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Heating & Air Conditioning

AC compressor draws 20+ amps and flips breaker

14 years ago

My AC tech said I had to buy a new one or help it limp along by changing the breaker to 40amp and prob replacing the disconnect switch which was pitted and black. It worked after he left for a few days but this morning the breaker was tripped. I replaced it with a 40amp. Didn't help. I replaced the disconnect. Still trips the breaker. What should I do?

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Comments (5)

zl700
14 years ago
2 choices:

1. Keep resetting the oversized breaker and step back and watch the home burn down
2. Get a new AC unit (But please hire a competent installer, the one that advised the 40A breaker is dangerous)
maryland_irisman
14 years ago
Call another A/C Tech who will find the cause of the excessive amperage draw. No credible A/C Tech would recommend putting in a larger rated breaker to offset breaker tripping. It may be the problem is as small as needing a capacitor to as large as a defective compressor and any number of things in between the two.
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RLA is rated, and as mentioned above the circuit wiring is sized to 125% of this number. LRA isLocked Rotor Amps and is used to size the overcurrent protection for the circuit. When an induction motor start up it pulls current that easily approaches the LRA since the rotor is not turning yet. This is often the cause of the dimming in other lights that briefly occurs when the unit starts. If the POCO feed does not have enough excess capacity, the voltage falls slightly for a few cycles of 60 Hz until the rotor in the compressor comes up to speed. If the unit nameplate or installation instructions specify a circuit size, those numbers are used. many electricians size the wires for the breaker for a compressor, but this is not required. The wires are 125% of the RLA. The unit will only draw this much current when it is loaded to full capacity, so the current will be lower on all but the worst days of heating or cooling.
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Artilce 430 covers motors and is applicable to industrial type delta saws. A 3 HP motor has a full-load current of 17 amps at 230 V (Table 430.248). It may start using a standard 20 A inverse-time breaker, but it may trip the breaker if started repeatedly in a short period of time. A circular saw is actually a continuous duty load (it could run indefinitely) and it will have either integral thermal overload protection or a rated controller. The conductors need to be 125% of the full load current, or 21.25 A. #10 or larger is typically needed here. A 30 A dryer circuit should have #10 at least so you should be compliant here. The breaker can be up to 250% of the full load current. There are a couple more exceptions that allow even larger breakers (430.52(C) Exceptions #1 & 2) if the motor still trips the breaker on normal starting. 250% would be 42.5 A, and exception #1 allows the next larger size standard value, or 45 A. If starting still trips the breaker you can go up top 400% for nontime-delay fuses, and 400% for inverse time breakers (68 A). The breaker or fuse is providing only short circuit protection for motor loads since the motor already has overload protection.
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DavidR, Phases are nothing more than comparing multiple electrical waves with respect to time. So assuming two waves, if they both peak at the same time, they're said to be "in" phase, or 0 degrees "out of phase". When the power company creates power, they do exactly what you suggest, and put the windings 120 degrees apart, and deliver 3 phases of power. That's why if you look at overhead transmission lines, they're always in groups of three. However, that's not the only way to create a phase difference. As Brickeyee pointed out, single phase motors use capacitors to start (and sometimes run) the motors. What they're doing with the capacitors is creating a second phase. Residential US power comes from a single phase which is delivered to the house via a center-tap transformer. By definition, the two hot legs are 180 degrees out of phase with each other. By convention, though, we don't call this 2-phase, because at one time, power was delivered as 90-degree out of phase, and was called 2-phase. (I'm not sure if this still exists anywhere). One thing that you might find interesting is that the fans on many computers and audio equipment are 3-phase to both prolong their lives, and to make them quieter. All it takes is a little bit of electronics to transform single phase into multiphase power.
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"Joe surely you don't believe that it is ok to run two separate 20 amp breakers on a single neutral and ground? What if you put a15amp load on each breaker the neutral wire is being asked to run 30 amps. I know this is not safe. Leave the second breaker off. My guess it was originally a single 220 circuit. Somebody swapped out the dual breaker with two single ones. They also changed the outlet". This is why I questioned your qualifications. No offense but I am trying to tell you in a nice way that you are obviously not qualified to be telling others how to do electrical wiring. A multiwire circuit that is properly wired cannot overload the neutral. The neutral only carries the unbalanced current. In your example of 15 amps on each leg, the neutral will see no current at all. Ever notice that the neutral coming into your house is smaller than the line conductors? Same principle. Multiwire circuits have been around since the beginning of electricity and are perfectly legal. They are also known as "Edison circuits" and there are millions in use, probably a few in your house. I am suggesting in a nice way that you do your due diligence and study up before giving out advice. This post was edited by joefixit2 on Wed, Jan 14, 15 at 21:41
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creek_side
14 years ago
What the M_I said. And turn your system off now. Wiring sized for 20 amps or less on a 40 amp breaker with a known defective unit is asking for a fire.
hvac_grow
13 years ago
Check the unit Data Plate and find out the min breaker and wire size and make sure you have the proper protection. In general when an HVAC tech encounters this problem it is due to a loose connection in the main power being delivered to the unit. Look for wire that is discolered inside the unit. Tighten all connections....connections that are most suspect are the ones right at the breaker and outside disconnect. Then check the main unit contactor connections after that procede to the compressor and check those connections. This should be done by a licenced HVACR contractor
brickeyee
13 years ago
"replacing the disconnect switch which was pitted and black"

This is most likely from a loose connection.

The disconnect switch is not normally used under load, it stays closed.

A loose connection will extend the starting time and result in the high starting current being prolonged.

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