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Thread: How many amps?

  1. #1
    Join Date
    Mar 2011
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    SouthWestern Pennsylvania
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    Default How many amps?

    I'm installing new recessed low voltage lights in my kitchen and want to know how many amps they will draw so I don't overload the circuit. The lights are 120V stepped down to 12V and can take a max of a 50W light. How do I calculate the amps used by each fixture?

  2. #2
    Join Date
    Dec 2010
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    Houston Texas
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    Default Re: How many amps?

    The amp rating is marked on the transformer, no?

  3. #3
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    Default Re: How many amps?

    Actually 50/120 = .42 amps which is the circuit load plus the power supply loss.

    Jack
    Be sure you live your life, because you are a long time dead.-Scottish Proverb

  4. #4
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    Default Re: How many amps?

    A lot depends on the efficiency of the transformer. Each bulb will draw 4.2 amps from the output of the transformer. If the transformer were perfect, then it would draw .42 amps from the circuit for each bulb that is attached to it.

    In addition to the .42 amps to support the bulb, it also draws power to support its losses. you can add .2 to 1% to allow for load losses. Core losses are a constant loss regardless of the number of bulbs attached or turned on.

    If your on/off switch is on the input of the transformer, core losses won't occur when the lights are off, but if the on/off switch is on the output of the transformer, then there will be some current flowing 24/7.

  5. #5
    Join Date
    Mar 2009
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    Default Re: How many amps?

    Howdy, have you determined the existing load on the circuit that you are considering? I will regret this- you likely are adding such a small additional load that it will not matter ...But better to compute it being smartly safe. For example if your loading a 15 amp circuit only load at 75%to 80% of the total circuit load- safe operating amperage being no greater than 12 amps. Thus maximum circuit wattage would be 1,800 watts thus safe wattage usage would be 1,440 watts.
    Any an all of my comments are just my opinion and not to be confused with facts.

  6. #6
    Join Date
    Mar 2008
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    42

    Default Re: How many amps?

    Or you could just look on the box that the lights come in . It will be clearly labelled .

  7. #7
    Join Date
    Apr 2011
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    Default Re: How many amps?

    using ohms law, just divide wattage/voltage = amps
    so if wattage = 48 and voltage = 12, then 48/12 = 4 amps

  8. #8
    Join Date
    Jan 2011
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    Columbiana, Alabama
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    Default Re: How many amps?

    Since the wattage on the secondary can be assumed to be the same on the primary (neglecting losses):

    I=P/E = 50/120= 0.4166 A (solved long ago)

    And, since the NEC only allows 80% continuous loading if we assume this to be a 15A 120V circuit:

    15A * .8 = 12A available for the transformer primary.

    The max. number of lamps which could be powered by this will be the lesser of:

    12.0 A (available) / 0.4166 (each lamp)= about 28.8 lamps

    Most likely the limiting factor will be how many primary amps the tansformer is rated for (at 120V), as was already mentioned, check the label.

    Good Luck from Wilsonville, Alabama
    Maurice Turgeon, http://thesemi-retiredelectrician.com

  9. #9
    Join Date
    Mar 2008
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    42

    Default Re: How many amps?

    A whole lot of ego's in play on this one . Wouldn't be the first time .

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