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Thread: Watts or Amps for Wiring a Lighting Circuit

  1. #1
    DIY Junior Member hagler's Avatar
    Join Date
    Aug 2010

    Default Watts or Amps for Wiring a Lighting Circuit

    15 Amp circuit for lighting in a horse barn

    Fixtures will be as follows (from Orion West Lighting - Equine):
    Round Light Fixtures = 36 Watts 0.3 Amps (CFL)..........output = 200 watts
    4' Fluorescent Fixtures = 84 Watts 0.7 Amps (CFL).......output = 500 watts

    Using the Round Light Fixtures as an Example:
    Can I figure out how many fixtures to be placed on the circuit by keeping the ACTUAL Amp TOTAL below 12 Amps (if using 80% rule..there is a possibility of all of the lights being on continuosly in the barn) ??? ..... could put many lights on the circuit drawing only 0.3 Amps per fixture....

    Or, do I have to go by the output of 200 watts, even though CFLs are being used?
    ----this would limit me to about 7 Round Lights per 15 Amp circuit (1400 watts of 1440....80% rule)

    Thank you!

  2. #2
    Licensed Electrical Contractor Speedy Petey's Avatar
    Join Date
    Jun 2007
    NY State, USA


    The "output" watts is a rough equivalent of light output, NOT draw,. so that number is meaningless as far as circuitry goes.

    Also, a continuous load circuit is not one where the load may possibly be on for three hours or more. It is one where the load is expected to be on for three hours or more. If it were the form,er then every circuit would be continuous.
    That said, a lighting circuit in a horse barn may very well be considered a continuous load, so wiring it that way would never be a bad thing. Considering the low wattages involved here I don't think it is a big deal either.

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    Don't think your math is correct - not sure how you got 200W with the use of a 36W bulb. Power=amps*volts, or amps=power/volts. 36w is the 0.3A. If you use that bulb, you should be able to install 40 round fixtures and not exceed 80% (12A). Now, if the fixtures can support a 200W bulb and you actually used those, you'd be limited to 6 of them. 4' fluorescent tubes don't have as much of a variation on wattage bulbs you can use, so you should be able to just add up the amps for each fixture and stop when you get to about 12A.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    Plumber jimbo's Avatar
    Join Date
    Aug 2004
    San Diego


    The "200 watt" number is a manufacturers suggestion that he light output ( not electric watts or amps) is approx. equivalent to that wattage of "regular old light bulb"

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