decide - not diced.
Oh - and it's whether - not weather.
Oh - and it's please - not pleases.

Just kidding.

I do feel like a bit of a dufus though (is that a word?), and am perfectly capable of chastising myself ('Shoot me now' is pretty much what was going through my head as I started this thread).

Have a good day!

 

[Marcus Vaughan]

View attachment 42715

Thanks Ian,

That's helpful - I like the way the example re-arranges everything to give maximum cable length allowed. I know there are tables for such things but still good to understand the Maths.

I've saved that one on my PC.

Great.