0
\$\begingroup\$

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?

enter image description here

\$\endgroup\$
  • \$\begingroup\$ Google awg calculator \$\endgroup\$ – Passerby Jan 5 '15 at 3:58
  • \$\begingroup\$ 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. \$\endgroup\$ – Hugo Sereno Ferreira Jan 5 '15 at 9:27
1
\$\begingroup\$

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.

EDIT to answer additional questions

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.

\$\endgroup\$
  • \$\begingroup\$ 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)? \$\endgroup\$ – Hugo Sereno Ferreira Jan 5 '15 at 9:28
  • \$\begingroup\$ @HugoSFerreira See the answer in edid above \$\endgroup\$ – GR Tech Jan 7 '15 at 4:15
0
\$\begingroup\$

Use the AWG chart here:

http://www.powerstream.com/Wire_Size.htm

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.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.