How To Look At A House

McGarry and Madsen's home inspection blog for buyers of  

site-built, mobile/manfuactured and modular homes

What is a reliable way to tell if the electrical service is 3 phase or single phase?

Saturday, June 30, 2018

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 with answers to some Frequently Asked Questions (FAQ) about ELECTRICAL SERVICE:

What is the minimum overhead electric service drop height/clearance to a house?

 Why is a leaning electrical service mast dangerous? 

• What is the fireman’s switch emergency disconnect requirement for residential electrical service?

• What are typical copper service entrance wire/cable sizes for the electrical service to a house? 

What are typical aluminum service entrance wire/cable sizes for the electrical service to a house?

What is the minimum clearance of overhead electric service drop wires above a house roof? 

What is a reliable way to tell if the electrical service is 3 phase or single phase? 

What does it mean when I find buried yellow "CAUTION" tape when digging a hole in the yard?

How can I find out the size of the electric service to a house?

What is the difference between the electric service to a mobile home and a site built home? 

What are the clearance requirements for an overhead electric service drop that is directly over or near a swimming pool?  

• What is the minimum size electric service to a mobile/manufactured home?

• Can anything else be attached to a service mast for overhead electric service besides the service cables? 

• What is the electrical "service point" of a house? 

What is a service conductor?

• What is the difference between single-phase and three-phase electric service? 

• What is the minimum clearance under an electrical service drip loop of a house? 

     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.

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