A/C Compressor wireing

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GlennG

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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
 
GlennG said:
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
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You almost always use the name plate :)
 
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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?
 
GlennG said:
So if i am useing the nameplate amperage, 24 amps doesnt that get sized at 1.25%?
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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.
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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?
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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
 
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.
 
GlennG said:
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.
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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
 
GlennG said:
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.
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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.'
 
billsnuff said:
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
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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!
 
GlennG said:
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!
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#10 is fine and leaves a little wiggle room for a larger unit in the future.
 
infinity said:
#10 is fine and leaves a little wiggle room for a larger unit in the future.
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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
 
raider1 said:
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
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I've encounter many electricians who make this error.
 
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