Residential electrical service is single phase (3-wires, 240V) more than 99% of the time. But there is that rare exception to watch out for. If the house is from the mid-20th century, it may still have a type of three phase service called “high-leg delta” (4-wires, 208V), which was necessary for air conditioning compressors of the era; or, it could be a newer home built for a hobbyist whose man-cave requires three phase for heavy-duty machinery.
Here are four ways to figure it out:
1) Count the number of wires going into the weatherhead if the service is overhead. It will tell you at a glance whether the service is likely to be be single phase or three phase. Three wires means single phase (two hots and a neutral), and four wires is three phase (three hots and a neutral). But this is only accurate most of the time, since it is possible to have a three or four wire configuration for either type of service. So continue to #2 for verification.
2) Look closely at the meter for the listing of service type. It will state “3 WIRE” or “3W” and “240V” for single phase service, and “4 WIRE” or “4W” and “208 V” or “120-480V” for three phase service.
3) Check the breakers in the main service panel. Only a three phase panel can have 3-pole breakers that occupy three slots in the box, with a toggle connecting them, like the photo below.
4) Open the panel box and examine the wiring. It can provide further verification of service type. We only recommend doing this if you are an electrical or inspection professional. The panel below, shown with the dead front removed, is an unusual example of three phase service at a 1950s era home. Although it has the three hot wires (one black and two red) and a neutral (left wire, taped white) for the four-wire service, it is a small panel and has no 3-pole breaker. So a panel does not have to be large or have a 3-pole breaker to be three phase. This particular panel had an ominous “WARNING! - THREE PHASE SERVICE” message scrawled on the cover in black marker.
To understand how three phase service works, see our blog post “What is three phase electric service.” Also, it is possible to have three phase service to a panel with only two bus bars, using an older configuration called a “High-Leg Delta.” To learn more about it, see our blog post “Why is there a 3-phase breaker in a single phase panel with only two bus bars?"
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Here’s links to a collection of our other blog posts about ELECTRICAL WIRING:
• Which house appliances need a dedicated electrical circuit?
• How can I tell the age of a Square D electric panel from the serial number?
• Can a short circuit cause a high electric bill?
• Can you use a light switch for a water heater disconnect?
• When did the requirement for two 20-amp kitchen counter appliance circuits begin?
• What is the maximum spacing requirement for securing NM-cable (nonmetallic-sheathed cable)?
• Is it alright to just put wire nuts on the end of unused or abandoned NM-cable or wiring?
• What causes copper wires to turn green or black in an electric panel?
• Is the latest edition of the National Electrical Code (NEC) the standard used for the electrical system of new homes?
• What are typical aluminum service entrance wire/cable sizes for the electrical service to a house?
• Why is it unsafe to bond neutral and ground wiring at subpanels?
• When was it first required that neutrals and grounds be separated (not bonded) on any panel past the main service panel?
• Should I get a lightning rod system to protect my house?
• Why is a strain relief clamp necessary for the cord connection to some electric appliances?
• Does a wire nut connection need to be wrapped with electrical tape?
• What is the minimum clearance of overhead electric service drop wires above a house roof?
• What is the difference between a UL rating for dry, damp, and wet locations?
• Can NM-cable (Romex®) be used to make a cord and plug to connect an appliance?
• What are the requirements for NM-cables entering an electric panel box?
• What is the color code for NM cable (Romex®) sheathing?
• Can I remove a 240-volt range receptacle and hard-wire the range?
• Why do some wires in an electric panel have tape wrapped around them near their connections?
• Why is undersize electric wiring in a house dangerous?
• Why is bundled wiring in an electric panel a defect?
• What causes flickering or blinking lights in a house?
• What is the voltage rating of a house electrical system?
• Why are old electrical components not always "grandfathered" as acceptable by home inspectors?
• What is tinned copper wiring?
• What is a conduit body or condulet?
• How can I find out the size of the electric service to a house?
• What could cause an extremely high electric bill?
• Can old electrical wiring go bad inside a wall?
• What are the code requirements for NM-cable (nonmetallic-sheathed cable or Romex®) in an attic?
• What is the difference between "grounded" and "grounding" electrical conductors?
• What does it mean when a wire is "overstripped" at a circuit breaker?
• What is an open electrical splice?
• What are the most common electrical defects found in a home inspection?
• What is the life expectancy of electrical wiring in a house?
• Do any pre-1960 houses have aluminum wiring?
• What is the gooey stuff on some of the wire connections in the electric panel?
• How much does it cost to rewire a house?
• What is an "open junction box"?
• What are the clearance requirements for an overhead electric service drop that is directly over or near a swimming pool?
• How dangerous is old electrical wiring?
• What is a ground wire?
• What are the most common homeowner electrical wiring mistakes?
• I heard that aluminum wiring is bad. How do you check for aluminum wiring?
• What is "knob and tube" wiring?
Visit our ELECTRICAL page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.