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  1. #53
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    He is refering to section 240.4.D.5 and .7. Someone called it derating, but it is just known as code in the NEC. This section is refered to as a fine print note under table 310.15(B)(16). 240.4 (G) refers you to specific appliance permentations in table 240.4(G) on page 70-91 of the 2011 NEC.
    That takes us to article 440 parts 3 and 6 for A/C and ref. applications.
    From there, 440.32 and most of section 6 are interesting, but I don't really see anything poping out that would deviate from section 240.
    440.31 says the manu's don't have to go by the wiring sizes that we do, 440.32 says that we must use 125% of either the compressor rated load or the branch circuit selection current, whichever is greater. This is where some of our ramblings have came in on this post.
    440.34 says we have to take into consideration the amp draw of the fan and compressor when doing our 125% calc.
    It goes on and on, but there is no room for interpetation on the code. The FPN at the bottom of the 310 chart shows you where to go, and then that code shows you where else to go.
    And with all of this talk, it has been neglected that the posts have been referencing the use of 75 degree C wire, which is most likely THW. That is a very important component in the wire sizing.

    Guys on here used to refer to electricians as "Old Sparky". I don't hear that term used too much anymore. The NEC demands a little respect, attention and patience.
    I will write what the NEC says in 240 for our purposes for those without a copy. This is public info, so there should be no harm.

    240.4(D) Unless specifically permitted in 240.4(E) or (G)(E has nothing to do with AC for our purposes and G takes us to the table that lists article 440), the overcurrent protection shall not exceed that required by D(1) through D(7) after any correction factors for ambient temperature and number of conductors have been applied.
    240.4(D)(5) 12 AWG Copper. 20 amperes
    240.4(D)(7) 10 AWG Copper, 30 amperes
    So, backing out of this, our original poster said the minimum breaker size is 25 amps. The code says right there that 12 wire can only be protected by a maximum of a 20 amp breaker, so that leaves us having to use # 10 wire with either a 25 or 30 amp breaker. Can # 12 carry the 18 amps? Yes. Can it carry the 18 if that is what the machine actually uses and hasn't been calculated by the manufacturer? No, because we have to multiply by 125% for continuous load, and that gives us 22 amps. What the kicker is is the breaker size. When the manu stated a 25 amp breaker, they took us to #10 THW for outdoor use.
    Now, where is the translation? No matter what, remember this. The NEC or any other code is the bare ball minimum.
    Hope that helps.
    The main thing is to keep the main thing the main thing!

    If "the grass is greener on the other side", it likely has been fertilized with Bull$hit!

  2. #54
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    Jan 2003
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    SE Pa
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    http://www.iaei.org/magazine/?p=3944#more-3944

    The Branch Circuit Requirements
    The branch-circuit conductor size requirements for hermetic refrigerant motor-compressors are located in Part D of Article 440. The sizing requirements for the branch-circuit conductors are very similar to those requirements for standard motors. Basically, the branch-circuit conductors are required to be sized at 125 percent of the rated-load current of the single hermetic motor-compressor or 125 percent of the branch-circuit selection current, whichever is less. However, for combination-load equipment having a nameplate as required by Section 440-4(b), the branch-circuit conductors are required to be “not less than the minimum circuit ampacity marked on the” nameplate of the equipment (see Figure 2). See NEC Section 440-35.

    The manufacturer has already calculated the conductor size to be based on the total of all of the motor loads in the combination-load equipment times 125 percent. It is not necessary to do these calculations again. For this type of equipment, the installer and the inspector only have to install and verify that the branch-circuit conductors supplying the equipment have an ampacity equal to or greater than the minimum circuit ampacity marked on the nameplate of the equipment.


    Simply put:

    size your wire to MCA. on nameplate unless further derating is required for ambient temp or distance

    size breaker to MOCP on nameplate
    5 out of 4 people don't understand fractions

  3. #55
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    Quote Originally Posted by glennwith2ns View Post




    Simply put:

    size your wire to MCA. on nameplate unless further derating is required for ambient temp or distance

    size breaker to MOCP on nameplate
    Size the wire to atleast MEET MCA. On a long run, its better to meet MOCP size. Or calculate the size you need by allowable voltage drop.
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  4. #56
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    Quote Originally Posted by beenthere View Post
    Size the wire to atleast MEET MCA. On a long run, its better to meet MOCP size. Or calculate the size you need by allowable voltage drop.
    On a long run the MOCP still does not come into play for sizing the branch circuit conductor. Once you calculate the voltage drop you may need a wire several gauge sizes larger. glennwith2ns has it exactly correct. Read his posts and follow his instructions and you will have a code compliant job.

  5. #57
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    Lexington, NC
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    "125 percent of the branch-circuit selection current, whichever is less"
    I think that is which ever is more.
    You have to figure in for the fan motor as well per the 440 section.
    I think we need to define what the "branch circuit selection current" is.

    I'm just not seeing where 440 says we can overide the wire size 12/20 amps rule in 240. The wire size is still the wire size. The smaller wire sizes really aren't pointed out (that I saw last night anyway)in 440, but breaker size is.
    The main thing is to keep the main thing the main thing!

    If "the grass is greener on the other side", it likely has been fertilized with Bull$hit!

  6. #58
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    Nov 2008
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    SW Florida
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    Everyone should keep in mind that the NEC is only a minimum requirement in jurisdictions that have adopted it. If memory serves me correctly there are 8 states that have not adopted the NEC. That is not to say that lower jurisdictions in those states have not adopted it. Also some jurisdictions may have codes that excede the requirements of the NEC.

  7. #59
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    Sep 2010
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    Florida
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    Quote Originally Posted by nchvac View Post
    You are correct. But the minimum amperage rating that you all are talking about has to be multiplied by 1.25 to give you 22.5, rounded up to next breaker size because it is less than 800 amps will take you to 25, which the post posted as being the manufacturer's minimum breaker size.
    Maybe you haven't had a chance to read the link that [U]bigtime[U]posted earlier. It really does explain this. Here is the link again. http://www.mikeholt.com/mojonewsarch...t~20040102.htm . It is well worth the time it takes to read. Some of us have to read it a couple of times to understand it.

  8. #60
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    Quote Originally Posted by Gulfside View Post
    Everyone should keep in mind that the NEC is only a minimum requirement in jurisdictions that have adopted it. If memory serves me correctly there are 8 states that have not adopted the NEC. That is not to say that lower jurisdictions in those states have not adopted it. Also some jurisdictions may have codes that excede the requirements of the NEC.
    However, in Florida, we have the Florida Building Code that has adopted the NEC in its entirety without any changes and local jurisdictions are prohibited from admending the Code without bringing the proposed changes to the Florida Building Commission. It is next to impossible for local jurisdictions to justify why they are so different from the rest of Florida that they need admendments. That makes it a little easier for contractors that work in different jurisdictions around the state. You know the Code is the NEC so you don't have to try to find out what idiosyncrasies the locals might have.

  9. #61
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    I still am not seeing where they are taking into account article 240D. Kind of funny how he used a 24 amp circuit multiplied up to 30 instead of one that would fall under the limitations in 240D.

    I would also like to point out a major error in the article. He writes "Article 440 does not cover such household appliances as room air-conditioners, household refrigerators and freezers, drinking water coolers, or beverage dispensing machines, because these are appliances and must comply with Article 422."
    I knew when I read that the rest of the article should be read with caution because there is a section VII in 440 that deals just with Room A/C units. The author must not have proof read, because he points out that very article later in this document.

    We still have not been pointed to the code that states in any way shape or form that the limitation of 240D doesn't apply to A/C units. Yes, 240 does give exclusion to 440 if 440 differs, but no where in 440 does it say that you can run more than a 20 amp breaker on #12 wire. Please show us where it says that. That is where the confusion is.

    Can 12 handle more than 20 amps? Yes. But the NEC says that you must use a 20 amp breaker on that application.
    Again, this poster said the name plate said to use a 25 amp breaker, and according to article 240 you have to go to #10 wire because #12 cannot be protected with anything over a 20 amp breaker. It is that simple per the NEC. Once you go over #10 wire, you can go up for starting. But you aren't going to have that type problem on the small equipment. If so, you have a problem other than starting current.
    Since when do HVAC Talkers suggest doing the bare minimum, and in this case less than the bare minimum?
    The main thing is to keep the main thing the main thing!

    If "the grass is greener on the other side", it likely has been fertilized with Bull$hit!

  10. #62
    Join Date
    May 2010
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    Minnesota
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    This has been a very interesting thread.

    So, just to clarify somewhat in layman's terms, the NEC allows, in specific instances, such as the installation of dedicated A/C equipment (where the exact rating of the equipment is known), for a deviation in the overall requirement that the breaker should protect the downstream wiring.

    In this particular case, a 12-gauge wire can be used with a 25-amp breaker.

    As a layman, it is difficult for me to understand how anyone could guarantee that something could not happen to that device where it would somehow get locked into a continuous draw of 24 amps, not trip the breaker, but produce an extreme fire hazard in the overloaded wire.

    Can any experts enlighten me on this? Is there something about the combination of equipment and overload protection in the device that makes this virtually impossible?

  11. #63
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    Your compressor overload will protect or in the case of a motor starter setup, the heaters will protect. There is a section in 440 that, if I am not mistaken, spells out maybe 4 or 5 requirement scenarios for the type of protection you are speaking of if the breaker size is increased without increasing the wire size.
    Where the argument comes in is with the sizing of the wire to 12 gauge, and automatically uping the breaker. The code makes this variation to be able to be uped to 175% and then 225% succesively if the unit will not start. It does not allow this exclusion otherwise, so there is another "interpetation" to take into consideration, being can you automatically up the breaker size without knowing and having a starting problem. The fact is that the time delay should take care of any starting problems on residential type equipment in the 18 amp range.
    I have a feeling that this topic will be addressed in upcoming code issues.

    And as far as anyone helping write the code, everybody has the oppurtunity to send in suggestions with each issue of the code. I think the deadline has already passed for the 2014 code though, or at least the last CE instructor I had said it was.
    The main thing is to keep the main thing the main thing!

    If "the grass is greener on the other side", it likely has been fertilized with Bull$hit!

  12. #64
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    Quote Originally Posted by captain123 View Post
    On a long run the MOCP still does not come into play for sizing the branch circuit conductor. Once you calculate the voltage drop you may need a wire several gauge sizes larger. glennwith2ns has it exactly correct. Read his posts and follow his instructions and you will have a code compliant job.

    Did you miss the part where I said " or calculate the voltage drop".
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  13. #65
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    Dec 2003
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    Princeton Texas
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    12awg is good for 20amps. This we know but a 20amp breaker is only good for 80% of that load, which nocks you down to a continuous load of 16. So it would be good to know the exact amp draw of that unit running. Which by the way is going to be different in lets say Alaska as compaired to Here in Texas on a 115 dergree day with the sun shining on the unit. Thats why my AHJ and myself would up the wire to 10 and try a 25amp breaker. Have to remember that a breaker doesnt care about amp draw. It cares about heat, unless you get into commercial digital breakers and switch gear.
    Thats why code derating is done for ambient temps on roof tops, to account for heat.
    So for the one who ask me why my AHJ would make me change the wire, thats why. Its to close, yes it will work as is, but the purpose of the NEC now International code is for the purpose of life safety and the protection of equipment. When you are an electrician you are dealing with peoples lives. Same with a/c when you install a unit or hook up gas. So its better to error on the side of safety as compaired to: We now we have to spend more time pulling wire and the cost of copper is up and this loweres are bottom line. I like to sleep good at night knowing that I did a damn good job and the home owner has a good safe reliable system that will be trouble free. Not eeeewww weee hope that current hog of a unit wont trip that old 20amp breaker on that old 12wire when its 110 in the shade.
    When I do a/c work and electrical work in the country(rural areas) were there really is know code enforcement, some of the stuff I see is just scarry. In the fact that its actualy working and hasnt burned up. I worked on a crapy tempstar unit that the heat wasnt working . 3 strips each at 22amps. The guy before had used a 2pole 20amp contactor and bridged one pole. 22amps on one side and 44 on the other. It worked for one season and then fried the contactor and blew the fusses. This unit is also on 60amp breaker and was pulling 63 amps. It was working and the breaker was hot as hell. I told him that I left one strip out of the loop until he dicided if he wonted to leave it with two or upgrade the breaker and wire, and I placed a three pole 40amp contactor in the unit. I would not have sleeped knowing that I replaced a 20amp with a 20amp and pulling 22on one pole and 44 on the other on a 60amp main breaker.

    Redfive

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