conductor sizing for A/C compressors

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lawrence kany

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My view on sizing branch circuits for A/C are follow the nameplate from
the manufacturer. You size the wire by the min cir ampacity on the plate
and size MOP by the manufacturers instructions on the plate. Does that
follow general practice. :cool:
 
lawrence kany said:
You size the wire by the min cir ampacity on the plate
and size MOP by the manufacturers instructions on the plate. Does that
follow general practice. :cool:
Click to expand...

I would say it meets code, I am not as sure I would call it general practice.

It will work fine and is safe as long as distance does not cause to much voltage drop.
 
You are required to use the information on the nameplate if available. You're correct that the MCA is used to size the conductors. I would add that 240.4(D) does not apply to this installation so you possibly could use #14 on a 20 amp MCA and #12 on a 25 amp MCA, etc. Also the MOP can be sized anywhere between the MCA and the MOP. However, if you sized the short circuit and ground fault protection for the MCA the unit might trip on start up.
 
I'm curious, but when satisfying the requirement for the indoor unit ( evaporator) do you use Article 440 for AC or Article 430? Thanks.
 
240.4(D) does apply, it points to 240.4(G)

240.4(D) does apply, it points to 240.4(G)

infinity said:
You are required to use the information on the nameplate if available. You're correct that the MCA is used to size the conductors. I would add that 240.4(D) does not apply to this installation so you possibly could use #14 on a 20 amp MCA and #12 on a 25 amp MCA, etc. Also the MOP can be sized anywhere between the MCA and the MOP. However, if you sized the short circuit and ground fault protection for the MCA the unit might trip on start up.
Click to expand...

Hmmm. I would say that 240.4(D) does apply. It's just that, within 240.4(D), there is the following statement:

Unless specifically permitted in 240.4(E) or 240.4(G), the overcurrent shall not exceed...

240.4(E) is for tap conductors
240.4(G) is the section that includes packaged equipment like A/C units.

Eric

Now how do i get those cute little icons to appear by the text? When i click on one it goes up by the subject line
 
eric stromberg said:
Hmmm. I would say that 240.4(D) does apply.
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No, I can not agree with that.

If you removed 240.4(D), (E) or (G) would still be applicable.

The only reason (D) has this language ' Unless specifically permitted in 240.4(E) or 240.4(G)' is to allow us to use (E) or (G) with small conductors.


BTW.....I get all tense when I respond to Eric....I feel like I am questioning Spock.:grin:
 
general practice

general practice

As far as "general practice" goes, in this area it's rare that I see anyone follow other than the amapcities listed in 240.4(D) {15 amp/14, 20 amp 12, etc}

I have worked areas where the inspector holds you "breaker determines wire size" regardless of the nameplate MCA.
 
eric stromberg said:
Now how do i get those cute little icons to appear by the text? When i click on one it goes up by the subject line
Click to expand...

Go to My Profile, under control Panel click on Edit Options. Next go to Misc. options at the bottom of the page and select Enhanced Interface.

you will also be able to us bold and underline and different type also.;)
 
240.4(D); just say no!

240.4(D); just say no!

iwire said:
No, I can not agree with that.

If you removed 240.4(D), (E) or (G) would still be applicable.

The only reason (D) has this language ' Unless specifically permitted in 240.4(E) or 240.4(G)' is to allow us to use (E) or (G) with small conductors.


BTW.....I get all tense when I respond to Eric....I feel like I am questioning Spock.:grin:
Click to expand...


I see your point. I'm looking at from a starting place of Table 310.16. The little asterisks point me to 240.4(D) and this is what takes me to 240.4(G). In this sequence, i wouldn't have gotten to 240.4(G) without going through 240.4(D) first. Narrow-mindedness on my part.

I concur. Now, on to getting those icons to work
 
Cavie said:
Go to My Profile, under control Panel click on Edit Options. Next go to Misc. options at the bottom of the page and select Enhanced Interface.

you will also be able to us bold and underline and different type also.;)
Click to expand...

:) Thanks. ;)
 
eric stromberg said:
I'm looking at from a starting place of Table 310.16. The little asterisks point me to 240.4(D) and this is what takes me to 240.4(G). In this sequence, i wouldn't have gotten to 240.4(G) without going through 240.4(D) first.
Click to expand...

I see your point as well. :smile:
 
Cavie said:
Go to My Profile, under control Panel click on Edit Options. Next go to Misc. options at the bottom of the page and select Enhanced Interface.

you will also be able to us bold and underline and different type also.;)
Click to expand...

Thanks from me also:smile:
 
augie47 said:
As far as "general practice" goes, in this area it's rare that I see anyone follow other than the amapcities listed in 240.4(D) {15 amp/14, 20 amp 12, etc}

I have worked areas where the inspector holds you "breaker determines wire size" regardless of the nameplate MCA.
Click to expand...

You can tell the inspector to learn articles 430 and 440 about how to size breaker and conductors for motors and AC. The breaker shall be sized large enough to carry the starting current of motor. So it will be much larger than conductor size. That is why the circuit needs overload protection also.

Regarding the MCA and MOCP shown on name plate, I have done some calculation based on article 440, the number shown on the name plate is the same as my calculation. Actually the AC menufacturer has done the calculation for us so that we size conductors based on MCA and breaker based on MOCP. It is not general practice. It is code compliance.
 
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