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Electrical definitions: Amps, Volts, Watts ElectricAPedia © Enter your search terms Submit search form   Search This Website - InspectAPedia.com

• Some simple definitions and examples of electrical amps, definition of volts, & definition of watts
• How to determine the size, capacity, or ampacity of electrical service at a building

This article provides electrical definitions: definition of amps, definition of volts, definition of watts as part of our complete text which explains how to estimate the electrical service size, (or "electrical power" or "service amps") at a building by visual examination of the service entry cables, electric meter and meter base, electrical service panel, main switch, and other details. Visual inspection and use of digital multimeters(DMMs), Volt-ohm meters (VOMs), neon testers, and electrical inspection safety are discussed. Photographs and sketches illustrate electrical panels, meter bases, and electric meters. © Copyright 2008 Daniel Friedman, All Rights Reserved. Information Accuracy & Bias Pledge is at below-left. Use links at the left of each page to navigate this document or to view other topics at this website. Green links show where you are in our document or website.

How do we DEFINE Electrical AMPS, VOLTS, & WATTS - Definitions of Volts, Amps and Phase - clearing up some confusion about electrical terms

In most places in the world, electrical service brought to a building is at either 240V or 120V. These numbers are "nominal," meaning that the actual voltage may be vary. Most modern buildings receive 240V service, a total achieved by the provision of two individual 120V incoming power lines. Older buildings and electrical services often delivered only 120V. Knowing which voltage level is available is important, but knowing the voltage alone does not indicate the amount of electrical power available inside a building. For that we need to know both the service voltage at a building, and the service amperage (typically 100A or larger, but historically, 30A, 60A, 100A, 125A, and more recently 150A, or 200A depending on the power requirements at a building). Don't confuse service VOLTS (120/240 V) with service AMPS or WATTS - those terms are discussed next.

Amperage or Amps provided by an electrical service is the flow rate of "electrical current" that is available. Speaking practically, the voltage level provided by an electrical service, combined with the ampacity rating of the service panel determines how much electrical demand, or in another sense how many electrical devices can be run at one time in the building. Branch circuit wire sizes and fusing or circuit breakers used set the limit on the total electrical load or the number of electrical devices that can be run at once on a given circuit. If you have a 100A current flow rate available, you could, speaking roughly, run ten 10 amp electric heaters simultaneously. If you have only 60A available, you won't be able to run more than 6 such heaters without risk of overheating wiring, causing a fire, tripping a circuit breaker or blowing a fuse. Just as a 10 gpm flow rate of water through a pipe provides half the amount of water as a 20 gpm flow rate, 10 amps of current in a conductor provides half the energy as 20 amps of current. Some people find the "water analogy helpful in understanding these terms. "Amps" is a measure total current flow (or "gallons per minute" or "gpm" using the popular water analogy) available from an electrical service.

Volt, formally, is defined as the potential difference across a conductor when a current of one ampere dissipates one watt of power. This definition is not very helpful to consumers. Using the water analogy, volts is analogous to "pressure." Having higher "pressure" in a pipe (or electrical conductor) means that conductor is capable of delivering more energy to the user.

A ten-amp 240V electrical service is capable of delivering, speaking roughly, twice the energy to the end-user than a ten-amp 120V electrical service. So volts is a measure of the strength of an electrical source at a given current or amperage level. If we bring 100A into a building at 240V, we have twice as much power available as if we bring in 100A at 120V. Volts (continuing the same water analogy) is the "pressure" in an individual electrical conductor.

The total current ("gpm") that will flow through a conductor is doubled if the pressure is doubled. Twice the power or energy can be delivered on a # 12 conductor by doubling the voltage and holding the current to 20 amps. Doubling voltage and also doubling the amperage will deliver four times the power or energy.

In either case, if we exceed the current rating of an electrical wire, it will get hot, risking a fire. That's why we use fuse devices (or modern circuit breakers), to limit the current flow on electrical conductors to a safe level to avoid overheating and fires. - thanks to Lousi Babin for technical review and edits to this text.

Is "240V" really exactly 240 Volts?: Don't expect a "240V" circuit to provide that exact voltage level. We've already said that "120V" and "240V" are "nominal" ratings, meaning that the actual numbers may vary. In a three phase circuit, even if you are using only two phases, the voltage between the phases is 1.732 x 120 = 207.6, or approximately 207 Volts and not 240 Volts. In various countries the actual voltage level varies around the nominal delivered "voltage rating" and in fact depending on the quality of electrical power delivered on a particular service, voltage will also vary continuously around its actual rating. Most (but not all) modern electrical equipment can handle small voltage variations and differences without a problem. Sensitive electronic equipment may require that a voltage stabilizer be installed. For example a "240V" appliance can usually handle "208V" just fine.

The technical detail of how "240V" (or 207V if you prefer) is actually delivered to a building may be a bit confusing, so let's follow this carefully. In fact 240V delivered to a building does not mean that the individual service drop wires are carrying that voltage. Rather, 240V in the building is obtained as follows: the two "hot legs" are on different electrical phases provided by a step-down transformer at a neighborhood utility pole or box. Each service conductor on its own phase delivers 120V to the building. The two (in this case) phases are arranged so that connecting a circuit across the two "hot legs" produces "240V" in for that circuit. An electrician or engineer, trained in safe volt-ohm meter (VOM) or digital multimeter (DMM) use can easily demonstrate this fact. Connecting a voltmeter from either incoming service conductor to ground will display 120V, and connecting a voltmeter across the two incoming 120V service conductors will display approximately 208V or 240V depending on just how the supplying transformer is designed.
[Technical details: Three phase power with the star (Y-connected) connected secondary and the neutral grounded, you get 208 Volts line-to-line, and 120 Volts line-to-neutral.
With a single phase transformer (240 V secondary with a center tap and the center trap grounded), you get 120 Volts line-to-ground (neutral) and 240 Volts phase-to-phase (line-to-line). -- Thanks to N. Srinivasan for these clarifications].

Watts: How does "Watts" relate to the "Volts" and "Amps" discussed above? Briefly, Volts x Amps = Watts. To impress your friends, you can rearrange this equation using simple algebra or you can re-write it using Ohm's law. Ohms is a measure of electrical resistance, which also measures the heat that will be generated in a wire carrying a given current. Amps = Volts / Ohms. Given the two equations just cited, we can also write: Watts = Volts x (Volts / Ohms), which lets us write: Watts = Volts ² / Ohms. (Hansen's various publications and his upcoming book point out variations of these formulae which are useful in discussing the heating of wires carrying current.)

Summarizing: 240V power delivered to a building in the U.S., Canada, and Mexico, and some other locations, means that the building is receiving two 120V lines which provide 240V for circuits connected across these two incoming wires, and which provides 120V for circuits connected from either of the individual incoming lines to ground. For heavier and commercial electrical power requirements, three and even four-phase electrical service may be delivered to a building, and in some applications, electrical equipment is designed to be fed directly by multiple phases. You will not ordinarily see such service at a residential property, but one of the authors (DF) has encountered it in cases where there was a dental office in the basement of a home. The dentist's x-ray equipment required three-phase power. The tip off was the observation outside of four rather than three service conductors at the masthead, and in the main panel, the main switch was fed by three incoming service conductors rather than the usual two.

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Technical Reviewers

Particular thanks are due to experts and also consumers who read these articles and suggest corrections, changes, and additions to the material. Content suggestions, technical corrections and content critique are invited for any of the content at our website.

• The Original Authors: Alan Carson is an ASHI Member, national home inspection educator, author and building failures researcher in Toronto, Ontario. Daniel Friedman, an original author of this article and the editor and producer of InspectAPedia where this article now appears is an ASHI Member, first ASHI Technical Committee chairman, editor and publisher of the ASHI Technical Journal, licensed home inspector, educator, and building failures researcher in Poughkeepsie, NY. Robert Klewitz is a licensed professional engineer, a professional home inspector, an ASHI Member, and has served on the ASHI Technical Committee as well as in other ASHI activities. His practice is in Issaquah, WA.
• Daniel Friedman - InspectAPedia.com ^TM Website Author/Editor
• Douglas Hansen, Robert Stead. Mark Cramer. Photographs: Daniel Friedman.
• Critique, contributions wanted: Contact Us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
• N. Srinivasan, MSEE, is a senior member of IEEE with 30 years experience in the electrical industry. Mr. Srinivasan is in Vienna VA.
• Louis P. Babin generously contributed technical editing about the effects of doubling ampacity in an electrical circuit (September 2007)

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