AWG gauge for 10W cob leds and common ground

Two questions:

1. How can I calculate the appropriate AWG for a 10W cob led (~12V DC at 900mA)?
2. Suppose the appropriate AWG is 22, and I want to use a common ground for them (for the purpose of this example, let's assume I have three equal leds). How can I calculate the appropriate AWG for the common ground? • Google awg calculator – Passerby Jan 5 '15 at 3:58
• Actually, I was more looking for the "theory" behind it rather than just looking it up in a table. And googling for an awg calculator doesn't answer (and neither explains) point 2. – Hugo Sereno Ferreira Jan 5 '15 at 9:27

IEC recommends 30 degr. Cercius as a maximum temperature rise on a wire, under normal conditions.

You have a return of a total 2.7A. So the thinner wire that you can use is AWG24 (0.0511cm diameter) insulated straigt wire standing at free air.

Insulated AWG22 (0.003257 cm^2 cross section) will rise a temperature of 15 degr. Celcius @3A load under same conditions.

If you have to use a long wire, check the voltage drop as well.

IEE set maximum acceptable drop on conductors the 2,5% of the nominal input voltage. In case of DC apply Ohm's law to calculate Voltage drop i.e. :

**V=IxR**

I is the total current of your load(s)

R is the total conductor resistance.

**R=ρxL/A**

ρ= resitivity of the conductor. For copper at 25degr. Celcius is 0.023 Ωmm^2/m (or from manufacturers tables Ω/km for the specific AWG#)

L= the TOTAL length of the conductor in meters

A= the conductor cross section in mm^2

For AC some additional factors sould taking into account.

Having resistance and current, you can start calculating the power dissipation from conductor and using thermodymnamics to calculate temperature. There is a big difference between bare conductor (or PCB trace) and insulated one.

Common conductor (return) calculated using the sum of all currents.

• Thank you :-) Could you please elaborate on the voltage drop and on "how long" should a long wire be to b considered a long wire? Also, could you shed some light on the common ground wire? Is it simply done by adding up the current (as you've seem to have done)? – Hugo Sereno Ferreira Jan 5 '15 at 9:28
• @HugoSFerreira See the answer in edid above – GR Tech Jan 7 '15 at 4:15

Use the AWG chart here:

If you are constrained on your design and can't just oversize the wires remember to take into account de-ratings for ambient temperature and adjacent wires.