Is this normal for a 30 amp breaker

[Skipg]

The 20 amp breaker for the air conditioner get warm when the air is running. Is this normal?

[jseyfert3]

LoudRam wrote:
We're talking about TT AC units not household or commercial units. My trailer AC unit is on a 15 amp breaker. If that has a 50 to 60 amp draw at start up, there is something wrong.

Doesn't really matter what size motor it is, unless it has a soft start or VFD it's going to have a huge inrush current as a proportion of the running current. Here's a quote from Cooper (manufacturer of fuses and circuit breakers):

When an AC motor is energized, a high inrush current occurs. Typically, during the initial half cycle, the inrush current is often higher than 20 times the normal full load current. After the first half-cycle the motor begins to rotate and the starting current subsides to 4 to 8 times the normal current for several seconds. As a motor reaches running speed, the current subsides to its normal
running level.

So if the AC draws 10 A, 50-60 amps would be quite normal to see on turn on. Breakers can usually handle this inrush, and breakers can also have different trip curves, allowing more or less inrush than others. This is similar to a time delay fuse. A regular and time delay fuse of the same current allow the same long term current, but the time delay gives more time for motor inrush and similar. I remember as a kid the circuit with my dad's table saw needed a time delay fuse, as a regular fuse would blow the instant you switched the saw on.

It's really cool to put a scope on the AC lines and watch the actual inrush current and voltages. I've done that at work for several different devices. It's staggering how high the peak currents can be for the first full half cycle.

EDIT: Missed there was a second page of comments before posting, so this was already covered. Oops.

[kellertx5er]

Chris Bryant wrote:
Is it a single or dual breaker? They often use a 30/20 for main/air conditioner, which more than doubles the current carried.

Give this man a cigar! This breaker combo is all too common and puts most of the heat into one breaker package. When running a/c in 90+ temps that breaker pair will get pretty warm no matter how good the connections.

[ken56]

Do you know the voltage that is at the trailer? #1 is look for that loose connection if there is one, #2 is check the voltage reaching the trailer. If it is at the very low end of safe operation for the A/C like 105 volts then that could do it also. Operating the A/C on a low voltage will kill the unit over time.

[bucky]

Why is the title question about a 30 amp breaker and the whole thread is about a 20 amp issue?

[larry_cad]

ve7prt wrote:

AJR wrote:
ve7prt that was a very good layman’s description of how the math works with electric motors. Thank you.

No problem! I hope it explained to everyone just how this all works.

Cheers!
Mike

Actually, the back EMF generated when the motor is up to speed is always slightly LESS than the applied EMF. If it was the same, there would be no current flow. Actually the back EMF is constantly fluctuating, attempting to keep motor RPM constant. This variation in motors is known as “slip” and slip is what produces torque.

[ve7prt]

AJR wrote:
ve7prt that was a very good layman’s description of how the math works with electric motors. Thank you.

No problem! I hope it explained to everyone just how this all works.

Cheers!
Mike

[AJR]

ve7prt that was a very good layman’s description of how the math works with electric motors. Thank you.

[ve7prt]

LoudRam wrote:
We're talking about TT AC units not household or commercial units. My trailer AC unit is on a 15 amp breaker. If that has a 50 to 60 amp draw at start up, there is something wrong.

Nope. In fact, 50-60 Amps is probably uber conservative for initial in-rush current, and most ammeters cannot measure the instant current spike when the switch is closed. Keep in mind when the compressor is started, it is in a stalled rotor situation. If you think about it, the stator at this point can be considered just a really long wire, and at the moment of power-up, looks exactly like a dead short circuit. Once the rotor starts spinning, it creates what is called back EMF that fights the incoming current flow. Eventually, as the rotor gets up to speed, it creates a back EMF that equals the incoming voltage. This is why when you start your A/C unit, your room lights flicker hard. That compressor is a REAL BIG sponge for a second.

Now, as to why your unit can use a 15 Amp breaker without tripping it on startup? Well, other than a soft start kit, most breakers, as mentioned before, use bi-metallic strips that warm up with current flow. The more current, the hotter the strip gets, until the current flow exceeds the rating, hence heat in the strip, the strip flicks, and opens the breaker. However, the strip takes time to heat up, and the initial in-rush current of a starting A/C unit isn't present long enough to heat the strip to trip point. Usually. There are exceptions (like warm to hot ambient temperatures, worn strips, etc).

Cheers!
Mike

[LoudRam]

Again we're talking about small TT AC units not log splitters. I'm not going to argue about motor efficiency for every piece of equipment. For trailers the manufacturers use residential code and a 20 amp breaker is enough. Sometimes like with my trailer a 15 is enough. I'm trying to keep this on topic.

[LittleBill]

LoudRam wrote:
We're talking about TT AC units not household or commercial units. My trailer AC unit is on a 15amp breaker. If that has a 50 to 60 amp draw at start up, there is something wrong.

i have an electric log splitter, uses a normal 15 amp outlet. i have a inrush amp meter, that thing will pull 62 amps on startup all day long...

[LoudRam]

We're talking about TT AC units not household or commercial units. My trailer AC unit is on a 15 amp breaker. If that has a 50 to 60 amp draw at start up, there is something wrong.

[myredracer]

LoudRam wrote:
As per NEC you are only allowed to load a breaker to 80% of it's rated capacity that way you have room for start up current. My AC is only on a 15a breaker and draws 10 to 11 amps. A 20a breaker should be more than enough. And as it was stated earlier warm is normal but hot is a problem. However it never hurts to check the connections.

That's not quite right when it comes to motorized appliances and equipment.

An AC unit (and any motor for that matter) has the breaker "over-sized" (per NEC table) to ensure the compressor motor will start on the initial momentary inrush current of 50-60 amps of a typical AC unit. The 20 amp (or 15 amp) AC unit breaker isn't based on the full load running amps.

Another thing to note is that a code-sized breaker for an AC unit doesn't take into account what happens when the voltage goes down which causes an AC unit to draw more current which can cause the 20 amp (or 15 amp) breaker to trip. A compressor load stays relatively constant but as voltage goes down, the motor has to draw more current to try to keep horsepower output up. The motor can end up drawing much more than 80% of the breaker rating and eventually cause it to trip due to the bimetal element heating up more. Low voltage is common in CGs in the summer and is often much higher than the 5% recommended by NEC.

[LoudRam]

As per NEC you are only allowed to load a breaker to 80% of it's rated capacity that way you have room for start up current. My AC is only on a 15a breaker and draws 10 to 11 amps. A 20a breaker should be more than enough. And as it was stated earlier warm is normal but hot is a problem. However it never hurts to check the connections.

[DrewE]

Terryallan wrote:

cavie wrote:
no. a 20 amp breaker on a 12 amp load should not be hot. You have a loose connection or loose breaker on the buss bar.

I don't know. I really don't. But I'm thinkin the AC pulls more than 12 amps.

Starting, yes. Running, probably not generally, or at least not a lot more. 15 kBTU and 12 A works out to an EER of 10.4, which sounds about right. 13 A running would not surprise me. (EER is BTU per watt under some test conditions; SEER is the same, with a specified set of varying test conditions in an attempt to simulate a "cooling season".)