Hope i can put it into words?!

[So much to do]

Hello learned leccies and the like new on here so treat me gently

As i have said ill try and put it into words so here goes.

I would like to wire up 4 lots of three 400 watt luminaires.
ie

3 luminaires on switch one
3 luminaires on switch two
3 luminaires on switch three
3 luminaires on switch four

now the trickie bit the luminaires are mounted 20ft of the ground so i would like to use the minimum ammount of cable to achive the task
a loop in out circuit would be to much work as the switch wires would have to run about 100 ft to the switch i am stumped any ideas?

thanks in anticipation

working still!

 

[breezer]

assuming all switches are in same place, run a cablbe from each light to the switch, then to the supply.

not for a foot ball pitch / similar is it?

 

[Panjandrum]

Any objections to linking each set of three in parallel back to their switch? That sounds like a minimum-cable solution.
Edited to say that of course cable and switching will need to be up to the load.

 

[So much to do]

thanks guys

Thats sort of my thinking but i will only have one incoming cable to the switches how to terminate neutral side
do it inside the back box of the switch gear? or would you suggest 3 incoming supplies?
Again the breakers rcd kit are another 30 meters from the switch gear?
1200 watts per switch x4 one supply cable what size breaker or cable for that matter

 

[breezer]

you still have not said what it is for

 

[So much to do]

Sorry its for iluminating a very large barn about half the size of a footie pitch

 

[breezer]

this may sound silly but is the barn to be used by animals or is it a "barn conversion"

 

[steve190]

Is this a single or 3 phase installation?

 

[Big_Spark]

I do hope that you have calculated the cable size correctly, taking into account Volt drop etc.

The lengths of cable you are likely to use will have significant volt drops with the type of loading you are envisaging.

Personally I would recommend you call in a spark and have the job done properly.

 

[So much to do]

Only for animo's unless i end up kipping out there no different to animals i guess????

Single phase i am afraid

yea thinking that to cable size / run is the main problem being only a mere diy'er thats the problem and the main reason for asking the Question

 

[breezer]

as you are using it for animals, i think you will find that comes under a "differnt set of regs", something to do with earthing, cable protection.

can any one else confirm / deny this?

 

[securespark]

Is this FWL I see before me????!!!

 

[felix]

A quick calculation shows that you have a total of 4800 watts. This is way over the limit for a lighting circuit. You will need a supply of at leat 20 amps and that's pushing it. I suggest you start with a thirty amp supply with 4mm cable. That's from the consumer unit to the switches. Having got there you can't just wire up your lights with 1mm cable. You need more fuses. If the switches are in another building you need another consumer unit with four 5 amp fuses - and you have to figure out how to get the cable to it. You can use conduit along a wall - most definitely NOT a fence - or you might have to go underground with armoured cable. If everything is in the same building you can use switched, fused connection units for your lights. Put 5 amp fuses in these and you can carry on to the lights with 1mm cable.

Now for the voltage drops. I think I'm right in saying that you can only afford to lose 4% of your supply voltage; that's about nine volts. (If that's wrong I'm sure somebody out there will correct me.) From the resistivity of copper, 1mm cable is 32 milli-ohms per metre (round trip) and 4mm is 8. The drop in your 4mm run will therefore be (30 x 8 x 20) mV or 4.8 volts. That's about half your allowance gone. What happens after the switches is less clear but you seem to be saying that the furthest light is about 30 metres away. If you run a 1mm cable from a switch to the the first light, then on to the second and end at the third you will have a drop of no more than (30 x 32 x 5) = 4.8V as before. This is one of those annoying cases where you're right on the limit so you need to check carefully.

Increasing the big cable to 6mm would get this drop down to 3.2V. If it's in the same building you can also use two runs of 2.5mm cable as a ring main. I know that's twice as much cable but 2.5mm is cheap. If the 1mm wire has to run up to each light and down again then you would probably be better off with a separate wire for each light. Maximum drop here would then be 1.6V. Another possibility is to add one more cable from the last light back to the switch to form a ring. The best layout will depend on the actual positions of your lights and switches so I can't give you much more advice here.

What you need is some practice with Ohm's law; "voltage = current x resistance". The other law you need is "power = voltage x current". These will enable you to work out your own voltage drops.

 

[Lectrician]

He needs a spark.

Hearing 1mm so many times with this sort of install.....umm.

I would prefer to see 1.5 or even 2.5 to the lights, and a decent submain....how long before he wants a socket aswell