Good Sam Club Open Roads Forum: Is this normal for a 30 amp breaker
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 > Is this normal for a 30 amp breaker

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LoudRam

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Posted: 08/19/19 06:09pm Link  |  Quote  |  Print  |  Notify Moderator

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.


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ve7prt

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Posted: 09/02/19 07:12pm Link  |  Quote  |  Print  |  Notify Moderator

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).

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AJR

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Posted: 09/03/19 07:07am Link  |  Quote  |  Print  |  Notify Moderator

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


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ve7prt

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Posted: 11/11/19 08:23pm Link  |  Quote  |  Print  |  Notify Moderator

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

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Posted: 11/12/19 02:37am Link  |  Quote  |  Print  |  Notify Moderator

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.


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bucky

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Posted: 11/12/19 04:55am Link  |  Quote  |  Print  |  Notify Moderator

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


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ken56

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Posted: 11/12/19 06:50am Link  |  Quote  |  Print  |  Notify Moderator

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.

kellertx5er

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Posted: 11/12/19 01:46pm Link  |  Quote  |  Print  |  Notify Moderator

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.


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jseyfert3

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Posted: 11/12/19 05:42pm Link  |  Quote  |  Print  |  Notify Moderator

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):
Quote:

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.


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