Understanding the NEC Vol. 1 (2014) - Overcurrent protection and Article 440

SoonerEE

Member
I am going through "Understanding the NEC Volume 1" and I'm not understanding the correction to one of the example problems. It is on page 192 and can be viewed at the link below.

https://www.mikeholt.com/documents/catalog/14_UNEC1_Textbook/14_UNEC1_Correx_Directory.pdf

I understand the correction from 12 to 10 AWG. It looks like there may have been a change in the example from 75c to 60c terminals. If the terminals of all the equipment were 75c, then would 12 AWG be sufficient (rated 25A at 75c)?

I don't fully understand why the answer was changed from 40A to 30A protection. Isn't the overcurrent protection for air conditioning and refrigeration equipment just supposed to match the name plate? I see that multiplying the rated-load current (18A) by 175% yields 31.5A, which would require a 30A OCPD (440.22(A)). Are you technically required to try the 30A OCPD before applying the 225% exception?

Despite the corrections indicated in the question, and the first bullet of the author's note, the Figure 240-10 and the second bullet point in the author's note still seem to be incorrect.
 

Carultch

Senior Member
I am going through "Understanding the NEC Volume 1" and I'm not understanding the correction to one of the example problems. It is on page 192 and can be viewed at the link below.

https://www.mikeholt.com/documents/catalog/14_UNEC1_Textbook/14_UNEC1_Correx_Directory.pdf

I understand the correction from 12 to 10 AWG. It looks like there may have been a change in the example from 75c to 60c terminals. If the terminals of all the equipment were 75c, then would 12 AWG be sufficient (rated 25A at 75c)?

I don't fully understand why the answer was changed from 40A to 30A protection. Isn't the overcurrent protection for air conditioning and refrigeration equipment just supposed to match the name plate? I see that multiplying the rated-load current (18A) by 175% yields 31.5A, which would require a 30A OCPD (440.22(A)). Are you technically required to try the 30A OCPD before applying the 225% exception?

Despite the corrections indicated in the question, and the first bullet of the author's note, the Figure 240-10 and the second bullet point in the author's note still seem to be incorrect.
Sizes #14 thru #10 are a special exception, indicated with footnote 1 in Table 310.15(B)(16). This is known as the "small conductor rule". It reads as follows:

The load current rating and the overcurrent protection for conductor types shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been applied.
You can also find more detail on this rule in 240.4(D)(5). Here's a link to a white paper from Cooper Bussmann, which explains it as well:
http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/solution-center/industrial_control_panels/BUS_Ele_Small_Conductor_Protection.pdf
 
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SoonerEE

Member
The footnote to Table 310.15(B)(16) directs one to 240.4(D) for the "small conductor" limitations. However, the air conditioner falls under 240.4(G), which is one of the expressly permitted cases in which those limitations do not apply.
 

don_resqcapt19

Moderator
Staff member
Give the information in the question, there is no reason that you could not use #10 with a 40 amp OCPD.
I also agree that if you make the terminations 75°C you could use #12 with the same 40 amp OCPD.
 
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