What You Might Want to Know About Electricity Before You Talk to Your Electrician

Alternating Current

This is the most difficult section in this article, since it brings in a small amount of Physics. And you don't really need to understand this part anyway, so don't despair if you find it confusing.

The most common way that electricity is generated is by rotating a circular coil of wire about an axis through the coil's diameter so that the rotation is perpendicular to a strong magnetic field. This is done with turbine engines, or flowing water, or windmills.

According to Maxwell's equations, a current will be induced in the coil of wire as it rotates. The size of the current is proportional to the number of loops in the coil, and also to the rate at which the magnetic field is changing. (This is why the coil has to be spun - so that the magnetic field is constantly changing.) The figure below depicts this.

As should be clear in the drawing, when the coil is perpendicular to the magnetic field, the maximum amount of magnetic flux is passing through it. When the coil is perpendicular to the magnetic field, the maximum amount of surface area (inside the coil) is exposed to those magnetic field lines.

When the coil rotates to an upright position, no magnetic field lines go through it. As the coil rotates, the current oscillates between its maximum value and zero. Note also that when the coil is face up, the induced current will be moving in the opposite direction as when the coil is face down. This means that on every half rotation, the current will change from positive to negative, and back again on the next half rotation.

In fact, the magnitude of the induced current will oscillate as a sinusoidal wave as shown in the figure below. This figure shows two common voltages that we are familiar with in our households, and in office buildings. The large sinusoid shows "240 Volts Alternating Current," and the small sinusoid represents "120 Volts Alternating Current."

Power is transmitted from power plants at tens of thousands of Volts (up to four hundred-thousand Volts) along special cables to local distribution centers. It is much more efficient to transmit power at extremely high voltage, because it takes much less current to do so. As we will discuss below, "Ohm's Law" dictates that the higher the voltage, the lower the current, hence the less Voltage will be lost.

It is far too dangerous to transmit power at tens of thousands of Volts through residential neighborhoods on telephone poles. Generally, there will be a power distribution station somewhere near you, and power will be sent into your neighborhood at 7.5 Kilovolts (7,500 Volts).

If you look up to the top of the telephone pole near your home, you will see a large can-shaped gadget with wires going into it. This is a "stepdown transformer." What it does is to take energy from the 7.5 Kilovolt power, and step it down to the 120 Volts and 240 Volts that runs into your house. (You would not want 7.5 Kilovolts running into your house, just as you would not want a hundred-thousand volts running down your street.) Transformers are basically two intertwined coils of wire. The coils are made so that the ratio of the number of loops in the two coils creates the "stepdown" in Voltage.

Hertz, Building Ground, and Wires

Looking back at the figure above, note that an entire "cycle" of the Voltage wave (starting at 0, going all the way positive, then back through 0, then all the way negative, and back to 0) as taking 1/60-th of a second. This waveform is said to be a "60 Hertz" Voltage waveform. "60 Hertz" means that the waveform repeats 60 times per second. This is why the length of one cycle is 1/60-th of a second.

And of course the reason that the waveform is 60 Hertz is because the coils of the generator (at the power plant) are spun at this rate. This is the standard frequency used for power distribution in the United States. In Europe (where many of the commercial espresso machines are made), the standard frequency is 50 Hertz.

This difference in frequency can make a difference in fine electronic appliances that use small voltages and electronic filters to process things like audio and video signals. For the most part, motors and heaters (as are used in restaurant appliances) do not care too much what the frequency is, as long as it's in this general range. So what works in Europe will work here - provided that the voltage is in the right range.

Note that there is a "Common" signal shown in the figure in green. "Common" is the reference point for the voltage, and it is the return path for the current that will flow through appliances attached to the sinusoidal voltages. Nominally, "Common" should be at 0 Volts, which is why it is the reference point for the power line. Because "Common" it is nominally 0 Volts, it is sometimes casually referred to as "Ground," although it is not the same Ground as the building Ground (which is the real Ground).

Unlike the real Ground, the "Common" ground is an active part of the circuit, and it will have current flowing in it when an appliance is running. Every electrical plug will have at least two prongs, and usually three (and sometimes more). The first two are the "Hot" prong (120 Volts in the U.S.) and the "Common" prong (which is the returning path for the current flowing into the "hot" prong). A third prong (in all NSF-certified equipment) will be the building ground.

The building ground is not part of any circuit. It is wired to a metal electrode that is driven deep into the actual ground so that it makes good contact with the cold damp Earth. Within an appliance, the building ground is connected to the appliance chassis, and to all metal parts that a person could touch.

This keeps the chassis at "ground potential," which is the voltage that the people standing around the appliance should be at. After all, the people are standing on the ground (unless there is a carpet, in which case you can build up some charge, and get a "shock" when you touch the appliance). If anything goes wrong inside of the appliance, and the live voltage somehow gets connected to the chassis (usually because of a worn wire), the appliance will short-circuit the live current directly to the Earth, and a circuit-breaker will trip. No one who touches the appliance will be hurt.

If you open up the outer insulation on a power cord, you will find two or more insulated wires (that is, wires with plastic coatings) inside of it, and a single bare (uninsulated) copper wire. The bare wire is the building ground (real ground). It is always safe to touch. Among the insulated wires, there will be a white one, a black one, and if there are more wires, these should be colored too. If there are exactly three wires, the third one is usually red.

The white wire is the "Common" return. It should be at or near 0 volts, but it may have current flowing in it. The colored ones all can have high voltage in them. You should not touch them unless you know that they are not connected to anything, or unless you know that the circuit breaker has been turned off at the main panel.

Voltage, Current, Ohm's Law, and Real Wires

We've used "voltage" and "current" somewhat interchangeably above for ease of exposition. They are different things, but they manifest as one another in simple circuits, so are sometimes naturally discussed in this way.

The Voltage associated with an electron is its potential energy, whether or not it is actually moving (flowing). Voltage is denoted "V," and is measured in "Volts." It is exactly analagous to the potential energy of a drop of water. If the drop of water happens to be at the top of a waterfall, its potential energy is very large, because it can fall and exert force. If it is at the bottom of the waterfall, its potential energy is zero, although it is the same drop of water. Note that the potential energy depends only on the position of the drop of water, and not on whether it is in motion. The drop of water does not change when you change its position; only its potential energy changes.

Electrical Current is the rate at which electrons flow past a reference point (e.g., into your espresso machine). Current is denoted "I," and is measured in "Amperes" (sometimes just called "Amps"). Current is exactly analagous (again) to the rate at which water flows over the waterfall. In fact, a flowing stream of water is called a "current."

The power is the energy that is used up over time (power is the time-integral of energy), and is equal to the amount of electrons that flow times the potential energy of those electrons (which manifest as energy and make your appliance work). Power is denoted "P," and is measured in "Watts." Very simply, Power is given by P = IV. We'll say a little more about this later.

The medium through which electricity (or water) flows will resist that flow to a greater or lesser extent, depending on what the medium is. The amount of resistance is literally called "Resistance," is denoted "R," and is measured in "Ohms." For example, a narrow pipe will "resist" the flow of water more than a wide pipe, which will resist the flow of water more than a waterfall. If you try to push too much water through a narrow pipe too quickly, it will burst.

Most equipment (like espresso machines) have lots of resistance. Sometimes the resistance of an appliance is called the "impedance." (Impedance is a slightly more complicated construct that we will not deconstruct here.) Wires are made to have small resistances, since the purpose of wires is to deliver current (electrical flow) without depleting the voltage (potential energy) of the electrons in the flow. The resistance of wire will be specified in "milliohms per foot" (a milliohm is 1/1000 of an Ohm). To get the total resistance, you have to multiply this number by the length of the wire (in feet).

In fact, the energy lost within an appliance (or in a given length of wire) is determined by the Voltage, the Current, and the Resistance of that appliance (or length of wire) in accordance with "Ohm's Law." Ohm's Law states that: V = IR.

This means that the voltage across the appliance (which is the voltage difference between the electrical input ("Hot") and electrical output ("Common") of the appliance) is proportional to both the magnitude of the current flow, and the resistance of the appliance (or wire).

Any real wire has resistance. And a power line can run hundreds of feet from the transformer (up on the telephone pole, or in the ground on your street) to your house or building. The figure below shows what happens because of the resistance in the wire. The standard voltages used in the United States are 120 Volts and 240 Volts. The higher voltage, 240 Volts, is used for large appliances, like commercial espresso machines.

Because wire has resistance, and because it may run hundreds of feet, some of the voltage is lost in the wire by the current flow (as it flows to where it is needed) in accordance with Ohm's Law. Fat wire (having a lower "gauge" number) has less resistance than thin wire. Wires that have to handle large currents will be fat so as to mimimize voltage losses. Like the pipe that will burst if too much water is run through it too quickly, a wire will burn itself up if too much electrical current is run through it. Bigger wires are needed for larger currents.

Electricians decide what gauge of wire is required for a circuit depending on the peak amount of current that the devices on that circuit can draw, and on the length of the wire. They are allowed a maximum of an 8% voltage drop in the wire, after which they are required to use a heavier (and more expensive, and harder to work with) gauge of wire.

If the electrician uses a wire that is too thin for the amount of current needed at a given distance, not only will there be a large voltage drop in the wire (and the attached appliances may not work the way they should), but you can literally burn up the wire, and maybe start a fire. Have you ever picked up an extension cord with too many things plugged into it that felt hot?

In the figure above, the nominal 240 Volts and 120 Volts are the outputs of the transformer out on the street. By the time these voltages get to your building, go through the electrical panel, then get back upstairs and through the walls to your espresso machine, they could have lost as much as 8% of the voltage. Therefore equipment that is specified at 240 Volts should be expected to work at 220 Volts. Similarly, lights and appliances that are specified as using 120 Volts should be expected to work at 110 Volts.

So when some appliance specifications say "240 Volts," and other appliance specifications say "220 Volts," they are not referring to different power requirements. They are just making different assumptions about where the equipment is plugged in relative to the transformer. In practice, the equipment should work anywhere in the 220Volt - 240Volt range, since the manufacturer can't know the actual voltage at the socket into which you are going to plug the appliance.

Note that in Europe, a new standard that many countries (including Italy) are using is 230 Volts at 50 Hertz. We've already explained that the "50 Hertz" piece of it does not matter to the appliances that we are concerned with. In Europe, the 230 Volt appliances must also work with an 8% line loss, so they must work all the way down to 212 Volts. Therefore, they will also work in the 220 Volt - 240 Volt range. Running a little bit hot at 240 Volts won't hurt them.

So if you see an appliance specified at "230 Volts at 50 Hertz" there is no need to worry. The specifications were written for the European market, but the appliance will work just fine in the U.S. The only difference is that the power and/or current will be a little different than what the specification says.

Remember, the specification is telling you the power (Watts) and current (Amps) assuming that the appliance is running at 230 Volts. At 240 Volts, either the power will go up in proportion to 240/230 (which is 4%), or the current will go down in the same proportion, or a little of each will happen.

Standard Two Phase Power, and 240 Volts AC

So far, we have implicitly been discussing "single phase" power. That is to say, we talked about a single sinusoid of 240 Volts (or of 120 Volts), having a "Hot" voltage wire (or terminal) and a "Common" return wire (or terminal). In fact, the power coming out of the stepdown transformer on the telephone pole near your building has three different wires, but they are not 120 Volts, 240 Volts, and Common. Instead, they are two different "phases" of 120 Volts, and a Common return.

The figure below shows what is actually coming into your building. It is a first 120 Volt AC power line (shown in Blue), a second 120 Volt AC power line (shown in Red) that is running ½ of a cycle behind the Blue power line, and a Common return shared by the two AC signals. This is called "two phase power," because there are two "phases," which (in this case) are 180 degrees apart. Another way to think about this is that Phase 2 arrives ½ of 1/60-th of a second (which is 1/120-th of a second) after Phase 1.

In the main electrical panel in your building (where all of the circuit breakers are), Phase 1 and Phase 2 "zig zag" down the panel so that in each bank of circuit breakers (the left side, and the right side), every other breaker is powered by the same phase (that is, adjacent breakers are powered by different phases). The breaker outputs run to the various circuits in your house and building, and all share the "Common" return.

It is interesting (although not important to us) to note that if the current drawn in each phase is the same, then since they are exactly out of phase, the current flowing in the Common return will be zero. In fact, two-phase power was conceived partly for this reason, to save on the cost of the wires.

Each single circuit-breaker in the panel controls an independent (from the other breakers) 120 Volt AC circuit. The fact that half of your outlets are on one phase, and half on another doesn't matter, as long as you don't connect the two phases together. Each circuit breaker is designed to "trip" when a specified current limit is exceeded, which disconnects the circuit from the voltage source. If you look at the circuit breakers in your main panel, you will see numbers on them like "15" and "20." These numbers specify the amount of current (in number of Amperes) that will trip the circuit breaker.

The reason that you want a circuit breaker that will trip at a given threshold is that if something goes wrong in an appliance (e.g., it gets dropped into a sink full of water, or someone sticks a screwdriver into it and accidentally touches the live voltage), the circuit-breaker will immediately trip, thereby shutting off the power so that no one gets seriously hurt, and so that the wiring in your walls does not get damaged.

In all of the discussion and figures above, we spoke about 240 Volts as if it worked the same as 120 Volts. Specifically, we spoke about a "Hot" line (120 Volts or 240 Volts) and a "Common" line (which is 0 Volts). While each of the 120 Volt phases work this way, 240 Volts is different.

To deliver 240 Volt power to an appliance like a commercial espresso machine, we connect the "Hot" terminal of the appliance to one 120 Volt phase, and we connect the "Common" terminal of the appliance to the other 120 Volt phase. What the appliance "sees" is then the voltage difference between the two phases, as shown in the figure below.

Note that if you subtract the Red curve (Phase 1) from the Blue curve (Phase 2), you get the 240 Volt waveform shown in Black. There is no "0 Volt Common" return for the circuit using this waveform. (More accurately, the return is a "Hot" 120 Volt AC waveform.) From outside the appliance, we happen to know that we have hooked 120 Volts AC to the "Common ground" terminal of the appliance, but the appliance (which only "sees" the voltage across its two power terminals) can't tell the difference between this two-phase hookup and the kind of single phase 240 Volt hookup that we had been (hypothetically) discussing earlier. The appliance has no way of "knowing" that its common terminal is not sitting still at ground potential (0 Volts).

And by the way, if we return the discussion to the basement, the reason that the two phases are "zig-zagged" back and forth down the center of your main electrical panel is exactly so that any two adjacent slots in the panel will be on different phases. This allows you to put in a 240 Volt circuit breaker. A 240 Volt circuit breaker is twice the width of a 120 Volt circuit breaker because it occupies two (adjacent) slots in the panel. This gives the 240 Volt breaker access to both phases, which it needs for 240 Volts.

When we connect 240 Volts to an appliance in this way, we know that the "Common" terminal on the appliance is moving up and down with 120 Volt amplitude at 60 Hertz, but the appliance doesn't know this. As far as the appliance concerned, its "Common" terminal is standing still at 0 Volts. This makes the other terminal (to which we've connected the other phase of the 120 Volt line) "look like" it is connected to a 240 Volt AC power line. Basically, we have "faked the appliance out."

Because the 240 Volt AC difference is generated from two 120 Volt "phases," some people will call this "240 Volt two-phase power." This is meaningless. From the point of view of the appliance, the appliance "sees" the power as a single phase 240 Volt signal. Don't get hung up on how many "phases" a commercial espresso machine uses. From the point of view of the espresso machine, it is one phase. That the single phase 240 Volt waveform happens to be generated by the transposition of two 120 Volt phases doesn't matter to the appliance. This is simply our standard way of providing 240 Volts, and the machine can't tell the difference.

The plug for a 240 Volt appliance can have 3 or 4 prongs. If the appliance only uses 240 Volts internally, it will only have 3 prongs. Two of the prongs are "Hot" (the two phases), and one is the building ground. There is no "Common" return.

However, in many appliances, the motors and heaters use 240 Volts, but the control functions (e.g., microprocessors) run on DC voltage that is generated from 120 Volts AC. In this case, the appliance will use the phase going to its "Hot" terminal as the 120 Volt AC input, and will regulate this down to a few DC volts relative to the 0 Volt "Common" ground. These appliances will have plugs with a fourth prong which is the "Common" line.

Three Phase Power and 208 Volts AC

Some industrial facilities (and also some rural areas) have "three phase power" instead of two phase power. Three phase power is exactly like two phase power, except that there are three phases instead of two, and the three phases are 1/3 of a cycle apart instead of ½ of a cycle apart. At 60 Hertz, this means that the voltage waveforms can be thought of as being "delayed from each other" by 1/3 of 1/60-th of a second. In Trigonometry, we would say that they were "120 degrees out of phase" from each other as shown in the figure below.

Three phase power is generally used in applications where large electrical motors need to be driven. Three phase power is particularly efficient for driving large motors. This is irrelevant to commercial kitchen appliances, but if the building that you are in has three phase power (which is possible but unlikely), you need to understand it too.

The way that high voltage is derived from three phases is exactly the way it was derived from two phases. That is, two (out of three) of the phases are connected across the appliance. It does not matter which two. The third phase is unused, and just like for 240 Volts, the "Common" return is not used.

For the two phase system, the two phases are exactly out of phase (shifted by 180 degrees), so that the magnitude of the waveform becomes twice that of the phases: 2 X 120 Volts = 240 Volts. But in a three phase system, things don't work out quite as nicely.

The two selected phases are only 1/3 of the way out of phase (shifted by 120 degrees), so their "peaks" do not line up. The magnitude of the resulting waveform is then only 1.73 (which is the square root of 3) larger than the magnitudes of the phases: 1.73 X 120 Volts = 208 Volts. This is shown in the figure below.

Note that 208 Volts AC looks and behaves exactly like 240 Volts AC. It is just smaller. Most (but not all) 240 Volt appliances will work at 208 Volts, but you will not get the same performance out of them. If the manufacturer specifically mentions 208 Volts as an operating point (as does FAEMA), the equipment will certainly work. Note that machines that are built for the European market in which the standard voltage is 230 Volts must work at 212 Volts to allow for an 8% line loss.

While going down to 208 Volts is not too big of a stretch from 212 Volts, remember that we need to allow for an 8% line loss in the 208 Volt system as well. The real voltage could be as low as 191 Volts.

Except for manufacturers of large electrical motors, when 208 Volts is mentioned in the specifications, it is not that they are advocating 208 Volts as a "best" operating point. They are merely assuring you that if you have three phase power, the appliance will work. For commercial espresso machines, the difference is that the power will be lower (than what is specified for 240 Volt operation), and it will take the boiler longer to warm up in the morning.

Plugs for High Voltage Appliances

Standard 120 Volt household appliances (by and large) all have the same standard plugs on the ends of their power cords, and those plugs can be plugged into any standard household outlet. This is not true of high voltage commercial appliances. There are different kinds of plugs and different kinds of outlets for high voltage, and they are not compatible.

You must have a plug on the end of your power cord that fits the outlet. For this reason, many commercial appliances are shipped without a plug at the end of the power cord. The person doing the installation first looks at the outlet, and then provides a compatible plug which they put on the end of the power cord as part of the installation procedure.

The main reason for having different plugs is to ensure that the appliance does not exceed the current limit of the wiring in the wall. For high voltage outlets, there are 20 Ampere sockets, 30 Ampere sockets (these are what you will likely see), and various others.

If your appliance will draw 25 Amperes (and you should look at the specification), you would not want to plug it into a 20 Ampere circuit, because you will trip the circuit breaker (if you are lucky) or worse. The good news is that you will not be able to plug it into a 20 Ampere circuit because the 30 Ampere plug on your appliance will not fit into a 20 Ampere outlet.

Make sure that the circuit that your electrician installs will comfortably handle the current that your appliance will pull. By "comfortably" we mean that you should leave a little headroom. If your appliance will draw 18 or 19 Amperes, don't try to "squeak by" with a 20 Ampere circuit. Put in a 30 Ampere circuit or you will wind up tripping the circuit breaker occasionally.

You don't need to over-think this when having your electrician install a new circuit. For commercial espresso machines, 1-group machines will draw about 15 Amperes, and can work on a 20 Ampere circuit. 2-group machines will draw close to 20 Amperes, and should not be put on a 20 Ampere circuit. 3-group and 4-group machines will draw current in the mid-20s (of Amperes). Therefore, you can get by with a 20 Ampere circuit only for 1-group machines. All other machines will need 30 Ampere service.

When in doubt, have the electrician put in a 30 Ampere circuit. But be careful. If you intend to plug other appliances into this same circuit, you need to add up the currents from all of the appliances, and put in a circuit that can handle the total current.

Another reason (other than current) that you might have different kinds of plugs is that some plugs have a "key" (one prong with a different shape) if there are two "Hot" inputs and a "Common" input. This is to make sure that the "Common" input gets connected correctly, because the machine will know the difference.

And a final reason is that some plugs are made to "lock" into their socket with a slight twist after the plug is inserted. This will prevent the plug from being dislodged accidentally. Some zoning boards will require this as a "safety feature." They are concerned that if someone trips over the cord, the plug will get pulled out, and there could be a small spark (like miniature lightning) that would cause any nearby volatile gasses to explode.

In the first place, there will be plenty of equipment in a professional kitchen that will make volatile gasses explode just by using the equipment the way it is intend to be used. Spurious sparks will be the least of the problems in a professional kitchen. This "safety requirement" is just a rule that makes sure that if someone accidentally trips over the cord, the cord will hold firm so that they go down hard.

A Final Word About Power

When we had discussed power earlier, we had said that P = IV. If you look at some of the specifications of our commercial espresso machines (in which all three numbers are given), you will see that this equation doesn't seem to hold. (That is, if you plug in the value of V and the value of I, you will get a different number for P than the one in the specification.)

There are a few reasons for this that we'll touch on very briefly. The numbers in the specifications are the ones that you should use.

First, this is AC power. Since the 240 Volt voltage waveform is a sinusoid, it is not constantly at 240 Volts. To calculate power, we are really interested in the area under the curve, and not in the peak of the curve. We can adjust for this by dividing the peak number by 1.4 (which is the square root of 2).

Second, you are interested the current and the power for different reasons, and the manufacturer tries to give you the numbers that you probably want. You probably want to know the peak value of the current (that is, the maximum current that the machine could draw at any instant). This is because you want to make sure that your wiring will accommodate the peaks.

But you probably don't care about the instantaneous power. Instead, you'd like to know the average power, because this is what will drive your electric bill. So the two numbers given in the specifications are not meant to be consistent with each other.

And finally, most appliances appear (to the power delivery circuit) as things that are a little more complicated than simple resistors. You might recall that in a single instance above, we used the word "impedance" instead of "resistance," and said that we were not going to discuss it. We will now spend two paragraphs on it.

Resistance is a real number. In addition to resistance, appliances (and wires) also have capacitive and inductive elements that behave differently at different frequencies. "Impedance" is a complex number (having both a resistance, and also a reactive component that is expressed as the imaginary part of the complex number) that captures these notions. For example, your commercial espresso machine may have a resistance of 10 Ohms. This means that it "looks like" a 10 Ohm resistor to a DC current. That is, if we put a constant 240 Volts (DC) across its terminals, it would draw 240 Volts / 10 Ohms = 24 Amperes (DC). But its impedance might be 9.8 + 2i Ohms, where i represents the square root of -1 (which is imaginary).

The effect of the reactive component (2i Ohms in this case) is to put the voltage and the current out of phase with each other. So when calculating power, you can't simply multiply the amplitudes of the voltage and current, and divide by the square root of 2. If you really want to know the power, you have to know the complex impedance, the operating frequency (or the frequency components of waveforms that are not pure sinusoids - which can be found with Fourier transforms), and the phase shift that the impedance will induce in the current at the operating frequency (or frequency components). In our example above, where we posited an espresso machine having an resistance of 10 Ohms, and an impedance of 9.8 + 2i Ohms, the "imaginary" 2i part is the part that will shift the phase of the current. And the amount of the shift would be different in Europe (at 50 Hertz) than it would in the U.S. (at 60 Hertz).

If you are running a coffee shop, you really don't need to know this, or worry about it. In fact, your electrician probably doesn't know this either, and certainly doesn't worry about it. So when you are talking to him or her, don't bring this up unless you are trying to intimidate them. It will not be constructive.

Except for the previous few paragraphs, if you've read and (more or less) understood this article, you should be comfortable looking at the commercial espresso machines specifications, and you should have no problem speaking with your electrician about what needs to be done. If you have any questions, or if you'd like us to speak to your electrician too, just drop us a note, or give us a call.

...written by your friends at The Coffee Brewers