12v 20 watt downlights on fused spur confusion

[jazzyjeff]

i have fitted 5 x 20 watt (12v) downlights supplied from a fused spur (off a socket) in our kitchen including two transformers (max 4.9A) and a block connector to divide the power into the two transformers, i calculated that 8.3 A would be the current used and so replaced the spurs 5A fuse with a 13A... the lights work fine but is this right? also 8.3A seems a lot considering it is supplied off 32A breaker and also the whole light circuit is only a 6A breaker i am a little confused any input would be appreciated. cheers......[/b]

 

[OwainDIYer]

I think you're confusing Amps at 12V with Amps at 240V.

 

[jazzyjeff]

thanks i know wha you mean sort of but can you explain the calculations to me im just a bit unclear..

 

[ban-all-sheds]

5 x 20 watt = 100W.

100W @ 230V = ________

 

[Taylortwocities]

5 x 20 watts = 100watts

from ohms law

Watts = Volts x amps so

Amps = watts ÷ volts

Amps = 100 ÷ 12 = 8.3 amps

But that is the current on the 12 volt side
Your fuse will be on the 240 volt side.

So to calculate the fuse (assuming no losses in the transformer - there won't be enough to worry about) your calculation is realls

Amps (primary) = 100 ÷ 240 = 0.41 amps

so a 3 amps fuse is your fella.

PS The 8.3 amps is a significant figure when you start looking at how big the wire needs to be from the transformer to the lamps, especially if it is over any sort of distance.

PPS I always like to give a full response, rather than a terse one

 

[OwainDIYer]

I like to encourage people to worth the answers out for themselves through Socratic dialectics.

 

[jazzyjeff]

thanks mate appreciate your reply, very informative to a non-electrician

 

[ericmark]

What Taylortwocities says is close enough for most purposes pity others can't be that helpful.

It's not quite all there is to it but close enough. Most inverters and transformers will have some inrush when switched on and most fuses will allow a degree of overload so in most cases the two things cancel each other out. Also Watts = Volts x Amps x power factor correction but in most cases the power factor correction is nearly unity anyway so it does not matter.

But some times you do get 3A fuses to blow with less than 3 x 230 = 690 Watts. It is extremely unlikely with 100W one will get enough power factor or inrush problems to cause a 3A fuse to blow but other will read the post and relate it to their problem so thought it is worth saying why it is not always true.

Because of power factor you will find many items rated in VA rather than Watts. At unity power factor Watts = VA and in most cases one can consider VA to equal Watts. You may find the inverter/transformer ratted in VA rather than watts.

In general terms an inverter does same job as a transformer. In real terms an inverter turns the AC to DC charges a capacitor then turns it back to AC at a high frequency, Because of the high frequency it can use a very small transformer. It will then normally sample the output and alter the mark/space ratio to correct the output voltage. This is also known as switch mode. So not only does it reduce the voltage it also corrects the voltage to very close limits. The whole idea of using a quartz envelope on lamps is to use very high temperatures. If the quartz is hot enough then the tungsten will not be deposited on the quartz it will in turn return to the element. If too hot the element will burn out. So the whole idea is to maintain the temperature to within very fine limits.

As a result of course it would be folly to use a lamp dimming unit although they will sometimes work. And also the inverter should be kept as close to the lamp as possible to reduce any volt drop in the extra low voltage cables to a minimum. At 12 volt even 0.1 volt will make a marked effect.

So unless the lamps are very close one should consider using an inverter per lamp.

Where the mains voltage fluctuates then using extra low voltage and an inverter makes sense. But where the voltage is reasonable stable the extra cost of both installation and maintenance (As inverters do blow from time to time) tips the balance in favour of the low voltage (230 volt) lamps i.e. GU/GZ10 these unlike the extra low voltage version have a fitting that will allow a non Dichromic lamp (GU) to fit either a GU or GZ holder but the Dichromic lamp (GZ) will only fit a Dichromic holder. This is important as a Dichromic lamp allows heat to travel through the reflector so not over heating the item it's illuminating. But as a result of course gets things behind the lamp much hotter. Also it removes the problem in matching inverter to the lamp. Inverters unlike transformers often have not only a max output but also a min output.

Sorry didn't intend to write so much. But hardly worth deleting now written. Hope it helps you understand why we use extra low voltage (12 volt) lamps. Of course they are also in areas like bathrooms used for safety. Although one can some times replace a tungsten 12 volt lamp for a LED one (depending on min rating of inverter) one can't use cold cathode (florescent) so the extra low voltage is losing popularity.