120V or 240V

[Ray,IN]

Originally Posted by JFNM

... and the two hots are 110VAC.

I sure hope that doesn't cause confusion.

They should test 120VAC on each leg, 110 is already close to low voltage, 108 is the minimum acceptable voltage to prevent damage to motors and inductive electronics.

[Skip426]

Originally Posted by Ray,IN

They should test 120VAC on each leg, 110 is already close to low voltage, 108 is the minimum acceptable voltage to prevent damage to motors and inductive electronics.

Yes 120 volts is the new standard for AC power , but many of us still remember when it was just 110 and I often have to correct myself when posting and use the wrong #.

[Solo_RV_Guy]

Originally Posted by Skip426

X2. just as you can hook two 6 volt batteries together and get 12 volts; or four 6 volt batteries, and still get 12 volt, or 24 , properly combined the two 120 volt legs of power available at the RV power post can be combined to power an on board appliance that requires 240 volt power.

That's a gross mischaracterization.


Go back and look at the articles @twinboat linked.


One does not "combine" separate 120V sources to get a 240V service; rather the service starts out as "split phase 240/120V service." This works because of transformers (not the cartoon). A transformer is basically 2 coils of wire wound around an iron core (it's all about magnetic fields), with the Primary coil receiving a (usually) higher voltage and the Secondary creating a lower voltage - done through the magic of "turns ratio" between the Pri and Sec; i.e. if the primary has 200 turns of wire and the secondary has 100 turns, the secondary will have half the voltage of the primary.


Engineers can do several things with the secondary, including "tapping" (adding additional connections that are bonded to the secondary at various point in the length, say at 90%, 94%, etc). In the case of 120/240 service we get 120V because of a "center tap" exactly mid-way along the secondary winding.


This means we've created a secondary that will give you either 240 V (not using the center tap) or 120V. The center tap becomes the *neutral* and the neutral MUST be bonded to the electrical service ground (and that ground must be bonded to your RV frame).


As @twinboats point out, these 120V lines are "out of phase" with each other, courtesy of the center tapped neutral. When you use a voltmeter to measure between L1 & L2, you read 240V; if you were to measure between 2 120V outlets you'd get either 240V or 0V. Line voltage from the same phase leg is not additive...


Hope this helps. There's so much misinformation about electrical services spread about, and LOTS of shoddy wiring & maintenance practices at campgrounds and parks.


I also want to plug the writings of my buddy Mike Sokol, who writes info columns about electrical safety in "NoShockZone.org". I have no commercial affiliation with the site or Mike (I know him from my work in entertainment & live event electrical services). He identifies some more esoteric but dangerous RV conditions like "reflected hot skin" when 1 RV in a string with a defective water heater or shore power cord can create a situation where 120V is returned on the GROUND (that is common to all units plugged into a pedestal) and then energize the skin/metal parts of other units. Not common, but he's investigated and documented all kinds of strange electrical problems that turn out to have very simple causes.