A friend of mine has an outside unit that is intermittently tripping the breaker in the main panel. The unit is a Trane and only about 2 years old.

A Trane tech came by yesterday and did the following:
- Checked starting amps - came up with 71A
- Reported that the unit was a bit overcharged, left it alone
- Checked all wiring and ensured all connections were tight (which they were)
- Installed a new cap and hard start kit, dropped starting amps to 50A
- Running amps at 11A
- Reported that he felt ductwork was undersized
- Recommended a new breaker as the unit's MOCP was rated at 45A and the builder only installed a 30A breaker in the box
- Also said the wire size had to be changed to #8 since the breaker needed to be upsized (we'll discuss this one)

Inspected the unit and agree with everything the tech said/did with the exception of the wire size. The unit's minimum circuit ampacity is listed at only 26.1A which should be good for #10 wire per NEC articles 440/310.15. If anyone disagrees, please comment.

We changed the breaker to a 40A (45A not in stock) and restarted the unit with no issues. Also verified the 11A running current, though was not able to see any startup load - my meter immediately displayed 11A upon startup.

Finally, they reported that the unit was frozen up last night. Since the tech said the unit was overcharged, suspect a potential airflow issue. The owner did feel as though the flow was reduced at the time of freezing. The air handler is feeding two zones, one each for upstairs and downstairs. The zone damper motors appeared to be operating correctly when we looked today and air was flowing fine. The filter is also new.

Does anyone have any further ideas on what to monitor for other potential causes? If the ductwork is truly undersized, could the extra pressure drop on the fan reduce the airflow to the point it is creating issues on hot days?

 

Welcome to the forums.

To see startup load you need to set the amprobe to peak reading. Yours might just be an averaging type.

Although your assessment of the wiring is correct I go one step further. If the run is under 50' I stay with the smaller gauge..... over that I go up one size to reduce voltage drop and excessive sag on compressor startup.


Could one have been stuck last night and freed up ?
You should have a bypass damper and that may have an issue.

I've considered installing a zone system on my furnace-A/C system but the problem of airflow when the dampers aren't all open always causes issues.

As far as I know overcharging will cause hard starts but not the evaporator to ice up. That's usually an undercharge or low air flow problem.

 

I also thought of the damper being stuck and suggested that he remove the motor and see if it operates smoothly by hand. There appeared to be a bypass damper that simply had a weight attached to it - it moved freely thought I can't comment on if the weight is appropriately sized for the fan static.

The run is about 55' - owner feels more comfortable to change it to #8 so we are going to do that tomorrow.

Thanks for the help!

 

You're welcome and good move on the wire replacement.

 

The frost line on a over charged system starts at the compressor suction pipe and works its way to the evap. Frost line from having poor air flow at evap. heads towards compressor. You can see it move along.

 

Is the disconnect a 30 amp or a 60 amp unit ?

God bless
Wyr

 

Please explain why an over charged system will frost up , at any spot ?

I think I understand why a somewhat under charged unit will frost up .

Or if air flow is diminished .

God bless
Wyr

 

Temp. difference. It's not unlike charging a system of 22 with vapor and you see the frost line of what left in the jug.Makes me miss using my old charging tube.

 

I would prove that the damper is fully open when they have a demand with a manometer. A visual inspection of the damper would be even better.
I have seen this issue several times this summer.


Many old timers bypass the furnace with stat and condenser Y. This results in 50% airflow with many furnaces manufactured after 1992.

We should also verify that the thermostat Y is connected to the furnace.
If the furnace has a Y1 and a Y2 and the condenser is a single speed model, stat Y should land on furnace Y2.

I have seen this mistake a few times this summer as well.

 

Completed the rewire with #8 this afternoon. I found a possible explanation to the airflow issue. The damper motor for the upstairs zone is right behind the pressure tank for the well. The cover for the motor was being pushed back by the tank and it had the shaft in a bind. The damper moved fine yesterday, but did not today. I got it off of the tank and it opened freely again.

Also, the trips just seemed to start recently after another tech (not the one from the first post) charged up the system. Possibly the overcharge caused some hard starts and in combination with the undersized breaker, was causing the trip.