measuring cable
- Thread starter AS
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it depends on which calculations
earth fault disconnection calculations should be based on the longest path any live conductor takes from the CU (including going via the switches etc).
volt drop calculations should be based on where the load actually is on the cuircuit. However using the length of the run to the furthest fitting is usable as a conservative estimate.
earth fault disconnection calculations should be based on the longest path any live conductor takes from the CU (including going via the switches etc).
volt drop calculations should be based on where the load actually is on the cuircuit. However using the length of the run to the furthest fitting is usable as a conservative estimate.
Yea, unless you have insanely long switch drops, the measurement to the furthest fitting is usually fine.
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Not unless you are right on the limit - what are the other numbers? The switch drop length only affects the one light it controls of course - dont include the switch drops to lights that are not the furthest from the CU.
See the cable guide in 'for reference' section for the limiting numbers.
Note that the 4% voltage drop is a recommendation that applies if you can't show that the equipmnet will survive greater droops (and for sockets, where you don't know what will be plugged in its most apropriate). Filament lights generally can survive a greater drop, and in any case, not all the lights are at the far end of the circuit normally. (The last light is usually only 100watts or so, unless its a special fitting)
So in practice the length you get from 1mm cable, and 6A load is very conservative at 45m, and can be doubled in most houses before problems set in..
hope that helps
M.
See the cable guide in 'for reference' section for the limiting numbers.
Note that the 4% voltage drop is a recommendation that applies if you can't show that the equipmnet will survive greater droops (and for sockets, where you don't know what will be plugged in its most apropriate). Filament lights generally can survive a greater drop, and in any case, not all the lights are at the far end of the circuit normally. (The last light is usually only 100watts or so, unless its a special fitting)
So in practice the length you get from 1mm cable, and 6A load is very conservative at 45m, and can be doubled in most houses before problems set in..
hope that helps
M.
If you can bear with me so I can work out how this works!
Ok have measured 1 of the lighting circuits with own 5A fuse in 1.0mm
Total cable in circuit is 56m
Goes like this
CU to JB1 is 2.1m then JB2 with switch cable 60cm and light cable 1m
Feed from JB2 to JB3 is 4.7m at JB3 feed splits and 2 of 60cm feeds go to large JB4
From JB4 comes 2 light cables 6m and 8.95m operated from 1 (3core) switch cable 5.8 m long.
Also from JB4 come 3 light cables 9m, 7m, 4.8m and 1 switch cable (3 core) going to 2 gang switch 8.1m long.
So feed length total to last JB4 is 4.70m (not including+60cm+60cm where feed splits)
Longest light cable from JB4 is 9m
Longest switch cable from JB4 is 8.1m
If I add 4.70m,60cm,60cm for feed and 9m (being my longest cable length from JB4) = 14.9m is this the figure I am looking for or is it total curcuit cable length 56m?
Does this circuit sound fine to you 6 lights at max 100W
Having read other posts - do I have the leway to embed part of these cables in between floor thermal insulation?
The cable lengths come from the fact they wind down underfloors, etc from attic, the house length is actually only 11m. So the light furthest from the JB is not necessarily the one with the longest cable. But I guess we are looking at cable length.
Ok have measured 1 of the lighting circuits with own 5A fuse in 1.0mm
Total cable in circuit is 56m
Goes like this
CU to JB1 is 2.1m then JB2 with switch cable 60cm and light cable 1m
Feed from JB2 to JB3 is 4.7m at JB3 feed splits and 2 of 60cm feeds go to large JB4
From JB4 comes 2 light cables 6m and 8.95m operated from 1 (3core) switch cable 5.8 m long.
Also from JB4 come 3 light cables 9m, 7m, 4.8m and 1 switch cable (3 core) going to 2 gang switch 8.1m long.
So feed length total to last JB4 is 4.70m (not including+60cm+60cm where feed splits)
Longest light cable from JB4 is 9m
Longest switch cable from JB4 is 8.1m
If I add 4.70m,60cm,60cm for feed and 9m (being my longest cable length from JB4) = 14.9m is this the figure I am looking for or is it total curcuit cable length 56m?
Does this circuit sound fine to you 6 lights at max 100W
Having read other posts - do I have the leway to embed part of these cables in between floor thermal insulation?
The cable lengths come from the fact they wind down underfloors, etc from attic, the house length is actually only 11m. So the light furthest from the JB is not necessarily the one with the longest cable. But I guess we are looking at cable length.
More than fine for 600W of combined load (say 2 and a half amps between friends), the voltage drop across the length carrying all the current is the only the first 2.1 metres, then it splits (correct me if I am wrong) the first hundred watts gets off at 2.1m +1.6m The remaining 500watts continue to JB3, where another 100watts leaves and the to go to the second light and the rest (now only 400W ) goes via JB4. From JB4, if the longest switch run controls the furthest light, then add them, and add them to the feeds beteen CU and JB4, BUT, remember that most of the cable to JB4 is not carrying the full load.
If not already you will be seeing why we don't normally do these sums in too much detail...
OK, switch controlling it plus + longest lamp line + feed to JB4 grand total is 14.9m + 5.8m switch ~about 20m.
You have stacks in hand my friend. At 600W the max length for voltage drop would be nearly 100m, and that assumes that the long length caries all the current, here it doesn't. From a disconnection point of view you are well in side the limits too.
Insulation wise, if 1mm cable is dangling in the open it is rated at well over 10A, this derates to about half if it is stuck between insulation and a wall or ceiling. Only the 2.1m length is carrying the full load, and this is only circa 2.5A, once again you should be well inside the safety limits, by nearly a factor of two. -Have a look in the Kevinborn guide in the for reference section to see your installation method, but I'd be most surprised if you had any problems at all.
Regards M.
If not already you will be seeing why we don't normally do these sums in too much detail...
OK, switch controlling it plus + longest lamp line + feed to JB4 grand total is 14.9m + 5.8m switch ~about 20m.
You have stacks in hand my friend. At 600W the max length for voltage drop would be nearly 100m, and that assumes that the long length caries all the current, here it doesn't. From a disconnection point of view you are well in side the limits too.
Insulation wise, if 1mm cable is dangling in the open it is rated at well over 10A, this derates to about half if it is stuck between insulation and a wall or ceiling. Only the 2.1m length is carrying the full load, and this is only circa 2.5A, once again you should be well inside the safety limits, by nearly a factor of two. -Have a look in the Kevinborn guide in the for reference section to see your installation method, but I'd be most surprised if you had any problems at all.
Regards M.
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