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Discussion Forum

Electrical Circuit Capacity

ripmeister | Posted in General Discussion on March 10, 2011 12:30pm

So I have done a thorough search of the site and found no definitive answer on the capacity of electical circuits.  There is some general stuff and I realize a certain part of the answer is it depends.  Having said that lets look at a 20 amp general circuit wired with 12 gauge.  If we start with the general rule of not exceeding more than 80% of the circuits capacity we arrive at a recommended max load of 1920 Watts (= 20 amps x 80% x 120 volts).  I’ve seen various posts for 20 amp circuits not having more than 10 or 12 duplex outlets on them.  But we also have to deal with fixtures and their wattages.  In addition if you try to design based on the 1920 Watts what wattage value do you assign an outlet, or do you?  Obviously that depends on what is plugged in to the outlet.  I would think there would be a general rule for the wattage applied to a duplex outlet but I don’t know.

So, I’d like to get the electricians explanation on this by listing a couple of mock circuits and trying to come up with a forumla that can be applied to these situations.  I know its not black and white but for example how can we apply a general rule to say a recepticle counts for this much wattage so we can then design a circuit to take full advantage of the available 1920 watts?  Thanks

Circuit 1-         6 dual recepticles

                          6 can lights with 50 watt bulbs             300 Watts total

                          Ceiling  fan                                               25 Watts total

                          Chandelier (3 lights x 60 watts ea      180 Watts total

                           Total Fixture Wattage                           505 Watts

 

Cirucit 2-         10 dual recepticles

                          Two ceiling lights at 100 watts each         200 Watts total

                           Total Fixture Wattage                               200 Watts

 

Circuit 3-           3 dual recepticles

                            Ceiling fans (x3) @ 25 watts each          75 Watts total

                            Ceiling fan lights (x12 @ 60 watts ea.     720 Watts total

                            4 Ceiling lights @ 100 watts each           400 Watts total

                            2 Outdoor floods at 150 watts each          300 Watts total

                             Total Fixture Wattage                                  1495 Watts

                           

                            

         

 

 

 

 

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  1. User avater
    xxPaulCPxx | Mar 10, 2011 01:13pm | #1

    Don't mix lights and outlets

    Keep them both on seperate circuits.  I know this is just a mental exercize, but I think it's flawed at the outset.

    One thing that has changed over time since we started putting outlets in homes is that each individual appliance that gets plugged in gets more powerful AND uses less energy.  The problem the modern home has isn't dangerously overloaded circuits, it's not having enough outlets to provide power to all the devices we use!

    Let's take a look at my kitchen as an example:

    I've got 20 seperate devices plugged in, spread among 32 outlets, divied up among three 20 amp circuits.

    In the bedroom we use as our office, I have 17 devices plugged in, spread among 28 outlets, all on one 20 amp circuit.

    Just going on how many outlets or recepticals are on a circuit really means nothing, unless you are running a shop that actually uses him amp gear.

    When I switched over from our old service panel to the new one I installed, there was a transition period as I cut circuits over from one side to the the other, then I had to wait to get the inspection.  During that time - about 4-6 weeks, this family of 4 with an electric oven, microwave, 2 refrigerators, 3 computers, laser printer, and a guy who never shuts off a light (me) managed to never trip the 40amp breaker that backfed out new panel and provided ALL the power our home used.  Never once.

    1. ripmeister | Mar 10, 2011 02:03pm | #2

      Capacity

      Paul,

      I appreciate what you are saying but that really isn't a practical approach and as far as I know its not a code issue.  You certainly could rewire or reroute everything but you could also do a teardown and start from scratch.  What I need is a practical approach to assessing a curent system and what can be done to expand that system it the most efficient manner and that would seem to start with how you look at or measure  the load on a circuit relative to its capacity.

      1. Tim | Mar 10, 2011 02:42pm | #3

        Separating lighting from

        plug loads is not a residential code requirement. But it really is a very practical and universally accepted as "good practice" approach. Not doing so, is generally considered unwise.

        The code (the National Electrical Code) limits the number of openings in a circuit (each and every switch, junction, receptacle, light or similar wired device counts aas an opening). This and practical wiring considerations will limit the number of receptacles "designed" into a residential circuit. Kitchens and laundry rooms have specific requirements. Get a copy of the latest NEC and read it. Ask questions about the parts that aren't clear.

        Can you legally have twenty (20) 15A duplex receptacles in a single 20 circuit? Yes. Is it wise? Probably not, but depending on the specific circumstances, might be acceptable.

        Looking to evaluate how much existing circuits are loaded and how much, if any, suplus capacity is available? Evaluating load vs capacity is a matter of simple math and you seem to have that under control.

        Maybe I don't fully understand what's being asked.

      2. DaveRicheson | Mar 10, 2011 02:51pm | #4

        Practical

        Do a load check on each of your three circuites. Use calmp on meter and check the amp draw with everything normally plugged in and operating. Then  plug an additional device, like vacum cleaner, into each circuit and check the load agian. 16 amps is the 80%  "practical" load you are talking about, but is not a hard ceiling. We have pushed many 20 amp circuits to 18 amp loading with no nusient tripping durring nermal useage. It does become an issue however when someone plugs in a 1500 watt space heater. :)

        As pointed out most devices, such as electronics, produce a very small increase in loads. It is the large irregular loads that will cause your problems. So what is the chance you will have one, two or three of that type of load operating at the same time, on the same circuite? What is practical for your life stile is a more realist question.

  2. junkhound | Mar 10, 2011 03:38pm | #5

    What Paul and Tim already said, be practical.

    cr@p on the code, imo just for schmoos, mfg, and the insurance companies

    1. ripmeister | Mar 10, 2011 03:54pm | #6

      Capacity

      Thanks All.  You pretty much said what I expected that there is the code and then the real, practical world.  A good example I guess would be my shop circuit where I put in a bunch of recepticles for the convencience of plugging in a tool without  a bunch of extension cords.  I'm not going to use more than one tool at a time so the actual number is moot.

  3. gfretwell | Mar 10, 2011 11:02pm | #7

    "The code (the National

    "The code (the National Electrical Code) limits the number of openings in a circuit (each and every switch, junction, receptacle, light or similar wired device counts aas an opening). "

    I would like to hear a cite for that.

    The fact is in residential construction there is no limit to how many outlets you have on a circuit.

    The general lighting circuits (120v circuits, not in the kitchen, bathroom or laundry) are based on an ampacity calculation of 3VA (think watts) per square foot and the only requirement is that the outlets be evenly distributed across the number of circuits the calculation requires. Outlets are actually placed by wall space and interuptions along the baseboard (no more than 6' from a receptacle). In essence the limit is really on square footage served, not the number of outlets.

    1. Scott | Mar 11, 2011 12:33am | #8

      >>>The fact is in residential

      >>>The fact is in residential construction there is no limit to how many outlets you have on a circuit.

      Dunno about your code (I'm in Canada), but we tend to be fairly parallel to the NEC.

      Our code has a hard-and-fast limit of 12 devices for any circuit that includes plug-in receptacles. I'm guessing that this is because there's no telling what load someone's going to plug in. Makes sense to me.

      Otherwise, we're allowed to calculate the number of devices based on ampacity, with no limit to the number of devices.

    2. DanH | Mar 11, 2011 06:45am | #9

      I'm thinking there is a limit, but when you read the fine print it only applies to commercial.  It's not obvious that it's a commercial-only limitation, though, until you do that fine-print reading.

      The NEC is one of the most obscure and impenetrable documents in existence, outside of a sports or movie actor's contract.

      1. gfretwell | Mar 11, 2011 10:42am | #10

        When you understand how receptacles are required to be placed it becomes clear why they use the square footage to determine the ampacity of the circuits that supply them. If you have a chopped up room (lots of doors and wall segments) you will end up with a lot of receptacles that may never be used. Personally I think they should raise the VA per square foot in a residence but there does not seem to be a big problem with the way they do it now. Most houses do end up with more general lighting circuits than are required in the code. A 1600 sq/ft house could legally be served by just two 20a circuits plus the laundry, (2) kitchen and bathroom circuits.

        1. ripmeister | Mar 11, 2011 11:41am | #11

          Thanks Again

          Thanks again for all the responses.  I agree that the code is impenetrable.  When you look at most aspects of the building code things are pretty clear and specific.  That doesn't appear to be the case with electrical with there being a lot of "it depends" situations.  I guess one needs to just fall back on common sense in this case.

      2. Scott | Mar 11, 2011 02:13pm | #12

        >>>The NEC is one of the most

        >>>The NEC is one of the most obscure and impenetrable documents in existence,

        Really?

        I read ours, the residential section only, from cover to cover and actually enjoyed it. Guess I'm just a sick fooker.....

        :-0

        1. gfretwell | Mar 11, 2011 03:01pm | #13

          The NEC has a vertically integrated development process. It is run by NFPA by electricians for electricians. They do not have the same situation that you see with the various building organizations. You also find the NEC is adopted as a standard when most building codes are excerpted from other documents and written as black letter law. That is why you can usually download your building code right from the "statutes" pages of your local/state government web site but the NEC is still a copyrighted document.

          My 2008 CD (code & handbook) cost me $185 and there is very little wiggle room in that price.

          I can say that once you get comfortable in the code it becomes natural. They do rewrites every few cycles, trying to make it more consistent and easier to read. Sometimes that works, sometimes they fix one thing and break something else.

          1. Scott | Mar 11, 2011 03:14pm | #14

            >>>the NEC is still a

            >>>the NEC is still a copyrighted document

            Yeah, ours is copyrighted too, so I borrowed it from a library.

            Interestingly, I remember Bill Hartmann (in the "old days") posting a link where you could download the NEC. I think it was from somwhere in N. Dakota.

        2. DanH | Mar 11, 2011 09:08pm | #15

          There isn't a "residential

          There isn't a "residential section", at least not in the 20-year-old version I have.  Rather, residential and commercial/industrial are mixed through the first several chapters, with all sorts of exceptions and special cases.  Only the back few chapters are all specific to commerical or industrial.  And no sort of index or reasonable table of contents, so often impossible to find the section you're looking for.

          I read most of the first half, but after a while you realize that in one section they're contradicting what they said in another section.  So you have to go back and carefully compare the sections and try to divine what conditions make one or the other apply.

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