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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?

Re: How many amps?

The amp rating is marked on the transformer, no?

Re: How many amps?

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


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.

Timothy Miller
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.

Re: How many amps?

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

Re: How many amps?

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

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

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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