View Full Version : Convert candlepower to amp draw
01-21-2012, 11:50 AM
Is it possible to calculate the amperage draw of a 32 candle power/6 volt bulb? Thanks
01-21-2012, 10:41 PM
You can't calculate it. candlepower is a measure of light intensity, while current draw is directly related to the amount of electrical power the lamp(bulb) consumes.
Different lamps will be more or less efficient at converting electrical power to light. And different lamps will also possibly have an effect on where the light they do produce is sent, for instance a headlight with a silvered reflector sends all of it;s light in a very narrow angle, thereby creating a greater intensity than an otherwise identical, same-efficiency lamp that radiates in all directions.
Your best bet will be to try to find a similar lamp to the one you are trying to figure out, and look at the candlepower and current ratings. For instance, the 6V, 4.1A, 32 cp #1680:
This lamp uses about 24.6 watts of electrical power to produce the 32cp (sometimes shown as MSCP or Mean Spherical Candlepower).
Some other 6V lamps are rated at as high as 5.3A for 32cp, see:
It get s even more fun when you start looking at lamps rated for 6.3V. I personally use 6.8 or 7V rated instrument panel lamps on my '52 FL, because of the lack of a voltage regulator.
01-22-2012, 07:04 AM
Another import factor is the available voltage. If the voltage to to lamp is low, the filament wion't reach the proper temperature and the resistance will remain low causing very little light output for the amount of wattage consumed, i.e. low effeciency. That's why a voltage regulator is so far superior to a fixed output system. If you can't maintain a steady, correct voltage, changing lamps won't solve power issues.
01-24-2012, 10:27 AM
All the above comments are correct. However, I found the following information on a web site that sells Halogen lights for old cars and motorcycles: www.classicandvintagebulbs.com
" Watts Vs. Candle power: As a guide, the following can be made between thes two measurments of power or brightness: 2 W= 3 CP 6 W= 6CP 10 W= 15 CP 16 W= 21 CP 25 W= 32 CP 35 W= 50 CP"
Again remember that Canddlepower is a measurment of brightness (the light of 1 candle being the basis) and Watts is a measure of electrical power, in this case power consumption by the bulb. Wattage of any given bulb can be calculated by using the simple formula of Watts = Volts x amps. Use the voltage applied to the bulb and the actual amperage passing through the supply wires to get these #'s. If you cannot measure the amperage passing through the wire, measure the resistance of the filament in the bulb in Ohms. Apply this to the following formula to calculate the current flow: Current = voltage/ resistance. Then use the calculated current figure to calculate wattage.
Need more? Review Ohms law! It's really quite elementary my dear WATson!
01-24-2012, 07:30 PM
If you use an ohmmeter to measure the resistance of the lamp, you won't get an accurate calculation of the power draw because the resistance changes drastically when the filament heats up. The only accurate way is to check the wattage is to multiply the voltage and amperage as you stated.
You can work backwards using Ohm's law to compute the resistance during operation: resistance = voltage/current.
I checked a springed headlamp bulb, and the resistance with an ohmmeter is almost zero. With 6 volts applied, it draws 2.6 amps, or 15.6 watts. The operating (dynamic?) resistance at 6 volts is 2.3 ohms. I don't have a variable power supply handy to check the amperage at higher or lower voltages.
01-26-2012, 05:48 AM
Thank you guys for the information!
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