230, 231, Whatever It Takes

Question: My understanding is that if you have a 230V, 3-wire device like a motor and you feed it two 120V hots and earth, then it is running at 208V.  Alternatively, you can use a step up transformer and feed it one 230V hot, one neutral, and earth and it is running at a full 230V.  Is there a scenario where this device is fed with two 120V hots and it is running at 240V? For example, if both 120V taps come off the same phase leg (or if they come off different phase legs?) Or is 120V + 120V and earth always 208V?

Answer: Yeah, it can be confusing. Here’s the lowdown. Most likely your 230V device is European; otherwise, it would be a 208V device or a 240V device. In Europe, the hot-to-neutral voltage is 230V (except in the UK where it is actually closer to 240V, but they still call it “230V.”) So the three wires on this device are probably one hot, one neutral, and one earth conductor.

In North America (and other parts of the world), if you feed it using two hots from a 3-phase system, it will get 208 volts. 

On the other hand, a lot of homes have “split-phase” service, meaning the phase-to-phase voltage is 240V and the phase-to-neutral voltage is 120V. That’s not a 3-phase system, but it allows you to use 120V for everything in your house except the washer and dryer, which are probably 240V devices. Also, some venues have “delta power,” which means the phase-to-phase voltage is 240V, but one of the windings in the transformer has a center-tap, which allows you to pick off 120V from phase to neutral. Delta power is more common in industrial parks where they have light manufacturing because they use the 240V for heavy machinery like drill presses, punching machines, CAD/CAM, etc., but they also have 120V for their computers and microwaves. You can identify delta power by the orange marking on the “high” leg, and it’s important to know that the voltage from the orange leg to neutral in delta power is 208V. That’s the only time you’ll find 208V from phase to neutral in North America, which is why is has to be identified by orange as a warning to users. 

Lastly, most 3-phase generators can operate in “single-phase zig-zag” mode, which gives you 120V from phase to neutral and 240V phase to phase. 

So there are a couple of different ways to get 240V in North America but it depends on the service.