Transformer secondary/cable sizing

[Neon Vagabond]

6x 50watt halogen downlights, 12V, 400 VA transformer.(central transformer)
Given that the current on the secondary side of the transformer will be 25A am i right to install 2.5mm cable to feed my spots?

 

[Darkwood]

I assume the transformer has multiple connection points for its outputs so you would only be running one wire per point so its is sized in accordance with 1 x 50watt load but you will need to be careful regarding volts drop as will have more implication on the cable size than you think so pick your flex size and work out your voltage drop and that it complies with app'12

 

[Neon Vagabond]

ok cool,my bad, i was taking the total power of the secondary side (300W), thats fine 50W at 12V is 4.1A per fitting, 1mm should cover it, the runs about 6-7 metres

 

[Darkwood]

I have just given you a pre-empt' on what to be careful off so please work out your VD for your suggestion of 1mm doing 7m, you are using 12v so your results will be applified by a factor of approx 20 so even a short run can turn out not to comply.

Ill happily guide you but ill stop short of doing the calcs for you.

 

[Neon Vagabond]

hi again, i am getting confused with these volt drop calculations, can you help? heres what ive got so far:
Ib=50w/12V = 4.2A
max volt drop = 4% of 12 = 0.48V
run is 7 metres so:
mV/A/m = 480/29.4 = 16.33
table 4d2b of bs 7671 says a 4mm cable has 11mv/a/m
just seems a bit big?
am i missing something?

 

[Knobhead]

The joys of low voltage? :seeya:

 

[HandySparks]

It's a pretty long run of ELV cable. Can't you move the transformer nearer the load or get rid of it all together and use 230V lights?

 

[Darkwood]

Firstly im unsure where you get 4% from as lighting max VD is 3% from table 4Ab.
I would assume flex would be a better option and this would be table 4F3A.

Although 0.36v seems a drop in the ocean you must consider the voltage is only 12v and lamps attached can suffer issues if even a small VD is present, if you were to throw 1mm in for 12m on a halogen 50w lamp you would notice a dimming effect compared to a lamp supplied from a 2m run, also to think about is cfl lamps and more so LED lamps as undervoltage may cause premature lamp failure as the control gear struggles with the VD but this will depend on the tolerances of the lamp.

When fitting a 12v system its always best to have a dedicated transformer adjacent to the lamp to eliminate these issues, supplying multiple fittings with a higher rated transformer should not be a system of choice due to the limitations of distance before cable size get stupidly large and dosnt fit the terminals.... IMHO i stopped fitting LV yrs ago as GU10 now have more advantages and lamp options...
Read the thread 'fully!' below to see a discussion that was had on this forum not to long ago but ignore any mention that 3% dosn't apply to 12v.

.http://www.electriciansforums.net/e...trical-forum/37453-12v-cable-lengh-csa-2.html

 

[Darkwood]

As you've shown you have tried to do the VD calc i'll help you out ....

Allowed volts drop is 3% of 12v = 0.36v

To find VD (mV/A/m) but as we already know the permitted VD we can transpose this to give max length per given cable size thus :-

0.36 / (mV x A) = mm

0.36 / (46 x 4.2) =mm (46 from table 4F3B for 1mm flex)

0.36 / 193.2 =0.00186 and to change to meters 0.00186 x 1000 =1.86

therefore 1mm flex would satisfy up to 1.86m but you also must consider all other applicable calcs in appendix 4 that could reduce the CCC of the cable before applying it to work out VD although its unlikely to be an issue with the given CCC of the cable to start with.

 

[Darkwood]

And finally to put the cherry on the cake here's the permitted max length of flex for 12v 50w load.

1.00mm = 1.86m
1.25mm = 2.30m
1.50mm = 2.60m
2.50mm = 4.50m
4.00mm = 7.10m
To note above 1.5mm i would assume you may have trouble with termination as fittings don't allow for the larger sizes usually hence i said earlier its a method you should avoid always keep runs as short as possible.

 

[Neon Vagabond]

yeah thanks darkwood, good noises from you. Truth be told this post doesnt actually refer to an ongoing job. Im goin over stuff that im not too sure about the now, trying to get clued up, qualified in febuary and im quickly finding that theres a lot of things i dont know about the trade. I'll read that post cheers

 

[Darkwood]

To tell you the truth its good to see someone dotting the i's and crossing the t's then amount of so called sparkies installs ive come across who have thrown 1mm in just because its ccc is well above requirements and ignoring or not knowing its not sized in accordance with VD requirements is more than you would believe....

What you will find though regarding regs it expresses the VD is determined from the origin of the installation but it is unclear about multiple voltage circuits and i make an educated guess that the transformer can be classed as the origin of the Selv circuit as long as the transformer 230v supply complies to the Regs im in believe that the 2 parts of the circuit can be treated and worked out seperately but im open to other opinions on that point.
Reg 6.4
'The voltage drop between the origin of an installation and any load point should not be greater than the values in table 4Ab expressed with respect to the value of the nominal voltage of the installation'

This is ambiguous and dosn't give any indication it would apply to the selv circuit attached to a 230v circuit but it could be argued that the selv is its own circuit with its own origin at the transformer where as it would require to comply with table 4Ab..... either way it should be subject to VD calculations as it effects the operation of the circuit when it is ignored.

 

[Neon Vagabond]

yeah i know what you mean, seems to be a bit grey in this area, i think to be on the safe side i would go with the secondary circuit being the start of seperate circuit, its fused and it operates at a different voltage, thats my view anyway.
i just think that with the way the job market is today people need to know the score, the better you know your trade the more chance you have of securing a job. Its harder for new electricians these days