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.
Dear Mike,
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
Dear 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.Â
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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
Dear Mike,
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
Dear Dan,
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.
Upcoming videos
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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1) Despite what you may think, YOU ARE OVERLOADING YOUR 30A CIRCUIT/EQUIPMENT because you fail to understand the definition of the way it is ‘rated’ to operate.
2) Contact cleaning is a key and often neglected issue and a likely contributor.
NEC limits a ‘continuous duty load’ on a circuit to 80% of its capacity. Continuous duty is considered anything more than 3 hours in a 24-hour period. An air conditioner is a continuous duty load.
A ’30A circuit is rated to carry a combination of continuous and intermittent loads totaling 30A, it can only carry 80% (24A) continuously and the remaining 20% (6A) intermittently. The on/off nature of intermittent loads will cause heating but because they are off most of the time, the heat they generate can be dissipated.
Copper is not only electrically conductive, it is thermally conductive. Plastic is not only electrically insulating, it is thermally insulating. The heat generated in the conductor travels along the wire.
Well said. One thing that we are all assuming is that receptacle is in pristine condition, what we do not know is the internal condition of the receptacle, the dog bone however appears to be new. Keeping this in mind I believe I would error on the side of caution and do a self derate and monitor the temperature of the dog bone.
I’m not criticizing the SoftStart product but cautioning the expectations. Yes, SS might allow 2 ACs to run on 30A, BUT…..
As Mike said, two 15,000 BTU ACs require close to 30A, leaving no allowance for running your inverter, microwave or other appliances. So yes, it might run 2 ACs on a test bench but not practical in real life. You will probably need to turn off one AC to do anything.
Same comment regarding running AC on one small generator. One AC plus your onboard inverter could very well exceed the capacity of your 2000W generator. Mine won’t, therefore not worth the expense for me.
Just about everybody has seen a plug or two get hot. You know what might sell? A mechanical electrical receptical cleaner in the shape of 30 amp and/or 50 amp plugs with harder-than-copper blades with sandpaper-like surfaces. One push in and pull out of a receptical with its breaker off and any heavy oxidation, if present, would be reduced.
I thought that you had to take a 20% derate on continuous loads. Meaning 24 amps on a 30 amp curcit.
Like you said, the outlet caused the burning. They burn up the contacts, the present user see’s that complains to the office and thats it. Nothing changes. I always use a 50 amp adapter if available. I’ve never seen a burnt 50 amp outlet.
Thomas you are right. As an electrical engineer one of the first things I learned while designing power systems is the 80% rule. Never load a breaker more than 80%, never load a wire more than 80%, never fill a conduit more than 80%, and so on. This being said ambient air temperature will play a huge role in this, the need to run 2 air conditioners means hot weather and you may need to derate the circuit a little more. Before plugging into a pedestal and turning the breaker on I always spray the receptacle with deoxit and put a little bit of contact grease on the male plug and run it in and out a few times, then turn it on. My surge protector shows the voltage and current for each leg so I know what the current draw is for all appliances and watch my load when on a 30 amp plug. I have been doing this for years both on our sailboat and the motorhome and have never had a plug issue.
Holy cow!
Is it just me or why don’t you just ensure that only one A/C unit is run when on 30amp?
Go read the posts on facebook “Fulltime RV” groups We seem to have a few hundred thousand new users that have bought RV’s in the last few years and are mostly clueless about RV electricty, plumbing, towing rigs etc. If a rv has 2 ac then get a adaptor and plug it into a 15amp plug and its supposed to work flawless. After posting about what not to flush down a rv toilet ( Handy Wipes ) and people replying they never had any problems I just stopped trying to help them. Shaking my head is all I can do…..
Most “regular” or residential electricians go the other way when they hear RV. If not they try what they normally do and do not take into account it’s an RV. Very hard to find a “true” RV electrician.
Almost impossible to find any true technician. Most are down to remove and replace skill sets.