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Low Voltage Transformer Quiery #143143 05/16/05 09:11 PM
Joined: May 2005
Posts: 12
B
Brite Spark Offline OP
Member
Hey everyone, I'm an aussie electrician and i was asked by my apprentice, how many 12V low voltage (50W) downlights can be put on single phase 10Amp circuit breaker. Since i have not encountered this problem in all my years, anyones help would be much appreciated.

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Re: Low Voltage Transformer Quiery #143144 05/17/05 05:29 AM
Joined: Dec 2004
Posts: 354
K
kiwi Offline
Member
G'day Brite Spark, I thought the watts at the lamps were the same as the watts at the circuit breaker, and the VA rating of the transformers took into account the losses. I suppose the best way of checking is by getting the current clamp out.

Next thing you should do is clip your apprentice round the ear and tell him to stop thinking about his cash-jobs on company time.

Re: Low Voltage Transformer Quiery #143145 05/17/05 06:30 AM
Joined: Jul 2002
Posts: 8,354
Trumpy Offline
Member
Hi there Brite Spark,
Welcome to ECN, mate. [Linked Image]
A standard electronic transformer draws about 250mA from a 230V supply.
And to my reckoning, that's 57.5W a piece.
Now, divide 2300W by that and you get something like 40 transformers. [Linked Image]
However, lets be realistic about this, no-one should load a circuit up to that extent.
You still need a certain safety margin.
Generally I wouldn't put any more than 20 on the one circuit.
There was at one time here in NZ, a maximum number of luminaires allowed on the one circuit, but I guess that's all gone now and I don't see that AS/NZS 3000 makes any mention of lighting apart from in a Maximum Demand scenario.
AS/NZS 3018(Domestic Electrical Installations) may have different ideas about that though.
Hope that this helps. [Linked Image]

Re: Low Voltage Transformer Quiery #143146 05/18/05 08:09 AM
Joined: Jul 2002
Posts: 8,354
Trumpy Offline
Member
Just a little point Brite Spark,
Was your apprentice referring to Electronic or the (older,supposedly banned) Wirewound type transformers?. [Linked Image]
The wire-wound type draw quite a bit more current, because of the Hysteresis current in the transformer.

Re: Low Voltage Transformer Quiery #143147 05/18/05 12:29 PM
Joined: Dec 2002
Posts: 1,250
D
djk Offline
Member
Trumpy,

The rules over here set a maximum of 10 points (lights or sockets) on any one radial. (you can't mix lights and sockets either. 10A for lighting, 20A for sockets)

Obviously, you're supposed to design the radial's load in such a way as to avoid overloading it too, but they're the maximum numbers allowed by the wiring rules.



[This message has been edited by djk (edited 05-18-2005).]

Re: Low Voltage Transformer Quiery #143148 05/19/05 12:59 AM
Joined: May 2005
Posts: 12
B
Brite Spark Offline OP
Member
Hello everyone, i normally load 10A circuit breakers at 2200W. i havent had any problems. 2300w is the max loading on a 10A breaker. Why would you only load the breakers at 50%?

Re: Low Voltage Transformer Quiery #143149 05/19/05 02:07 AM
Joined: Jul 2002
Posts: 8,354
Trumpy Offline
Member
Just my own opinion Brite Spark, but what would happen if most of the Lighting within an installation was on one or two breakers?.
If one trips, you have quite an area of darkness.
I would like to spread the lighting load over 3-4 Circuit Breakers to make sure that if there was a fault, people would still be left with some light to get to the Switchboard.
It also depends upon what sort of installation you are talking about.
Sure a Minature Circuit Breaker of 10 Amperes will take 25A before it trips under Magnetic trip, but it is the Thermal current you have to be careful of.
Thermal trip will occur at 12.5 Amperes.
Run any circuit-breaker close to that level and you run the risk of Nuisance tripping, due to Thermal effects.
It also depends upon the brand of Circuit-Breaker you are using too.

Re: Low Voltage Transformer Quiery #143150 05/19/05 03:25 AM
Joined: May 2005
Posts: 12
B
Brite Spark Offline OP
Member
Thats true Trumpy, but i've neva had a problem with thermal tripping in domestic situations. Also i put lighting on a separate RCD to Power, just so that if an appliance becomes faulty that there is still lighting to get to the switchboard.

I was at a job last week, they were complaining about the RCD tripping. There was only one RCD for both power and lighting. what do u think is it wise to separate power and light?

Re: Low Voltage Transformer Quiery #143151 05/19/05 04:18 AM
Joined: Jul 2002
Posts: 8,354
Trumpy Offline
Member
Personally I've always seperated Lighting and Socket-Outlet loads as far as an RCD goes.
AS/NZS 3000 requires that you have at least 2 RCD protected Lighting circuits and at least 2 RCD protected Socket-Outlet circuits.
Considering that most home-owners don't even know that thier house even has RCD's in it,
it seems rather rich that most of the call-outs here are to do with RCD's.
In the worst case scenario, we have had them by-passed here. [Linked Image]

Re: Low Voltage Transformer Quiery #143152 05/20/05 01:02 AM
Joined: Mar 2005
Posts: 1,803
Alan Belson Offline
Member
You only need one for safety, provided it breaks the whole house supply on detection of a fault. Fitting them to undividual cicuits costs more, but if a power circuit trips you could still have lights I suppose? Can't see the point of more than one on whole house power, except a small advantage in fault location.
Only idiots bypass safety features. Problem is there's a lot of idiots. They're not all locked up yet!
Alan


Wood work but can't!

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