I have been getting an education from all of you on the electrical aspects of our hobby. So I'm good with what mA is and volts, and am now working on layout lighting for everything but the track - I have that covered.
Most products come with the obvious voltage to run bulbs on, but what they are lacking is the mA maximum they draw so I can determine what the transformer draw is.
Any suggestions on how to determine the mA are much appreciated! Terry
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If you are looking for what mA a particular light bulb draws, you can take an ohmmeter and measure the resistance of the bulb. Then using ohm's law ( Volts = Amps x Ohms) divide your 12 volts by the ohm reading of the bulb. If you are powering them with a DC power pack, that how many amps the bulb will draw. To figure out mA divide your answer (amps) by 1000. AC is a bit different, but close enough for incandescent light bulbs.
Or if you are using a common GE lamp, you could look up the data sheet on line for that number (like 53, 57, 1445, etc.) and that will tell you the mA it draws.
Or you could just err on the side of caution and call each lamp 5 watts like they used to. Then add up all your watts and size your transformer according to that.
So there's three different ways; there will probably be dozens more here by the time all is said and done 
J White
If you are looking for what mA a particular light bulb draws, you can take an ohmmeter and measure the resistance of the bulb. Then using ohm's law ( Volts = Amps x Ohms) divide your 12 volts by the ohm reading of the bulb. If you are powering them with a DC power pack, that how many amps the bulb will draw. To figure out mA divide your answer (amps) by 1000. AC is a bit different, but close enough for incandescent light bulbs.
The resistance of an un-powered bulb filamant will be many times lower than when it is powered. Also, to figure out mA, you multiply your answer by 1000. So 1 Amp becomes 1000 mA.
I suggest you use a 12V supply to power the bulb and use a meter's current measurement function to directly measure current flowing from supply to bulb. Some hobby-grade meters don't offer the AC-current measurement function so it's fine to use a 12V DC source and the DC-current measurement function for the purpose of estimating a bulb's current requirements.
I use Stan's method for unknown bulbs. The DC supply will make it easy to measure the current with any common multimeter.
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