The reality is that your calculated current will in fact be higher than the real current, The below example is talking about dumb RGB lights. Intelligent pixels use constant current to drive the LEDs, what this means is that the optimal current for that LED is maintained even if the voltage drops a volt or 2, but there are limits and if the voltage drops too low then the constant current driver cannot maintain that current as volts and current are linked and then you will see the effects of voltage drop.

The remainder of info has been copied from this thread https://auschristmaslighting.com/threads/27-channel-dc-dmx-rgb-controller.741/page-16#post-25206

........What will in fact most probably be the issue is the voltage drop created when using white which may not actually produce white because of the voltage drop because the more you load a cable up to its maximum rating the higher voltage loss

This can be easily show using Ohms law with the example below

So if your cable is rated at 18 amps then it would be around a 14 guage or 2mm2 cable with an approx resistance of 0.095 Ohms per metre

So that cable with 50% load will have a voltage loss over a 2 metre run of

2 metres x 2 (2 metres for +V + 2 metres for GND)

4 metres x 0.0095 ohms/m = 0.038 Ohms of resistance for the 2 metre length of cable

Volts = Current x Resistance

so at 50% load of 9 amps

0.038 x 9 = 0.342 volts loss

so now the same cable at 100% load

0.038 x 18 = 0.684 volts loss

So as you can see the higher you load up your cable to it maximum load the higher the voltage drop. The lower the voltage the higher % of volatge drop for the same distance.

LEDs are current devices, whch means that current determines the output of an LED and not voltage. But voltage and current are linked together through Ohms law. A strip that uses 3 x 5050 LEDs per metre will have a calculated current of 0.6 amps per metre for a total current draw of 3 amps for a 5 metre strip. but this is calculated and does not take voltage drop into consideration. Dumb strips are voltage controlled through a current limiting resistor, so the supplied voltage plays a very important part in this as shown with the Ohms Law calculation below

So lets look at the 3 LED/m dumb strip you are using.

Each section is made up of 3 x 5050 RGB LEDs connected in series, the 5050 LED is rated at 20mA per colour

With LED circuits

Series = Current is the same, voltage is divided between the LEDs in that circuit

Parralell = Current is added with each additional circuit, voltage is the same

So a dumb strip is made up of individual sections with 3 LEDs in series which is made of 50 parrallel section.

So a 5050 LED voltage rating will vary between colours as shown below

Red - 2.3volts

Green - 3.2volts

Blue - 3.2volts

When these are in series the voltage is added together

Red - 12 volts - (2.3 x 3) = 5.1 volts required to be dropped

Green - 12 volts - (3.2 x 3) = 2.4 volts required to be dropped

Blue - 12 volts - (3.2 x 3) = 2.4 volts required to be dropped

So to maintain 20 mA for the LED circuit voltage required a limiting resistor is used

Resistance = voltage/current

Red - 5.1 volts / 0.02amps = 255 ohms (normally a 270ohm resistor would be used)

Green - 2.4 volts / 0.02amps = 120 ohms

Blue - 2.4 volts / 0.02amps = 120 ohms

So the dumb LEDs current of 20mA from a 12vdc supply is maintained by these reasistor values.

So what happens when the voltage is 1.5 volts lower due to voltage drop (12 volts -1.5 volts = 10.5 volts)

Red - 10.5 volts - (2.3 x 3) = 3.6 volts required to be dropped

Green - 10.5 volts - (3.2 x 3) = 0.9 volts required to be dropped

Blue - 10.5 volts - (3.2 x 3) = 0.9 volts required to be dropped

So now that same strip that is designed for a 12 volt input is now only getting 10.5 volts input

current = volts / resistance

Red - 3.6 volts / 255 ohms = 14mA

Green - 0.9 volts / 120 ohms = 7.5mA

Blue - 0.9 volts / 120 ohms = 7.5mA

So now you see that with just a 1.5 volt loss you then under drive the LEDs and because the difference between the red LED and green and blue LED output, the more voltage drop the lower the light output, the lower current drawn and the more red you will start to see in your white light

Sorry for the long winded technical answer but I thought showing the theory behind this may help people understand the importance of good design and how voltage drop effects the use of dumb RGB lighting.