I just had a phone call from a nervous camper that went something like this:
Hey Mike, Guess I should have read your columns more closely, but just what are the lowest and highest voltage readings acceptable at a campground pedestal? —Chuck
Chuck, it does get a bit confusing at times since the acceptable voltage in the U.S. has varied through the years. Edison’s original design called for 100 volts DC, with a 100-watt light bulb drawing 1 ampere of current. It later changed to 110 volts for many years, and when I was a young electrical apprentice in the ’70s it was set to 115 volts. Nowadays, the official voltage of the U.S. is 120 volts, with an allowed voltage drop of 6% under static loads. That works out to a deviation of around 8 volts, so as low as 112 volts is considered acceptable, and up to 128 volts is OK on the high side. But campgrounds are sort of special since there’s such a huge swing in loads from RV air conditioning and such, so let’s consider 110 volts to be the low side.
Now, in reality most of your modern appliances will have switching power supplies so even 100 volts would work. However, getting your air conditioner to start properly at 100 volts is questionable, so I like to see nothing less than 110 volts for RVs with HVAC units. New campgrounds need to comply with the latest electrical codes that will guarantee nothing less than 110 volts under the most severe loads, but older campgrounds may not be up to the task. So you can expect drops down in the low 100s on a hot day where everyone is running their air conditioners. Basically, if your air conditioner compressor seems to start up normally without any locked-rotor problems (it hums, but doesn’t actually start up), then you should be OK. But a steady diet of 100 volts for your air conditioner can be hard on the compressor and starting relay.
Of course, if you have a 50-amp shore power connection the line-to-line voltages are doubled. But you really should be measuring from neutral-to-line to test for acceptable voltages. Below are two diagrams reminding you on how the voltages are measured in various shore power outlets.
BTW: The voltage measured between ground and neutral will only be zero under no-load conditions. So if other RVs are plugged into pedestals on your campground “loop” you can expect that reading to be up to 5 volts and still be acceptable.
Let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 40 years in the industry. Visit NoShockZone.org for more electrical safety tips. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.