Low Voltage Outdoor Transformer Current Draw

[Tom in PA]

The missus wants outdoor lighting, with a number of fairly bright spots
on the house, so I'm looking into doing this. I'd be adding a new
outdoor outlet to an existing 15 amp circuit for the power source. I'd
like to know how much of a transformer this circuit can handle. For
example, can I put in a 600W transformer and load it up with lights?

Sorry if this is a repeat question. I surfed the old posts a bit but
ended up confused!

Thanks! Tom in PA

[RBM]

Tom, you have to figure out what is on the circuit you are going to tap. If
it's a 15 amp circuit, its good for about 1200 watts. See what is currently
on it and add it up. If you've got a spare 600 watts, go for it

Post by Tom in PA
The missus wants outdoor lighting, with a number of fairly bright spots
on the house, so I'm looking into doing this. I'd be adding a new
outdoor outlet to an existing 15 amp circuit for the power source. I'd
like to know how much of a transformer this circuit can handle. For
example, can I put in a 600W transformer and load it up with lights?
Sorry if this is a repeat question. I surfed the old posts a bit but
ended up confused!
Thanks! Tom in PA

[SQLit]

Post by Tom in PA
The missus wants outdoor lighting, with a number of fairly bright spots
on the house, so I'm looking into doing this. I'd be adding a new
outdoor outlet to an existing 15 amp circuit for the power source. I'd
like to know how much of a transformer this circuit can handle. For
example, can I put in a 600W transformer and load it up with lights?
Sorry if this is a repeat question. I surfed the old posts a bit but
ended up confused!
Thanks! Tom in PA

What is on the circuit now? 600 watts at 120v could be a problem.

A 600 watt transformer at 12v or 120v?

I suggest you go to the store and read some boxes for a few minutes.

[Tim Fischer]

Post by SQLit
What is on the circuit now? 600 watts at 120v could be a problem.
A 600 watt transformer at 12v or 120v?

Watts are Watts are Watts.

A 15A circuit fully loaded is about 1800 watts. 80% loaded (which is a good
safety threshold) is 1440 watts. Doesn't matter if those watts are at 120V
or 12V, they're watts.

(it's AMPS that depend on the voltage, eg. 600W of lighting at 12V is 50A,
which would require some pretty hefty wire).

-Tim

[Kevin Ricks]

Post by Tim Fischer

Post by SQLit
What is on the circuit now? 600 watts at 120v could be a problem.
A 600 watt transformer at 12v or 120v?

Watts are Watts are Watts.
A 15A circuit fully loaded is about 1800 watts. 80% loaded (which is a
good safety threshold) is 1440 watts. Doesn't matter if those watts are
at 120V or 12V, they're watts.
(it's AMPS that depend on the voltage, eg. 600W of lighting at 12V is 50A,
which would require some pretty hefty wire).
-Tim

If the OP's transformer is like mine then the secondary is split into 2
outputs. Maybe the OP has 3?
My Malibu transformer is sold as 500W unit which is referring to the 12V
output. The label on my transformer says INPUT: 3.5A max @ 120VAC which
would calculate to be 420 Watts.
The OUTPUT is labeled: 500W total @ 12V, BUT it is really two 250W outputs
or about 20.8A each output.
If the OP has 3 equal outputs of 200 Watts each then that would be 16.6 amps
each which would be reasonable for the 12ga low voltage wire they sell.
Kevin

[Ralph Mowery]

Post by Kevin Ricks
My Malibu transformer is sold as 500W unit which is referring to the 12V
would calculate to be 420 Watts.
or about 20.8A each output.
If the OP has 3 equal outputs of 200 Watts each then that would be 16.6 amps
each which would be reasonable for the 12ga low voltage wire they sell.
Kevin

Thats a good one, good for more watts out than the input wattage. The
trasnformer could not be more than about 90% efficiant so 380 watts should
be about the maximum output of the transfromer.

The 120 volt circuit should be good to power atleast 2 of the transformers
if that is the only load on them. If there is some other load but not much
he should still be ok with it.

[Kevin Ricks]

Post by Ralph Mowery

Post by Kevin Ricks
My Malibu transformer is sold as 500W unit which is referring to the 12V
would calculate to be 420 Watts.
or about 20.8A each output.
If the OP has 3 equal outputs of 200 Watts each then that would be 16.6

amps

Post by Kevin Ricks
each which would be reasonable for the 12ga low voltage wire they sell.
Kevin

Thats a good one, good for more watts out than the input wattage. The
trasnformer could not be more than about 90% efficiant so 380 watts should
be about the maximum output of the transfromer.

Just guessing but It may be that the stated output wattage is based on the
advertised wattage of the intended low voltage bulbs, which may or may not
be what they say, AND maybe with the cable voltage drop factored in, rather
than the actual transformer secondary wattage. The word 'nominal' is printed
somewhere on the label so that means it could be anything.... They give
examples on the label: Use ten 25W bulbs on each output etc. I know if you
don't have enough cable, 10'+ to the first connection, then the bulb will
burn out fast. Also if I am not about 50% loaded the bulbs burn out fast,
like in 2 or 3 weeks. As one bulb blows it makes things worse for the rest
and they blow faster etc.
I didn't mention that my box has a 125V timer motor that I assume is added
in as part of the input current but not the output current. That would make
things a little bit worse as far as the actual input/output wattage
comparison is concerned.
Kevin

Post by Ralph Mowery
The 120 volt circuit should be good to power atleast 2 of the transformers
if that is the only load on them. If there is some other load but not much
he should still be ok with it.

[Tom in PA]

I'm the OP and this discussion is proving to be very helpful. Thanks!
It looks like if I go with the 600 W transformer it will use a
substantial chunk of the capacity of that 15 amp circuit. The only
other non-trivial item on the circuit is a refrigerator in my garage.
Do y'all think the circuit can handle 600W transformer + fridge?

Thanks again.... Tom in PA

[RBM]

Without knowing the amperage of the particular fridge, my guess is that
you'd be close but Ok. It is likely that other outlets are on that circuit
that you are not aware of though

Post by Tom in PA
I'm the OP and this discussion is proving to be very helpful. Thanks!
It looks like if I go with the 600 W transformer it will use a
substantial chunk of the capacity of that 15 amp circuit. The only
other non-trivial item on the circuit is a refrigerator in my garage.
Do y'all think the circuit can handle 600W transformer + fridge?
Thanks again.... Tom in PA

[P***@comcast.net]

what is the formula for figuring out Amps?

phil

[Dr. Hardcrab]

Post by P***@comcast.net
what is the formula for figuring out Amps?

E=MC2

No wait....

;-]

Amps=volts/ohms

[Kevin Ricks]

Post by P***@comcast.net
what is the formula for figuring out Amps?
phil

Here is an Excel file I made to that will do the calculations.
http://pages.prodigy.net/klricks/OhmsLaw.xls
Kevin

[Tim Fischer]

Post by P***@comcast.net
what is the formula for figuring out Amps?

Watts = Volts x Amps

Ohms don't really come into play for this situation...

-Tim