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Old 06-12-2013, 10:18 AM   #1
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AC blowing breaker in coach

SO it was 97 yesterday when I got home. The Coachmen is sitting in the cul-de-sac between trips and was 104 inside (DW closed all the windows vents etc., as it was thundering at 5:30 am but never rained).

Then found out the neighbor's A/C was out. Wanted to offer the camper to them, but I was uncertain if the A/C would run off the house. I have it plugged into a 20 amp run in the garage, but with 100ft of regular outdoor extension cord (not the heavy 30 amp cord) I sincerely doubted I was going to get enough juice out to the camper to urn the A/C. Went ahead and turned it on anyway.

It ran for about 10 minutes, and then blew the breaker inside the camper. Not the house...

This confused me. I would assume the 20 amp breaker in the camper would allow the A/C to pull 20 amps through it. Since I had at max 20- amps hitting the camper, and probably less, I assumed the weakest link was the breaker in the house.

The more I thought about it, the more I thought maybe the breaker in the camper is the problem. We have only ru the A/C a couple times, just have not been in hot weather, yet. I tried it again after letting it sit for a while and did the same thing. By that time the camper had cooled off a little from all the windows open and dinky little fan in the bathroom running.

Any thoughts as to why the A/C breaker in the camper blew but not the supply breaker?
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Old 06-12-2013, 10:29 AM   #2
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What size cord and how long.

Cord should say awg rating on it
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Old 06-12-2013, 10:31 AM   #3
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cords are your standard home depot outdoor extension cord. 16/3 - 2 50 ft cords.
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Old 06-12-2013, 10:32 AM   #4
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This will help.

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Old 06-12-2013, 10:35 AM   #5
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As seen you need at least 12 ga ext cord no longer than 50'

Standard cords are 16 -18 ga.


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Old 06-12-2013, 10:38 AM   #6
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those posts suggest that no way am I getting 20 amps, or close to it, to the camper...which makes me wonder even more why the breaker would blow. As I understand it, breakers blow when overloaded, not under loaded, right?
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Old 06-12-2013, 11:18 AM   #7
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Quote:
Originally Posted by jeeplj8 View Post
those posts suggest that no way am I getting 20 amps, or close to it, to the camper...which makes me wonder even more why the breaker would blow. As I understand it, breakers blow when overloaded, not under loaded, right?
My point is if you have a 16-18 ga cord @ 100' then that cord will not carry the amps required to run your ac therefore heating up breaker and tripping it.
Cord is prolly hot to!

The hoter it gets outside the more amps the a/c will require.

A 50' 12 ga cord would be at the least minimum.
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Old 06-12-2013, 11:20 AM   #8
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http://www.forestriverforums.com/for...dge-41766.html


Read towards end.
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Old 06-12-2013, 11:21 AM   #9
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Originally Posted by f1100turbo View Post
My point is if you have a 16-18 ga cord @ 100' then that cord will not carry the amps required to run your ac therefore heating up breaker and tripping it.
Cord is prolly hot to!

The hoter it gets outside the more amps the a/c will require.

A 50' 12 ga cord would be at the least minimum.
there is what I was missing, so the breaker will blow in an under-current situation?!? Now it makes sense.

Looks like 100 ft of 10/3 is $133 at Home Depot...oh boy.
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Old 06-12-2013, 12:00 PM   #10
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Quote:
Originally Posted by jeeplj8 View Post

there is what I was missing, so the breaker will blow in an under-current situation?!? Now it makes sense.

Looks like 100 ft of 10/3 is $133 at Home Depot...oh boy.
Now your talkin!
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Old 06-19-2013, 08:44 PM   #11
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Quote:
Originally Posted by jeeplj8 View Post

there is what I was missing, so the breaker will blow in an under-current situation?!? Now it makes sense.

Looks like 100 ft of 10/3 is $133 at Home Depot...oh boy.
I still don't think it will work. The house wiring is only 14awg. To the panel on a 20 amp circuit. If you try this then your house breaker would most likely trip.
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Old 06-19-2013, 08:52 PM   #12
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Quote:
Originally Posted by jeeplj8 View Post
cords are your standard home depot outdoor extension cord. 16/3 - 2 50 ft cords.
Your voltage was probably 100 volts due to that flimsy extension cord.
You had to be trying to pull 30 amps of start-up through that straw.
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Old 06-19-2013, 08:56 PM   #13
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Nice Chart to follow Turbs! You can be pretty smart...not as smart as Herk, but you are earning points!
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Old 06-19-2013, 09:02 PM   #14
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Remember that induction motors like the fan and compressor in your air conditioner require WATTS to start turning. The lower the voltage coming in the MORE amps are needed to generate the watts (power) to turn the motor over.

Since watts = incoming voltage times amps.

A device that needs 18 amps to start and 12 amps (at 120 volts) to run, like an RV air conditioner, it needs MORE amps to run at lower voltages.

Example: 18 amps times 120 = 2160 watts.

If the voltage is reduced to 100 volts due to the cheap cord and length, to get the 2160 watts the unit needs to turn over, the device has to suck

2160 = 100 times amps or 21.6 amps (blown breaker)
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Old 06-19-2013, 09:19 PM   #15
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As Herk posted, the real problem with under sized conductors in a standard drop cord on high amp draw applications is that the resistance in the conductor cause the voltage to drop (thus causing the conductors to heat up, just like the wires in a toaster.

Now, a motor is rated to run at a certain power level, whither expressed as kW or HP. Based on Ohms Law, as the voltage drops, the amp draw must increase to satisfy the motor power requirement.

So, you get caught in a catch 22 loop. The amp draw increases, causing the voltage to drop, causing the amp draw to increase, causing the amp draw to increase - etc. etc. Eventually you reach an equilibrium and things may run for a while.

So, at the time your circuit breaker tripped, the voltage had dropped to the point where you exceeded the 20 amp rating of the circuit breaker. Considering that an AC draws 13 to 15 amps at 120 VAC, it doesn't take much voltage drop to exceed the circuit breaker 20 amp rating.

Why did the AC run about 10 minutes? This is the characteristic of circuit breakers. If there is a direct short or a very-very high amp draw, a circuit breaker will trip almost instantly. However, if the amp draw is at or only slightly above the circuit breaker rating, the circuit breaker will trip based on a time curve. So, if you are only 1 amp above the circuit breaker rating, it might take 10 minutes to trip. 2 amps above the rating, 3 minutes. 3 amps above the rating, maybe only 1 before it trips.

The long and short of it, with a 100 foot drop cord, you're going to need a 8 or 6 awg cable tied directly to your home distribution panel to run your AC.

Rick

Looks like Herk and I were typing at the same time. Herk either types faster then me, or I was too long winded!
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Old 06-19-2013, 09:19 PM   #16
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For completeness, resistance loads just become "less efficient"

Your water heater on AC (with a 1000 watt heating element) for example will draw 8.33 amps at 120 volts. That 8.33 amps is constant because the resistance is constant. (E=IR)

If the incoming voltage drops to 100 volts, your 1000 watt water heater element will only deliver 833 watts. (switch to propane for hot water).

Thanks to BarryD for the following correction:

You got the part right about constant resistance. You extrapolated that to mean constant current, which is wrong.

I=E/R
P=I*E = (E/R)*E = E*E/R

In your example, the current doesn't stay at 8.33 amps. It drops, along with the voltage, to 6.95 amps at 100 volts. Power at 100 volts is 694.4 watts.

Which is much worse...
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Old 06-19-2013, 09:44 PM   #17
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I doubt you are doing your AC unit any favors , with the cpmpressor trying to run at those voltages. I used to try to run my AC unit off a 20 amp circuit and a short 12 gauge cord and it pops after about 20 minutes. so I put another breaker in the box for 30 amps and wired the correct plug and voila no problems any more.
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Old 06-19-2013, 10:05 PM   #18
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More info:

OK, here is the plate off MY Coleman Mach III 15K RV air conditioner:

Note the Compressor only ratings first:

Compr LRA (locked rotor amps) - This is the amps at 115 volts that a "stuck" compressor will draw. In my case 58 amps at 115 volts. This is bad and means the compressor is locked up and without any way to rotate it (since it is single phase and you can't switch wires around) you are stuck with replacement.

Compr RLA (Rated Load Amps) - This is the UL rated load for the compressor "doing its job" It has started and is up to speed. Also called the Maximum Continuous Amps (MCA).

In my case, the RLA is 11.9 amps at 115 volts AC.

BUT

This is not the START UP amps. To find the start up amps (at 115 volts AC) you must multiply that number (MCA/RLA) by 1.56 (For UL rating) to find the nominal start up current FOR JUST THE COMPRESSOR.

In my case, the compressor will draw 11.9 amps times 1.56 or 18.56 amps.

Determining Compressor Amperage Ratings

To these numbers you must add the fan.

My fan is a 1/3 HP fan with a FLA (Full Load Amps) of 3.4 amps. That means if the fan motor is running at 100% capacity it will draw 3.4 amps at 115 volts all by itself. The GOOD news is most fan motors are sized to deliver between 80 to 90% of capacity when running at 115 volts.

So say the fan alone draws 85% of its FLA when running, or 2.9 amps in my case. (Again the start up current for UL rating can be no higher than 1.56 times the running current or 4.5 amps) This is very close to what I get on my AC ammeter in the camper.

HOW TO MEASURE BLOWER MOTOR AMP DRAW | Archive content from Contracting Business

The theoretical start up amps for my unit would be 18.5 + ~4.5 or 23 amps and the running amps should be 11.9 + ~3 or 14.9 amps.

Remember that these numbers are at 115 Volts! If available voltage is LESS, these numbers can go out of sight! (If you are getting 120 volts the amps are lower)

My unit typically draws 14.3 amps running and I have seen (before my mods) as high as 25 volts momentarily on start up. The Hard Start mods have reduced start up amps to around 16-17 amps. I am VERY happy with the results so far.
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Old 06-19-2013, 10:17 PM   #19
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Good info Herk! I keep forgetting to bring my clamp-on meter home from work to check inrush current (fancy new Fluke clamp-on with inrush metering).

I just did the hard start kit on my AC. Noticeable reduction in compress startup time and no noticeable voltage drop on start.

Rick
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Old 06-20-2013, 09:07 AM   #20
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OK - before everyone gets too carried away, and since of you clearly did not read the original post very well, couple things:

My questions was why one particular breaker blew before the other. Again, I knew one was going to blow, I just wanted a technical explanation as to why.

This was an off the cuff thing to try it and see what would happen. Got my results and shut it off. I considered it more of a controlled experiment, and as I was surprised by the results (I expected a breaker to trip within 30 seconds) I was looking for some explanation. Which I got...in the first page of posts, but thanks for everyone else teaching that dead horse a real lesson!

Oh and by the way, despite the fact I didn't understand the relationship of conductor and heat on a circuit breaker, I do know that solid copper wire can carry more load than stranded. Hence the 14 or 12 gauge home wiring going to any number of outlets does not cause a problem, but a 14 or 12 gauge stranded wire extension cord cannot carry the same load.

As I did want to to run the A/C for a little while packing up, I went ahead and dropped 133 on a 10/3 extension cord at HD. I justified that expense by buying a new grill while I was there.

The 10/3 is capable of carrying 20 amp that far, and it ran fine for the hour I wanted to load everything in. It is heavier per foot than 30 amp RV cord, which I thought was interesting.
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