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Old 01-05-2009, 05:12 AM   #1
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Can this circuit handle the load


Hi there - first time poster here. I reviewed the archives and could not seem to find an answer that made me comfortable with moving forward in either direction here - so I am hoping you guys can help me out.

I am looking to add some exterior lighting to my existing home that consists of the following:

- 600 watt transformer for low voltage lighting (12v) which will be about 65% loaded
- 2 exterior fixtures at the driveway entrance (110v)


Based on the convenient location of a couple existing lines and switches - I am wondering if a circuit with the following load can handle the additional items above. The circuit has a 15 amp breaker and powers the following:

- Front Stairwell - 2 x 60w fixture
- Front Interior Chandelier - 6 x 60w fixture
- Front Exterior Chandelier - 3 x 60w fixture
- Exterior Spotlights - 2 x 75w fixture
- Dining Room Chandelier - 9 x 60w fixture
- Laundry Room - 2 x 65w can lights (floodlights)
- Front Hallway - 2 x 65w can lights (floodlights)

These lights are not all on at the same time - but my gut says there is no more room on this ciruit as I believe I am already drawing 9 amps or so. But I thought I would see if someone with more experience could help clarify this for me.

Can the circuit handle the additional load or some subset of what I want to add? Any help is appreciated.

Thank you
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Old 01-05-2009, 05:53 AM   #2
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Can this circuit handle the load


Looks like your already over the maximum watts allowed on the 15A circuit - without adding to it.

115 volts x 15 amps = 1725 watts - derated to 80% (required) = 1380 watts allowed on the circuit.

Some are more conservative and only use 110 volts in the calculation.
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Old 01-05-2009, 06:38 AM   #3
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Can this circuit handle the load


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Originally Posted by Rivethead View Post
Looks like your already over the maximum watts allowed on the 15A circuit - without adding to it.

115 volts x 15 amps = 1725 watts - derated to 80% (required) = 1380 watts allowed on the circuit.

Some are more conservative and only use 110 volts in the calculation.

What code section makes you derate a circuit to 80%? and its 120 volts not 115... so a 15 amp circuit can be maxed out to 1800 watts.

Re-read the definition of Continuous Load.

To the OP: I say your good to go!
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Old 01-05-2009, 03:56 PM   #4
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Can this circuit handle the load


I say you will be overloaded. Did you include the fixtures at the driveway in the wattage you listed?
You must include the full wattage of the transformer regardless of the intention of using it at 65%.

I come up with 1610 watts already on that circuit. add 390 for the 65% use of the transformer and the two fixtures outside( based on 4, 60 watt lamps) and I come up with 2240 watts.
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Old 01-05-2009, 04:24 PM   #5
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Can this circuit handle the load


"600 watt transformer for low voltage lighting (12v) which will be about 65% loaded"

so the voltage will be above 12.0v due to the poor regulation of the transformer.

I'd put a series resistor upstream of the lamps to give you longer lamp life and reduce xformer loading.


There is a correction factor in the NEC that roughly says: the more appliances you have, the less likely they will all be on at once. I forget what its called or if it applies here.

Last edited by Yoyizit; 01-05-2009 at 04:54 PM.
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Old 01-05-2009, 05:08 PM   #6
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Can this circuit handle the load


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Originally Posted by jbfan View Post
I say you will be overloaded. Did you include the fixtures at the driveway in the wattage you listed?
You must include the full wattage of the transformer regardless of the intention of using it at 65%.

I come up with 1610 watts already on that circuit. add 390 for the 65% use of the transformer and the two fixtures outside( based on 4, 60 watt lamps) and I come up with 2240 watts.

I re did my math and I came up with 1610 as well, so my mistake and the circuit would be overloaded.
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Old 01-05-2009, 05:47 PM   #7
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Can this circuit handle the load


Thanks everyone for the input and confirming my fear here. I think I will just pull another 20 amp circuit off the sub-panel and use it dedicated for all outdoor lighting (and run a couple outdoor outlets for holiday lighting while I'm at it).

This is a bit more work (having to add breakers and more switches) - but appears to be the right way to go.

Thanks again.
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Old 01-05-2009, 08:10 PM   #8
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Can this circuit handle the load


youre over for a 15Amp breaker and before you slap in a 20 Amp, what size wire is at the panel and how far is the run?
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Old 01-05-2009, 08:12 PM   #9
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Can this circuit handle the load


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Originally Posted by II Weeks View Post
youre over for a 15Amp breaker and before you slap in a 20 Amp, what size wire is at the panel and how far is the run?

The OP is going to pull another circuit.
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