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e57

Senior Member
If you mean maximum breaker 50a, then you're correct: #10 wire and up to a 50a breaker.

When MCA and MOP are listed, you use them, and do no math.
IMO there is a GAP between the #10 on T310.16 and the 50A "min" OCP.

Even in the 90 column the ampacity is 40A, the next common size is 45A per 240.6 (if you can find one :roll:), but the conductor needs to be protected at the rating of the OCP per 240.4(B). (Which apparently is a MIN OF 50...)

Additionaly #10 is a "small conductor" listed in 240.4(D)

In any rate - next size #8... (Right?)

At that point you are no longer sizing the OCP and conductors for the load - but sizing the conductors for the OCP....

(For ease of looking it up I pulled this out of the '02 code on my drive in case anything changed???)
 
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28.5 minimum circuit ampacity ,minimum HVAC breaker 50 amps. The AC guy insists that it needs number 8 romex I say 10 romex ,10 romex at 60C is rated 30 amps.
Have I missed something?
At minimum HVAC breaker being 50A, wouldn't you size the feeder to the 50A, so your feeder is protected? Don't forget the ambient correction if it is applicable! (A #8 @ 40*C would only be rated to 44A)
 

e57

Senior Member
At minimum HVAC breaker being 50A, wouldn't you size the feeder to the 50A, so your feeder is protected? Don't forget the ambient correction if it is applicable! (A #8 @ 40*C would only be rated to 44A)
I agree, and would put you in the 75 column depending on the termination rating right at 50A, and 40A in the 60 column for #8.... #6 in the 60 column is 55A.... (With no corrections)
 

Smart $

Esteemed Member
Location
Ohio
I agree, and would put you in the 75 column depending on the termination rating right at 50A, and 40A in the 60 column for #8.... #6 in the 60 column is 55A.... (With no corrections)
Backtracking a little, shouldn't the breaker rating on the nameplate be a maximum, not a minimum!!!
 

Smart $

Esteemed Member
Location
Ohio
That's the disconnecting means, not the branch circuit.
Yeah, as you can see with my follow up post I caught that, but the edit option had already timed out :mad:

Regardless, what about this requirement?
440.32 Single Motor-Compressor. Branch-circuit conductors
supplying a single motor-compressor shall have an ampacity
not less than 125 percent
of either the motorcompressor
rated-load current or the branch-circuit
selection current, whichever is greater.
...
...and it's in Part IV: Branch-Circuit Conductors :cool:
 

augie47

Moderator
Staff member
Location
Tennessee
Yeah, as you can see with my follow up post I caught that, but the edit option had already timed out :mad:

Regardless, what about this requirement?

...and it's in Part IV: Branch-Circuit Conductors :cool:
As I understand it, 440.4(B) covers that by requiring a mfg nameplate giving the min branch circuit rating as determined by Part IV.
The nameplate date should already include the necessary multipliers.
 

e57

Senior Member
Backtracking a little, shouldn't the breaker rating on the nameplate be a maximum, not a minimum!!!
I have seen both listed....

I mistyped MAX CIRCUIT BREAKER 50A ,MIN CIRCUIT AMPACITY 28.5 using 10/2 romex based on 60 c table rated 30 amps.
Then.... if no other calculations are required, the next size is 30A, or more if other calcs are needed. See chapter 4 for "air conditioning " in applicable code cycle. Part 4 - I think???

But - FYI I have had problems before in going for min OCP on AC units... Trips at start up...
 
Standard overcurrent protection of conductors is found in 240.4.
In the paragraph it references 240.4(A)-(G).

240.4(G) Overcurrent Protection for Specific Conductor Applications.
See that Article 440 is referenced.

440.6(A) is the reference for sizing of conductors for A/C.
Notice that is states the conductor size (in general) is sized per the nameplate ratings.
One of the reasons for this, is the A/C manufacturers (unlike motor manufacturers) are required to perform all the calculations and use the results as part of the labeling. Hence there are no calculations for the electrician, just use the label.

Notice we are not following the requirements of 240.4(D) Small Conductors.
So, if the manufacturer says a 50A CBer and 28.5 Amps, then a 10 AWG sized conductor is permitted on the 50A CBer.

The reason for the 50A CBer is for motor startup inrush current.
 

220/221

Senior Member
Location
AZ
28.5 minimum circuit ampacity ,minimum HVAC breaker 50 amps
I think someone misread the plate.

Generally they will say "min circuit amps and max fuse/breaker size"

Saying min circuit and min breaker seems redundant and conflicting.
 

e57

Senior Member
See that Article 440 is referenced.

440.6(A) is the reference for sizing of conductors for A/C.
Notice that is states the conductor size (in general) is sized per the nameplate ratings.
One of the reasons for this, is the A/C manufacturers (unlike motor manufacturers) are required to perform all the calculations and use the results as part of the labeling. Hence there are no calculations for the electrician, just use the label.
That is IF it hermetic? Otherwise it would still fall under 440.22(B) - right? :confused:

I think someone misread the plate.

Generally they will say "min circuit amps and max fuse/breaker size"

Saying min circuit and min breaker seems redundant and conflicting.
Yeah it was a typo that got straightened out....
 

Smart $

Esteemed Member
Location
Ohio
As I understand it, 440.4(B) covers that by requiring a mfg nameplate giving the min branch circuit rating as determined by Part IV.
The nameplate date should already include the necessary multipliers.
OK... I was having a difficult time finding where it actually said that... but now I found it.
440.35 Multimotor and Combination-Load Equipment.
The ampacity of the conductors supplying multimotor and
combination-load equipment shall not be less than the minimum
circuit ampacity
marked on the equipment
in accordance
with 440.4(B).
But now, the question I have is, where does it say the ocpd can exceed the next higher standard ocpd rating over the Article 310 ampacity of the conductors?
 
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OK... I was having a difficult time finding where it actually said that... but now I found it.


But now, the question I have is, where does it say the ocpd can exceed the next higher standard ocpd rating over the Article 310 ampacity of the conductors?

440.6(A) Hermetic Refrigerant Motor-Compressor.
For a hermetic refrigerant motor-compressor, the rated-loaded current marked on the nameplate of the equipment in which the motor-compressor is employed shall be used in determining the rating or ampacity of the disconnecting means, the branch-circuit conductors, the controller, the branch-circuit short-circuit and ground-fault protection, and the separate motor overload protection.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
...
But now, the question I have is, where does it say the ocpd can exceed the next higher standard ocpd rating over the Article 310 ampacity of the conductors?
240.4(G) tells you that the OCPD sizing for air conditioner circuit conductors are found in Article 440. Part IV of Article 440 tells you how to size the conductor and Part III tells you how to size the short circuit and ground fault protection.
 

Smart $

Esteemed Member
Location
Ohio
240.4(G) tells you that the OCPD sizing for air conditioner circuit conductors are found in Article 440. Part IV of Article 440 tells you how to size the conductor and Part III tells you how to size the short circuit and ground fault protection.
I understand this is how we typically view ocpd and conductor sizing, but I am just being critical (overly critical, perhaps :roll:) on the actual wording of the code and disregarding the unwritten intent for the sake of discussion...

If you'll note that 240.4(D) states, "Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied."

So we have one part of 240.4(A) through (G) that says it is rendered moot when permitted in 240.4(G)... so now I ask where is that same type of statement rendering 240.4(B) moot when the 240.4(G) comes into play.

Yes, Article 440 Part III is used to size the ocpd at a maximum. Yes, Article 440 Part IV is used to size the conductors at a minimum. The point here is that there is no direct correlation in Article 440 between the ocpd and the circuit conductor's ampacity. It seems to be assumed that any resulting combination of OCPD and circuit conductors will be compliant. Yet I see nowhere in either Article 240 or Article 440 which specifically relinquishes or amends the requirement of 240.4(B).

I understand how the code is currently interpretted. So this post is only intended as food for thought, and not to upset the status quo :D
 

Smart $

Esteemed Member
Location
Ohio
440.6(A) Hermetic Refrigerant Motor-Compressor.
For a hermetic refrigerant motor-compressor, the rated-loaded current marked on the nameplate of the equipment in which the motor-compressor is employed shall be used in determining the rating or ampacity of the disconnecting means, the branch-circuit conductors, the controller, the branch-circuit short-circuit and ground-fault protection, and the separate motor overload protection.
That don't get it... because it says only that the data "...shall be used in determining the rating or ampacity..." and thus leaves the finer points of the determination to other stipulations, namely but not limited to those of Article 440 Parts III and IV.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
...
So we have one part of 240.4(A) through (G) that says it is rendered moot when permitted in 240.4(G)... so now I ask where is that same type of statement rendering 240.4(B) moot when the 240.4(G) comes into play. ...
The specific rules rules in the Articles/Sections in Table 240.4(G) make all the rest of 240.4 moot for those specific applications.
 
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