Plain language definitions of electrical terms: definition of amps, volts, watts, resistance, current, ohms, electrical phases. We include basic formulas relating amps, volts, resistance, watts, and we explain what these electrical terms mean in practical applications such as for building or appliance electrical power, electrical wiring, and basic troubleshooting.
Photographs and sketches in this article illustrate and help explain concepts and definitions of electrical voltage, electrical resistance, and other electrical wiring concepts.
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How do we Define Electrical Amps, Volts (Current), Resistance (Ohms), Watts & Electrical Phase - clearing up some confusion about electrical terms
In most places in the world, electrical service brought to a building is at either of two voltage levels: 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 as we discuss below.
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.
Speaking practically, the voltage level provided by an electrical service, combined with the ampacity rating of the service panel determine how much electrical demand, or in another sense how many electrical devices can be run at one time in the building. Sketch courtesy of Carson Dunlop Associates.
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 100 Amp current flow rate available, you could, speaking very 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.
Also see AMPS MEASUREMENT METHODS for a description of using a clamp-on ammeter to make actual electrical usage measurements.
Ampacity, in the electrical code, refers to the current, measured in amperes, that a conductor (a wire) can carry continuously under the conditions of use without exceeding its temperature rating - in other words, the ampacity of a #14 gauge copper wire intended for residential electrical wiring is 15 Amps because that's the amount of current that the wire can carry without getting too hot. "Too hot" means a temperature that could damage the wire insulation and thus reduce its safety.
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 a water-in-pipes analogy, volts is analogous to water "pressure" in the electrical system. Having higher "pressure" in a pipe (or electrical conductor) means that conductor is capable of delivering more energy to the user. Later in this article we further explain electrical potential. Mathematically Volts = Watts / Amps. (Volts equals Watts divided by Amps).
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.
Amps,as we discussed above, is a measure total current flow (or "gallons per minute" or "gpm" using the popular water analogy) available from an electrical service.
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.
Some people explain volts as similar to water pressure in a pipe, and amps as water current or total quantity flow. We discuss volts and amps below and in detail at this website. Sketch courtesy of Carson Dunlop Associates.
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.
Why we need circuit breakers or fuses: 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 Louis 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.
Watts is a measure of the amount of electricity being used - a rate of electrical power consumption. Most people use a very simple mathematical formula to determine how many watts an electrical circuit can carry or how many watts an electrical device will require:
This formula shows how Watts relates to Volts and Amps. 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.)
Given those two equations just cited, we can also write:
which lets us also write Watts as
How do we calculate watts, volts, and amperage for an electrical device like an air conditioner?
Watts (W) as used in a simplified manner here and by electricians, is a measure of electrical power and is expressed by any of the formulas shown below. [All forms of power are measured in units of Watts, W, but this unit is generally reserved for real power (see definitions further below.]
W = V x I
W = I2 x R
W = V2 / R
W = Watts, V = Volts, I = Current or Amperage or Amps and R = Resistance measured in Ohms
Example: if we have a 50 watt light bulb running on a 120V circuit we can solve for the missing number, I or "Amps"
50 = 120 x I
50 / 120 = I
0.416 = I
Our 50 watt light bulb is drawing .4 amps of current.
Reader Michael V. points out that in the above watts, volts, amps calculations, these simplified formulas are for DC voltage.
In AC electrical systems, V*I=VA not watts. Watts is W=V*I*PF where PF =power factor.
At SEER RATINGS & OTHER DEFINITIONS we include additional examples of calculations of electrical usage by air conditioning equipment, including how we calculate watts, volts, and amperage for an electrical device like an air conditioner.
Also see AMPS & VOLTS DETERMINATION "How to estimate the electrical service ampacity and voltage entering a building".
Reader Daniel Mann adds that "Watts is correctly shown as Watts-Voltage times Current times power factor. Since power factor varies all over the place,..." [W = V x I] "perpetuates misinformation". We include additional more technical explanation of power factor, real power, apparent power, complex power, and reactive power just below.
Various sources offer further definitions of real power P, reactive power Q, complex power S, and apparent power |S|.
If pf - power factor - = 1, then 1 VA of apparent power transferred in a circuit will produce 1 W of real power. If pf = .5 then to produce 1 W of real power we would need to transfer 2 VA of apparent power. (1 W / .5 = 2).
It is interesting that in an electrical circuit, a load that causes a low power factor will draw more current than a load with a high power factor for the same a mount of usable or useful power actually transferred. If an electrical circuit is drawing higher current, there is more energy loss, larger transmission wires are needed, and thus costs of both the circuit and its operation are increased. Electrical engineers may design equipment to include components that improve the power factor to thus lower its operating cost.
Lots of electrical appliances include a label providing the appliance's wattage, and in the case of heating and air conditioning equipment, lots of other details are provided too.
Seer ELECTRICAL POWER ANALYZERS for descriptions of power analysers used to measure electrical energy usage, consumption, efficiency
See SEER RATINGS & OTHER DEFINITIONS and
also A/C DATA TAGS for an example.
What's a WattHour? Watt hours (Wh), sometimes written W.h, can measure either electrical energy produced, say by a power station, or Watts can measure the amount of electrical energy consumed (say at a light bulb or an air conditioner in our home). For air conditioners, the A/C units' total Wh is the energy used in running the air conditioning system for an hour.
If you turn on a 100-watt light bulb for an hour, you've used 100 Wh of energy. Or if you had a one-watt bulb and lit it for an hour, it'd use 1 Wh of energy. Thank James Watt (1736-1819), credited with developing a useable steam engine, for WATT which was named for him in 1882.
Watts is an instantaneous measurement, not related to time. To factor in time, as the electrical utility wants to do in sending us an electrical bill, the electric company's meter calculates the number. of watt hours (actually kilowatt hours) of electricity we use. If we run our 50 watt bulb for one hour, we've used 50 watt-hours. That's all the electric utility cares about.
As our sketch, courtesy of Carson Dunlop, shows, 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.
(Douglas Hansen's various publications on electricity and electrical inspections and his upcoming book point out variations of these formulae which are useful in discussing the heating of wires carrying current.)
Electrical Definitions of Wires & Components Used to Bring Electricity to a Building
We list these common electrical terms roughly in the order that they are observed, from the electrical utility company's overhead wires and pole to the building receiving electrical power to its electrical panel, and in the panel to individual circuit breakers which provide power to and protect individual electrical circuits that distribute electrical power throughout a building.
ELECTRICAL SERVICE DROP: the overhead electrical service conductors from the last electric utility pole (or other aerial support) to and including splices if any, connecting to the service entrance conductors at the building. These wires usually belong to and are the responsibility of the electric utility company.
Service-Entrance Conductors, overhead system: the service conductors (wires) between the terminals of the service equipment (main electrical panel) and a point usually outside of the building, clear of building walls (usually the electrical masthead, visible in this sketch), where the wires are joined by tap or splice to the service drop (the wires from the utility company). These are the electrical wires coming down the building exterior from a mast-head connection point to the electrical meter, and continuing inside to the main electrical panel or service switch. These wires normally belong to and are the responsibility of the building owner.
Service Conductors: wires connecting service point (such as the outside electric meter) to the main service disconnect (such as the main breaker in the main electrical panel). These are wires bringing electrical power from the electric meter into the electrical panel.
Service Equipment: usually the circuit breaker(s) or switches and fuses used to connect to the load end of service conductors coming to the building. This is the main electrical switch, fuse, or breaker, usually in the main electrical panel but sometimes installed as a physically separate switch before the main electrical panel.
Circuit Breaker: a device designed to open and close (turn off or on) an electrical circuit by non-automatic means (a physical toggle switch) and to open the circuit automatically (internal trip mechanism) on a predetermined overcurrent without damage to itself when properly applied within its rating. (That is, a 15A circuit breaker is expected to protect a 15A circuit, not something else).
Branch circuit: is the conductors (electrical wires, hot, neutral, ground) between the final overcurrent device protecting the circuit (a circuit breaker or fuse in the electrical panel) and the outlet(s).
A general purpose branch circuit is an electrical circuit that supplies two or more receptacles or outlets for lighting and appliances. In other words, the wires that bring power from the electrical panel to one or more points in the building where it will be used to power an light, power something plugged into an electrical outlet, or to an individual appliance.
Alternating current is almost universally used for home electric power and is, therefore, the kind this article is primarily concerned with. In an AC circuit, the amount of voltage applied to the circuit is constantly changing from zero to a maximum and back to zero in one direction and then from zero to maximum and back to zero in the other direction. The maximum voltage is set by the generating plant.
Because voltage is the pressure that causes current to flow, the current will also change from zero to maximum to zero and will reverse direction and repeat. The maximum amount of current, however, is determined by the load resistance and can vary as the load resistance varies. Each complete change from zero to maximum to zero in one direction and then zero to maximum to zero in the opposite direction is called one hertz (formerly cycle).
The term hertz implies "per second." So, 60 hertz means the same as 60 cycles per second. Hertz is abbreviated Hz. Cycles-per-second, which you will still see marked in some electrical devices, is abbreviated cps.
Direct current is most commonly found in homes in the form of electrical energy stored in batteries. In a DC circuit, the amount of voltage and the direction of application are constant. The amount of voltage is determined by the type and size of battery. The direction of current flow is also constant and, as in AC circuits, the amount of current flow is determined by the load resistance.
Batteries convert chemical energy to electrical energy. The chemical energy can be in wet form, as in your car battery, or in dry form as in flashlight and transistor-radio batteries. Some batteries are designed to be recharged from an AC source. The voltage from all batteries, unless recharged, will gradually decrease. AC power can be converted to DC power for some uses in the home. The conversion is performed by a device called a rectifier or current converter.
15-Amp 120V electrical circuits: typical U.S. 120V household electrical circuit uses #14 gauge copper wire and is protected (and thus limited) by a 15-amp circuit breaker. Such a circuit can deliver about 350 watts of electrical power to devices plugged into it, and another roughly 10 watts is consumed by the resistance of the circuit and its devices (receptacles and switches).
20-Amp 120V electrical circuits: typical U.S. 120V 20A household circuits use #12 gauge copper wire and are protected and thus limited by a 20-Amp fuse or circuit-breaker. A 20-amp circuit can provide about 2400 Watts. But as some writers have pointed out, for safety, household circuits are intended to carry less current (about 20% less) than their theoretical maximum. 80% of 2400 Watts is 1920 watts - that's about what you should expect to obtain from such a circuit.
How are we figuring these numbers? we use the formula from above, Watts = Volts x Amps and we plug in the nominal voltage and a guess at the resistance over the electrical circuit before we've plugged in anything:
W = 120V x 15 Amps so W = 1800 for our 15-Amp electrical circuit.
W = 120V x 20 Amps so
W = 2400 for our 20-Amp circuit.
If we have a 1500 Watt electric heater running on "high" and thus drawing a maximum of 1500 Watts, plugged into our 20-Amp circuit above, we've got another 420 Watts available on that circuit. So we could run maybe another four 100-watt lights on the same circuit.
The real world is a little more complex; lots of devices draw more current when they're starting-up, especially air conditioners and refrigerators. The electrical engineer (during design) or electrician (during house wiring) choose a circuit breaker that can tolerate that temporary high current but that will trip off if high current continues to flow on the wire.
The word potential is used to explain that the capacity to do work is present, but not that work is necessarily being performed.
Water in the bucket in our sketch at left is a capacity to do work (move water, or exert pressure) but until water actually flows out of the bucket (say when it's tipped), no water is moving and no work is being performed.
Until something has happened, it's just a potential.
Electrical Resistance is illustrated at left courtesy of Carson Dunlop Associates.
Watts = Volts 2 / Ohms
Current (Amps) = Potential (Volts) / Resistance (Ohms)
Electrical resistance can be thought of as how easily electricity flows through a material. Where resistance is high more effort is needed. A smaller-diameter electrical wire has more resistance to electrical flow than a larger-diameter wire.
A reason that the light bulb filament has high resistance is that it is very small in diameter. Beginning with Thomas Edison, researchers discovered that if resistance in a wire is high enough the wire will get hot enough to glow (produce light) or even to start a fire (which is why the inside of an incandescent light bulb is a vacuum - to deny oxygen and thus protect the filament from simply burning up).
Georg Ohm's Law, first published in 1827, I = V / R
tells us that the current (amps) through a conductor (wire) between two points on a circuit is proportional to the potential difference (voltage drop) across the two points and that the current between the same two points is inversely proportional to the resistance between them (ohms). We can re-write Georg Ohm's law to describe each of amps, volts, or resistance in terms of the other parameters, as shown below.
I = the current, measured in Amps; I = V / R and using simple algebra to re-write the Ohm's Law equation,
V = the difference in potential between the same two points, measured in Volts; V = I x R
R = the resistance in the conductor or circuit between the same two points, measured in Ohms; R = V / I
What is an electrical circuit?
Electrical circuits, alternating current (AC), direct current (DC), short circuits, and other basics of electrical wiring are defined further in our series of articles on electricity for homeowners starting at ELECTRICAL CIRCUITS, SHORTS.
What are the Definitions of Electrical Ground, Grounding Electrode, Grounding Conductor, Grounded Conductor, Ground Wire, Neutral Wire (Grounded conductor wire), Ground Rod?
Home inspector Arlene Puentes summarizes distinctions important in understanding the function of electrical grounding at a building:
Here are some elementary electrical ground, grounding, and ground wire definitions to help us get our terms straight when discussing electrical grounds, grounding, and ground bonds.
Also see Electrical Circuits, shorts, AC/DC.
Also see our Electric Power Frequency Table for a table showing the voltage and frequency for nearly every country in the world, provided courtesy of Paul Galow, Galow Consulting. Sketch courtesy of Carson Dunlop Associates.
Continue reading at ELECTRICAL GROUND DEFINITIONS or select a topic from the More Reading links shown below.
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Frequently Asked Questions (FAQs)
Question: What do I need to hook up DC running lights and a porch light on a gutted Airstream International trailer?
I have a 1967 Airstream International planted in the backyard and want to run an extension cord out to it to run basic electric. It has 5 amber running lights and a porch light on the outside that runs on DC. It is completely gutted and I have access to the single wire from each fixture. What do I need to do to hook up the outside running lights to a power strip. - Rob
Reply: AC - DC power converter, extension cord, connectors, caution
For just a few running lights and a porch light, most likely you won't even be drawing 10 watts, but to be on the safe side and to allow for expanded use of your power supply once you figure out how useful it is, I'd look at a unit with 35 watts or larger output.
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