# Cable/ connector ratings and voltage drop

Hello all. I am trying to use a string of addressable 3W LED modules (Adafruit Pixies), but it is a long run and a lot of pixels. I am concerned about getting my sums and circuit design right, perhaps you can help?

The circuit diagram and notes attached should sum it up, but note that what looks like a 78xx 5V regulator should actually be a 5V buck converter.

My questions are:

1. Does my design look sensible and are my calculations correct (bearing in mind I am short of time/funds!)?

2. Do I need cables and connectors rated for 25A in this scenario? It is quite unlikely it will draw that much, but I want to prepare for the max.

3. Am I right in thinking that if I use a 600W SMPS with a 24Vdc 25A output from a 240V AC UK mains socket, then the max input current is well within the 13A available? 600/240 + some = 3A odd?

4. The Pixies have a thermal cut out and I have some heatsinks I can use if necessary, but will anything else get hot in this scenario?

Many thanks if you can help.

OK, I understand I am using the wrong calculation for voltage drop! If I am correct:

1A at 5V = 5VA and this table tells me with 24V this would cover to 0.2A Volt-amps (VA) to amps (A) conversion calculator

Then I can use that figure to calculate my actual current draw as 25 * 0.2 = 5A (+ some for conversion)

Happy days... if I got it right this time.

Use this calculator to calculate the voltdrop

Voltdrop Calculator

You are correct on the AC input Value

I = W/V

I = 600W/230V

I = 2.6A

V = IR,
R = rho L / A

therefore V = I rho L / A

rho = resistivity of copper, 1.68 x 10^-8 ohm-m
A = area of copper in cable
L = length of cable.

For ac use rms values in these equations. You can also lookup ohms/m for the cable directly, but its
easy to calculate - do both and cross-check.

Once you know that the voltage drop is acceptable you can calculate the heat dissipated per unit length
of wire and check its reasonable (or just look up the max current rating for the wire guage).

Watch out for evil CCA (copper coated aluminium wire - much higher resistance).

Thank you guys, I will check all this out!

Just add to add. When I PAT test any appliance below 700W uses a 3 Amp fuse. Anything above uses a 13 Amp fuse. Rarely I would use a 5 Amp for computers etc with a high surge PS if it kept blowing 3 Amp fuses and it passed the tests

When calculating wire resistance it is easy for me to remember:

1000 Feet of #10 AWG = 1 ohm
Resistance doubles every 3 wire sizes

So:
1000 feet of #13 = 2 ohms

1000 feet of #14 = 2 ohms *( 2 * 2/3) = 2.66 ohms

1000 feet of #16 = 4 ohms

Crudely 1000 feet = 300meters

@ #14, 1 meter = 2.66/ 300 =8.86 milli ohms
@ #16, 1 meter = 4/ 300 =13.3 milli ohms

When doing your calculations remember there are two runs dropping voltage, the supply and return.

Note: I doubt any power supply capable of more than 10 amps will have a barrel type power plug.

Good luck.