Question about 2 problems in the 2008 Understanding the NEC, Volume 1 Articles 90-450

Status
Not open for further replies.

Eddy Current

Senior Member
Can anybody explain the 2 questions on pages 154 and 155 in the Mike Holt Understanding the NEC book?


In the question on page 154 i don't understand how they got the answer 14awg wire. I see how they are getting the amperages but how are they determining what wire to use and how do they know what type?



In the first question on page 155 im not understanding how they are getting 12awg, it says the minimum circuit ampacity is 23A, i thought you would have to use 10awg since it was good for 30amps?
 
Can anybody explain the 2 questions on pages 154 and 155 in the Mike Holt Understanding the NEC book?


In the question on page 154 i don't understand how they got the answer 14awg wire. I see how they are getting the amperages but how are they determining what wire to use and how do they know what type?



In the first question on page 155 im not understanding how they are getting 12awg, it says the minimum circuit ampacity is 23A, i thought you would have to use 10awg since it was good for 30amps?

14 awg copper is the smallest wire that be used for building wiring (this not including controls etc..) NEC 2008 310.5.

12 awg copper is good for 25 amps. Remember that the breaker can be higher on when dealing with refrigerators, motors etc.. Thats why its allowed to oversize it on a smaller conductors.
 

Eddy Current

Senior Member
14 awg copper is the smallest wire that be used for building wiring (this not including controls etc..) NEC 2008 310.5.

12 awg copper is good for 25 amps. Remember that the breaker can be higher on when dealing with refrigerators, motors etc.. Thats why its allowed to oversize it on a smaller conductors.


Isn't there supposed to be a certain percentage that you are to allow for i didn't thing 25 would cover that with it being 23 amps.





What about the other question on page 155 do you know how they are getting 22 amps?
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Isn't there supposed to be a certain percentage that you are to allow for i didn't thing 25 would cover that with it being 23 amps.

The MCA already has 125% added to it so the MCA is used for sizing the conductor.




What about the other question on page 155 do you know how they are getting 22 amps?


Do you mean Figure 240-8?

____________
 

cadpoint

Senior Member
Location
Durham, NC
The MCA already has 125% added to it so the MCA is used for sizing the conductor.
...

I've read many, many, great articles here on motors and their required sizing for their circuit service. I believe I've even lifted a few written motor articles for my own studying from this forum; they are sitting on the other CPU!

I just don't deal enough with motors to remember this one little fact about MCA, I will now, thanks for posting the obvious!
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
MCA?

I guess i was talking about the percentage like 16 amps is the most youre supposed to put on a 20 amp breaker and so on.


That only applies to a continuous load or a motor circuit etc. A 20 amp resistive circuit that operates continuously for 2 hours and 59 minutes can be loaded to 20 amps.

The MCA is calculated by the manufacturer of the unit to already include the 125% as part of the MCA so you only need to use the MCA ampacity to size the conductors. In Mike's example the MCA is 23 amps, a #12 conductor is rated for 25 amps at 60? C so it can be used for a 23 amp MCA.
 

jumper

Senior Member
MCA?

I guess i was talking about the percentage like 16 amps is the most youre supposed to put on a 20 amp breaker and so on.
I do not know what your exact question is, I do not have the book.

But

MCA minimum Circuit Ampacity.

A MCA has already been calculated using 210.19(A) 1

(1) General. Branch-circuit conductors shall have an ampacity
not less than the maximum load to be served. Where
a branch circuit supplies continuous loads or any combination
of continuous and noncontinuous loads, the minimum
branch-circuit conductor size, before the application of any
adjustment or correction factors, shall have an allowable
ampacity not less than the noncontinuous load plus
125 percent of the continuous load.

If you have a motor/compressor see 240.6 (G) for MOCP sizing.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Look at art. 240.4(D) which takes you to (E) & (G). G allows a/c to be exempt from the limitation of 240.4(D). Thus #12 is good for 25 amps-- notice the asterisk after #14-10 and the note at the bottom of T. 310.16
 
Status
Not open for further replies.
Top