12v transformers

[bondy]

ok, here goes. im preparing for my on site assesment. doing all my reading up of osg, guidance note 3, etc, etc. and i have thought of a question to which i do not know the answer. im sat here milling through the correction factor tables and volt drop tables, and was thinking about the power a 12v dimmable transformer draws. do you use the bulb size, ie 35w times the amount of light fittings, say 6. which would be 210w, and then work out Ib, In, It, etc. or do transformers have a formula of their own.

 

[RF Lighting]

It is worked out the same as any other circuit, using the total wattage of the lamps as the load, and the transformer output as the voltage (and correction factors etc). Personally if doing something like this I would rate the cables to the maximum output of the transformer rather that using say 6no. 35w lamps as the transformer will deliver its rated output if someone comes along and shoves in 50w lamps everywhere thus overloading the cabling. Personally though I always fit one transformer per light.

 

[Adam_151]

For the primary side, just treat the transformer as you would any load that draws 210w at 230v (actually will be a bit higher than 210w due to the transformer losses)

For the secondary side, remember that the current is higher (210w/12v = 17.5A) and the allowed volt drop is lower 4% of 12 = 0.48v

 

[bondy]

sorry, should of explained myself better. yes, 1 light fitting and transformer per jb. the transformers i normally use are rated at 60w.

 

[Adam_151]

sorry, should of explained myself better. yes, 1 light fitting and transformer per jb. the transformers i normally use are rated at 60w.

Well in that case, just ingore the fact that its a transformer and 12v lamp stuck together, just imagine its all a 230v lamp of the same rateing

EDIT: why did I write back instead of fact?

 

[bondy]

when you say the same rating, is that 35w per position or is it 60w which is the transformer max rating.

 

[RF Lighting]

You would rate it for 35W per fitting. I think I mis understood you to start with and assumed you were running six lights from one transformer, which with this setup the load increase at 12V is substantially more than at 230V
Upgrading from 35W to 50W
At 230V = extra 0.39A
At 12V = extra 7.5A

 

[bondy]

You would rate it for 35W per fitting. I think I mis understood you to start with and assumed you were running six lights from one transformer, which with this setup the load increase at 12V is substantially more than at 230V
Upgrading from 35W to 50W
At 230V = extra 0.39A
At 12V = extra 7.5A

that was what i was unsure about. i didnt know what end of the transformer was used for the calcs. otherwise, 6 35w fittings if calculated at the low voltage end would draw enough current to warant using a seperate supply. it didnt seem correct, but the question had to be asked.

thankyou for helping me out. gary.

 

[baldelectrician]

i would normally rate each lamp at 50 VA/W if you are installing one lamp/fitting per tranny. The lamps don't do any higher than 50W so you should be OK.

I think the trannys are usually over 90% efficient so remember to add in for losses.