# Low Voltage Outdoor Transformer Current Draw

• posted on September 17, 2005, 12:09 pm
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
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on September 17, 2005, 3:04 pm

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.
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on September 18, 2005, 2:12 am

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( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on September 18, 2005, 12:37 pm

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
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on September 18, 2005, 1:42 pm

amps
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.
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on September 18, 2005, 7:23 pm

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

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on September 21, 2005, 8:41 am
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
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on September 21, 2005, 7:53 pm
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

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on September 17, 2005, 4:36 pm
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

<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on September 18, 2005, 12:12 pm
what is the formula for figuring out Amps?
phil
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on September 18, 2005, 12:18 pm
E=MC2
No wait....
;-]
Amps=volts/ohms
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on September 18, 2005, 7:25 pm
Watts = Volts x Amps
Ohms don't really come into play for this situation...
-Tim
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
• posted on September 18, 2005, 9:18 pm

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