I've been tasked with designing a three phase system to be sent over to the US and have encountered a few questions along the way.

My system will be hooked up to a 208Vac three phase socket and most devices that I am using in my system are happy to be powered by the line to neutral voltage that you get from 208V three phase of 120Vac. However there are two devices that require a minimum of 180Vac (I guess they are not intended to be used in the states).

I understand that the line to line voltage in 208V three phase is 208Vac which would be be fine for these devices of which I have two.

My question is, are there any adverse effects when you wire up a system this way or is this perfectly standard practice. I guess that you can probably tell that my experience in this field is fairly limited.

Any help would be greatly appreciated thanks.

I have attached a picture of what I intend to do for clarity. enter image description here

  • \$\begingroup\$ Is all of this wiring inside your device? And you're using a standard three-phase connector? If so I don't see much problem with what you're proposing. You should take some care to balance the current in each of the three phases as much as is practical. \$\endgroup\$ – Nate S. Feb 5 '19 at 21:16
  • \$\begingroup\$ @Nate Strickland This question was asked nearly 2 years ago. The asker last logged on a few days after asking the question. \$\endgroup\$ – Charles Cowie Feb 5 '19 at 21:33

Technically, it does work. Basically this produces the same load as a broken 3-phase motor might. The RCD should be able to deal with this, too.

I dislike the solution, because this is unexpected for service personnel: "Two brown cables? huh?". Someone is bound to end up attaching a 120V-only device to those two leads at one point.

Using a transformer dedicated to the devices to step up one phase to the required voltage would be way clearer.

  • \$\begingroup\$ I don't think the colors on the diagram were intended to be wire colors, and wiring line-to-line is pretty standard in three-phase equipment. I don't think qualified service personnel would be confused by this scheme. \$\endgroup\$ – Nate S. Feb 5 '19 at 21:20

Three-phase with a neutral would only be wired to sockets in a special installation where it is required for a specific purpose. Most 3-phase equipment is wired directly to equipment terminals without a plug and socket. For single phase plug-connected equipment is preferable to use the standard single-phase plugs and sockets for the voltage and current required.

208 volts is a somewhat unusual service. 240 volts is much more common.

  • \$\begingroup\$ 120/208 Volt 3 phase power is common in commercial and industrial environments in the US and Canada. 120/240 volts single phase is normal in homes. \$\endgroup\$ – Peter Bennett Apr 5 '17 at 16:36
  • \$\begingroup\$ 208V is not at all unusual -- it's probably the most common three-phase voltage. And three phase connectors are extremely common as well, provided you're talking about commercial buildings and not residential. \$\endgroup\$ – Nate S. Feb 5 '19 at 21:22

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