Generator Power Output - 3-Phase 120v/208v vs. 1-Phase 120v/240v

Discussion in 'General Electronics Chat' started by Mr. Fahrenheit, May 27, 2015.

  1. Mr. Fahrenheit

    Thread Starter New Member

    May 27, 2015
    I am an HVAC technician in the Marine Corps. I have a basic understanding of electricity but I'm having trouble wrapping my head around this particular problem. I work for a Communications shop with techs that don't fully understand the power requirements of their equipment and are unable communicate (go figure) exactly what they need to power their equipment.

    The majority of our equipment is running off of a medium sized generator - we'll call it Generator A - that produces 3-Phase 120v/208v power. However, we have one shelter running on its own much smaller dedicated generator - Generator B - that provides 1-Phase 120v/240v. Or so it says on the data plate on the generator, anyway. The individual responsible for this shelter believes that the shelter requires 1-Phase 120v/240v for its built in air conditioner.

    I want to move the shelter to Generator A's power grid, but I'm trying to figure out if it would function properly on 3-Phase 120v/208v power. Generator B is designed to put out 3-Phase 120v/208v normally. It has 5 load studs: Ground, Neutral, L1, L2, L3. There is a selector switch inside the generator that allows me to change the output of the generator (presumably by opening and closing the circuit for different phases). There is a rather helpful data plate inside the generator that explains the output of each mode. This is what it says on the data plate:

    Mode :: Output

    3-Phase 120v/208v :: N, L1, L2, L3
    1-Phase 120v :: N, L1
    1-Phase 120v/240v :: N, L1, L3

    Currently the switch is set for the third option and the shelter is wired into Generator B with a 4-wire cable (Gnd, N, L1, L3). The shelter runs properly with this setup. However, this business with 1-Phase 120v/240v is where I'm having difficulty understanding.

    1 - What is the difference between 120v/240v and 120v/208v? Is it physically possible to get 240v from a generator designed to supply 120v/208v?

    2 - How is it possible for this output to be single phase? Wouldn't it be two-phase since it uses two lines (two phases)? I'm a bit skeptical 1-phase 120v/240v is even real.

    3- Could I run the built-in air conditioner on two phases of 3-Phase 120v/208v instead? The rest of the equipment in the shelter is single phase 120v, so no problems there.
  2. MaxHeadRoom


    Jul 18, 2013
    Connection across 2 phases is single phase, there is no 3rd reference for 2ph, the difference with 120/240 is this either has to be derived from a delta generator and a tap for 120 or a single phase transformer used, the 120/208 is a star or Wye arrangement where the 120 x sq rt 3 (1.73).
  3. Mr. Fahrenheit

    Thread Starter New Member

    May 27, 2015
    Thanks for taking the time to respond.

    Your link helped clear some things up for me - a single phase must have been routed through a transformer to provide 120v/240v. My meter confirms that it is 240v between L1 and L3 on the smaller generator.

    I just have one other question - could the air conditioner run on two legs of 120v/208v power? Currently it's running on this 1-Phase 120v/240v power. I would just plug it in and try it but I don't want it to fry anything.
  4. JWHassler


    Sep 25, 2013
    Consult the data on the A/C. Lots of motors are dual-rated: 240 or 208, with just a current-draw difference.