How about the power of the LED lamp?
The use of resistance but also according to your current input voltage and depending on LED, because LED rated current is 20MA, but the actual application generally do not use 20MA, usually 15--18MA! To the series resistance, by Ohm's law can be counted by U=IR!So first you decide what kind of LED you use!
In general, 30 meters of lamp / lamp with the larger current LED. such as 0.028A, calculate the power per meter (30 / 3: lamp lamp group) * (12V * 0.028A) =3.36W; and 60 meters of lamp / lamp with LED will use less current to produce, for example: 0.011A calculate the power per meter (60 / 3: lamp lamp group) * (12V * 0.011A) =2.64W.
LED has different chips that cause different currents. For example, there are 0.011A, 0.015A, 0.028A, and so on. It should be said that different batches of LED chips will have different values of current, so each batch should be measured by an instrument
