HVAC min. circuit amp

Status
Not open for further replies.
jxofaltrds said:
Please go to the link that I provided above. AND read it.
Click to expand...
I did read it.
suemarkp said:
Why are you multiplying by 125%? This factor is already included in the MCA value. Now "nameplate" is a bad term when parts of the code book tell you to multiply it by 125%, because there can be many terms on a nameplate -- LRA, RLA, FLA, MCA.
Click to expand...
This is what I was looking for. Thanks
infinity said:
Yup, the MCA already continuous the additional 25% so no further calculation is required. Take the MCA, then look up the minimum conductor size in 310.16 and you're good to go. The Max OCPD can be any size between the minimum and the Max OCPD listed on the nameplate.
Click to expand...

Thanks also Rob.

I Get the "Wire to the minimum/ fuse to the maximum" line of thinking. It was never a question for me, what I was looking for ( and it was answered ) is When the MCA is given is that 125% in it.
The scenario usually goes like this, I am roughing in a house no one can tell me the A/C unit information, So I know by experience the Minimum wire size of the largest unit that will get put in is a #8. So I rough in with 8-2 NM . Then the units get installed and the MCA can be any where from 20 amp to 40 amp. I see no need to run from the disconnect on the wall with a #8 if the MCA is 20.5 amp, but I just want to be sure that I am not going one size to small with a #12
 
Motors and A/C are treated similarly, take a look at the table of contents and the similar parts between the two Articles.


One big difference between motors with nameplates and A/C with nameplates is:
A/C manufacturers are required to test and include the testing results in the form of a nameplate many of the calculations that we in the field are required to calculate for motors.
Hence, the 125% and other factors are already performed and included as nameplate data.
 
I gave this thread 5 gold stars. It's very helpful knowlege that can be used to save money.
I just hope all the small town inspectors understand this. :wink:
 
buckofdurham said:
I gave this thread 5 gold stars. It's very helpful knowlege that can be used to save money.
I just hope all the small town inspectors understand this. :wink:
Click to expand...

That was nice of you, This is one of those back and forth issues that takes a little beating to get it to sink in through the thick crust of what we have been told over the years that may not be true or apply. :smile:
 
So In this case we would have a 40A OCPD and 10awg copper conductors.
johnston_fig2.gif
 
acrwc10 said:
I disagree, look at article 220.5(B)
(B) Fractions of an Ampere. Where calculations result in a fraction of an ampere that is less than 0.5, such fractions shall be permitted to be dropped.

I am interested in hearing your oppinion on this.
Click to expand...

Its akay to disagree and many others will disagree also. When something says minumum it means it can not be less than.

If it came out to be .001 it still would have to be rounded up because it has the word minimum in the statement.
 
jwelectric said:
Its akay to disagree and many others will disagree also. When something says minumum it means it can not be less than.

If it came out to be .001 it still would have to be rounded up because it has the word minimum in the statement.
Click to expand...

So are you saying disregard 220.5(B) ? It does say we can drop any fraction of less then 1/2 an amp.
 
220.5(B) permits the .4 and less to be dropped from load calculations.


240.4 requires the conductor to be protected at it's rated ampacity. A conductor rated at 25.1 in regards to A/C wiring requirements would be required to go up to the next size OCPD.
 
acrwc10 said:
So are you saying disregard 220.5(B) ? It does say we can drop any fraction of less then 1/2 an amp.
Click to expand...

You can only drop it after the calculation is complete and you are looking at your final answer. 220.5(B) applies to the calculations of branch circuits, feeders and services.
 
acrwc10 said:
So are you saying disregard 220.5(B) ? It does say we can drop any fraction of less then 1/2 an amp.
Click to expand...

This is what I am saying and maybe it will help you to better understand the point I am trying to make

Article 220 Part I is for general information concerning branch-circuit, feeder, and service calculations.
Part II of Article 220 covers calculations for branch circuits and Section 220.14(C) covers branch circuits for motors and air-conditioning and refrigerating equipment that have hermetic refrigerant motor-compressors.

The reference to motors and compressor motors found in 220.14(C) will have a statement somewhere in those sections that sound something like this;
?not less than? and this is the minimum.
This minimum or not less than will not allow a fraction no matter the size to be dropped or it would be less than or below the minimum. Therefore a nameplate that has Minimum Circuit Ampacity of 25.1 the branch circuit conductors can not be less that 25.1.
To just drop the .1 would make the conductors to small for minimum circuit ampacity.
 
jwelectric said:
This is what I am saying and maybe it will help you to better understand the point I am trying to make

Article 220 Part I is for general information concerning branch-circuit, feeder, and service calculations.
Part II of Article 220 covers calculations for branch circuits and Section 220.14(C) covers branch circuits for motors and air-conditioning and refrigerating equipment that have hermetic refrigerant motor-compressors.

The reference to motors and compressor motors found in 220.14(C) will have a statement somewhere in those sections that sound something like this;
?not less than? and this is the minimum.
This minimum or not less than will not allow a fraction no matter the size to be dropped or it would be less than or below the minimum. Therefore a nameplate that has Minimum Circuit Ampacity of 25.1 the branch circuit conductors can not be less that 25.1.
To just drop the .1 would make the conductors to small for minimum circuit ampacity.
Click to expand...

I agree that we can not ( and should not ) drop the fraction of an ampere when it is shown on a name plate of an appliance, mainly because the manufacturer has put it there for a reason. I just doubt that 0.01 of an amp is going to burn up the conductor.:smile:
I so appreciate the explanations that are being introduced into this thread, I think there have been some very good opinions added to it. Thanks for your input.
 
acrwc10 said:
I just doubt that 0.01 of an amp is going to burn up the conductor.
Click to expand...
I will agree that 100 milliamps will not burn a conductor into but if we look at this as though it was only one motor involved the locked rotor of that one motor would be somewhere around 120.5 amps. Should this hold for a few seconds could burn a #12 up.

It most certainly burn a #12 quicker than it would a #10

There is much more to look at when dealing with a motor than just one tenth of an amp under normal conditions
 
We cannot round down when it comes to the actual data plates on listed equipment.

We can only round down at the end of a calculation for determining the load to be served.
 
Status
Not open for further replies.
Top