Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM. Today I discuss a 30-amp plug with two air conditioners.
I purchased and installed two compact SoftStartRV modules in our Dometic rooftop air conditioner units about a month ago. Until recently, we haven’t had the opportunity to test the SoftStarts using 30A service.
A few days ago we stayed at a park that only had 30A service, so we tested the SoftStart by running our two 15K BTU A/C units and it worked! But, doing so caused us to pull all 30A available from the power post just to run both A/C units. I use a Hughes 50A Autoformer when connecting to shore power, including to 30A service.
We have an all-electric coach, so doing anything other than running the A/C units (like using the induction cooktop or the microwave or the residential fridge) means we’d have to shut one of the two A/C units down.
Plug on dogbone started melting
When breaking camp the next day, I noticed that one of the blades on the 30A dogbone to 50A plug to the power post was burnt and the insulation at its base was starting to melt. The air conditioner test on 30A circuit lasted less than five minutes. But it looks like the dogbone couldn’t handle maximum 30 amps for extended periods.
So, what’s the point of trying to run two air conditioning units if you’re ultimately going to fry the 30A side of the power cable? Are there any 30A to 50A dogbones that can handle the increased current for extended periods? —Dan
Remember that SoftStart technology only helps reduce the inrush current when the compressor starts up. It does nothing to reduce running current. So if you have a pair of 15kBTU air conditioners running you can be pulling very close to 30 amps.
But that doesn’t leave any headroom for other electrical appliances, so you’ll need to shut off one air conditioner if you want to run an electric appliance. That’s the reality of trying to power an RV from a 30-amp circuit that can only supply 3,600 watts. Be aware that a 50-amp circuit can supply 12,000 watts, which is a huge difference in available power.
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Why the 30-amp plug started to melt
As far as your 30-amp plug meltdown, that was most likely caused by a poorly maintained pedestal outlet, not your own plug. Essentially, your 30-amp plug was damaged by a worn or oxidized pedestal outlet. However, campgrounds will rarely admit that.
I should note that while the Hughes Autoformer does indeed increase your amperage draw from the pedestal by up to 10% for resistive loads, it also helps reduce the 10% extra current draw from an air conditioner at low voltage. So while the Autoformer may allow you to run closer to the 30-amp limit, it won’t allow you to pull more power from the pedestal than the service can provide. That’s because the 30-amp circuit breaker in the pedestal should prevent you from drawing over 30 amps of current for extended periods.
While UL does rate the TT-30 connector for 30 amps continuous current and tests them at 125% of rated load (37.5 amps) for extended periods with only a minimal temperature rise, their testing assumes the pedestal outlet contacts are clean and tight. But that’s not always the case in older campgrounds.
Question from reader, part 2
Thanks for the explanation on why my 30A to 50A dogbone adapter overheated and about the 3600-watt limit on 30A sources.
That brings me to a another question. Would changing out my Magnum MS 2812 Pure Sine Wave Inverter for Magnum’s hybrid MSH3012RV 3000W 12VDC Pure Hybrid Inverter Charger allow the hybrid inverter to draw power from the battery bank when running two air conditioners on a 30A pedestal and we need to microwave something or the residential fridge turns on, etc.?
I think this hybrid can be configured to start the generator when power consumption gets near the limit of the input shore power. But, I’d prefer to prioritize the house batteries supplying power before resorting to starting a generator during quiet hours in an RV park.
We have four 100W solar panels on the roof which would be used to assist in recharging the batteries after a short term increased amp draw pushes the total current above 30A. If the hybrid inverter works this way, it could prevent tripping a 30A circuit breaker.
The last time I checked on the internet, the 3012RV 3000W inverter retailed for around $2,200. Yikes! —Dan
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Mastervolt Power Package
You could be correct with some rewiring to allow the inverter to supply power to one of your air conditioners while the other half of the RV is powered by the incoming shore power. I have a prototype Mastervolt Power Package in my Geo Pro toy hauler that I’ve been experimenting with. This package includes a 3,000-watt hybrid inverter, a 400 Ah lithium battery, and 570 watts of solar panels. You can set exactly how much current can be used by the shore power cord.
For example, if I’m connected to a 20-amp outlet in a driveway, I can set the controller for a maximum of 15 or 20 amps. Then when the RV is drawing 30 amps, the battery/inverter adds its own 15 amps to the 15 amps supplied from the driveway outlet for a total of 30 amps. The solar panels are always making up the extra current needed, and when your RV requirements fall below that 15 amps, the extra current then recharges the battery. BTW: It will charge this 400 Ah Mastervolt 12-volt lithium battery at up to 230 amps of current. So it can be completely recharged in under 2 hours if you’re on a 30-amp shore power connection, or 3 to 4 hours if you’re powered by a 15- or 20-amp outlet.
Tony Barthel, from RVtravel.com and StressLess Camping, and I will be discussing how the Mastervolt Power Package works in a few videos we’ll be live-streaming from the FROG Rally in Goshen, IN, starting August 14th. Yes, it will be available to the public in recordings, so stay tuned.
OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.
Let’s play safe out there….
Send your questions to me at my new RVelectricity forum here.
Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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