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Old 03-24-2010, 12:18 PM   #1
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How to wire three switches on one circuit


I am not sure what the best way to wire my theater room.

I am going to have 6 can lights, rope light, and floor lights all controlled by separate switches and I am not sure how to wire this correctly on one 20 amp circuit. Does anyone have a diagram showing this or could they explain it to me.

Thanks.

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Old 03-24-2010, 01:37 PM   #2
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How to wire three switches on one circuit


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I am not sure what the best way to wire my theater room.

I am going to have 6 can lights, rope light, and floor lights all controlled by separate switches and I am not sure how to wire this correctly on one 20 amp circuit. Does anyone have a diagram showing this or could they explain it to me.

Thanks.
The most straighforward way electrically is to run the power feed (from the panel) to the switch location, aothough it may not be the shortest in terms of feet of cable. Continue the power feed to the next switch box if more than one.

From each switch, run a cable to the first of that group of lights and continue the cable to daisy chain all the lights in that group.

At a switch box connect together the black wire of the incoming power, the black wire of the continuing power if any (not to light fixtures) and short lengths (say 6") of black wire one for each switch.

The black wire of the cable going to a group of light fixtures is connected to the other terminal of the respective switch.

All of the white wires are connected together.

All of the bare wires are connected together with a wire nut, separate from the white wires. You will need extra short lengths of bare wire so each switch has one attached to its green screw and also the box itself, if metal, has one connected directly to it.

All wires must be 12 gauge or fatter if the circuit is to have a 20 amp. breaker.

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Last edited by AllanJ; 03-24-2010 at 01:45 PM.
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Old 03-24-2010, 01:41 PM   #3
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How to wire three switches on one circuit


You have 2400 watts available to you on a 20 amp branch circuit. be sure you stay within those limits with your lighting. Just add all the maximum fixture wattage ratings together.

A diagram is difficult with out more detail all the switches in one 3 gang box or are they in different locations. Any 3 way switches?

If single pole switches in 3 gang box this would be one way of doing things...these could be dimmers also.
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Old 03-24-2010, 02:37 PM   #4
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How to wire three switches on one circuit


Thanks for the info guys and Stubbie, awesome diagram, that is what I was looking for.
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Old 03-25-2010, 10:14 AM   #5
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How to wire three switches on one circuit


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You have 2400 watts available to you on a 20 amp branch circuit. be sure you stay within those limits with your lighting.
Isnt it true that you have to multiply that 2400 by 80% which would then be 1920 watts available?

Please correct me if I am wrong.
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Old 03-25-2010, 10:18 AM   #6
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How to wire three switches on one circuit


If it's simply a lighting circuit (known load) it's ok to come pretty close to the actual wattage limit. If you have outlets on the circuit, different story, because you never know what you will plug in. It might make sense to leave a little headroom, though, because maybe you'll want to add to it someday.
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Old 03-25-2010, 10:22 AM   #7
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How to wire three switches on one circuit


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Isnt it true that you have to multiply that 2400 by 80% which would then be 1920 watts available?

Please correct me if I am wrong.
Yes you are wrong. A 20 amp 120 volt circuit will provide 2400 watts. A continuous load for 3 hrs or more is limited to 1920 watts.
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Old 03-25-2010, 10:39 AM   #8
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How to wire three switches on one circuit


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Yes you are wrong. A 20 amp 120 volt circuit will provide 2400 watts. A continuous load for 3 hrs or more is limited to 1920 watts.
Thanks Brric for answering that question, it is a very common misconception by a lot of people. We often have to claify when the 80% deration applies to branch circuits as this question keeps popping up.....
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Old 03-25-2010, 10:42 AM   #9
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How to wire three switches on one circuit


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Thanks Brric for answering that question, it is a very common misconception by a lot of people. We often have to claify when the 80% deration applies to branch circuits as this question keeps popping up.....
YW. It's misunderstood by a lot of people including a lot of experienced electricians.
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Old 03-25-2010, 10:48 AM   #10
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How to wire three switches on one circuit


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A continuous load for 3 hrs or more is limited to 1920 watts.
Thanks for the clarification. But what would be considered to need a continuous load for 3 hours or more?

Only things I can think of 'continuously' would be TVs and computers and maybe electric heating.
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Old 03-25-2010, 11:04 AM   #11
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How to wire three switches on one circuit


Any thing that is on for 3 hours or longer at a time. Not really relevant to residential other than perhaps heating. Lighting in a store would be an example of a continuous load.
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Old 03-25-2010, 01:07 PM   #12
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How to wire three switches on one circuit


I should be safe. My can lights are 75 watt max, my rope light is 3 watts per foot, and my floor lights are 3 watts per light (3). I'll be under 1000 watts. I might add some wall sconces as well. This will be in my home theater room, so the lights won't always be on as well.
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Old 03-25-2010, 04:15 PM   #13
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How to wire three switches on one circuit


In a single family dwelling it is virtually impossible to control what people may operate on a general purpose branch circuit with multiple receptacles. There are NEC rules that cover this but the average homeowner would have no understanding of them.

A branch circuit breaker and branch circuit conductors must be sized to carry 100% of the non-continuous load plus 125% of the continuous load. For "small conductors" on a general purpose multi outlet cord and plug branch circuit there is no practical way to allow for continuous loads so we just use the conductor size and NEC rules for the circuit breaker that protects these small conductors on these types of branch circuits. That is 30 amps for 10 awg, 20 amps for 12 awg and 15 amps for 14 awg.

Continuous loads generally don't come into the picture on residential applications as brric mentioned till we are running individual (dedicated) branch circuits to single pieces of equipment like fixed resistance heaters or combo equipment like bathroom combination devices like heaters, fan and light units.

As an example if I was running a circuit to a fixed 120 volt bathroom heater mounted on a wall and lights. The heater was rated at 1800 watts and the light fixture was rated 100 watts. The required circuit breaker must be able to carry the amperage or wattage at 125% of the 1800 watts and 100% of the 100 watts so 1.25 x 1800 = 2250 watts plus 100 watts = 2350 Watts or therefore a 20 amp breaker is sufficient because the 1.25 compensates for the 80% loading buffer for the continuous load both for the circuit breaker and the branch conductors. Looking at this you will notice that 125% is the inverse of the 80% requirement or 1/1.25 = .80.

To see this more clearly consider only a continuous load of 1920 watts on a 120 volt branch circuit. 1.25 x 1920 watts = 2400 watts so a 20 amp breaker is all that's needed.

Remember though that I can still add a non continuous load to that 1920 watts as long as I do not exceed 2400 watts..

My point being that it is not a requirement to stop at 1920 watts period or the 20 amp branch circuit...it simply means that 1920 watts of continuous load is all that I can place on that branch circuit.

It might be helpful to look at only a 2400 watt continuous load at 120 volts on a branch circuit. This requires a conductor that can carry 125% of the continuous load and a breaker that can carry the same. There is no non continuous load so 1.25 x 2400 = 3000 watts or 25 amps. For a resistive load I would need 10 awg to comply with 210.19(A) and a minimum 25 amp breaker. Even though 12 awg is rated 25 amps at 60C and 75C I cannot breaker the 12 awg with a 25 amp breaker. In accordance with 240.4(D) a 20 amp breaker must be used for this type of load. So I must use 10 awg to allow me to place at least a 25 amp breaker on the circuit with the continuous load.

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