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Thread: Breaker size

  1. #1
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    Breaker size

    not to beat a dead horse but i am going to install a 2.5 ton unit where a 2 ton was the mca on new unit is 15 with a max breaker size of 30.There is a 20 amp breaker installed with 12 guage wire already there and changing it would be next to impossible as its running in a crawl space with limited room and was put in during the construction.i am just wondering about breaker tripping on a hot day even with the time delay on stat.

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    Your answer lies in article 440 of the NEC.

    In it, you will find requirements for overload and overcurrent protection, wire ampacities, and other requirements.

    One interesting aspect of this article is that you will find that wire ampacities can be significantly smaller than the breakers that protect them, since the breakers must not open due to startup current. The wire is sized for the current drawn during running, and not starting, and the breaker is sized larger so it does not open during starting.

    Most techs oversize the wiring when it is not required, because it seems out of place to use the MCA ampacity with the MOP breaker, however, it IS permitted by code.

    Check your equipment label. If FUSES are mentioned, then FUSES must be used, and not breakers. In that case, I put a fused disconnect at the unit, and use breakers to feed that circuit in the panel.

    MCA is your wire size ampacity. It should be at least 125% of FLA for the unit. However, this calculation is done by the manufactiurer, and the unit is listed for that value.

    MOP (maximum overcurrent protection) is the breaker or fuse size. Again, if the label specifies fuses, fuses must be used.

    Spend some time in this article. There is a lot of info there.
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  3. #3
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    don't forget to include the conductor length in your calculations.

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    Thread Starter
    Its 40 feet total from breaker to disconnect

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    Depends on the wire type also. If it's THHN in conduit then a 20 amp would be fine. I'd install a hard start kit (a real one) not a huge fan of time delays in situations like this.

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    While the 20A breaker is ok, using a smaller breaker than the "MAX fuse/breaker" listed on the name plate can result in occasional nuisance tripping on start-up.

    Go with the 30A breaker, no need to alter the wire size.

    If you read the code reference Timebuilder was talking about, you will find that there is an exception to the normal rules for over current protection size vs wire size for air conditioning equipment containing inductive loads with overload protectors.
    Condenser fan motors and compressors are inductive loads with overload protectors.

    The simple answer is that as long as the wire meats the minimum circuit ampacity requirements for the unit, you can use the maximum fuse/breaker size listed on the units name plate, even if it would not normally be allowed for the size of wire used.
    The overload protectors in the compressor/motors will protect the wiring from overcurrent due to the operation of the equipment, and even an "oversized" breaker will trip instantly due to a short.

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    Quote Originally Posted by mark beiser View Post
    The simple answer is that as long as the wire meats the minimum circuit ampacity requirements for the unit, you can use the maximum fuse/breaker size listed on the units name plate, even if it would not normally be allowed for the size of wire used.
    The overload protectors in the compressor/motors will protect the wiring from overcurrent due to the operation of the equipment, and even an "oversized" breaker will trip instantly due to a short.
    Wow, that would fail inspection in my area no doubt, no questions asked.
    I'd pay big money to watch you try to talk this jackass into passing something on that theory.

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    Quote Originally Posted by truck12 View Post
    Wow, that would fail inspection in my area no doubt, no questions asked.
    I'd pay big money to watch you try to talk this jackass into passing something on that theory.
    It isn't a theory, it is spelled out in the NEC, and multiple official explanations of what the code means.

    Here is a very good article that explains overcurrent protection for "Combination load equipment", which is what the typical condensing unit or heat pump is.
    http://www.iaei.org/magazine/2000/07...tion-equipment

    I've run into inspectors that didn't seem to know what the code books say. Even to the point of never having actually read one, and were just using some pocket guide to the NEC that was mostly pictures...

    Oddly enough, they rarely seem to appreciate the education when you pull out the actual NEC book and show them what it says.
    The biggest problem is that some inspectors are stuck on article 240 of the NEC and don't know about article 440, or that conductors and overcurrent protection for most air conditioning and refrigeration equipment falls under article 440 instead of 240.

    My personal favorite is all the inspectors that never seemed to notice the first paragraph in the combustion air section of the IMC that says it does not apply to fuel gas burning appliances, and to use the IFGC, which has more options for how to do combustion air intake.
    Last edited by mark beiser; 04-07-2012 at 11:42 PM.

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    Quote Originally Posted by timebuilder View Post
    Your answer lies in article 440 of the NEC.

    In it, you will find requirements for overload and overcurrent protection, wire ampacities, and other requirements.

    One interesting aspect of this article is that you will find that wire ampacities can be significantly smaller than the breakers that protect them, since the breakers must not open due to startup current. The wire is sized for the current drawn during running, and not starting, and the breaker is sized larger so it does not open during starting.

    Most techs oversize the wiring when it is not required, because it seems out of place to use the MCA ampacity with the MOP breaker, however, it IS permitted by code.

    Check your equipment label. If FUSES are mentioned, then FUSES must be used, and not breakers. In that case, I put a fused disconnect at the unit, and use breakers to feed that circuit in the panel.

    MCA is your wire size ampacity. It should be at least 125% of FLA for the unit. However, this calculation is done by the manufactiurer, and the unit is listed for that value.

    MOP (maximum overcurrent protection) is the breaker or fuse size. Again, if the label specifies fuses, fuses must be used.

    Spend some time in this article. There is a lot of info there.
    I'm curious. Does this article jibe with what you're saying?

    Importance of nameplate information

    The nameplate may provide information for sizing and selecting major components that are necessary for safe and reliable operation. Therefore, it is easy to understand why it is so important to those with the task of installing and maintaining these units. For this reason, the nameplate should never be covered or removed from the unit.

    When required, electricians use the nameplate information on an AC unit to determine the ratings of the branch-circuit conductors, are ground-fault protection devices, short circuits and overloads. The disconnecting means as well as other components of the circuit are selected.

    Sizing matters

    The conductors supplying HACR equipment as with cord-and-plug type are sized from the full load current (FLC) listed on the compressor’s nameplate. For a hard-wired AC unit, however, the condenser motor is also considered. The NEC requires these FLC ratings of the compressor to be increased by 125 percent plus 100 percent of the condenser motor’s FLC rating per 440.32 and 440.33. For example, 125 percent × 21.5 amps (compressor FLC) plus 2.5 amps (condenser FLC) at 100 percent is equal to 29.4 amps. Referring to Table 310.16, 10 THWN, AWG, copper conductors are required.

    The overcurrent-protection devices (OCPDs) protect the branch-circuit conductors and compressor windings from short circuits and ground faults. When calculated, the OCPDs are sized from the provisions listed in 440.22(A) and (B), which requires the FLC values in amps to be increased from 175 percent up to 225 percent to allow the starting and running of the unit and prevent nuisance tripping of the upstream OCPD.

    For example, 21.5 amps × 175 percent plus 2.5 amps equals 40 amps. Therefore, a 40-amp OCPD is permitted per 240.6(A). If the unit fails to operate properly, multiply 21.5 amps by 225 percent plus 2.5 amps. That equals 50.9 amps. This rule permits a maximum 50-amp circuit breaker (CB) or set of fuses to be installed.

    The NEC in 440.51 permits the overload protection (OLP) for a compressor to be accomplished by installing OCPDs in separate enclosures, separate overload relays or thermal protectors that are an integral part of the compressor. For example, a disconnect switch can be installed with time-delay fuses that either back up the OLP in the AC unit or serve as the OLP scheme. In other words, time-delay fuses sized at 125 percent of 21.5 amps per 440.52(A)(3) produce 26.9-amps. Therefore, 25-amp fuses as the OLP for the AC unit are permitted. Often, installers use 30-amp fuses that the manufacturer provides in the compressor of the AC unit as outlined in 440.52(A)(1) and 440.53 to back up the OLP.

    The FLC ratings on the nameplate or the nameplate branch circuit selection current of the compressor and condenser motor, whichever is greater, must be used at 115 percent to size the disconnecting means. For example, 21.5 amps plus 2.5 amps times 115 percent is equal to 27.6 amps. Therefore, the NEC requires a 30-amp nonautomatic CB or 30 amp nonfusible disconnect.

    A horsepower-rated switch, CB, or other type switch may be used as the disconnecting means per 440.12(A) through (E) of the NEC.

    A minimum amperage is derived when applying 115 percent per 440.12(A)(1) and (B)(2) to size the disconnecting means, and it may not be capable of starting larger AC units. In such cases, the disconnecting means may have to be determined by the horsepower rating outlined in the NEC per 440.12(A)(2) and (B)(1).
    http://www.ecmag.com/?articleID=10667&fa=article
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    Quote Originally Posted by SandShark View Post
    I'm curious. Does this article jibe with what you're saying?
    Yes, but for most of the equipment we encounter, the manufacturer has already determined the the minimum circuit ampacity the wire needs to be sized for, and the maximum fuse or breaker size allowed for ground fault/short circuit protection, and listed them on the nameplate as they are required to do.

    None of the multipliers listed in the code should be applied to the values listed on the equipment data plate.

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    I would leave the wire and the breaker alone. As long as the breaker is above the MCA it shouldn't give you problems.

    The smaller breaker is more likely to trip if there is a legitimate problem such as a failed capacitor. With the larger breaker the compressor could just sit there cycling on the internal overload.

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    Quote Originally Posted by mark beiser View Post
    It isn't a theory, it is spelled out in the NEC, and multiple official explanations of what the code means.
    I don't care, we have one inspector over here that would fail it in a heartbeat. You could point to it in the book, wouldn't matter.

    He won't pass it if you have a 30 amp breaker on 12 wire feeding into even 10 amp fuses at the unit.

    Remember, there is a difference between what the book says and what happens in the real world.

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    Quote Originally Posted by SandShark View Post
    I'm curious. Does this article jibe with what you're saying?



    http://www.ecmag.com/?articleID=10667&fa=article
    Yes. With a big caveat.

    it is important to not requote copyrighted information. The NEC IS copyrighted, so I do not cut ands paste it in anything but an authorized forum for electricians, as they have limited access, and the NEC approves of those discussions.

    Second, not every Authority Having Jurisdiction has adopted the same version or end material relating to electrical installations. The IRC imports much of the NEC into their own code, and some jurisdictions use that version of the rules.

    So, it is imporetant to not give a single answer with specific values as a catch-all when someone has a specific question.

    Instead, each tech or company should consult with the authority that issues permits, ask which docs they have adopted for their local code, and then follow those docs. They chances are good it will indeed be a word-for-word of article 440, 430, and 422.

    So, find out what docs are in use, and consult them. Don't shoot from the hip and just say, "do it this way" or "I've been doing it like this for 30 years." Frankly, a lot of what was done 30 years ago was dangerous, and is no longer an approved method.

    Let me give an example of how bad information seeps out into the ether..

    When calculated, the OCPDs are sized from the provisions listed in 440.22(A) and (B), which requires the FLC values in amps to be increased from 175 percent up to 225 percent to allow the starting and running of the unit and prevent nuisance tripping of the upstream OCPD.
    Nope. It allows the increase in those values. it does NOT require them.

    Do NOT use republished info as your guide!!!!!


    Consult the right code, and you can't go wrong.
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