Resistance on Low Voltage Circuits

[mobiledynamics]

I remember someone advising me to run 10 AWG, so I ran it for my Low Voltage Undercabinet Lighting. I'm curious on the reasons behind it. Something about resistance if I recall.

In my application, I had a low voltage transformer mounted in one of my kitchen cabinet bases. I had 3 short runs for the low voltage (12) undercabinet lighting. I initially was planning to run 14AWG.

I just read a generator inlet thread and more than once, the pros had mentioned that bigger is not always better. For a standard 30 amp 120/240 inlet, they were advising the short run of 50 feet or so, that 10AWG would be better than 8AWG - resistence as well/

Can anyone shed some light/insight on this - upsize versus no-upsizing

 

[justsam]

Re: Resistence on Low Voltage Circuits

In terms of reducing resistance, larger wire is always better. The greater the circular mils, the lower the resistance per foot. In US wire that means a smaller wire gauge number for larger wire.

Low voltage lighting typically takes more current than 120V lighting. In the past, and even today, lights have been rated in Watts. Here the higher the number, the brighter the bulb. Since Power in Watts is equal to current in Amps times voltage in volts, a 100Watt rated bulb running on 120V would draw about 0.8 Amps. The same 100 Watt rated bulb running at say 20V would need to draw 5 Amps. The greater the current in Amps through a given wire, the greater the voltage drop across the wire. The formula here is Voltage drop = Current in Amps times the resistance of the wire. Remember the smaller the wire the greater the resistance.

Thankfully we are now going more towards rating lighting in lumens or an actual light output as opposed to bulb wattage.

 

[mobiledynamics]

Re: Resistence on Low Voltage Circuits

While 1 Wire Size up is better given the fill factor is not exceeded ---I'll have to find that thread. A couple pro sparks kept advising that upsizing is not always a good thing. I'm trying to understand why not....in the electrical world of things.

 

[rockwithjason]

that is very unusual. the only thing you have to watch out for in upsizing is that the wire becomes so large it won't fit in the equipment enclosures or termination lugs. 12v lighting is very susceptible to voltage drop. generally the best way to control voltage drop is to use short runs. the second best way is to upsize the wire.

 

[ddawg16]

What kind of current are we talking about?

 

[n8n]

Voltage drop over a run of wire is dependent only on the resistance of the conductor and the current passing through it. So a given run may drop one volt. For, say, a 10AWG wire you can safely pass 30 amps through it. But let's say that the length of the run is such that at that current you're dropping one volt at the end of the run. At 120V that's insignificant; the main driving factor at "high" voltages is how much current you can pass without having significant heating. At 12V however, a drop of one volt is almost ten percent of the original voltage!

 

[Mustang51js]

Usually need to plan out your runs for low voltage stuff so you don't need to oversize the wires. What happens when you daisy chain them from one to another depending on how many lights you have the first one will be bright and get dimmer as you go down the line. What I like to do is run one or two lights up to a corner of the top cabinets or in the basement. Then run the switch leg to them so the transformer would be where all the splices are. So if you have 8 lights you would have 1 120volt line and 4 wires going to 2 lights each. If you decide to go 120 volt lights later you just need a j-box