[Joeywhat]

OK, I'd like some help troubleshooting this issue. Sometimes my window A/C unit (120v, 10k BTU) will trip the breaker for my master bedroom. It says it draws 8.3 amps max. This happens only sometimes, and only when the compressor first kicks in. When it does happen, everything on the circuit dims for a few seconds (usually I have a fan or two running, which slow down considerably) until the breaker finally trips. Sometimes the compressor will try to start a few times before finally tripping the breaker.

I'm just curious if this is an issue with the window unit (bad compressor) or with my electrical (bad breaker or lose wiring somewhere). Since it only happens intermittently I wasn't entirely convinced it's the compressor. Will a bad compressor try and draw significantly more power leading to this issue?

 

[surferdude2]

It sounds like the circuit may be overloaded. The wire size and breaker rating will determine what load can be tolerated. The starting surge for a small 120 volt a/c can be as high as 25 amps. That alone isn't a problem unless it's on a 15 amp circuit (14 awg wire) or if there's other loads on the circuit.

Test it by turning all other loads off when it's having the problem.

An amperage/voltage test of the circuit when fully loaded would be helpful in diagnosing the problem.

 

[Joeywhat]

How would I test the amperage/voltage on that circuit?

Also to clarify, before the circuit trips the condenser is trying to turn on, but running real slow. I'm not certain what that indicative of, but could a bad compressor (or maybe some bad capacitors) cause the compressor to be pulling too much power?

I guess I'm just not certain why this only happens some times... Not every time. I usually only have the same couple of things running on that circuit (a ceiling fan and plug in floor fan, both on medium). Sometimes it starts right up, other times I can hear it strain for a bit then it shuts off and tries again and starts up, other times it just tries for a few seconds then the breaker trips.

 

[surferdude2]

You'll need some way to measure voltage. The easiest way to do a meaningful test is to connect your voltmeter to the other outlet on the duplex plug that the a/c is plugged into. (use a 3 way adapter if it's not a duplex rcpt) Keep your voltmeter connected and note the voltage before and when the unit is struggling to start. Post back those readings.

 

[user_12345a]

You could have a failing compressor run capacitor. Also possible the compressor is getting a bit tight/worn out - in this case a cheap hard start kit can be added.

When the compressor motor is in the circuit but isn't turning, it draws a heck of a lot of current. The thermal overload is supposed to cut the compressor out but it's possible for the breaker to trip before that.

If you measure voltage when the compressor is trying to start and it's lower than normal, doesn't mean there's a problem on the circuit. The draw can be 30 to 40 amps trying to start a compressor and that's bound to produce a voltage drop.

A better test is would be putting a 1500w-1800w load on the circuit and seeing if the voltage drops below normal limits.

You can also try the window unit on another circuit.

 

[carmusic]

if the ac tries multiple time to start, you have a problem with the ac, perfectly normal the breaker trips if the compressor is locked for few seconds

 

[Joeywhat]

You'll need some way to measure voltage. The easiest way to do a meaningful test is to connect your voltmeter to the other outlet on the duplex plug that the a/c is plugged into. (use a 3 way adapter if it's not a duplex rcpt) Keep your voltmeter connected and note the voltage before and when the unit is struggling to start. Post back those readings.

Well, I have schrodingers air conditioner apparently... I started the compressor several times but it clicked right on each time. I'll check it again over the next couple days and see if I can catch it when it struggles. Voltage was 118 with just the room fans on, 116 with the A/C compressor running normally.

 

[HotRodx10]

You may need to change the breaker. Breakers get more sensitive (trip at a lower current draw) the more times they're tripped. Several electricians I know automatically replace breakers after they've tripped 3 times.

 

[Joeywhat]

The whole panel is being replaced sometime this summer, so that will fix all of that. It's the original panel from 1964, so it's got some miles on it.

 

[seharper]

That doesn't mean it's bad. Panels don't really get "miles". I have some 60s era CH panels that are tip-top.

Being split-bus, dangerous (Zinsco, FPE), just obsolete (Pushmatic), or too little spaces, or ampacity... are all good reasons for an upgrade, though.

It seems like you're getting voltage drop at the moment the compressor tries to start. That could be a loose connection anywhere in the chain. Replacing a panel is an expensive way to check the cleanliness/tightness of all the connections.

 

[Joeywhat]

That doesn't mean it's bad. Panels don't really get "miles". I have some 60s era CH panels that are tip-top.

Being split-bus, dangerous (Zinsco, FPE), just obsolete (Pushmatic), or too little spaces, or ampacity... are all good reasons for an upgrade, though.

It seems like you're getting voltage drop at the moment the compressor tries to start. That could be a loose connection anywhere in the chain. Replacing a panel is an expensive way to check the cleanliness/tightness of all the connections.

It's a federal pacific.

The issue is largely that it only struggles to start maybe 10% of the time. Every other time it seems fine. Makes it difficult to diagnose.

 

[seharper]

It's a federal pacific.

<Emily Litella mode> Oh, that's very different. *pause* Nevvvvermind! </Emily>

The issue is largely that it only struggles to start maybe 10% of the time. Every other time it seems fine. Makes it difficult to diagnose.

Nah, I think you had it on the first sentence. Unsafe at any speed, that one!

 

[Joeywhat]

OK, I tried again today and managed to catch the compressor struggling to start. While struggling to start it read 92 volts on the outlet. It tried for about 3-4 seconds before I shut the unit off, and during that time I could audibly tell that the compressor was trying to start but couldn't (like trying to start a gas motor with a dying battery).

After turning the unit back on and waiting for the compressor to cycle again it fired right up this time, dropping to 116 volts.

 

[seharper]

92 volts sounds like a conductivity problem at a connection somewhere, caused by a bad connection resulting in overheating or arc-fault. Possible candidates are:

- Backstab connections at any receptacle or switch in the chain
- Loose side-screw connections, ditto
- Loose or sloppy wire-nut splices. Watch for wire nuts with tape added after the fact; that's a hallmark of people who suck at wire-nutting taping them so they don't fall apart, but what makes them fall apart also makes them a bad connection that will overheat or arc-fault.
- All neutrals must also be checked.
- The wire landing on the neutral bar on the breaker panel
- The wire landing on the circuit breaker
- where the breaker bus stab and clip meet, especially on alien breakers (wrong brand wrong panel) and bus stab problems are exactly why FPE panels must be fully replaced and can't be fixed by just changing to non-fraud breakers. However, while this is the prime suspect merely based on prejudice/history, you should still check the other stuff.

 

[RAL238]

It's possible that the compressor is having a hard time starting because it is short cycling. If the compressor runs for a while, then stops and tries to start again too soon, it will be fighting against high pressure it previously built up in the system and won't be able to restart.

There are several things that can cause short cycling, such as dirty filters, frozen evaporator coils, low refrigerant, or the unit being too large for the space you are cooling.

I would check for those problems before you spend too much time chasing an electrical problem.

 

[seharper]

Good point RAL.

Short-cycling is very bad and can damage the A/C. Most A/C units have a protective circuit involving a timer/delay to prevent short cycling. However that timer can be defeated if power is cut and restored (i.e. breaker off/on). The bad connection may be having that effect!!!

 

[Joeywhat]

When I observed it this morning the unit hadn't been run in over a week...so I don't see how it could be from short cycling.

I'll check the outlet connections. Still seems odd that the connection is poor only some of the time.

 

[user_12345a]

OK, I tried again today and managed to catch the compressor struggling to start. While struggling to start it read 92 volts on the outlet. It tried for about 3-4 seconds before I shut the unit off, and during that time I could audibly tell that the compressor was trying to start but couldn't (like trying to start a gas motor with a dying battery).

After turning the unit back on and waiting for the compressor to cycle again it fired right up this time, dropping to 116 volts.

Can you try it on another circuit?

It's possible there's a problem in the circuit but it could also be due to drawing very high current trying to start, not turning over.

All the connection on the circuit should be checked if the unit starts fine on another - the plugs are usually daisy chained.

Can your meter read capacitance?

 

[Jim Port]

I'll check the outlet connections. Still seems odd that the connection is poor only some of the time.

Connections under load heat up, when they cool off they can become loose.

 

[Missouri Bound]

Any idea what the breaker size is on that circuit and the wire size?
Dimming lights usually relates to a shortage of power, usually overloaded circuits or too small a circuit.