Although people commonly refer to the voltages as “110” and “220” for household current, that was actually just the original voltage for early alternating current systems. The two “hot” (ungrounded) wires serving the first systems were 110-volts each and, when both were used for a major appliance, the total voltage was 220. Voltage was set at these numbers early in the 20th century so the newer alternating current (AC) systems, designed by Nikola Tesla and manufactured by George Westinghouse, could be compatible with the existing wiring of the older direct current (DC) systems installed in many communities by Thomas Edison. Edison’s DC system suffered from rapid voltage drop over even short distances, and was eventually completely abandoned.
The voltage provided by local utility systems was ramped up to 115/230 by the 1930s, and then edged up again to 120/240 by the 1980s. The American National Standards Institute (ANSI) now requires that electric utility voltages not deviate more than 5% from 120/240 volts, and the voltage delivered to the home can be adjusted at the transformer to accommodate voltage loss by a longer distance from transformer to house.
So, the voltage delivered to homes with single-phase service today is 120/240, and some are rated at 125/250, except for an occasional large house or apartment building with 3-phase service (3 hot wires). This type of system provides 120 volts to the neutral or ground, and 208 volts between the hot conductors.
But when our customers refer to the “220 breaker for the dryer” or the “110 wall plug,” we don’t correct them. Everybody understands exactly what they are talking about.
Also, see our blog posts How can I find out the size of the electric service to a house? and What is a reliable way to tell if the electrical service is 3 phase or single phase?
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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?
• 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?
• What is three phase electric service?
• 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?
• 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?
• 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.