Volts, Amps & Watts
December 1, 2005
The words volts, amps and watts -- or voltage, amperage and wattage -- float around in the world of electricity. In this column we'll look into what they mean to you, besides the feeling, “Geez, I never could keep that stuff straight.”
Before we begin, volts or voltage is abbreviated as V. Amps or amperage is A. Watts or wattage is W.
They're really just terms of measurement. No one has any confusion about the measurement terms inches or gallons or pounds. They, too, are just different measurements.
My favorite analogy for electricity coming from the power plant is an electricity train. (see Figure 1). This “train” is loaded with electricity that's full of “hot” energy. The hot electricity coming from the power plant is pressurized. The measurement of the pressure is voltage.
Ordinary house wiring is standardized at 120 volts (V), called line voltage. This is the voltage you can expect to have in your house, plus or minus 5 percent. It may not be actually 120V. If you use a meter to test the voltage in a wall outlet, you'll find that it can vary by time of day or the season of the year, but rarely more than by a few volts.
For example, in many locations voltage may be lower in late afternoon in the summer when demand for air conditioning peaks. In the mountains of Colorado, it may be lower in the winter because of the demand of ski lifts. But there isn't any significant demand for air conditioning in cool high mountains. These slight voltage differences don't affect the performance of lights or appliances.
Sometimes you hear of 110V or 115V. These simply represent “standard line voltage.” Over the years, standard voltage has moved up from 110V to 120V.
There's also 240V for large appliances. An electric water heater or clothes dryer is standardized at 240V. Over the years this standard has grown from 220V, to 230V, to 240V. This 240V electricity is available at the service panel. Pairs of red and black wires are usually used for 240V.
If you're curious about the voltage in the building where you are, it's simple to measure. An inexpensive voltmeter or multimeter will tell you. Simply set the meter on V for voltage, and AC for alternating current. That may be represented on your meter as V~. The squiggle stands for alternating current (AC). The setting you wouldn't use is V DC or V with a straight line or dashed line over it. Those settings are for direct current (DC).
Voltage is a measurement of pressure. Amperage is a measurement of flow, or how much.
Every circuit has an amperage, or amp, rating. Amperage rating is also called current rating. The rating is limited mainly by the size of the wire. A larger wire can carry more amperage, like a larger pipe can carry more water.
An ordinary 120V household circuit is rated at 15 amps (A). Circuits also can be 20A or 30A. There also are 240V circuits rated from 30A to 60A.
If you try to get or “pull” too much amperage from a circuit, there is a switch in the service panel that will open (switch off) to turn the circuit off. Depending upon the age of the building, the switch is either a fuse or a circuit breaker. We have this safety precaution because trying to pull too much amperage can make the wiring heat up and cause a fire.
More amperage requires heavier wire to carry it. Wire is measured by the term “gauge.” The smaller the gauge, the larger the wire. It's the same idea as shotgun shells, where 12 gauge is larger than 20 gauge.
For wire, No. 14 gauge could be used for 120V, 15A wiring of light fixtures and outlets. For a 240V, 40A electric range, No. 8 gauge wire could be used. Wiring problems are often related to too small a gauge wire being used, probably with the intent of saving money.
You can tell by looking at an outlet what its voltage and amperage limitations are. Figure 2 shows some common ones.
Many loads are rated in watts (W). An ordinary light bulb might be 60W. A household appliance might be 1,500W. Wattage is simply volts multiplied by amps.
W = V x A
If an electrical device has only an amp rating and you want to know its wattage, simply multiply the amps by the voltage (most likely 120V).
The Difference Between Watts & VA
But wait a minute. If wattage is volts multiplied by amps, then what's VA? Both are figured as volts x amps. I asked several engineers, who were very gracious and sent me pages and pages of small-print explanations and charts. Despite all the explanations, it still comes down to
W = V x A and VA = V x A
So you know what the answer is? Watts and VA are the same!
Perhaps the determinate of whether a device is rated in amps or watts is whether it is a resistive or an inductive load. Resistive loads, such as light bulbs, are rated in watts -- a 60W bulb. Inductive loads, such as motors, generally have an amp (A) rating -- a 3A electric drill, a 13A radial arm saw, a 0.3A zone valve, etc. Multiply the amps by the volts and you have both watts and VA. Call it whichever you want, the number's the same.
Of course, there's the confounding fact that small, motorized household appliances, such as mixers and hair dryers, are rated in watts. So maybe the deciding factor is where the motor is used: the house gets watts, the workshop gets amps. If you know, let me know!
Regardless, it's easy to translate the amps to watts. The electric drill is 3A x 120V = 360VA or 360 watts. The zone valve is 0.3A x 24V = 7.2VA or watts. You might try the math for a 0.8A zone valve.
Figuring Circuit Capacity
For the low-voltage zone valve circuit, a transformer is the power supply. Its capacity is measured in VA. To figure how many zone valves you can put on a 40VA transformer, divide the transformer's 40VA by the zone valve's 7.2VA. That means you can put five 7.2VA zone valves on a 40VA transformer if there's nothing else on it.
Now let's look at the capacity of a house circuit. Maybe you wonder if you can safely plug in a new appliance. Or you're finding that your circuit breakers or fuses keep going out. Here's what you can to do figure the circuit capacity. Look on the circuit breaker or fuse box for that circuit and get the amp rating. Let's say it's 15A. Multiply that by the circuit's voltage (120V, unless it's a dedicated heavy appliance circuit). That gives you 1,800W. To give a margin of error, let's assume just 80 percent of that. Multiply 1,800W by .80 and you get 1,440W.
If there are loads that have an amp rating rather than watts, multiply amps x volts to get watts. Now add up the wattage of every load (fixtures, plug-in lamps and appliances). If the total is less than 1,440W, you're in business. If it's more, you need to remove loads until it's less.
And in case you're wondering, when a device such as a radial arm saw is rated in horsepower, 1hp = 745 watts.