MCA High Efficiency AC Units

[augie47]

I don't have any data to support the claim, but a HVAC contractor was questioning me as to what he states are "higher" MCA requirements on more efficient HVAC units. After being rejected for inadequate size conductors on a 10 kw air handler (MCA = 58) he claims he can supply the same air handler in a "less efficient" model and the MCA is on 54 amps.
He went on to say the condenser units with a better SER rating has higher MCA.
My only thoughts were they were possibly using motors similar to the "Design B" with higher starting current.

Has anyone found this to be an accurate portrayal ?

 

[Jraef]

I don't have any data to support the claim, but a HVAC contractor was questioning me as to what he states are "higher" MCA requirements on more efficient HVAC units. After being rejected for inadequate size conductors on a 10 kw air handler (MCA = 58) he claims he can supply the same air handler in a "less efficient" model and the MCA is on 54 amps.
He went on to say the condenser units with a better SER rating has higher MCA.
My only thoughts were they were possibly using motors similar to the "Design B" with higher starting current.

Has anyone found this to be an accurate portrayal ?

He seems to be implying that MCA has some connection to efficiency. It does not. A long time ago the calcs for MCA were very simplistic, but they have not been that way for a long time. The MCA has nothing to do with efficiency other than in a vaguely indirect effect on one component of the determination. MCA is determined from (nutshell version) FLA and RLA of connected loads, expected lifetime of the equipment and expected increases in those values as it ages. Then there are different calculation methodes depending on what TYPE of AC unit it is; direct expansion or chilled water, etc. etc. etc. So it's entirely possible that a unit with lower efficiency may have a lower MCA value, especially if it is not expected to survive as long! I wouldn't lose a lot of sleep over it.

And to your idea, the difference in the type of motor might make a difference in the MOP for sure, because newer energy efficient motors tend to have higher inrush current and there are issues with breakers and fuses sized to close to the bone popping on the higher inrush, so the NEC rules have had to make additional allowances that may end up affecting the MOP rating on a piece of equipment. But that doesn't go into the process of determining the MCA as far as I know.

 

[steve66]

Could be that the more efficient units use larger HP motors and don't drive them as hard.

For example, a condenser fan could be connected to a 1 HP motor, but the fan design might be such that it only requires 3/4 HP to run.

I'm assuming the full load amps (and the MCA) of the unit would be based on the full HP capacity of the electric motors, even if for example, an attached fan or compressor only loaded the motor at 75%.

 

[sgunsel]

Higher SEER units optimize everything, and that includes NOT oversizing motors. Motor efficiency drops as the load drops, especially in relatively small motors. It is a combination of more efficient heat exchangers (both sides), more efficient motors, better controls, and different refrigerants. The highest SEER unit compressors also operate at two or more speeds. I just replaced my 35 year old AC with a new 16 SEER system (single speed), and the MCA went from 30 amps to 15 amps! It definitely cools as well, if not better than the old one with a very noticeable drop in the electric bill, and this summer was a good test.

 

[steve66]

Higher SEER units optimize everything, and that includes NOT oversizing motors. Motor efficiency drops as the load drops, especially in relatively small motors. It is a combination of more efficient heat exchangers (both sides), more efficient motors, better controls, and different refrigerants. The highest SEER unit compressors also operate at two or more speeds. I just replaced my 35 year old AC with a new 16 SEER system (single speed), and the MCA went from 30 amps to 15 amps! It definitely cools as well, if not better than the old one with a very noticeable drop in the electric bill, and this summer was a good test.

I'm just guessing why a more efficient AC unit might have a higher FLA rating. But according to the DOE, motors are usually most efficient in the 60-80% range, with most motors usually performing best at about 75% rated load.

Also, according to the DOE, motors with a service factor higher than 1.0 can be overloaded for short periods of time, driving them into a even less efficient operating range.

 

[iwire]

As far as I know the design of higher efficiency motors and transformer results in higher inrush currents during start up.

 

[sgunsel]

I deal mostly with small AC motors, 0.5 - 5 HP. Efficiency is always lower at 75% than at full load, and actually continues to improve as load increases above rated full load. I think DOE most likely stated that most motors actually operate at moderate load since it appears to be a common belief that it is better to overpower a load and 1) efficiency is not important, 2) the motor only produces the HP it needs for the load so it doesn't matter, or 3) better to run the motor at partial load ("the motor doesn't have to work as hard"). That covers sloppy design but hurts efficiency. And as mentioned above, most high efficiency motors do have higher starting current.

 

[Jraef]

As far as I know the design of higher efficiency motors and transformer results in higher inrush currents during start up.

Yes, but as I said, inrush is a factor used in the MOP, but it is NOT factored into MCA.

Let's not get our TLAs all FUBAR here.

 

[Mgraw]

These are numbers from two Carrier units.
13 SEER 3 ton condenser MCA 21.9 MOCP 35
16 SEER 3 ton condenser MCA 18.1 MOCP 30

 

[topgone]

Yes, but as I said, inrush is a factor used in the MOP, but it is NOT factored into MCA.

Let's not get our TLAs all FUBAR here.

I agree with your statements jeff.

The confusion really is because in an AHU, we have the pump, the motor and a fan. When a drive motor is replaced with premium efficiency ones (leaving the pump in the unit), the SEER can improve (lesser electrical input for the same work - lesser current draw at running speed). That is assuming that the pump design is not sensitive to slight increase in motor speed (high-efficiency motors operate at slightly lesser slip than the standard efficiency motors). If the HVAC tech tweaks the pump/ the heat exchangers or the fan, the SEER will also improve (more work done for the same electrical input). I guess we need to ask the original poster to clearly state which item/s was changed for us to synthesize our ideas here.

 

[kbsparky]

These are numbers from two Carrier units.
13 SEER 3 ton condenser MCA 21.9 MOCP 35
16 SEER 3 ton condenser MCA 18.1 MOCP 30

Mine is a 18 SEER 3 ton unit:

MCA = 23.7
MOCP = 40

The beast draws less than 10 Amps while running. THAT has made a significant impact on my electric bill this summer.

 

[Mgraw]

Mine is a 18 SEER 3 ton unit:

MCA = 23.7
MOCP = 40

Probably a two stage unit.

 

[steve66]

These are numbers from two Carrier units.
13 SEER 3 ton condenser MCA 21.9 MOCP 35
16 SEER 3 ton condenser MCA 18.1 MOCP 30

Yes, but you are comparing apples to apples. Where is the fun in that?

Maybe the variation in Augie's first post might be more a difference between manufacturers than a difference in just SEER's effect on MCA.

 

[kbsparky]

Probably a two stage unit.

Not that I'm aware of ....

(I'll ask the fellow who sold it to me and report back here)

 

[GeorgeB]

The highest SEER unit compressors also operate at two or more speeds.

I'm certainly not an expert, but did a lot of research with my HVAC contractor when I replaced my old, freon leaking unit with a new unit last year.

The fans are multiple (usually variable) speed, both for moving conditioned air and for cooling the condenser (compressor). All the compressor data he gave me and that I found in research only switched compression gas paths, operating the compressor at a fixed speed. This may be in error, and certainly I may read and comprehend incorrectly, but that's what we THINK we found.

 

[kwired]

Not that I'm aware of ....

(I'll ask the fellow who sold it to me and report back here)

Not a refrigeration expert but am around it a lot. I don't recall ever seeing anything over 14 SEER that was not multistage, but I suppose that it could exist. Running on the low stage the majority of the time is what gives it the higher SEER rating.

 

[sgunsel]

My new AC unit is a 3 ton, 16 SEER, single stage, MCA 14.9, MOCP 30. The unit was also available as a two stage unit for a few hundred dollars more. The unit it replaced was 35 years old, MCA 29, Max fuse 45 (no fuse was ever installed, circuit breaker only).