Volt Drop

[switched on]

Hi I have a query one of you may know the answer to. I have installed some lights in the woodlands for a customer and he is using 12watt LED lights he is only installing a few i think it was 186 watts or something. I have used SWA cable 1.5 mm as it is about 200metres and worked out the voltdrop. He now wants to extend it another 100m and add on a few more lights!! I have told the customer that it will incur too much volt drop as the resistance on the cable.

For my own sake was wandering the dangers of volt drop obviously appliances wouldnt work correctly as the volt drop and the resistance is high so surely it cant do the cable any good.

Was wandering what you guys would do and if you have any past experiences or knowledge.

Thanks

 

[telectrix]

the only problem you'll have is a lower voltage. it won't do any damage to the cable, but the more distant lights might be a bit dim. depends on the minimum voltage the LED lights are happy with . if it's 220V you might just get away with it. rough figure with 1.5mm at the load you are at, is around 3-5 volts/100m

 

[ackbarthestar]

"......it is about 200metres and worked out the voltdrop. He now wants to extend it another 100m and add on a few more lights!!"

With many garden lighting designs you often end up extending the supply line until you are more or less back where you started from. With that in mind, can you run a separate cable to feed the new lights?

 

[telectrix]

or make it into a ring final them he'll have lights all round the garden

 

[ackbarthestar]

or make it into a ring final them he'll have lights all round the garden

Agreed, a ring would do the job, but if you supplied a radial with a larger csa supply then it might be used to supply other things in the undergrowth!:shades_smile:

 

[telectrix]

just more tat for the gardener to put a spade through.

 

[ackbarthestar]

just more tat for the gardener to put a spade through.

Its an acceptable risk for this type of job. A separate circuit will help with the fault discrimination.

He should also make sure that he gets the maintenance contract as well as the installation. :clap:

 

[JUD]

Just out of interest how did you work out the volt drop?

Remember that this is a distributed load and the amount of volt drop on cables will decrease as you get closer to the end of the circuit i.e the last length of cable will only carry the load for the last light so volt drop on that length of cable will be lowest, whereas the first length of cable will be carrying the load for all the lights and the volt drop on that length of cable will be highest.

It's a common mistake with lighting circuits to just work the volt drop out on the total load i.e 186W ÷ 230V = 0.81A, VD = (0.81 x 29 x 200) ÷ 1000 = 4.7V. If you break the circuit down and work the voltage drop out for each individual length of cable you will find that the total volt drop will be lower than this.

 

[ackbarthestar]

I agree with this, but remember as the load decreases the length increases.

 

[telectrix]

yeah, but as jud has so eloquently put it, that incresased length is only carrying the load of the last lights in circuit, so VD on thast length will be a lot less.

 

[ackbarthestar]

Yes but the volt drop is additive that means you add each section.

 

[telectrix]

yes, but as the load decreases the further you get, so the VD between each load decreases. a bit like watching the fuel gauge on a 4litre cherokee jeep

 

[ackbarthestar]

yes, but as the load decreases the further you get, so the VD between each load decreases. a bit like watching the fuel gauge on a 4litre cherokee jeep

Yes I agree that each section will have reduced volt drop. However the overall volt-drop seen at the end of the line will be the addition of each section.

 

[Engineer54]

Just out of interest how did you work out the volt drop?

Remember that this is a distributed load and the amount of volt drop on cables will decrease as you get closer to the end of the circuit i.e the last length of cable will only carry the load for the last light so volt drop on that length of cable will be lowest, whereas the first length of cable will be carrying the load for all the lights and the volt drop on that length of cable will be highest.

It's a common mistake with lighting circuits to just work the volt drop out on the total load i.e 186W ÷ 230V = 0.81A, VD = (0.81 x 29 x 200) ÷ 1000 = 4.7V. If you break the circuit down and work the voltage drop out for each individual length of cable you will find that the total volt drop will be lower than this.

This is the only way to calculate VD on a multi point constant load circuit, such as longer length lighting circuits. It will give you a much more realistic value on the total VD afforded to such a circuit.


Out of curiosity, is the SWA cable used in this installation direct buried?? If so, because of the long distance (greater chance of installation sheath damage) of the direct buried cable, have you done an overall sheath test??

 

[JUD]

Take a look at the lighting section of Table 7.1 in the OSG.

Maximum length for a 1.5mm² circuit on a 6A breaker is 90m.

Now work out the volt drop for 90m carrying 6A. (6 x 29 x 90) ÷ 1000 = 15.66V (6.8%). This is way over the 6.9V (3%) limit.

Now imagine you've got 11 x 100W lamps with equal lengths of cable between each (90m ÷ 11 lamps = 8.2m) and then work out the volt drop of the individual lengths of cable and add them together.

Your volt drop will be around 3% and this is how the lengths in the table are worked out.