TRANSFORMERS - loading etc

[happyhero]

Hi can anybody advise me. We are looking into doing quite a bit of garden lighting at low voltage, and I heard some where to be aware of loading the transformer correctly ie obviously not to overload but also not to underload it, if there is such a word. I think it said something about not doing the transformer any good. If this is so what percentage should I aim to load the transformer to so that I can select correct transformer sizes? What harm does underloading it do anyway?

Also is it ok to mix wattages of fittings on a single transformer, eg could I have 5x20w lamps, and 5x10w lamps and 1x50w lamp on a 200w output transformer?

And lastly does anybody have any guidance on maximum cable run lengths so as not to get any significant volt drop or any other problems?

Any help appreciated please.

 

[Adam_151]

You should consult the manufactorers specification as to what the minimum loading is

Volt drop calculator: http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html (make sure to change the default voltage)

No problem with maxing wattages, but I would suggest everything is split into serveral branches, or even from serveral transformers, or you'll end up with cables as thick as your arm to keep the voltdrop in spec

Oh, and this is notifiable work btw, so be sure to tell your LABC before you start...

 

[RF Lighting]

Most transformers tend to have a minimum value printed on them as well as the maximum i.e. 20...60 VA. Be careful as significant voltdrop will occur in a reasonably short length of cable. Also another thing to consider is that much higher currents are used by low voltage fittings compared to normal mains fittings. Your example setup of 200W of lighting would pull over 16 amps requiring at least 2.5mm2 cable.

Rob

 

[happyhero]

Most transformers tend to have a minimum value printed on them as well as the maximum i.e. 20...60 VA. Be careful as significant voltdrop will occur in a reasonably short length of cable. Also another thing to consider is that much higher currents are used by low voltage fittings compared to normal mains fittings. Your example setup of 200W of lighting would pull over 16 amps requiring at least 2.5mm2 cable.

Rob

Thanks for the answers. Following up my example setup, so what would you do to improve situation. I thought you used special cable for LV lighting that you have connectors that twist and bite in to make a connection a bit like scotch locks, so what can you do if you need bigger cable? Can you just buy this special cable in different thicknesses and it works with all fittings or do they only take one size? With a house fitting you have some scope for different cable sizes ie as long as it fits in the hole and you can tighten the screw on to it but how do you achieve this on these twist and bite LV fittings? Is there a way to use other cables with these fittings, ie surely you could not use something like Twin and earth or flex?

I read somewhere that you should try to load the transformer 80% upwards for best performance.