CARRIER TAG: A data tag mirroring a 50-amp max breaker with a 31.4 minimum ampacity, i m sorry still enables a No. 10 circuit through a 75°C wire and termination rating.

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WIRE CONTACTOR: A wire and contactor displaying the 90°C rating top top the wire and 75°C rating top top the contactor (when properly torqued).



NO. 10 top top A 50-A BREAKER: The data tag and also the wiring of the air conditioner in Orr’s home, which contains a No. 10 wire on a 50-amp breaker.


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My an initial job in the trades was together an electrician’s apprentice doing tasks, such together pulling wire, structure light fixtures, and moving scaffolding in grocery store renovations. Ns moved right into HVACR and now offer as one HVACR contractor, instructor, and also electrician.

Growing up in a household surrounded by electricians, we were constantly discussing codes and theory about the house. Much of what I understand came second-hand through my dad but additionally through Mike Holt, a well-known electric author and instructor i now have the privilege of calling a friend.


Over and over again, I have heard HVACR technicians and also electricians quote rule of thumb regarding wiring sizing, and I have witnessed and participated in numerous heated disputes on the topic.

Before we gain too deep right into this subject of wire sizing, let’s recognize that while comprehensive conductor sizing is the task of engineers and electricians (not HVACR technicians), there space many situations where contractors are tasked with assessing even if it is a breaker or conductor (wire) is enough in both retrofit and also service situations.

During organization calls, gift able to determine undersized conductors can help resolve compressor start worries or intermittent trips of breakers or fuses. Discovering appropriate and inappropriate breaker and also conductor size will conserve you and your client time and money.


I am definitely not suggesting HVACR techs engage in performing work outside of their an abilities or licensures, but, in numerous cases, being able to identify the problem and also making the repair space two various things.

RULES of THUMB

Many techs will repeat this rules that thumb and rely on lock in every circumstances:

“Twelve-gauge wire is great for 20 amps, 10-gauge wire is an excellent for 30 amps, 8-gauge is great for 40 amps, and also 6-gauge is an excellent for 55 amps,” and “The circuit breaker or fuse is always sized to safeguard the conductor .”


These rules of thumb are referring to the amperage capacity or “ampacity” that the conductor (wire) and also are often correct. Sometimes, these basic rules lead technicians to think that if an appliance is rated because that 50-amp breaker/fuse best overcurrent circuit protector (MOCP), you need to use 6-gauge wire and also a 50-amp circuit breaker.

It’s no that an easy according come National electric Code (NEC) NFPA 70 for number of reasons. Right here are some added factors to be considered:

What is the conductor make of? Is the aluminum, copper, or miscellaneous else?What is the ambient rating of the conductor (wire) and its termination (connection) points?What form of load is gift controlled?What other thermal de-rating problems exist? space there many conductors in a raceway, high ambient conditions, etc.?What is the allowable voltage drop based upon wire length? This is not dictated by the NEC but rather by the demands of the associated load.

PROPERLY SIZING A CONDUCTOR

When we use a dominance of thumb, us are missing two main locations as that relates come the conductor.

Is the conductor aluminum or copper, and also is the insulation rating 60°C (140°F) or greater?

Aluminum wire has actually a reduced ampacity than the exact same gauge copper wire, an interpretation aluminum should be bigger to accomplish the same ampacity project as copper wire. The rule-of-thumb sizes count on the shortest allowable temperature rating of the wire; in some cases, the circuit may have actually a higher ampacity if the insulation top top the wire and the link endpoints are all rated at 75°C (167°F) or 90°C (194°F).

All of this ratings deserve to be discovered in NEC (NFPA 70) Table 310.15(B)(16) in detail, yet as one example, follow to this NEC chart, a 6-gauge copper circuit rated in ~ 90°C (194°F) has actually an ampacity the 75 amps while a 6-gauge aluminum conductor rated in ~ 60°C (140°F) has actually an ampacity that 40 amps. The course, there are other considerations in enhancement to this, yet it is clear the rule of ignorance we often use can gain us in trouble if us aren’t conscious that exceptions exist and also what those exception are.

MOCP VS. MCA

These ratings top top the data tag call us specifically what minimum ampacity the circuit have to be capable of carrying and also the maximum size the circuit breaker or fuse may have the ability to protect the circuit versus in one overcurrent condition.

Take a look at the Lennox air conditioning condensing unit tags on web page 19. An alert that the MCA is 28.6 amps and also the preferably fuse or circuit breaker is 50 amps. This way it is allowable to install this unit top top a circuit breaker rated at 50 amps and also a conductor rated at 28.6 amps, follow to the NEC’s NFPA 70.

I recognize this goes versus what many of you have actually been told and understood your entire career, but take a deep breath and also read on.

NEC short article 440 was added because wait conditioning and refrigeration systems are different from typical loads, prefer lights and also heaters. Wait conditioner compressor and also fan electric motors are largely inductive (magnetic) loads and have different characteristics than completely resistive circuits. Write-up 440 has details guidelines because that HVAC and also refrigeration devices manufacturers to follow as soon as writing the tools data tags as well as for electricians as soon as sizing conductors.

There is a lot there, yet here are two highlights the directly use to you together an HVACR professional. This originates from the 2017 version of the NEC NFPA 70.

440.32 solitary Motor-Compressor — Branch-circuit conductors giving a single-motor compressor shall have actually an ampacity not less than 125 percent of one of two people the engine compressor-rated load present or the branch-circuit choice current, whichever is greater.

And the best breaker dimension (MOCP) is dictated through the following short article from the 2017 NEC.

440.22 application and choice (A) Rating or setting for individual Motor-Compressors — The motor compressor branch-circuit short-circuit and also ground-fault protective maker shall be qualified of transferring the starting current that the motor. A protective machine having a rating or setup not exceeding 175 percent that the engine compressor’s rated-load existing or branch-circuit selection current, whichever is greater, shall it is in permitted, noted that, whereby the defense specified is not adequate for the starting current the the motor, the rating or setup shall be permitted to it is in increased however shall no exceed 225 percent the the motor rated-load existing or branch-circuit an option current, whichever is greater.

This 225 percent of motor load/branch circuit pin money for breaker sizing is to allow for motor beginning without nuisance trips while still offering circuit protection versus short circuit conditions.

If girlfriend hesitate in ~ the believed of doing this mathematics for every mechanism you occupational on, the NEC write-up 110.3(B) states: noted or labeled tools shall it is in installed and used in accordance with any instructions contained in the listing or labeling.

For every item of equipment that perform MOCP and MCA, friend are offered the breaker size and minimum wire ampacity. As above, the MOCP is usually significantly greater than the MCA, however this is to allow for the spike in amp attract upon engine start.

Many will certainly ask what protects the circuit in one overload condition in between the MCA and also MOCP ratings? In these overload conditions, the overload protections top top the electric motors themselves space designed to protect the circuit. If the overload protection on the compressor or pan motor fails, the windings in the motors have actually a far lower ampacity 보다 the branch circuit and will fail open up or shorted before the branch conductor will certainly fail. In the situation of any significant ground-fault short, the overcurrent protector (breaker or fuse) will certainly still protect the circuit.

In part cases, building contractors have declared that certain municipalities are requiring that conductors it is in sized based upon the breaker size rather of the MCA. This is not uncommon, however it is generally a matter of a respectful conversation around the above NEC articles. The “authority having actually jurisdiction” or AHJ is responsible for interpreting the code, and also they have the right to make any standard they see fit. While specific local regulation may supersede the NEC with extr regulations, the NEC is a nationally well-known standard throughout the U.S. That is rare that a structure department will overlook entire articles of the NEC as soon as it is brought to their attention in a respectful manner.

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The key is to referral NEC NFPA 70 Table 310.15(B)(16) to discover the ampacity that a conductor and also size the conductor follow to the MCA listed on the unit label. Once this is done, the conductor will be properly sized follow to the NEC.

While no traditional is perfect, i have found that learning the NEC and also looking closely at manufacturer listings and also labels can save friend time, her customers money, and help keep everyone safe.