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Is 240 V single phase power common in the United States?
November 5, 2007

The full text of this question, which came in response to my October 2007 cover article entitled “Getting Control of Electric Power” in the print version of Control Engineering, is: 

On your diagram "typical power monitoring points" you show a service entry and then 240 V ac 3-phase going to a panel with 240 V ac single phase coming out. Is this 240 V single phase power common in the United States? If so, where is it used? This is the voltage used in Europe. I have not been able to find it in use in the US. What is available is 220 V which is created using two 120 volt lines, which are not in phase.

Actually, standard service for incoming mains in U.S. residences really is 240 V ac single phase. Your confusion may stem from the fact that it is a balanced line, meaning that the conductors both carry voltage with respect to ground. I know this for sure because I’ve spent a lot of time over the past year with my nose stuck in the power panel of my antique house trying to chase down gremlins in the patchwork electrical system. I’ve identified the original wiring installed in 1914, along with upgrades made in the 1960s, 1980s, and early in this century.

As Wikipedia points out, this voltage comes from tapping across two phases of a 240 V ac delta-connected 3-phase step-down transformer set up by the utility company outside the residence. The ground/neutral is supplied by a center tap off the secondary winding. This center tap sits (phase- and voltage-wise) directly in the middle between the two poles of the 240 V ac single-phase line, so the typical 120 V ac unbalanced “house” voltage is available by simply tapping between either pole and this neutral.

Typical appliances that use 240 V ac single phase include electric clothes dryers and electric stoves. I have a 240 V ac arc welder sitting unused in my garage because I don't have the service wired out there.

240 V ac single phase comes from tapping across one phase winding of a 240 V 3-phase transformer.

The main differences between residential and commercial electric supplies are: 

• There are often 3-phase loads in the commercial building, so 3-phase current is typically brought right into commercial buildings; 

• Commercial buildings typically require much more power, so the current rating is typically much higher; 

• Commercial buildings often have additional higher voltages (such as 480 V ac) to support especially high-powered equipment.

Depending on the power requirements, commercial buildings can have 480 V ac service or more. It all depends on the equipment being used. Manufacturing facilities are usually such power hogs that the electric utility is happy to install just about any service required.

Historically, there was a 220/110 V ac standard in the U.S. When I first started learning about wiring back in the mid-20th Century, many folks talked about “110 Volts,” when actual measurements showed 115 V ac at typical outlets. This is probably where the idea of 220 V ac being available comes from. I believe that the actual standard at that time was 230/115 V ac, but I might be mis-remembering. I haven’t seen those voltages in decades.

Posted by Charlie Masi on November 5, 2007 | Comments (0)