A/C Compressor wireing

Status
Not open for further replies.

GlennG

Member
Location
Hicksville, NY
I dont have too much experience with air conditioning and I have a few questions about properly designing the installation of 2 new 5 ton AC compressors going in place of one 10 ton. The new units are 3 phase, minimun circuit ampacity is 24Amps and the max breaker size is 40Amps. Any info or code references would be helpful. .

Do i need to have a 40 amp breaker to handle any inrush current or do i size the breaker based on the wire size needed to handle the min. amps (24) for my application?

Also I have an existing 1" run of EMT feeding the old unit that i would like to pull my new circuits through, would i still only need one ground sized to the largest conductor in that pipe?

This is something i was never really sure about and i am starting to do more AC units and want to make sure I am doing whats best. thanks
 

Buck Parrish

Senior Member
Location
NC & IN
I dont have too much experience with air conditioning and I have a few questions about properly designing the installation of 2 new 5 ton AC compressors going in place of one 10 ton. The new units are 3 phase, minimun circuit ampacity is 24Amps and the max breaker size is 40Amps. Any info or code references would be helpful. .
You must use the name plate for the breaker and wire size :)440 A exception 1

Do i need to have a 40 amp breaker to handle any inrush current or do i size the breaker based on the wire size needed to handle the min. amps (24) for my application?
Same as above :)
Also I have an existing 1" run of EMT feeding the old unit that i would like to pull my new circuits through, would i still only need one ground sized to the largest conductor in that pipe?
Yes 1 is okay , 250 Part IV, VI, VII
This is something i was never really sure about and i am starting to do more AC units and want to make sure I am doing whats best. thanks

You almost always use the name plate :)
 
Last edited:

GlennG

Member
Location
Hicksville, NY
So if i am useing the nameplate amperage, 24 amps doesnt that get sized at 1.25%? That would make it 30 amps with 40 amp breaker? Thats not taking into account any other de-rating or voltage drop. Then looking at 440.11 (A) , i would be able to use a 30A unfused disconnect?
 

raider1

Senior Member
Staff member
Location
Logan, Utah
So if i am useing the nameplate amperage, 24 amps doesnt that get sized at 1.25%?

No, the 125% is already taken into account when the manufacture comes up with the minimum circuit ampacity. So you would use the 24 amps and find a conductor that is rated for at least 24 amps. So #12 THHN at 75 degrees C is rated at 25 amps. (Under the 2008 NEC #12 is also rated at 25 amps at 60 degrees C but was changed for the 2011)

That would make it 30 amps with 40 amp breaker? Thats not taking into account any other de-rating or voltage drop.

Nope, that would make it 24 amps protected by a maximum 40 amp breaker.

Then looking at 440.11 (A) , i would be able to use a 30A unfused disconnect?

I think you mean 440.12(A)(1) so if you apply the 115% to 24 amps you get 27.6 so a 30 amp unfused disconnect would be acceptable.

Chris
 

GlennG

Member
Location
Hicksville, NY
Thanks for the info. That breaker size was throwing me off also. I am gonna be running #10 THHN protected with a 30A breaker, i just dont want any problems with the motor tripping the breaker on startup.
 

raider1

Senior Member
Staff member
Location
Logan, Utah
Thanks for the info. That breaker size was throwing me off also. I am gonna be running #10 THHN protected with a 30A breaker, i just dont want any problems with the motor tripping the breaker on startup.

You can use #12 THHN with an ampacity of 25 amps (Small conductor rule of 240.4(D) does not apply to A/C units) and protect those conductors with a 40 amp breaker if the nameplate of the A/C unit says MCA 24 and MOD 40.

Chris
 

dana1028

Senior Member
Thanks for the info. That breaker size was throwing me off also. I am gonna be running #10 THHN protected with a 30A breaker, i just dont want any problems with the motor tripping the breaker on startup.

Ditto what Raider says: lots of HVAC installers go with the max overcurrent device [40A] because 'they don't want any problems with the motor tripping on start up.'
 

GlennG

Member
Location
Hicksville, NY
The information in the attached pdf covers MCA and MOP. It is a pretty good explanation of the stuff behind MCA and MOP. Enjoy.

http://www.apcmedia.com/salestools/KSIH-6Y6UU5_R0_EN.pdf

thanks for all the input, this link was especially helpful in explaining the reasoning of everything.

The reason i was saying 10 awg rather than 12 awg was to compensate for the distance of the run, although that doesn't change the bottom line of the MCA and MOP. I feel like i have a much better understanding of this now, thanks to all!
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
thanks for all the input, this link was especially helpful in explaining the reasoning of everything.

The reason i was saying 10 awg rather than 12 awg was to compensate for the distance of the run, although that doesn't change the bottom line of the MCA and MOP. I feel like i have a much better understanding of this now, thanks to all!

#10 is fine and leaves a little wiggle room for a larger unit in the future.
 

raider1

Senior Member
Staff member
Location
Logan, Utah
#10 is fine and leaves a little wiggle room for a larger unit in the future.

I agree, my point with the the #12 was to illustrate that with A/C units you don't have to size the conductors to the breaker. That is a common misconception when wiring A/C units.

Chris
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I agree, my point with the the #12 was to illustrate that with A/C units you don't have to size the conductors to the breaker. That is a common misconception when wiring A/C units.

Chris

I've encounter many electricians who make this error.
 
Status
Not open for further replies.
Top