3 Phase High and Low Voltages

In layman's terms-

First- In the US, there are three basic "high" voltages in three phase supply- 208, 240 and 480. The decision on which voltage to use is typically based on the overall power requirements of the site. The type of voltage depends on the design of the generator and how the wires are "connected" to the end-user devices.

The connections are called Wye and Delta, because of their "shape" electrically.

208 and some 480 volts systems are Wye connections- 3 power contacts at 120 degrees separation with a central "neutral" connection between each of the 3 power connections- forming a "Y" shape. The voltage between any leg and neutral is either 120 volts (208) or 277 volts (480)- this is called the "single" phase voltage. For Wye systems, the single phase voltage is the 'primary" voltage divided by the square root of 3 (1.73). Any two of the power legs create a "rated" voltage (208 or 480) in single phase as well, or all three legs connected yield rated voltage (208 or 480) in three phase. The NEUTRAL connection is designed to "drain away" any imbalanced flow between the three power connections=> say each leg had, 20, 25 and 28 amps of load and there was NO "low" voltage single phase connections. The neutral would show a current of 8 amps (28-20). If there were low voltage connections as well, the amperage of those would also be applied to the neutral.

240 and other 480 volt systems operate on a Delta configuration. They have no neutral connection so their operating voltage is always either 240 or 480, either single phase (between two contacts) or three phase if all three contacts are connected. The connections are also at 120 volt separation, so the wiring description looks like a diamond, or "D" and is called Delta. House wiring is a bastardization of this system because homes are normally fed with two phases of a 240 system BUT they have a neutral connection because the neutral goes back to the transformer grounding lug on the pole where the transformer is located. This allows homes to have either 120 volts (one power and a neutral) or 240 volt power for big users (air conditioner, stove, dryer or water heater). The

As an aside- based on your comment- the voltages I described are the "true" voltage names, but sometimes people refer to them as 220, 230, 440 or 460 volts. These nicknames came to be because often, the actual available voltage (due to power line losses) may not actually be the official voltages. Additionally, the "rated" voltages for motors WILL be 200 or 230 or 460. This is because the motor manufacturers have designed their motors to run at full power with "likely" voltages (due to power transmission line losses) to minimize any likely overheating due to "lower" voltages- if they see "rated" voltages, the motors can do a little more work and/or will run a little cooler because the amperage will be lower than at "motor-rated" voltages. See the following discussion.

The WORK done by each is the same for any motor or other device they are connected to. WORK being defined as WATTS (with motors, it is also called horsepower, where 746 watts equals 1 HP).

Watts is defined as volts X amps. Volts is like pressure and Amps is like Flow, so the work is V units of pressure working on A units of flow- V x A = Watts. For a given power output, if the voltage (pressure) is higher than design, the amperage (flow) will be lower than design (or nameplate). Higher amperage (flow) causes more "friction" so the wires connected to the load will be hotter than if the amperage was lower.

Back to the decision on which voltage to use. First- will there be any significant single phase (120 or 277 volt) loads. If YES- then it will be either 208 or 480 volt. Next decision is what will be the maximum peak power load supported by the feed? Higher loads using higher voltages can install smaller wires because the amperage will be lower, so smaller wires can carry that load. BUT- higher voltages require higher cost components because the short circuit potential is higher (higher pressure) and better devices are required to sustain and contain the operation.

Hope this answers your questions and makes you a "smarter" end-user.