By Mike Sokol
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.
Dear Mike,
I’ve got an EMS surge protector that keeps cutting off our power in the afternoon. It usually comes back on in a few minutes but gives us a low-voltage error. Isn’t it supposed to be 110 volts? So just how low is too low, and what does it hurt? —Estelle
Dear Estelle,
Those are some great questions. First of all let’s correct a common error. In the USA the nominal voltage is 120 volts, not 110. That’s a pretty common mistake because it actually started out at 100 volts DC in the time of Thomas Edison, was moved up to 110 volts for the longest time to compensate for voltage drop in the wires, then it was boosted to 115 volts for many years that I was working on industrial power, and finally it’s now at a nominal 120 volts. But you can call it 110 volts if you like since I know dozens of electricians who can’t kick the 110 habit.
Next, the National Electrical Code allows power to vary by plus/minus 5% of 120 volts and still be in compliance. Since that’s plus/minus 6 volts, it could be as low as 114 volts and as high as 126 volts at your home or office electrical outlet and still considered to be in compliance.
However, campground pedestals are another creature entirely, so a lower limit of 105 volts is the threshold where your EMS surge protector will decide to cut off the power to your RV. The 180-second delay before it comes back on is there to protect your air conditioner compressor from stalling due to excessive head pressure on startup.
Now, what gets damaged with too low of voltage? Contrary to urban myths it’s not your laptop computer or home entertainment system or even your RV’s 12-volt DC converter/charger or other electronics. All of those devices probably have a universal power supply that can be plugged in anywhere around the world, and thus are rated for anything from 90 to 250 volts. And your microwave oven probably won’t be damaged even if the voltage dips below 100 volts, even though it will take longer to heat up your coffee. Same for your coffee pot or electric water heater. They usually don’t care about low voltage, they just take longer to do their job.
The real areas of concern for low voltage are the compressors in your rooftop air conditioner and residential refrigerator. Any electric motor is rated for a specific voltage range, and once you get below that voltage it begins to draw excessive amperage. And that extra amperage turns into heat that can damage the (expensive) compressor. The industry has decided that anything below 105 volts is too low for these compressor motors and can cause damage, which is why that’s the low voltage trip setting on EMS surge protectors.
So what happens if you bypass your EMS and run your air conditioners below even 100 volts? I’m not sure, but that’s why I’m setting up a test next month in my lab. I have a huge VariAC (variable transformer) that I can vary the voltage to anything I want to test from 0 up to 140 volts. Next month I’ll do that on a Dometic Penguin II 15kBTU air conditioner while monitoring all kinds of electrical parameters. I’m sure there will be some interesting results to report.
In short, your EMS surge protector is just doing its job to keep your RV appliances from damage. Yes, campgrounds should supply you with safe power but many times that’s not possible. However, I’m working on that issue and may have a few tricks in place for next season. So please stand by…
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….
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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Those “power savers” that unsuspecting homeowners buy to reduce their fridge’s power consumption also destroy the compressor through undervoltage. They work by chopping off the end of the sine wave, which reduces the RMS voltage. Unfortunately, this increases the current, overheating the compressor while not saving any power (P=VI).
Mike, along a similar line, we have solar cells, charge controller and batteries as an emergency backup at home. We also have a 2,000 watt inverter. The plan is that we could use this setup to power the fridge and freezer (not at the same time) in case of a power outage. The inverter is a “modified sine-wave”, meaning that it puts out a square wave, not a pure sine wave.
Will the modified sine-wave inverter work on the fridge and freezer, or do I need to upgrade and purchase a “pure” sine-wave inverter?
Thanks, in advance.
I recently experienced an issue with low voltage. I have a motor home with 2 ACs and during a very, very hot day they basically stopped cooling. Even the AC fans were not blowing the normal amount of air. My RV was parked at my home RV pad and connected to a standard 50A pedestal. I tested the voltages and they were reading 113v and 114v. I reported low voltage to the power company and they came out, analyzed the issue and installed a new transformer feeding my residence. That fixed the issue.
Mike what about high voltage. Twice I had my progressive EMS-HW30C shut down for it being 1-2 volts over. I had to use the Bypass so I could have power. What do you recommend?
I think a key word in your response is “nominal” voltage. I assume that means the voltage the power company tries to maintain in it’s lines to your house or the breaker box in the campground. Beyond that point, I assume the power company is not responsible for how you use the power. If you put too much demand on the line, or if the wiring/connections are corroded, the voltage will drop & could cause problems, even though the power company is providing at least the minimum required 114V to the park’s main breaker box. So, if 115V is coming into the park, you could easily lose another 10V from aged wires, old weak breakers, corroded connections & heavy demand from others on the same line.
Yes, that’s true. But large amounts of current draw by RV’s with huge electrical demands and campgrounds with substandard wiring makes it even worse. I’ve seen state campgrounds that had as low as 94 volts in the afternoon when everyone was pulling power for their air conditioners. Their wiring infrastructure was never designed for that much demand.
Excellent concise article on the impacts of low voltage and history lesson. Looking forward to your findings. Thanks.
I never have my Progressive Industries EMS-HW-50C shut down for low voltage any longer once I installed the Power Master VC-50 Voltage Controller that I purchased years ago from Todd Coffelt. Unfortunately these devices are no longer being sold but you can purchase a Hughes Autoformer which will do the same function.
Be aware that as of the 2020 edition the NEC (National Electrical Code) has declared all autotransformer products a code violation in campgrounds since they may create additional voltage sag due to drawing more amperage from the pedestal to boost the voltage. But Hughes says just the opposite occurs, that it will actually reduce extra amperage draw due to low voltage being supplied to the air conditioner. I’m beginning a study next month that will either prove or disprove these conflicting theories. Please stand by….
Thanks, Mike! Looking forward to your study and results.
Will your research show up on this site? If not where will you publish it?
Since I now have the Soft Start gizmo installed in my a/c, would this eliminate the danger to it that you mention? Our reefer is the ‘customary’ RV reefer.
The Soft Start controllers are only in-circuit for the fraction of a second that your air conditioner compressor starts up. They don’t do anything at all while the compressor is running. So even though they can help your compressor start up more easily with low voltage, they won’t stop the overheating that seems to occur from low voltage. More testing for me to do to see just how much damage is done by low voltage to air conditioners, but realize I’ll probably have to destroy a new Dometic air conditioner in the process. Stay tuned….
Mike. Is there any way to bench check a roof air before going thru all the troubles of installing it on roof. I did this one time and received a bad unit and got replacement that worked. My other unit is original and I’m sure it’s a matter of time. What would I need as far as controls to do the bench test if possible
It was pretty simple. I used a pair of portable contractor tables spaced far enough apart for the air duct and wiring to hang out of the bottom. I hooked it up to a standard Dometic thermostat to run it, but needed to add a little 12-volt DC wall-wart to power the thermostat. I connected everything up with WAGO lever connectors, and wired it to a short 12-gauge extension cord that I sacrificed the female end off so I could apply power via the WAGO connectors. Pretty easy once I realized that the thermostat needed a 12-volt DC supply. I’m moving it outside of my lab space this week since it’s too noisy for my extended tests schedule to begin later this week.
Well Thanks Mike, that answers a question I had, How low is to low to run the AC. Last month at a camp ground voltage was hanging around 103 to 105. Had me worried. So turned if off.
That’s the lowest voltage anyone will recommend that air conditioners are used at. Interestingly, Dometic has said they have no actual data below 110 volts, which is why I’m setting up this test. I’ll take the voltage down in steps 1 volt at a time (probably down to 90 volts or so) and watch what happens for 5 minutes or so. I’ll repeat these steps until the compressor simply stops running or trips its over-temp breaker or just catches on fire. Along the way I’ll monitor voltage, current, power factor and compressor temperature. I can’t really test for efficiency since that would require a temperature and humidity controlled chamber, but I will monitor outside air and cooled air temps and humidity. And, of course, HRDL can look for any weird things happening with the power.