If that stuff really is 24W/metre, it would need 2A/metre at 12V to run correctly. To run the entire strip ought to require 10A....
Calculating the Amps drawn by LED ribbons is straight forward when powering them with DC, but when you need to plug them into the wall things get complicated because a lot of LED AC power supplies are not power factor corrected. Which means that if you do not take their poor power factor (which can be as low as .45) into account when running LED Ribbons you can find yourself tripping breakers because the lights will draw twice the AC current you would think. A good example of this was a Christmas movie shot here this last January. The set decorators dressed a large tree (20’ high) with hundreds of LED strings. Where each string was just a few watts they figured they could plug them all into one circuit. After the breaker tripped several times, only then did they call in the electricians to sort out the mess. But of course now the production is waiting for the tree lights to come on before they can shoot and the electricians caught the blame.
(Even the premium priced LiteGear RGB Lite Ribbon pictured above has a pf of .45)
Unfortunately, the manufacturers of LED Light fixtures generally do not give power factor specifications for their products. One would think that the less expensive LED lights would not be pfc, while the more expensive ones would, but that proved not to be the case in recent testing of fixtures. Over half of the fixtures that I tested at random (from the inventories of Boston area rental and lighting sales companies) were not pfc. With power factors ranging from .45 to .63, these fixtures generated considerable harmonic distortion (THD ranged from 75-85%.) Even those that were pfc generated harmonic currents when dimmed. For instance, the pfc of the new Litepanel Astra 1x1 below dropped from .99 to .54 when dimmed 50% (THD increased to 83.2%.)
One could argue that the wattage of LED fixtures is so low that the harmonics they draw and kick back into the electrical distribution system is so low that it does not matter from a practical standpoint.
While that is true to a degree, when used as the predominant source of light in a studio, like the new BSkyB studio pictured above, or for a night exterior powered by a portable generator, the harmonics generated by even small LED fixtures will accumulate and effect their power source adversely. To see which LED lights are power factor corrected or not, use this link to see some of the results of my tests.
Guy Holt, Gaffer
ScreenLight & Grip
Lighting Rental & Sales in Boston