Three-Phase Power Basics

Most commercial and industrial buildings run on three-phase power. It feeds motors, chillers, compressors, welders and large lighting systems. If you know only 120/240-volt single-phase, three-phase looks strange at first: more conductors, unfamiliar voltages, and a square root in the math. The ideas are simple once you see how the three phases fit together, and getting them wrong can burn up equipment or get someone hurt.

What You Will Be Able to Do

• Explain why industry uses three-phase power.
• Calculate line-to-line and line-to-neutral voltage on a wye system.
• Tell a wye, a delta and a high-leg delta system apart.
• Calculate full-load current from a kVA rating.
• Recognize single-phasing and voltage imbalance, and what they do to a motor.

What Three-Phase Means

A single-phase source gives one AC wave. Three-phase gives three waves on three conductors, each 120 electrical degrees behind the last. The three are called phases A, B and C, or L1, L2 and L3.

Because the waves peak at different times, power never drops to zero the way it does in single-phase. That gives three real advantages:

• Constant power delivery. The load sees a steady flow of energy, not a pulsing one.
• Less conductor material for the same amount of power.
• Simple, efficient motors. Three-phase naturally creates a rotating magnetic field, so an induction motor starts on its own with no starting winding or capacitor.

Line Voltage and Phase Voltage

Two measurements matter:

• Line-to-line voltage is measured between any two phases.
• Line-to-neutral voltage is measured from one phase to the neutral.

In a wye system they are related by the square root of 3, which is about 1.732:

Line-to-line = Line-to-neutral x 1.732

Line-to-neutral = Line-to-line / 1.732

Worked Example: 480Y/277V

1. Line-to-line voltage is 480 volts.
2. Line-to-neutral: 480 / 1.732 = 277 volts.

Worked Example: 208Y/120V

1. Line-to-neutral voltage is 120 volts.
2. Line-to-line: 120 x 1.732 = 208 volts.

Notice that two hots on a 208Y/120V system give 208 volts, not 240. A 240-volt appliance on this system runs at reduced voltage unless it is rated for 208.

System Types

Wye

In a wye, or star, the three transformer windings meet at a common center point. That point is usually grounded and brought out as the neutral. A wye gives you two voltages from one system:

• 480Y/277V: 480 volts for motors and large equipment, 277 volts for lighting. This is the most common US industrial system.
• 208Y/120V: 208 volts for equipment, 120 volts for receptacles. This is common in offices, schools and stores.

Delta

In a delta, the windings connect end to end in a triangle. No center point exists, so there is no neutral unless one is created. A 480-volt delta gives 480 volts between any two phases and suits motor-only loads.

High-Leg Delta

A high-leg delta, also called a wild-leg or red-leg system, takes a 240-volt delta and center-taps one winding to create a neutral. You find it in older shops and small commercial buildings, where one service feeds 120-volt lighting and 240-volt three-phase motors.

Warning: Never connect a 120-volt load to the high leg. About 208 volts will destroy it. NEC 110.15 requires the high leg to be marked orange or identified by another effective means. Always measure to neutral before connecting a single-phase load to a panel you do not know.

On a high-leg panel, the high leg is normally placed in the B position, in the center, though a meter is the only reliable proof. Testing live equipment is work for a qualified person.

Identifying the Phases

The NEC sets only a few color rules: white or gray for the grounded conductor, green or bare for the EGC, and orange for a high leg. Phase colors for other conductors are usually industry habit, and they appear in your local rules or employer's standards. A building with more than one voltage system must post how its conductors are identified.

Note that orange means different things in different systems. Older buildings may not follow any of this. Verify with a meter before you trust a color.

Calculating Three-Phase Current

Three-phase power carries a 1.732 in the formula too:

Current = VA / (Volts x 1.732)

kVA means thousands of volt-amps, so multiply by 1,000 first.

Worked Example: A 75 kVA Transformer

Find the full-load current of a 75 kVA, 480-volt, three-phase transformer.

1. Convert: 75 kVA = 75,000 VA.
2. Multiply volts by 1.732: 480 x 1.732 = 831.
3. Divide: 75,000 / 831 = 90 amps, about.

The same transformer's 208-volt secondary carries more current: 75,000 / (208 x 1.732) = 208 amps. Lower voltage needs more current for the same power.

Phase Rotation

The order in which the phases peak, such as A-B-C, is the phase rotation. It sets the direction a three-phase motor turns. Swapping any two phases reverses the motor. Before you connect a motor, or after any work on its supply, use a phase rotation meter or a short bump test, according to your procedure. A pump or compressor run backward can be damaged in seconds.

Single-Phasing

Single-phasing is the loss of one of the three phases while a motor is connected. It can come from a blown fuse, a burned contact or a loose connection.

• A motor at rest usually will not start. It hums and draws locked-rotor current.
• A running motor often keeps turning, but the two remaining phases carry much more current.
• The motor overheats and burns out if the overload does not trip.

If a three-phase motor hums, runs hot and will not reach speed, measure all three phases. Zero or a low voltage on one of them points to single-phasing.

Voltage Imbalance

Phase voltages are never exactly equal, but they must be close. A small voltage imbalance causes a much larger current imbalance and extra heating in the motor. As a rule of thumb, a 2 percent voltage imbalance can raise motor heating by around 8 percent, and that heat shortens insulation life. Many motor makers recommend staying under 1 percent.

Worked Example: Percent Imbalance

You measure 460, 467 and 450 volts between the phases.

1. Average: (460 + 467 + 450) / 3 = 459 volts.
2. Largest difference from average: 459 - 450 = 9 volts.
3. Percent imbalance: 9 / 459 x 100 = 2 percent, about.

That is enough to investigate. Look for uneven single-phase loads, loose connections, or a problem on the utility side.

Common Mistakes

Check Your Understanding

1. You measure 277 volts from a phase to neutral. What should you expect between phases?

About 480 volts: 277 x 1.732.

2. A 45 kVA three-phase transformer has a 208-volt secondary. What is its full-load secondary current?

45,000 / (208 x 1.732) = about 125 amps.

3. A new 120-volt receptacle added to an old panel burned out a tool. The panel has an orange conductor on the center bus. What likely happened?

The circuit was connected to the high leg of a high-leg delta, giving about 208 volts to neutral. Move the circuit to a 120-volt phase after a qualified person verifies it.

4. Reversing two phase conductors at a motor's terminal box has what effect?

The motor turns the opposite way.

Key Takeaways

• Three-phase gives constant power, uses less conductor and runs simple, efficient motors.
• On a wye, line-to-line is line-to-neutral times 1.732.
• A delta has no neutral unless a winding is center-tapped.
• The high leg of a high-leg delta is about 208 volts to neutral and must be marked orange.
• Three-phase current is VA divided by volts times 1.732.
• Losing one phase or having a few percent of imbalance can destroy a motor through heat.