12V DC wire size question

[Mikey B]

I wish to install an LED light strip (1M). The run to it from from my Solar Control panel fuseboard is in the order of 25M. I am confused by the online table in which we enter current draw (say 7A), nominal voltage (12V), circuit length (25M) and conductor area (1.5mm2) - this gives an answer of a Voltage drop of 37.1% or 4.45V
My query is when I attach a 25M length of cable (with no load) to the fuseboard - as a test (supply 12.8V) and measure the voltage at the end of the cable it shows 12.7V - only a loss of 0.1V where I was expecting it to be 8.25V? where am I going wrong or do I just ignore the table for resistance in the wire?

 

[Richard-the-Ninth]

You have answered your own question without realising it.
Voltage is the potential difference between two points. As you said, there is no load. If you connect your 7A load then check the voltage, you will probably find it is 8v or even less. Also, you will be lucky if the far end LEDs glow.
When LED tape is installed, it is better to have the power supply as close to the LEDs as possible to avoid problems.

 

[Mikey B]

OK thanks Richard, I will do a trial connection of the LED strip on the 25M cable before I wire it in and also try an increase in the size of the conductor to see the effect.

 

[Mikey B]

Richard, I connected a short LED strip (probably around 1A) to my fuseboard (Voltage measured 12.6V at the fuse) and measured the voltage at the strip and it showed 11.1V on my multimeter. So exactly as you advised. Thanks for the advice.

 

[steptoe]

How big is your LED strip that it's drawing 7amps?

 

[Mikey B]

This was an estimate for the use of two 5M strips plus a bit of leeway

 

[Bruspark]

Put the transformer(s) as near to each strip as you can
To restrict voltage drops it’s better to feed each section of strip with a separate cable back to its driver transformer if possible.

Even 2 cables with one from each end of a very long strip helps keep them all bright and not fading along the length (effectively put the strip in the middle of a DC ring circuit

 

[Mikey B]

Thanks for your reply Buspark,
I don't have transformers, The 12V connection is direct from my Fuse board fed from my 12V solar battery. In my test with a 25M cable to a 1M length of LED strip there is no noticeable dimming of the LED's and only a small drop in voltage. However turning the connections into a DC ring main looks good advice.
Cheers Mike