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Old 02-16-2009, 05:18 PM   #1
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How Many Lights on a 15 amp Circuit?


Thanks in advance for taking the time to answer my question.

I have a 15 amp circuit that goes into the attic and I want to install a series of screw-in type lightbulb fixtures.

I am planning to run those new 27-watt CF bulbs.

How many outlets could I feed on that one circuit?

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Old 02-16-2009, 05:38 PM   #2
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How Many Lights on a 15 amp Circuit?


A 15 amp circuit can handle 1440 watts, or equal to 24-60 watt lamps. Add up the wattage and do not exceed 1440 watts and you will be fine. This is for non- residential installations, but is widely used as a good guide, because the 1440 is really the max a 15 amp circuit is allowed to be loaded.

The code takes into consideration that it would be very unlikely most homeowners will not, and should not need to, worry about how much load is on a circuit.

Residential lighting circuits are size according to the square foot, using 3 watts per square feet of livable space , so a 1200 Sq ft home has an estimated 3600 watts of general lighting load. Next 3600 / 120 volts = 30 amps. The home would be wired with at least 2-15 amp circuits.

Even though you are using the less wattage ones, you need to size it for normal load that someone, other than you, might use the circut for.

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Old 02-16-2009, 05:48 PM   #3
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How Many Lights on a 15 amp Circuit?


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Originally Posted by ctsmiths View Post
This is for non- residential installations, but is widely used as a good guide, because the 1440 is really the max a 15 amp circuit is allowed to be loaded.
Actually a 15A circuit can be loaded to 15A, or 1800 watts for a 120v circuit.
The 1440 watt number is for continuous loads, and like was mentioned, residential lighting loads are not typically considered continuous loads.

You really can't say a 15A circuit is only supposed to be loaded to 80% period. It all depends on the load and the type of circuit.

See NEC Table 210.24 and section 210.23(A)
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Old 02-16-2009, 05:51 PM   #4
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How Many Lights on a 15 amp Circuit?


In a residential home I've always loaded a 15a circuit to 1800 watts based on the MAX bulb allowed by the fixture
So 18 100 watts fixtures

Residential lighting is not considered continuous & can be loaded to the allowed wattage of the circuit. I use almost all CFL's, so loading to 1800 watts by the fixture I still only use about 1200w if every light was on at the same time. I'm lucky to have 300w of lights on at any given time

As stated it doesn't matter what bulb you are using
It depends upon the Max wattage bulb that could be installed according to the fixture. IE my cans usually have a rating of 100w, but I only use the 23w CFL's
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Old 02-16-2009, 06:52 PM   #5
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How Many Lights on a 15 amp Circuit?


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Originally Posted by ctsmiths View Post
A 15 amp circuit can handle 1440 watts, or equal to 24-60 watt lamps. Add up the wattage and do not exceed 1440 watts and you will be fine. This is for non- residential installations, but is widely used as a good guide, because the 1440 is really the max a 15 amp circuit is allowed to be loaded.

The code takes into consideration that it would be very unlikely most homeowners will not, and should not need to, worry about how much load is on a circuit.

Residential lighting circuits are size according to the square foot, using 3 watts per square feet of livable space , so a 1200 Sq ft home has an estimated 3600 watts of general lighting load. Next 3600 / 120 volts = 30 amps. The home would be wired with at least 2-15 amp circuits.

Even though you are using the less wattage ones, you need to size it for normal load that someone, other than you, might use the circut for.
Here we go again. The old 80% rule...
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Old 02-16-2009, 06:57 PM   #6
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How Many Lights on a 15 amp Circuit?


Which begs the question

If the breaker is rated for 15a, the wire is rated higher
Then why can't you run it at Max capacity - or very close to it ?
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Old 02-16-2009, 08:26 PM   #7
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How Many Lights on a 15 amp Circuit?


The answer is mainly because of manufacturing tolerances.

No two breakers will trip at exactly the same amperage. Some 15's will hold 15 forever, others might trip at 14. The 80% rule is to minimize nuisance tripping. This is why it generally applies only to continuous loads.

Certainly, there are other factors, but this one makes the most sense.

Rob

P.S. Some large industrial breaker are listed for 100% continuous, and code allows their use at full rating.
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Old 02-16-2009, 08:41 PM   #8
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How Many Lights on a 15 amp Circuit?


I guess they can only go so far in codes
But its quite possible to load any 15a or 20a circuit to near capacity & leave it that way. The only time I ever unplugged anything from a circuit ( as a kid/young adult) was if I kicked the breaker out

I guess I just find it weird that they would regulate lights commercially when they can't regulate what will be plugged into the outlets. You just know if a business has a choice between paying an electrician to come in & add a circuit VS loading outlet circuits to close to 100% they will in most cases just load the circuits up

One place I worked they only ran new circuits when the breakers kept tripping - too many computers added
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Old 02-16-2009, 08:46 PM   #9
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How Many Lights on a 15 amp Circuit?


With commercial lighting the key is the word "expected" in the definition of "continuous load".
Commercial lighting IS expected to be on for three hours or more.
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Old 02-16-2009, 09:05 PM   #10
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How Many Lights on a 15 amp Circuit?


[quote=ctsmiths;231372]A 15 amp circuit can handle 1440 watts, or equal to 24-60 watt lamps. Add up the wattage and do not exceed 1440 watts and you will be fine. This is for non- residential installations, but is widely used as a good guide, because the 1440 is really the max a 15 amp circuit is allowed to be loaded.

The code takes into consideration that it would be very unlikely most homeowners will not, and should not need to, worry about how much load is on a circuit.

Residential lighting circuits are size according to the square foot, using 3 watts per square feet of livable space , so a 1200 Sq ft home has an estimated 3600 watts of general lighting load. Next 3600 / 120 volts = 30 amps. The home would be wired with at least 2-15 amp circuits.
quote]


Even though you are using the less wattage ones, you need to size it for normal load that someone, other than you, might use the circut for.[/


I was looking at a book about home wiring the other day and when I got to the chapter about sizing the load the author used the same example about residential lighting. He continued on with all the other required calculations for the laundry circuit, appliance circuit and so on and so forth. I got to wondering though what about the other general use receptacle load.

I happened to look at another book that I had and it stated that the general receptacle load was included in the lighting load calculation. Is this correct? Is this usually sufficient for todays homes?
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Old 02-16-2009, 09:08 PM   #11
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How Many Lights on a 15 amp Circuit?


Quote:
Originally Posted by Bill_in_NY View Post
Thanks in advance for taking the time to answer my question.

I have a 15 amp circuit that goes into the attic and I want to install a series of screw-in type lightbulb fixtures.

I am planning to run those new 27-watt CF bulbs.

How many outlets could I feed on that one circuit?
Any receptacles going on this circut as well, or other things such as ceiling fans or a whole house fan?
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Old 02-16-2009, 09:16 PM   #12
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How Many Lights on a 15 amp Circuit?


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Originally Posted by Steelhead View Post

I happened to look at another book that I had and it stated that the general receptacle load was included in the lighting load calculation. Is this correct? Is this usually sufficient for todays homes?
Sure it is. Keep in mind this is for load calculation purposes.

Sure it could be used as a BARE bones unrealistic minimum of circuits to run, but the real world is much different.
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Old 02-16-2009, 09:28 PM   #13
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How Many Lights on a 15 amp Circuit?


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Originally Posted by jerryh3 View Post
Here we go again. The old 80% rule...
Well it does apply if you are going to supply other equiptment on this circut, thats why I asked later if he was going to be feeding any thing else with this circut.
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Old 02-16-2009, 09:33 PM   #14
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How Many Lights on a 15 amp Circuit?


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Originally Posted by ctsmiths View Post
Well it does apply if you are going to supply other equiptment on this circut, thats why I asked later if he was going to be feeding any thing else with this circut.
You are misreading 210.23(A)(1). The 80% rule in this case is for any ONE piece of equipment. NOT for the whole circuit.

(1) Cord-and-Plug-Connected Equipment Not Fastened in Place. The rating of any one cord-and-plug-connected utilization equipment not fastened in place shall not exceed 80 percent of the branch-circuit ampere rating.
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Old 02-16-2009, 10:45 PM   #15
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How Many Lights on a 15 amp Circuit?


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Originally Posted by Speedy Petey View Post
You are misreading 210.23(A)(1). The 80% rule in this case is for any ONE piece of equipment. NOT for the whole circuit.

(1) Cord-and-Plug-Connected Equipment Not Fastened in Place. The rating of any one cord-and-plug-connected utilization equipment not fastened in place shall not exceed 80 percent of the branch-circuit ampere rating.
You misunderstand me, I am talking about the other equiptment. I was wondering if anything else was going on the circut and if so what it was. I dont dispute that the 80% rule is a rule of thumb for lighting for res work, not a hard and fast NEC rule.

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