I moved these posts from the television discussion in the Podmod sub-forum.

Most residential service in North America has 2 parts (legs), each one is a separate 120 volt supply from the transformer outside.  These can be combined to create 240 volts for the big stuff, like dryers and water heaters, and/or they can be used separately for the 120 volt loads.  Some appliances like clothes washers use both at the same time.

The proper explanation for why you can get 120 and 240 at the same time is head-exploding stuff, so I won't get into it unless you want me to.  What is important is that inside your circuit panel you have the 2 separate 120 volt “legs” (circuits).

Now let’s say you have a generator that only outputs 120 volts, and you don't want to spend the big bucks to buy a transformer that would convert this to a proper 120 + 240 volt output (properly referred to as 120/240 split-phase).  For most of the stuff in your house that is no big deal - most outlets, lights and appliances only need 120 volts.  Half of them are wired up to one of the legs inside the circuit panel, the other half are wired up to the other leg.  Electricians do this to balance the load between the two legs.  Each leg operates independently from the other, and in a special situation (half of the utility transformer outside fails) you actually could lose just one of the legs so half the stuff in your house would work, and the other half wouldn't.  I experienced this once in a commercial building, and my brother experienced this a few years ago at his home.

In order to make all the connections into (or out of) your circuit panel you need 4 wires.  Leg 1 (similar to the + connection on a battery) usually colored red, leg 2 (ditto) usually colored black, neutral (similar to the - connection on a battery) colored white, and the ground wire is always green.  If you put a volt meter on the red and white wires you'll see 120, black and white 120, black and red you'll see 240.  Technically you'll see the same thing if you used the green wire instead of the white wire - but for safety reasons you never use the green wire for a load (that said, some clothes dryers in older homes, do...).

So, most people would call an electrician and have them install a transfer switch plus a twist-lock plug which would allow you to run your house in an outage.  That twist-lock plug has 4 connections... leg 1, leg 2, neutral, and ground.  Medium-sized generators supply all 4 wires via a twist-lock outlet and you use a special heavy-duty extension cord to attach it to the twist-lock plug the electrician installed.  Start the generator, plug in the extension cord, flip the transfer switch and now everything in your house will run normally - in theory.  NOT getting into how you will only have enough power through a 30 amp twist-lock to run some of the stuff at one time, and big things like permanent A/C units won't be useable.  You need a big permanently-wired generator like a Generac to actually run everything normally.

So, back to the small generator.  Since it only outputs 120 volts you will only see a standard 120 volt outlet or a smaller twist-lock outlet on it.  Either way, you only get 3 wires... Leg 1, neutral, and ground.  No leg 2.  How do you connect those 3 wires to the 4 wires?  For MOST things in your house, you can just feed leg 1 from the generator to leg 1 and leg 2, plus the neutral and ground.  Both legs (black and red wires) in the circuit panel get 120 volts like they are expecting.  But, due to the head-explody stuff I didn’t get into, if you put a voltmeter on the black and red wires you would see zero volts instead of 240 volts.  Because of this...

There are 3 potential problems with this scenario, which is why you can’t go to Home Depot and buy a cable that would let you do this.  Obviously 240 volt appliances won't run, you only have 120 volts.  Some of them, like water heaters, only use 240 volts, they don’t use that neutral wire at all, and due to the head-explody stuff they will actually see zero volts.  This is safe, they will effectively be completely off.  No safety problem here, you just don’t get hot water.  Some big appliances use 120 and 240 volts, usually clothes dryers and combined washer/dryers, possibly stoves and ovens.  They use 240 for the big heating elements and motors and 120 for the control board and the little light just inside the door.  No 240 so they won’t run, but they could be damaged since the control board will still be active.  Its possible this would be a safety issue, too.  So there is a problem here due to possible safety and damage issues (unlikely, if they are designed properly, but still possible).  The real safety issue is the final potential problem, those multi-wire branch circuits (MWBC).  MWBCs are used for lighting and outlet circuits.  Many homes don’t have them, but many homes do.  They let you create 2 separate circuits somewhere in the house using fewer wires, but they use 2 circuit breakers.  The simple explanation is they save money during installation since you use less copper in the walls, and they can reduce voltage drop if you get into some head-explody stuff.  You have to know all the head-explody stuff to understand why these can be a real safety hazard using the “trick” I described above, just understand that they can cause a fire in a worst-case scenario.  It can be hard to figure out if you have them at all, even an electrician would have to poke around in the circuit panel for a while to find out.  Since they can be hard to identify and can cause a fire, I don’t recommend using this “trick” unless you really know what you are doing.  There are multiple ways around this problem, but since you’ll need an electrician they aren’t cheap.

In my case I installed a sub-panel which my big UPS powers all the time.  There are no 240 volt appliances on this subpanel (although ironically my big UPS outputs 120/240 split-phase so I could), and no MWBCs.  I actually did transfer a MWBC to the subpanel but I combined the 2 circuits to one circuit breaker, which eliminates the safety hazard.  So that leaves me with a sub-panel that is safe to power from a 120-volt-only source like a smaller generator.  I wired up my own special heavy-duty extension cord that combines leg 1 + leg 2 and will plug into the small generator and the sub-panel.

Questions?