Can I Run My 120/240V Wired House With A 120/208V Three Phase Generator?

Below is a great video that concerns itself with answering this oft-asked question. Many military generators are only configured to output three phase electricity, especially the larger units including the MEP004A, MEP005a and up.  Note that the MEP002A and MEP003A models output single phase 120/240V single phase power just fine and are not the focus of this post (That is the main reason we focus on those units.)

So the short answer to this question is, YES!  The medium answer is YES, BUT!  The long answer is the following:

Single phase power that typically comes to a home has two hot legs of 120v each.  You can use either leg to power 120v devices or combine both hot legs to power a 240v appliance or motor (e.g. for a well-pump).  This power is all 60Hz.

Three phase power has three hot legs, each of 120v.  You can use any one hot leg to power 120v devices exactly the way you do with single phase power.  So you would typically have four wires coming to your home with three hot and one neutral.  In the more typical single phase scenario, you would have three wires coming to your home, two hot and one neutral.  In either case any one of these legs gets you 120v power so any and all devices that run on 120v power will be fine. This includes all devices plugged into your standard 120v outlet.

Note that in either of the scenarios above, single or three phase, the frequency remains at 60Hz (in the US).

The wrinkle with three phase power comes when you would like to power 240v devices (e.g. a well pump, stove, dryer). If the device’s nameplate states that it can run at 208v (in addition to say 220v or 240v), you are fine.  Sometimes the nameplate will state 208-240v as acceptable voltage ranges.  Most modern devices can support this, but you must check the nameplate of each device to be powered.  So under almost all (typically 90%+) of everything you run in your home or shop will support being run at 208v (which is associated with a three phase generator output).

So what’s the big deal or difference with using a three phase generator output to power your home?  Since a three phase generator has three hot legs and you are only using two of the three to wire to your typical transfer switch or two bus breaker panel, you are only extracting 2/3 of the total potential capacity of the generator.  One hot leg of the generator is simply not used as your home is wired for only two hot legs.  Who cares?! Since generators in general run on fuel (diesel, gas, propane) and a generator running at no load and 100% load consume similar amounts of fuel to stay running at 1800 or 3600 rpm (for 60Hz applications), you are running at 2/3 of efficiency (power output per unit of fuel), which may or may not be of great concern.  Put another way, generators are generating electricity, whether you use it or not, so if you want maximum efficiency (power output per unit of fuel), you want to run at 80-100% depending on the make and model of the generator. For example, most military diesel units like to be run in the 80% capacity range to maintain the ideal engine temperature and operating conditions.  If you only utilize two of three hot legs, you are essentially only using 2/3 of this 80-100% capacity.  Doing the math would be 0.667 x 0.800 = 0.534 or 53% of designed capacity.  So if you have a 30kw generator and it’s configured for three phase and you are only using two of the three hot legs, you would have  revised capacity of 20kw and generally use only 80% of this or 16kw.

So fuel efficiency drops dramatically with the above configuration, but electricity-wise, you can generally run what you need and want to. So, for the occasional use scenario, there is no problem, but the more often you run this configuration, your fuel bills might start climbing! For most of us, fuel is a real cost.  So in the end, the final determination will relate to your application, needs, and desired fuel efficiency.

Watch this great video from You Tube to learn more!


8 thoughts on “Can I Run My 120/240V Wired House With A 120/208V Three Phase Generator?

  1. john says:

    consult your local Edison provider ,codes etc. required something as small as my friends sandwitch shop to get 477v service witch is a common and convient senerio . As this accodmates Edison’s one meter policy . It doues require the use of a poast meter transformer and breaker boxes for each of the output configurations, each of wich will match the requirement of your equipment. all that being said, your equipment will run more effentely. ovoid forced lockout till complince by local agency, leastley poast fire noncompension by your insurance compeny

  2. gmg says:

    3ph = 208v typically. Look at the nameplate on the specific piece of equipment you want to run. Often the motors/compressors will state that any 60hz electricity in the range of 208v to 240v will be fine for the motor/compressor. The voltage is what you want to match more than the phase.

  3. gmg says:

    400 amp service is provided by your electric company. We do not have generators with 400 amp capacity. We can acquire on a custom basis. Please fill out a contact form. GMG

  4. gmg says:

    Are you asking about a generator? If so, our units can handle 120/240 or 120/208 so our units would help. The amp service is up to you and your electric company.

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