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.
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
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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