how many light fixtures per switch / circuit?


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Old 12-15-07, 06:52 AM
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how many light fixtures per switch / circuit?

I have a contractor finishing out a somewhat large room in my basement. It is going to be used for a gym and we've decided to make the lighting very bright, but put it on a dimmer.

Currently he has 22 can lights in the ceiling and we were going to run this on one dimmer switch.

a few questions:

1) If we use 45 watt or 90 watt bulbs (or whatever you usually use in a can light) how many fixtures can be used safely in this situation?

2) Does this need a dedicated circuit? how many amps?
 
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Old 12-15-07, 07:20 AM
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Most standard dimmers are only rated at 600 watts. That is with all the heat sink tabs attached. If you put these dimmers in a multi gang box you will have to break off tabs on the side which will reduce the amount of watts that the dimmers are allowed to handle. There are dimmers that can handle up to 1500 watts I believe but they are very large and not so pretty. Also they can only be put in a single gang box.
So, 22 cans on one dimmer is not going to fly no matter what size lamp you use. You have to figure the largest wattage that you can use and that will determine how many dimmers you will need.
The standard can PAR 30, 65 watt lamp, you could put a max of 9 on one dimmer. Remember that is with all the heat sink tabs attached. It will be less if you put them all in the same box. So you will need 3 maybe 4 dimmers to control the lights.
You will also need one 15 amp dedicated circuit for sure. I would pull 2 circuits just in case some day down the road you want to lamp the cans with even higher wattage lamps.
 
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Old 12-15-07, 08:15 AM
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If I used this 1000 Watt Insteon switch, could we do it with 2 switches?

http://www.smarthome.com/2476dh.html

How many different breakers would we need if they are 15amp?
 
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Old 12-15-07, 10:05 AM
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Don't try to see how few switches you can get by with. That doesn't make any sense. Don't focus on what you "can" do, but what you want to do.

Stay well below the wattage rating of a dimmer. The closer you get to the rating, the hotter the switch will be.

Also, the more switches, the more control you have over the lighting. Control is always good.

I'd probably stick to no more than six lights on any one switch. That means four switches. And don't put them all in the same electrical box.

22 lights times 65 watts is 1430 watts. That theoretically fits on one 15-amp circuit.
 
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Old 12-15-07, 11:03 AM
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That many lights, I'd consider dimmable fluorescents irrespective of how they were wired.
 
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Old 12-15-07, 06:15 PM
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to Michael Thomas: do you mean compact fluorescent bulbs? if so, are you suggesting this to use less electricity?

--------------------------------------------------------

to Anyone: here's the dumbest couple of questions ever, but here goes:

1) when a lightswitch is rated 600 watts, then the maximum number of 60 watt bulbs that it should be attached to is 10? is it that simple?

2) when John says "22 lights times 65 watts is 1430 watts. That theoretically fits on one 15-amp circuit", how do we figure that? I remember Ohm's law (V=IR) but how do we figure watts vs amps?
 
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Old 12-15-07, 08:15 PM
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1) Yes, it's that simple. Add up the inoput wattage of the bulbs and do mnot exceed 600 watts if the dimmer is a 600 watt dimmer.

2) P = VI. Power = voltage times current. Power in watts equals volts times amps. Power = 120 volts times 15 amps. 120 volts times 15 amps = 1800 watts.
 
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Old 12-16-07, 09:03 AM
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by code you are only allowed to use 80% of the circuit. so if it is 14 gauge/ 15 amps circuit. 120 volts x 15amp = 1800 watts x.80 1440 watts.

Then you have to go by the can ratings not light bulbs, which are either 75 watts or 150 watts. Now you divide that into the 1440 watts so @ 75 watts rated cans will allow you 19 cans. @ 150 watts you are allowed 9 cans.

you can feed a three gang box and use one branch circuit and use three 600 watt dimmers to split the 19 cans on a 15 amp circuit.

I'd use more dimmers then fewer with three every third light could be controlled by one dimmer

if you use 12/2 nmb w/ground that is a 20 amp circuit 120 vac x 20 = 2400 watt x.80 = 1920 watts @ 75 watt cans is 25 @ 150 watts cans is 12
 

Last edited by brewaholic; 12-16-07 at 09:57 AM.
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Old 12-16-07, 11:24 AM
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Originally Posted by brewaholic
by code you are only allowed to use 80% of the circuit. so if it is 14 gauge/ 15 amps circuit. 120 volts x 15amp = 1800 watts x.80 1440 watts.

Then you have to go by the can ratings not light bulbs, which are either 75 watts or 150 watts. Now you divide that into the 1440 watts so @ 75 watts rated cans will allow you 19 cans. @ 150 watts you are allowed 9 cans.

you can feed a three gang box and use one branch circuit and use three 600 watt dimmers to split the 19 cans on a 15 amp circuit.

I'd use more dimmers then fewer with three every third light could be controlled by one dimmer

if you use 12/2 nmb w/ground that is a 20 amp circuit 120 vac x 20 = 2400 watt x.80 = 1920 watts @ 75 watt cans is 25 @ 150 watts cans is 12
Okay, thanks. I think this is what I'm going to ask the contractor to do. (they are 75 watt cans... 20 amp circuit... 22 cans... 3 dimmers)
 
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Old 12-16-07, 11:42 AM
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Originally Posted by someguy in KC
Okay, thanks. I think this is what I'm going to ask the contractor to do. (they are 75 watt cans... 20 amp circuit... 22 cans... 3 dimmers)

three gang deep box to make sure the cu inches meet the wiring and device space requirements.

this contractor knows what he is doing electrically,doesn't he?
 
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Old 12-16-07, 12:31 PM
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Remember a 20A circuit requires 20A wire so be sure he is using 12/2 wire.
 
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Old 12-16-07, 02:20 PM
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For residential wiring you can use up to the full capacity of a lighting circuit. That means all the way up to 15 amps or 20 amps. Residential lighting is NOT considered continuous use.

That being said, I would not exceed 80 percent, especially in the design phase as you never know what may change in the future.
 
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Old 12-16-07, 02:34 PM
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where did you get you license from??

residential branch circuits can not exceed 80% of their rating
 
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Old 12-16-07, 02:40 PM
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Originally Posted by someguy in KC
to Michael Thomas: do you mean compact fluorescent bulbs? if so, are you suggesting this to use less electricity?
Less heat, more lumens per watt, less frequent maintenance (significant if the cans are in a high ceiling). You still have to spec the circuits for the maximum requirement of the fixtures per their labeling, however.
 
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Old 12-16-07, 02:46 PM
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brewaholic, please help us learn. I also believe the same as Bob that the 80% rule only applies to certain circuits, not to all residential circuits. I would like to learn more. Can you provide us with the code section to which you refer? Thanks.

Also, brewaholic, are you in the United States?
 
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Old 12-17-07, 04:02 AM
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nec 210.19 [a] [1] for one
 
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Old 12-17-07, 04:45 AM
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Originally Posted by brewaholic
nec 210.19 [a] [1] for one
Lighting in a residence is NOT considered continuous, so this section does not apply.
 
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Old 12-17-07, 06:54 AM
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There are many different 80% rules in the code, including the one cited above. Each of them applies only to the specific situation described in its context. There is no universal 80% rule, so these things need to be taken on a case-by-case basis.

Even when not required, staying within 80% of the circuit's capacity is usually a good idea. However, proper application of the code requires that we clearly differentiate between what is required and what is recommended.
 
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Old 12-17-07, 08:20 AM
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210. 23(a) 15 20 amp branch circuits

the whole thing with this guy trying to formulate his branch circuit at 100% is ill advised. 80% of a 15-20 amp lighting branch circuit is the way to formulate.

if he leaves the lights on for 3 hours that itself meets the continuous load you speak of.
 
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Old 12-17-07, 08:33 AM
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I allowed that I would never design a lighting circuit to be more than 80 percent loaded.

The code does not consider a lighting circuit in a residence to be a continuous load. Period.

The NEC does not take into account how you are going to use a lighting circuit. Even if the homeowner leaves the lights on 24 hours a day, the NEC does not consider the circuit to be a continuous load as implemented.
 
 

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