ELV lighting amp draw?

[Dirt-Monkey]

I am about to fit 6 12v 35watt downlighters in the bathroom, i will be using two 105va loop-in transformers with 3 lights per transformer (all from screwfix). This is at the end of the existing 5amp lighting circuit which is in 1.0mm t&e. the bit i don't understand is when you work out the total amp draw for 210 watts of lighting at 12v it comes out at 17.5amp. So how does this work on a 5amp circuit? could anyone clear this up for me please?

Also, the 3 terminals on the secondry side of the tranny's only looks big enough to fit flex into them. what size of flex would be ok? (from tranny to light)

 

[Pensdown]

Dirt-monkey,

Sorry to be the bringer of bad news but your bathroom is "a special location" so I assume by your question you should not be carrying out the work. Please refer to other posts on this site for more information on Part P

 

[Adam_151]

210 watts is 210 watts, on the secondary side of the transformer, the voltage is 12 volts, apply watts law (P=IV), you find the current on that side as you have correctly stated is 210/12 = 17.5A , on the primary size the voltage is 230, so we have 210/230 = 0.91A (actually slightly higher than this, becuase the transformer takes a few extra watts and turns them into heat due to inefficency)

A bit of electricial theory to help you understand: power (measured in watts) is the rate of energy transfer in joules per second, one watt is equal to one joule a second, and as engery cannot be created or destroyed thats why this is the same both sides, less whats lost to heat, current (measured in amps) is the transfer of charge in coulombs per second, 1 amp = one coulomb per second, voltage (measured in volts) is the enegry per coulomb, 1 volt is one joule per coulomb, hence watts law of power = voltage x current *

I personally would use one transformer per light however, 60va each one, easier for maintainance as the transformer can be pulled though the hole the light sits in, and if one fails you only loose one light, and also you might only use 35w lamps, but someone else might instal 50w

*ignoreing power factor for these perposes, its irrelevant for a purely resistive load such as a filament lamp anyway

 

[Pensdown]

Great advise as always. Also, use regulated electronic transformers as the design life quoted for LV lamps of around 4000hrs is based on something like 10,000 lamps tested at 12.4Volts. If you over volt the lamps by something like 5% you decrease the lamp life something like 15%. However, if you decrease the voltage by around 5% (dimer) you can increase the lamp life by loads.

Its late tonight & I don't have the actual figures to hand.

However, it's still a bathroom

 

[ban-all-sheds]

However, if you decrease the voltage by around 5% (dimer) you can increase the lamp life by loads.

And if you decrease it by 10% you start to shorten the life...

However, it's still a bathroom

Yes, but that doesn't mean he can't do the work, it just means that he is required to notify LABC first.

 

[Pensdown]

I'm no expert on bulbs and wonder why the lamp life decrease if the voltage goes down by 10% According to some basic documentation I have on lamp life, it quotes "as a general guide, a 10% increase in voltage causes a 50% reduction in bulb life and the opposite applies for a reduced voltage"

 

[Adam_151]

I'm no expert on bulbs and wonder why the lamp life decrease if the voltage goes down by 10%

its for halogen lamps as opposed to 'normal' filament lamps, its to do with a filament thinner than normal, and it wouldn't last long normally, but there is something called the 'halogen cycle' where the halogen returns vaporised atoms of tungston back to the filament, it does however require a certain temperature to work properly, so if the lamp is run cooler the bits of tungston that vapourise are not returned to the filament and the lamp burns out quicker

link here explains the chemistry part of it better than I can: http://www.goodmart.com/facts/light_bulbs/halogen_cycle.aspx

(I didn't take chemistry beyond the combined GCSE science course)

 

[Pensdown]

Cheers Adam, great link!

 

[Dirt-Monkey]

so my lighting circuit should be fine then yeah?