dual AC on 30 amps - I think I made it work

Splitshaft wrote:
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You plug in with a good, but not so shiny and or clean 30-amp RV plug or worse yet, one that has copper oxidation already coating its blades. Yes, the copper color, not brass color you see on 30-amp blades is copper oxidation from previous overheating and results in extra electrical resistance. Clean it off or risk a meltdown.

And the best way to clean the blades?

I generally set my demand to a max of 24 amps.

myredracer wrote:

philh wrote:
why would pulling 30a on a circuit rated for 30a be a problem?

Not sure if you're being sarcastic or serious. Technically and by code you should be able to draw 30 amps indefinitely.

Yes and there is also a provision in the code that relates to continuous draw circuits. Code requires a supply 125% of the rated load in this circumstance. (load 80% of supply is mathematically the same). Continuous load is defined as 3+ hours.

This code provision does not apply to a variable load such as an RV. However if you are going to run equipment for 10 hours at this level I and several previous posts recommend following the continuous load rule and limit power to 24 amps.

OK to run at 30 amps too as long as you are not disappointed if something heats up. Running at the very max will test any deficiency.

The number of current drawing devices on a RV 30 ampere circuit and the standard plug connection is dependent on the condition of the electrical connections involved as long as the “average” current draw for a 30 ampere RV circuit is not exceeded. Average current draw is the Key and what matters, “when the current that can flow and electrical components that are protected by the circuit breaker.” 30-amp circuit breakers do not trip immediately at 31 amps, or even 40 amps, and one can trip at only 25 amps under the right conditions. Electrical current flow results in heat and heat is the most common cause of a tripped circuit breaker, however, magnetism can also trip circuit breakers when the current flow is great enough to cause a strong magnetic field in the breaker such as a direct short.

That being covered, burned 30-amp electrical connections are simply due to electrical resistance in a plug and socket and nothing more at or near full current flow supported by the circuit protection device “the circuit breaker.” A basic understanding of OHMs law is necessary to understand the cause of burned and failed 30-amp RV plugs and sockets. Electrical resistance creates heat, oxidized, corroded, unclean, otherwise loose or damaged plugs and sockets will burnout or melt every time they are called upon to carry the full rated load of a 30-amp circuit. Have you ever wondered why your 30-amp cloths dried plug and receptacle and plug never burnout and melt down? Electrical resistance is the key! Look at the blade size of the 30-amp drier plug, it is the same size as 50-amp RV plugs! The 30-amp RV plug is grossly undersized for the job it is required to do and as such, is doomed to constant burnout and melt down especially when connections are loose and dirty. And 30-amp to 50-amp adapters only hasten the demise of the 30-amp receptacle in a campground power pedestal when power hungry RVs stress them.

Now consider this: even before plugging into a 30-amp power pedestal, 120V are measured with a volt meter under no load. The power pedestal receptacle is loose due to wear and has possibly been over heated before so its internal contacts are oxidized creating electrical resistance. You plug in with a good, but not so shiny and or clean 30-amp RV plug or worse yet, one that has copper oxidation already coating its blades. Yes, the copper color, not brass color you see on 30-amp blades is copper oxidation from previous overheating and results in extra electrical resistance. Clean it off or risk a meltdown. OK, so the AC is running, the refrigerator is running and the water heater is heating and the converter is charging our batteries and powering our 12-appliances all from 120V AC. We are near the rated 30-amps, so why does this stuff always burn out?

Simple, too much electrical resistance. If we measured the voltage drop though the power pedestal receptacle and the RV cord plug and realized a 10-volt drop, so what? 110-volts inside the RV is plenty good to power all of our equipment, right? Well what is happening out at the power pedestal and your RV plug? OHMs law is at hard work heating, melting, and burning stuff up. Voltage x Amperes = Watts (electrical heat) 10 volts drop x 30 amps = 300 watts of power dissipation or heat. Converting watts to heat (BTU) that stuff that comes out of you RV furnace, equals roughly 1,000 BTU! Wow and all that heat has to be dissipated in the 30 amp plug and receptacle. Rubber and plastic melt, burn, and char, brass contacts get hot and oxidize, causing copper oxides, first copper color and then becoming black in color as the copper oxide resistance and heat increase and the 20-percent zinc in the brass contacts vaporizes. That is our 30-amp melt down, until something changes in the industry, we are stuck with it.

I would put hard start capacitors in each unit. This will reduce start up current in case they both happen to fire up at once. This could happen easily if your short power is lost for a short period of time and then comes back on.. WHAM! A hard start cap will only help with startup, once running they are out of circuit.

Beyond that you have a potential problem. The hotter the ambient temperature the more current the AC's will draw. The motors simply have to work harder and the heat causes a loss of efficiency.

If your main breaker is old Id replace it with a quality unit because you are going to be asking a lot of it. At the campground I am at over memorial day weekend thay ran fans on the breakers. If I were you I would install a fan behind the breaker and run it off of a temperature switch.

Fun project... keep modding! Get the hard start caps! they are cheap!

My 04 Pace Arrow came with two a/c's on 30 amp service. Had ems and never a problem.

Depends on your ACs. I have 2 Atwoods and I can run both with no problem, along with the converter, light and TV. Generator or Campground. Both units are rated at 12.5A and they are 13.5K
The Fleetwood units that come with 30 Amps and 2 ACs use the High efficiency ACs that consume only 9 amps. My 36ft Fiesta LX did 106K miles running 2 ACs on 30 amps, never a problem.
As you see, the answer to "Can I run 2 ACs with 30 amps?" could be depending on a lot of factors.
I say "Yes you can, just check voltage and/or temperature on the plug"

I ran two A/C's last week, but I was connected at home, with a solid electrical connection, 30 AMP connection in my garage, on a separate, dedicated circuit, Fridge and HW heater OFF, and not using any other high amp items.

Voltage drop was only 3 volts, steady at 117, normally steady at 120. Cord and plug cool to the touch. I probably wouldn't do it with a campground connection or any electrical connection I didn't trust, but it worked fine for me in my situation.

Only did it one other time, at a camp ground, years ago. Ran both AC's and fridge.. 30 amp breaker tripped. Turned fridge to gas, and ran both AC's that evening fine.

Not something I would routinely want to do, but it can be made to work with a solid connection.

Mike

Conditions as I said two AC’s are not a problem on 30A.

Cummins12V98 wrote:


The idea is to turn off all high draw items and the two AC's on 30A will be fine. Yes low voltage could be an issue! There are variables in all situations. The 30A plug should be of good quality and it's receptacle on the pedestal.

A 12.5K AC unit has a full load draw of 12.5 - 13.1 amps so 2 ACs is already at 25 - 26 amps. The converter is going to be drawing at least a few amps and you can't turn it off, so you're pretty close to or even at 30 amps. If you're running the fridge on electric, that's another 2.5 amps which will put you over 30 amps. Turn on any other loads, high draw or not and you are def. over 30 amps.

Don't forget that if the voltage drops much, the AC units are going to be drawing even more current.

I hate to repeat myself, but running two AC units simultaneously is asking for trouble.

philh wrote:
why would pulling 30a on a circuit rated for 30a be a problem?

Not sure if you're being sarcastic or serious. Technically and by code you should be able to draw 30 amps indefinitely. But the receptacles in pedestals, especially 30 amps, are often in poor shape with either loose contacts or dirty/corroded contacts or both. And many times, RV-ers don't look after their shore power cords and the plug blades can also be dirty/corroded. A sloppy connection and dirty contact surfaces creates enough resistance to cause overheating and a meltdown which is not uncommon. Lots of pics are on the 'net.

The problem with 30 amp receptacles and connectors (on cords) is that you can't see inside them and it isn't easy to clean the contact surfaces, esp. on twistlock connectors.

This youtube vid shows what can happen to the connector on a twistlock detachable cord when running 2 AC units.