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Why 15A for #14, 20A for #12, and 30A for #10?

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    Why 15A for #14, 20A for #12, and 30A for #10?

    I never really put much thought to it that the NEC states that #14 is limited to 15A, #12 is limited to 20A, and #10 is limited to 30A. Amazingly then, through divine intervention, suddenly #8 is good for, not 40, not 50, but 60-70A or more, depending on the temperature rating. The limit on #14, #12, and #10 has been in the NEC as long as I can remember (early 80's, anyway). But another post regarding the 80% limit on breakers got me thinking about it.

    In a nutshell, why? Where does this come from, and what documentation substantiates this limit? The conductor is by calculation capable of carrying more then the NEC limit of Table 310.16. I looked on manufacturers website, and I looked through IEEE 835. The tables in IEEE 835 appear to have some similarity between them and the NEC, but not exactly, such as ambient temperatures are 40 deg C not 30 deg C. In reviewing IEEE 835, the tables are much more detailed, and specific, whereas the NEC tables seem to be more of a summary, one size fits all.

    So, the question is, can anyone provide some insight as to the engineering validity of the NEC cable ampacity tables? Otherwise it seems #14 should be able to be used on a 20A breaker for house wiring. After all, you can only load the circuit to 16A, and the conductor is actually good for 20A.

    I hope this brings some good discussion.
    "Just because you're paranoid, doesn't mean they're not out to get you"

    #2

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      #3
      Remember that conductors #1 and smaller are limited to the 60 degree ampacity unless you know that the terminations are rated for 75 degrees. See 110.14(C). Also keep in mind that NM cables are always limited to the 60 degree ampacity - see 334.80. This may be due, in part, to the fact that these conductors are often installed where we have little control over the loads to be placed on the conductors, such as residential general use receptacles. It may also be due, partly, to the fact that small conductors are less likely to disapate heat, especially at the terminations.

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        #4
        UL test breakers in open air, but when they are installed they are placed in an enclosure. This enclusure has other breaker in it which are a source of heat. This is why the 80 percent limit come to place

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          #5
          A 100%-rated CB and the end use equipment have been tested to verify that the additional heat generated by the 100% continuous loading conditions is safely dissipated. Other equipment specifications also are driven by the need to dissipate the heat associated with the level of heat rise achieved during 100% rated testing. In cases where the temperature at the CB wiring terminals exceeds 50 [degrees] C during 100% rated testing, UL 489 requires the use of 90 [degrees] C insulated wire (sized at the 75 [degrees] C ampacity) with these CBs, and the CB must be marked as such by the manufacturer. UL 489 also specifies minimum enclosure size and venting requirements if needed for heat dissipation. A CB that successfully has passed these additional tests is still not listed for application at 100% of its rating for continuous loading unless it's marked as such by the manufacturer.
          Ed Samples - Florida

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            #6
            I think we are getting away from the intended question. We can easily verify that a #12 THHN conductor has an ampacity, at 90C, of 30 amps. However, 240.4(D) limits its overcurrent protection to 20 amps. Why? What is the historical basis for 240.4(D)?

            Kingpb: Did I summarize your question correctly? If so, I do not know its answer.
            Charles E. Beck, P.E., Seattle
            Comments based on 2017 NEC unless otherwise noted.

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              #7
              Could it be over caution? I mean these are the most popular wire sizes for every day uses. We know people in general do not know anything about electricity and they keep over loading circuits with plugging in to many devices.

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                #8
                Originally posted by kda3310
                Could it be over caution? I mean these are the most popular wire sizes for every day uses. We know people in general do not know anything about electricity and they keep over loading circuits with plugging in to many devices.

                Isn't that why we install overcurrent protection?
                "Electricity is actually made up of extremely tiny particles called electrons, that you cannot see with the naked eye unless you have been drinking."

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                  #9
                  I want to get back to the whole 80% thing. I know what the code says. Is it true that a 20A breaker loaded to exactly 20 amps will eventually trip? I asked this because i'm being pulled both ways in school!

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                    #10
                    I have seen so many devices on one circuit that it was reading 22 amps on a 20 amp breaker that was not tripping in an office. The office is open over eight hours and was currying the load for days and was not tripping. I am just saying there are something's that can happen when you use something that often.
                    Last edited by kda3310; 10-01-07, 04:38 PM.

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                      #11
                      Originally posted by kda3310
                      Could it be over caution? I mean these are the most popular wire sizes for every day uses. We know people in general do not know anything about electricity and they keep over loading circuits with plugging in to many devices.
                      This sounds most plausible. General-purpose receptacle circuits are the easiest type to abuse, and are the circuits that use these smaller conductor sizes.

                      Anything larger than #10 is almost always used for specific-purpose circuits, such as those described in Art.'s 240.3 and 240.4 ('02 NEC), and easy to predict.
                      Master Electrician
                      Electrical Contractor
                      Richmond, VA

                      Comment


                        #12
                        Originally posted by LarryFine
                        This sounds most plausible. General-purpose receptacle circuits are the easiest type to abuse, and are the circuits that use these smaller conductor sizes.

                        Anything larger than #10 is almost always used for specific-purpose circuits, such as those described in Art.'s 240.3 and 240.4 ('02 NEC), and easy to predict.
                        I have to agree. The other day i inspected an (industrial) MCC room. The electricians were pulling 12 AWG circuits for the receptacles and lighting. I looked in a box and it had three circuits in the same conduit. I said to the foreman "OK, let's see. THHN; 30 Amps. 6 current carrying conductors; derated to 80%, that's 24 Amps Ampacity. 20 Amp breakers; good to go."

                        IMHO, it's back to the fudge factor again. There is simply, probably, a good historical basis for keeping these at 15,20, and 30. :-?
                        Eric Stromberg, P.E. Texas

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                          #13
                          opps, misread
                          Last edited by K2500; 10-01-07, 07:35 PM.

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                            #14
                            rating

                            I may be mistaken, but I believe it has to do with the fact that most 15 and 20 amp devices being rated at 60 deg. In accordance with 110.14(C)(1) . If the termination device is 60, then you have to use the wire at 60.
                            At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

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                              #15
                              Is it true that a 20A breaker loaded to exactly 20 amps will eventually trip?
                              The trip curve for QO 20 amp breaker shows that it will trip between 300 seconds and never with a 20 amp load. It also shows a trip time of 80 seconds to never for a 24 amp load.
                              Don
                              Don, Illinois
                              (All code citations are 2017 unless otherwise noted)

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