10 on 40 amp

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440.22 tells you how to size the branch cicuit, short circuit and ground fault protective device. 430.31 and sections following it tell you how to size the conductor. In the case of the package unit all the work is done and the information is shown on the nameplate.
 
OK let me try to explain myself better. Just because the nameplate says 27 mca and 40 mocp what says I have to use both of those. 40 is the max. 27 is the min. That doesnt clear me to use 10 on a 40. 10 is fine for the 27. A 40 hacr is fine for the ocp. But I dont have to use them exclusively. So what I am saying is the nameplate is a parameter to use. Not that we can blindly use it without some knowledge. I am not saying that it isnt ok. I am saying I need more.
 
lmchenry said:
Just because 10 works for 27 amps and the max ocp is 40, doesnt mean that you cant put a 30 for the 10.
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True, but the compressor may trip the breaker trying to start.

OK let me try to explain myself better. Just because the nameplate says 27 mca and 40 mocp what says I have to use both of those. 40 is the max. 27 is the min. That doesnt clear me to use 10 on a 40. 10 is fine for the 27. A 40 hacr is fine for the ocp. But I dont have to use them exclusively. So what I am saying is the nameplate is a parameter to use. Not that we can blindly use it without some knowledge. I am not saying that it isnt ok. I am saying I need more.
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We're saying you don't.

The wire must suit the minimum-circuit ampacity, period. It can be bigger.

The OCPD must suit the maximum-OCP rating, period. It can be smaller.
 
I understand that the wire must be 10 or bigger and that the breaker must be 40 or smaller. But that does not mean that they are completely independent. What code allows them to be done this way? Why shouldnt the 40 be a 30 since I am using a 10? I'm not saying you are wrong, I am just trying to cover myself.
 
lmchenry said:
What code allows them to be done this way?
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The codes have been stated numerous times already in this thread, and they have been explained as well.

You're over thinking this. The bottom line: Size the conductors for the minimum circuit ampacity, and size the OCPD for the max. This is code legal. There is absolutely no simpler way to explain this.
 
Start by spending some time reading article 430 for electric motors. Then 440 for air conditioners will make more sense. For a motor or a/c compressor the fuse or circuit breaker only provides short circuit and ground fault protection NOT overload protection. Overload protection is provided by another device (thermal protection, impedance protection, or motor "heaters" for example). In the event of a short circuit or ground fault, due to the low impedance (low resistance) of the current path the circuit breaker will see hundreds or perhaps thousands of amps and will open very quickly, so whether it is a 20-amp circuit breaker or a 60-amp circuit breaker will make little difference. Either of these circuit breakers would trip very, very quickly when subject to an amperage draw of several times their rated ampacity. In the case of a motor, using an inverse time circuit breaker, you are allowed to "oversize" the breaker by 250%, see 430.52; so the HVAC manufacturer has actually been very conservative in only allowing a 60-amp breaker for a 27-amp load. 440.22(A) allows you to size the branch circuit short-circuit and ground-fault protection at 175% of the rated load current of the motor. 440.32 allows you to size the conductors at not less than 125% of the rated-load current. 440.22(C) advises you to follow the Manufacturer's Values as they have already done these calculations for you. When wiring a motor or compressor you must forget everything you learned about wiring a lighting circuit, it is a whole 'nuther ballgame.
I hope that helps.
 
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lmchenry said:
What code allows them to be done this way? Why shouldnt the 40 be a 30 since I am using a 10?
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Lets try this another way, what NEC code section prevents using a 40 amp OCP with 10 AWG?
 
lmchenry said:
I understand the concept that there can be more on the circuit for a short period of time and I understand the ratings of the a/c equip. However, I wonder why there is an exception for a/c compared to any other circuit. The conductor is what we are trying to protect, so why give exception in this case?
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In certain motor load calculations. The NEC will allow a #14 awg to be protected by a 40 amp. breaker.
 
crossman said:
How about a #10 on a 100 amp breaker?
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Unless it's an instantaneous trip breaker I suspect that may be beyond code compliance. (I did not do the math)

That is smaller then the EGC normally required for a 100 amp breaker.
 
430.52 C(3) Excp 1 allows some very large breakers as compared to wire ampacity. In one case 1700 percent. So, under the proper conditions, 100 amp OCP for #10 would be easy. - Instantaneous trip as Iwire mentioned
 
Lets try this angle.

240.4 is the general rules about protecting conductors.

What most of us in the industry have ingrained upon our brains is 240.4(D) Small Conductors. - where we have been taught to fuse 14 AWG at 15 amps, 12 AWG at 20 amps and 10 AWG at 30 amps.

Table 310.16 has 3 columns, 60C, 75C and 90C, based upon the parameter in the title of the table and Conductor Types.


240.4(G) and table, provides us with some options for relief from 240.4(D). For A/Cs we can reference 440.22 & 440.31. Basically those references tell us we can use the manufacturer's nameplate ratings to size the overcurrent device and conductor size (Which is most likely different than what you may be use to in general wiring applications).

The manufacturer tests the motor and applies that information to the nameplate rating. As has been discussed, in general for motors and A/C units (A/C is mostly a motor), we protect the conductors for short circuit and the A/C unit protects for overload via the design of the particular unit.

Don't forget to take a look at the different ampere values for the different temperature columns in Table 310.16.
Also take note, that there are other parameters to be aware of, such as for NM cable 334.80.
 
Errr... I can't believe no one has mentioned 240.3 where it expressly tells us to go to other articles for certain equipment, for instance, OCP for air conditioning equipment ----> Art 440.
 
Ok I am not arguing that this can be done! Just trying to understand. 310.16 allows 35 amps on thwn #10. Correct me if I am wrong but on a package unit there are other than motor loads. So would that be 440.35 in the 05. So what reference would that be for the breaker?
 
crossman said:
Errr... I can't believe no one has mentioned 240.3
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It was mentioned. :)


Regarding the instantaneous trip breaker, as you know they can only be used if part of a listed combination motor starter. For many of us that would make it a very unusual breaker, I don't think I have ever seen one.

The majority of us will be using fuses or inverse time breakers.
 
iwire said:
For many of us that would make it a very unusual breaker, I don't think I have ever seen one.

The majority of us will be using fuses or inverse time breakers.
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Yep, I haven't ever seen it done either, just saying it is possible. ;)
 
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