Voltage Drop - help!!

[scotteng]

hi all,

i think i must be doing something wrong, im trying to work out cable sizes for some swa to supply some lights and double socket in a garden, but im getting crazy cable sizes due to the voltage drop...im sure im doing something wrong.

runnng swa from consumer box firstly to two lights 38m away via fused spur. then running another 12m to summer house to cater then for double socket and another fused spur to inside light and security light.

all run from 20amp breaker with rcd of course. burried cables

with only 3% volt drop allowed for lighting im fining it hard to get a sensible cable size!! the max demand is very low so that isnt affecting the cable size just the bloody voltage drop!!

what do you think i should do! (no sarcy comments please) )

 

[ban-all-sheds]

What is the load?

 

[LoveRocket]

1) As Banal has said, what load are you expecting? A few hundred watts for the lights, what do you expect to be plugged in? As long as you're not going to have a big heater or tumble drier down there, it shouldn't be huge.

2) Measure your supply voltage in the house. (Also measure it with a heavy load like a shower to see how 'stiff' it is) If you've got a good 240+ volts, then don't worry about the daft 3% limit for lighting circuits.

You've got to be realistic with things like this, and assuming a healthy supply to your house, 2.5mm SWA would be fine. (or you may consider 4mm depending on the cost difference)

 

[ban-all-sheds]

Even sticking to the 3% VD limit, and assuming all the load is at the far end (which it isn't) 2.5mm² will handle nearly 1.8kW and 4mm² nearly 2.9kW, so I don't know what load you were assuming, scotteng.

However - 50m of trench? Do you ever want to have to dig that again if things change?

Stick in 10mm² or 16mm², and be done with it.

 

[scotteng]

that sounds much more sensible, i take it regs are based on full load then, and we can reduce the load depending on whats required?

can you just clarify how your calculating 4mm can take 2.9kw

cheers

 

[LoveRocket]

that sounds much more sensible, i take it regs are based on full load then, and we can reduce the load depending on whats required?

can you just clarify how your calculating 4mm can take 2.9kw

cheers

What Banal means is that 50m of 4mm SWA gives a volts drop of less than 3% with a 2.9kW load. - check it out here. (when using that calculator choose 2core even if using 3, as it assumes a 3-phase supply if you choose 3core)