# How many light/switches per circuit?

Discussion in 'Electrical and Wiring' started by leeym, Aug 13, 2006.

1. Aug 13, 2006

### leeym

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Is there a way to calculate how many switches or lights I can attach in series to one single circuit in my house? How does this calculation relaye to the typs of romex cabling I'm using?

Thanks

2. Aug 13, 2006

### Square Eye

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I try to keep my 15 amp circuits as close to or below 1000 watts as I can, although, it will hold 2,000 watts with little trouble.
You will have to check the watts ratings on your switches also!
Just add up the total of the wattage of the bulbs you intend to use.
Match that to switches and circuits that can handle the watts.
For a 20 amp circuit, You could possibly go as far as 3,000 watts with no problems.
These figures are at 120volts with a little under the 80% of the circuit amperage tolerance allowed by code.
NEVER operate a circuit at full rated amperage. The rated values are there so you can make responsible and safe decisions about what you install in your home.

Once again, check the switch watts rating. Keep the total at 80% or below the rated watts of the switch.

Welcome to the forum,
Tom

3. Aug 14, 2006

### petey_racer

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For 120v lighting circuits, a 15 amp circuit can handle 1800 watts. A 20 amp 2400 watts.

For a continuous load you must use the 80% loading rule. Almost nothing in home is considered a continuous load, especially lighting. Common sense does tell us though to keep loads under these full current numbers.

Circuits for lighting MUST be sized according to the maximum wattage a fixture is rated for. If you use 150 watt recessed fixtures, but are using 65 watt lamps, you still use 150w to figure the circuit load.

Switches are rated in amperage, NOT wattage, and must be sized according to the load they control. So do not switch fifteen 150w fixtures with a 15 amp switch.

Residentially I use 15 amp circuits almost exclusively. I HATE having to fight with #12 wire in multi-gang boxes with 3&4 ways and dimmers.
So I lay out my lighting circuits to have under 1800w worth of fixtures. I generally go much lower to allow for future changes/additions.

4. Aug 14, 2006

### Square Eye

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Petey,

Nearly every dimmer switch has a maximum watt rating. The most common being 600watts. If you tell this guy that any 15 amp switch will work, what if he assumes that he can use any dimmer on any 15amp circuit?

A 15 amp switch speaks for itself. Anyone who knows anything about electricity can understand that a 15 amp switch can't handle a 20 amp load for very long.

5. Aug 14, 2006

### petey_racer

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OK, that's dimmers. He specifically mentioned switches so that is what I stated. If I was referring to dimmers I would have stated so.

YES, dimmers are rated in watts. Keep under the rated wattage for dimmers so they don't get too hot, and don't forget to derate dimmers when ganged.

How's that?

6. Aug 15, 2006

### manhattan42

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The National Electric Code does not restrict the number of switches or luminaires that can be placed on any circuit.

The rule is that the wattage rating of all luminaires does not exceed the maximum wattage of the circuit.

For a 15 amp circuit, the total amount of load drawn by all luminaires cannot exceed 1800 watts.

Volts X Amps = Watts
120 Volts X 15 Amps = 1800 watts.

For a 20 amp circuit, the total amount of load drawn by all luminaires cannot exceed 2400 watts.

No limit on the amount of switches.

A switch is a device. A device is defined by the Code as "a unit of the electrical system that is intended to carry but not utilize electrical energy."

Since switches do not (normally) consume energy, their amount is not restricted.

7. Feb 10, 2009

### dtnagle

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hey guys, good info so far. I am rewiring a old house with tube and knob wiring. I ran 14/2 from the new 100amp circuit box. Going to the bedrooms upstairs for the lights. I had the same question as above. But we are planning to put ceiling fans with lights in the bedrooms. How do you figure the wattage usage for that combination (not bought yet). Also is 14/2 heavy enough? One light/fan per room and one closet light x four bedrooms. Can I use one breaker for the whole upstairs? what size? 20amp?
Dan

8. Feb 10, 2009

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