current before transformer??

[chrisreid186]

on down lights how do you figure out the current before the transformer?

12v down light

50 watts

50/12 = 4.1 amps

also wondering why 2 down-lights (8.2 amps) dont pull a 6amp mcb?

 

[telectrix]

because it's 4.1A at 12v which equates to 4.1/20 ( divide 240 by 12 ) = 0.205A @240v ( assuming 100% efficiency of TX )

 

[baldsparkies]

Yep its down to good old ohms law. The higher the voltage the lower the current and vice versa. So that's why the national grid transfers power at such high voltages !! Keeps the cable sizes down.

 

[telectrix]

and the pylons keep 'em up

 

[nickblake]

if you have 2 down lighters at 50 watts thats 100 watts so divide that by 230 100/230 = 0.43 amps the current you have mentioned is the secondary current of the transformer especially with electronic transformers which are normall 100% ish efficient how ever if using wound transformers which are less efficiant you use the correction factor of 1.8 as with chokes and ballasts so will become 100x1.8 /230 = 0.78 amps

 

[chrisreid186]

cheers guys easy now you put it like that

 

[telectrix]

one thing you have to remember is that the cable sizes at 12volt need to be far larger. a. to carry the current, and b. volt drop at 12v is far more critical than at 230v. 5% of 12v is 0.6v compared to 11.5 at 230v.