Industrial Controls - Electricity 101: Basic Fundamentals

Three-phase circuits are not designed for 115 volt single-phase use. While about 115 volts can be obtained from a hot leg to ground, using this circuitry is forbidden in electrical codes. It is a dangerous practice.

There are three basic three-phase distribution systems in use today. Small commercial buildings and some small industrial plants that have about 50 percent of the electrical load as 120 volt single-phase will have 208/120 volt, three-phase, four wire system. See Figure 5.

Figure 5

There are three hot lines (A, B, and C) along with a neutral (N), which is grounded. Single-phase 120-volt loads are fed from line-to-neutral (C to N, A to N, or B to N), and three-phase 208 volt by lines A, B, and C.

Ametek Common AC Distribution

In the USA the most common configuration is 208 Y/120. This indicates that line to line (L-L) voltage is 208 Vac in a WYE configuration and the line to neutral (L-N ) voltage is 120 Vac . It is also sometimes designated 120/208 Vac, 120/208 WYE, 208/120 WYE, 4 wire WYE or 120/208 Y. See Figure 4. Most Ametek supplies > 5 kW will operate on this configuration.

Figure 4

Research

Practical Electrical Wiring

• 15th Edition p. 57, ch. 3
• 18th Edition p. 527, ch. 28

3 Phase in the wood shop lumberjocks.com forum

TopamaxSurvivor

• delta - 240 3 phase
• Y - 208 3 phase

3 phase comes in 2 common models in the US for small motors. Delta, which is a triangle transformer connection with the power wires coming off the corners of the triangle. The other and most common, at least in the NW, is a Y. The transformer is connected with the 3 phases coming off the ends of the Y. The center of the Y is grounded so as to create a neutral. From each corner to the grounded center is 120 V. From any end to any other end is 208. All 3 ends make 208 3 phase. Any 2 corners are 208 single phase.

The delta gives 240 volts from any corner to any other corner. All three corners make 240 3 phase. Any 2 corners of the triangle is 240 single phase. You ground the center of any side of the triangle to get access to 120 volts. When you ground the one side, you have 120/240 single phase with those 3 wires. The 4th wire coming from the odd corner gives what is commonly known as a “high leg.” It is supposed to be marked Orange. The voltage to the neutral on the high leg is approximately 190 volts. If you see a panel with a 120 volt circuits and every third breaker is missing, it is 120/240 3 phase with a high leg. The high leg can only be used for 240 single phase or 240 3 phase connections. If it gets connected on a 120 volt circuit, it will probably cause the smoke to come out of the equipment. We all know, electrical devices work best with the smoke left intact!! ;-))

The other common 3 phase voltage is 480/277 Y. Most 3 phase motors will have 9 leads in the connection box for dual voltage connections; either 208/480 or 240/480. If the motor is marked 240, it will run on 208, but it needs a Service Factor (SF) of 1.25. Motors used to have SF of 1.15 or 1.25. I have seen them with 1.0 lately. The SF tells you how much of an overload the motor will handle without any damage.