12v circuit design (Voltage drop correction)

[kiwi32]

Hello !
I am new to this forum and signed up specially for this question.I do have a pretty good electrical knowledge but I could not get this one sorted out without doubts.

The problem takes place in a 2-level home in Canada (same electricity than US).I have a cheap 400W PC Power supply located downstairs that I use to get DC 12V to power local (less than 20ft away) CCFL inverters among other things.I used 18AWG Speaker cable (is.gd/PGXrvI) to carry the electricity from the PS to the lights.I heard a pair of ccfls draw less than 0.6A,and this system works perfectly.I would now like to power 12V Leds and maybe a CCFL upstairs,drawing 3A max in total.It would be a cable run of about 100ft from the PS on a separated rail(not at the end of the existent ccfl circuit).It is not possible to use another PS upstairs,and I was planning on using 18AWG speaker wire for this too.

I am concerned about voltage drop.Apparently low voltage is highly prone to drops in the long run.According to this calculator (is.gd/yBDM2p),the VD would be 3.83V for 12VDC over 100ft 18AWG with a load of 3A.Now,this is a domestic application that does not need to meet any efficiency code,but since a high VD migh cause fire I would like to make sure this would not be dangerous in this situation.Also,since the voltage at the end could cause problem with the lights,I was thinking of using a device like this (is.gd/pOKgzN) at the start of the speaker wire run to correct the voltage.I would put about 16V on it at the start and get exactly 12V at the end,verified by a multimeter, according to this calculaton: is.gd/PZBgqA.

Will the wire take it ? Does it sounds safe to do this ? Is there any risk of fire with such a setup ?

Thank you for the help !

 

[davelerave]

i would fit a 12v power supply adjacent to load or abandon the project

 

[kiwi32]

Thanks for the quick reply.
So you think such a setup could be dangerous,due to the voltage drop ?

 

[Marvo]

The volt drop of 3.8 volts on a 12VDC supply sounds way too high to me, you're dropping nearly 25% of your voltage across the cable or wiring. If the powersupply output is fused it shouldn't be dangerous as such but I doubt the LED lamps will run very well on the remaining 8.2 volts.

ELV (extra low voltage) systems such as this are very prone to fires but that risk can be almost completely eliminated by a correctly sized fast-blow fuse on the ELV output (12 volt side) of the power supply.

 

[davelerave]

i think he's planning on using overvoltage to compensate for the VD

it's not standard practice ime to design things in this way anyhow

the power losses on supply cables seem to be about 12watts but the cable is too small if it's even the right type
American Wire Gauge (AWG) Cable Conductor Size Chart / Table


as marvo said you quickly run up large percent voltage drop as currents are higher and supply voltage is lower
power losses are greater too for ELV(i think)

i think you'd need voltage regulation at the load end if you were using overvoltage like this

 

[kiwi32]

Hmm... Interesting points !
Marvo,you mention using a fast blow fuse.How do you choose one ? I think it would be a great idea to install one on each 12v rails just in case,and it is certainly nice to know that if anything ever go wrong with this long cable run I would be protected.
davelerave is right,I was planning on using a DC-DC step-up to boost the voltage at the start.I also think I could manage to use 16AWG instead.I was planning on using 18 because its easier to pass thru walls,especially since there is a prehmade hole that is very thight already.But I think 16 should still be fine.
I am confused about the voltage regulator.I think I will need it as soon as I install more than one device at the end.Lets say I install only the leds at first.All of them would be together,on the same switch.If I power the leds,then set the booster so I get exactly 12v at the end under load(while the leds are on),the voltage will rise a bit as soon as they are off but would be back at 12v as I turn them on.If I add the ccfls then I would defenitly need a VR since the load would change depending on whats on.Is that a correct reasoning ?

Thanks.

 

[Marvo]

You would select a value of fuse (in Amps) according to the current carrying capability of the cable or wiring you're installing. I wouldn't worry too much about the physical size or technical characteristics of the fuse, in the application you're talking about any fuse with the right amp rating is a good fuse. The fuse is just there to prevent cable overheating damage or a possible fire and nothing else.

The bottom line is your load (in Amps) will determine your wiring size. You can't select your wire size based on the size of hole you can conveniently drill....it's just not done that way I'm afraid.

Stepping up the voltage to compensate for massive losses in the cabling is definitely not the road you want to take. Without tight stabilization and regulation once your cable becomes a considerable part of the load then things are no longer predictable. Consider for example if you step up the voltage by 30% to compensate for transmission losses in the overloaded wiring. One day two of your lamps blow (open circuit), this reduction in load would cause an increased voltage to the remaining lamps because the wiring losses have suddenly decreased correspondingly with the reduced load. You now have the beginning of a snowball effect where lamps start blowing due to the higher voltage and every time one blows the voltage on the remaining lamps gets even higher.

If you go the route where you're trying to work around an inherent shortfall in the wiring size it's going to get complicated and expensive. I don't really want to get any deeper technically with this problem when the best solution is as easy as using the correct sized wiring.