July, 2012
Inspection News and Views from the American Society of Home Inspectors

Let's Talk Three-Phase


 Just when you thought you had this electrical gig kinda figured out, someone has to put a fly in the ointment and start talking about that three-phase stuff!

 Well, chances are, sooner or later, you are going to be confronted with a three-phase service. More inspectors are doing commercial inspections where three-phase is a common system. Even some residential inspections will involve a three-phase service from time to time for wood shops, pottery hobbies, etc.

 The ASHI Standards Practice® requires an inspection of the service and internal components of the service panel, and a description of the amperage and voltage rating. There is no specific requirement for determining between single-phase and three-phase; however, the ability to identify and understand a three-phase system is a great asset for an inspector. But until you are comfortable with these systems, it's OK to say you are not familiar with the system and refer for further evaluation.

 The good news is that, with some training, most experienced inspectors can become qualified to properly inspect and describe a three-phase service. Most of the primary issues you'll need to look for are the same ones you look for in a single-phase service: conductor size, over-current protection, bonding and grounding and so on. The differences between the single- and the three-phase service are the number of service entrance conductors (normally three ungrounded and one grounded as opposed to two and one), the presence of 3-pole breakers and common voltages of the systems.


 In a standard single-phase system, we usually describe the voltage as a 240/120-volt system. The 240 volts is the measurement from line to line and the 120 volts is measured from either line to the neutral or grounded conductor.

 A typical three-phase service could be a 208/120-volt wye system, which would be 208 volts from any line conductor to either of the other two line conductors and 120 volts is measured from any of the line conductors to the neutral or grounded conductor. Another standard configuration is a 480/277-volt wye system. The 480 volts is usually for motors and some appliances and the 277 volts is used for lighting. A transformer is needed in these systems to obtain 120 volts for receptacles.

The mathematical formula for a single-phase service is to divide the ungrounded conductor voltage line to line by 2 to arrive at the one line to neutral voltage (240/2 =120).

A three-phase system uses 1.732 (square root of 3) as the divider (480/1.732 = 277.14). It is important to note that single-phase power is available from any three-phase system.

Most single-phase appliances, motors and HVAC systems are rated at 208/230/240 volts. The full load-amp rating will adjust, depending on which voltage is used. As the voltage goes down, the amperage rises, which could affect wire size and over-current rating. 208 volts is used to obtain the common voltage of 120 volts for receptacles and lights. Using the formula in reverse: 120 x 1.732 = 207.84

 The previously described three-phase systems are the most common ones that home inspectors will encounter. They are known as wye-type systems, which is a reference to the configuration of the power company transformer windings.

A less common configuration is known as a delta hookup. This type of system is generally older and less often found, but they still exist. One advantage of a delta system is that a 240/120 voltage is available. Two of the line-to-neutral voltage is 120 volts. The main disadvantage in this system is that the third line-to-neutral voltage will be 208 volts and is known as the "high leg." This voltage is basically useless in single-phase applications unless converted to 120 volts with a transformer.

The NEC requires identification of the "high leg" phase with orange labeling. Some three-phase services have no neutral, which then would have only a three-wire service. Ungrounded three-phase systems are also available. These most often are seen in industrial plants. Three-phase power is used in industry and commercial facilities because it is more efficient than single-phase, particularly in applications with large inductive loads such as motors.

Photo: A typical 208Y/120 volt 3-phase 225 amp sub panel.
Photo courtesy of Mike Twitty.

Multi-unit buildings such as apartment buildings commonly use three-phase power for supply. At each individual unit, a single-phase panel is installed. This could confuse an inspector who is not familiar with three-phase systems because the line-to-neutral voltage will be the normal 120 volts and the line-to-line voltage will be only 208 volts.

 It is not necessary for a home inspector to fully understand the characteristics of all electrical systems, but being able to identify unique systems or less common systems gives our profession more credibility. Continuing education never stops in learning about all building systems and components. The more we learn, the better we are as a profession.

 This is a brief, general overview of three-phase power. For those who would like to review a detailed, in-depth explanation of three-phase systems, there are many excellent sites on the Internet. A recommended electrical reference for inspectors is the Ugly's book. It has many common formulas and diagrams in a pocket-size book. If you don't already have one, try it. I think you'll like it! www.uglysbook.com.