Adding Outside Lights to Existing Tranformer but is it Powerful Enough?


Postby transform » Sun Apr 23, 2017 4:17 pm

Hi.

I have 4 outdoor ,20w mr16 lights, hooked up to a low voltage transformer.

I have bought another 3 lights and was wondering if I can attach them to the existing transformer or will I need to attach them to a new transformer. Is it powerful enough!

Transformer details:

Input: 230v-240Vac 50Hz
Output: 12v-8.75 A 105VA

Many thanks!
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Postby ericmark » Sun Apr 23, 2017 8:48 pm

7 x 20 = 140 you have 105 simple maths no you can't power them from the same transformer. 105 / 7 = 15 normally rated 35 ~ 105 so 35 / 7 = 5 so if you used LED lamps between 5 and 15 watt it should work as long as they are designed for that transformer, many G5.3 lamps are marked 50 Hz and many electronic transformers give out power in the MHz range so they may not work.

In the main people today shy away from extra low voltage because of compatibility issues, today most use low voltage with GU10 bases.

Note
MR16 is multifaceted reflector 16/8th inch across does not matter which base.
Extra low voltage under 50 volt ac.
Low voltage 50 to 1000 volt ac.
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Postby kbrownie » Mon Apr 24, 2017 6:36 am

The existing transformer is under rated for the required load, each lamp require 20W, the existing transformer can only deal with 105W.

So a maximum of 5 X 20W lamps only, then I would also recommend leaving a 15% to 20% excess on the transformer demand.
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Postby transform » Mon Apr 24, 2017 8:42 am

Thanks guys.

Im not a electrictician, so didn't really understand the info on the transformer, as it didn't specifically say watt, so I was unsure.

Many thanks for your help, I shall just rig them up to the additional transformer.

Cheers
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Postby kbrownie » Mon Apr 24, 2017 9:43 pm

105VA is equivalent to the maximum demand/load in watts the transformer can deal with. But it would be foolish to have the transformer working at maximum output, I would recommend that you leave at least 15% unused.
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Postby ericmark » Mon Apr 24, 2017 10:49 pm

Watts = Volts x Amps x Power factor correction, in the main the power factor is around unity so VA and Watts means nearly the same, I do think it is poor how the transformer is rated in VA and the lamp in Watts, but the EU rules say the bulb has to be labelled in watts, and to ensure a transformer is not overloaded it has to be labelled in VA. So typical electronic transformer is 105 VA and two G5.3 bulbs are likely 50W each so there is 5 VA to spare in case PF is not unity.
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