What is a reliable way to tell if your electrical service is single phase or three phase?

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.

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.

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