Maximum cable length lighting circuit

[Mabro22]

Been out the domestic game for a long time, but now having to rewire my own house.

Was wondering what the maximum cable length would be for a 1.0mm lighting circuit with 100w of leds?
Is the max cable length the ‘maximum’ length of r1+r2 from conumser unit - up and down switch drops - to the furthest away light?

Any help would be Greatly appreciated.

Thanks

 

[timhoward]

How long do you need it to be?!
Assuming a TNS earthing system with highest permitted Ze and protection by a B6 breaker using 80% OSG Max Zs values, about 119m in theory.
Volt drop also needs checking but with the load you mention it wouldn't be an issue.

Yes, R1+R2 is measured at the furthest point in the circuit which will be the furthest light fitting (which will include it's switching path).

 

[Mabro22]

How long do you need it to be?!
Assuming a TNS earthing system with highest permitted Ze and protection by a B6 breaker using 80% OSG Max Zs values, about 119m in theory.
Volt drop also needs checking but with the load you mention it wouldn't be an issue.

Yes, R1+R2 is measured at the furthest point in the circuit which will be the furthest light fitting (which will include it's switching path).

Ive basically used 100m of 1.0mm, looped the feeds at each switch then switch wire to light.
Just confused with max cable length as it’s been a while.
The 100metres includes each switch wire, hence I asked if the max cable length was from consumer unit to last light fitting (including switch drops)
Thanks again

 

[Deleted account]

Basically the maths will let you know.

on site guide will give you the milli ohm per meter value for each cable size.

multiply that by the length you need will give you the total ohms. Add the Ze bobs your uncle.

 

[DefyG]

And 2 way switch circuits can make a big difference (eg 3 story house with intermediates on each landing eats up the cable

 

[timhoward]

And 2 way switch circuits can make a big difference (eg 3 story house with intermediates on each landing eats up the cable

and the opposite applies if you use Quinetics wireless switches for these situations, they can save a heap of cable (but are a fair price)

The 100metres includes each switch wire, hence I asked if the max cable length was from consumer unit to last light fitting (including switch drops)

It's the complete current path of the R1 including switching. As you've looped at the switches, the R1 path is from consumer unit via each switch and to the last light fitting, but not including the cable from every other switch to every other light fitting. Same for the R2 path back. So it won't be as bad as the total cable used multiplied by the resistance / meter.

 

[Mabro22]

And 2 way switch circuits can make a big difference (eg 3 story house with intermediates on each landing eats up the cable

Yeah, I’ve used 100metres in total.. longest run including all switch wires to the last fitting is 55metres.. that sound ok?

 

[Mabro22]

and the opposite applies if you use Quinetics wireless switches for these situations, they can save a heap of cable (but are a fair price)


It's the complete current path of the R1 including switching. As you've looped at the switches, the R1 path is from consumer unit via each switch and to the last light fitting, but not including the cable from every other switch to every other light fitting. Same for the R2 path back. So it won't be as bad as the total cable used multiplied by the resistance / meter.

Understood. Thanks a lot for your help, much appreciated

 

[DefyG]

Yeah, I’ve used 100metres in total.. longest run including all switch wires to the last fitting is 55metres.. that sound ok?

If your longest run is 55m including the switch circuit for the 'furthest' point then yes sounds alright as OSG states about 120m [post #] but double check it with a MFT.

 

[buzzlightyear]

multiply that by the length you need will give you the total ohms. Add the Ze bobs your uncle.

a what about aunty nelly will she work that out .

 

[davesparks]

Yeah, I’ve used 100metres in total.. longest run including all switch wires to the last fitting is 55metres.. that sound ok?

It sounds like you've already installed the cable, so you can just test it to check compliance.