Power Quality in Your "Electronic" Home
Copper Applications in Electrical Innovations
Proliferation of Sensitive Devices Makes Good Electrical Power A Must-Have.
About 1.5 million new homes will be built in the USA this year, maintaining a pace that has gone on for about a decade. But while the number of new homes built each year has remained fairly constant, one thing about the homes has been continually changing: it's the amount of copper they contain, up to 20 to 30 pounds more per house in some cases.
Does this look familiar? New homes today have become increasingly packed with sensitive electronic devices. Aside from audio/video entertainment centers, homes might contain one or more computers and their peripherals, data ports for security systems, several telephones, plus fax machines and copiers for home-office use. Structured communications wiring, shown schematically here, is a valuable addition if all devices are going to work together. In order for the" electronic" house to work reliably, its equally important electrical system must be designed to ensure high power quality.
The additional copper is needed for the most part because consumers are increasingly demanding the integration of, and provision for, modern electronic devices. For example, more than one-half of American homes are now equipped with at least one computer, along with its usual peripherals, and if there's a home office present, it will almost certainly contain a fax and maybe a copier, as well. The stereo and TV have long since given way to an "entertainment center," which might have its own Internet connection. Computer networking, whole-house audio, home theater, video distribution, home controls and security systems are becoming increasingly commonplace.
All of this electronic gadgetry requires proper wiring if the devices are to function in harmony. Accordingly, it is estimated that about 15-20% of new homes are now (2002) being fitted with what the industry calls "structured" wiring, and perhaps 42% of new homes will be so equipped by 2004. Fully one-third of all U.S. home builders offer this as a feature, either standard or optional. (Structured wiring refers to wiring systems in which high-bandwidth "Category" communications wire, coaxial cable and other conductors are routed to each room from a central control point. There is more information about structured wiring and its impact on copper elsewhere on The Copper Page.)
This phenomenon isn't limited to new homes. It is surprisingly affordable to retrofit structured wiring systems into existing structures, thereby increasing their functionality as well as their value.
But What About Power Quality?
A proper communications wiring system allows the home's electronic devices to work together, but keeping those devices working reliably makes it equally important to pay close attention to the house's electrical wiring system. Why? Because we are dealing here with all sorts of what the Institute of Electrical and Electronic Engineers (IEEE) refers to as "sensitive electronic equipment," and such equipment absolutely needs good quality electrical power in order to function properly.
Power quality is an issue that is ordinarily associated with commercial structures such as office buildings that contain large numbers of computers, printers and other electronic equipment, along with heavy machinery that wouldn't be found in a home. Still, the proliferation of electronic equipment in home settings has come to the point where power quality considerations are becoming increasingly germane.
The term power quality simply describes how well the power available at the outlet conforms to the electrical standards we all take for granted. The power should be continuous, uninterrupted by gaps, "notches" or other outages, even momentarily; it should be delivered at 120 V, plus or minus only a few volts; its frequency, 60 Hz, and waveform should likewise be invariant and undistorted. Except for occasional hiccups, electric utilities generally do a good job of supplying high quality power.
If you're building a home, now is the ideal time to make certain your electrical system is designed and installed such that your sensitive electronic equipment receives the power quality it needs inside the home. If you're upgrading the electrical system in an older home, you can easily and relatively economically accomplish the same goal.
To keep abrupt positive changes in line voltage (spikes or surges) from damaging sensitive equipment, install transient voltage surge suppressors (TVSSs). These devices are available in a range of sizes to match the size of the load(s) being protected. A TVSS large enough to protect an entire house can be installed at the service entrance panel for about $150. However, many experts recommend that TVSS protection be installed in layers; that is, a combination of one large TVSS for the entire service, plus individual units for each sensitive branch circuit, followed by smaller units directly upstream of the sensitive equipment itself.
To make sure your equipment continues to receive power in the event of an outage, install an uninterruptible power supply (UPS). UPS devices contain batteries and an inverter to maintain a flow of ac power for a few minutes or longer, enough time to shut down the connected equipment in an orderly fashion. Units of the size needed for most home offices can be purchased for about $200 or less.
Many UPSs contain built-in TVSS circuitry, as do better-quality "power strips." In some cases, however, TVSS power strips and UPSs should NOT be installed in-line on the same circuit since power disturbances known as harmonics, generated by either or both devices (or by downstream electronic equipment), may interfere with their proper operation.
Branch Circuit Considerations
It's best to wire your house so that your computer system is not on a circuit that also serves motors like those found in air conditioners, refrigerators, home workshop power tools, etc. Energizing the motors can cause sizable voltage fluctuations. Industrial and commercial installations often go so far as to have dedicated circuits and separate electrical panels for sensitive equipment, but this precaution usually isn't needed in homes.
Limit the number of outlets on the "sensitive" branch circuits. While the National Electrical Code®(NEC) permits an unlimited number of general use outlets on branch circuits, it is advisable to install no more than six outlets on circuits containing computers and related equipment. Doing so may require a bit more wire, but think of it as inexpensive insurance to guarantee that the equipment will function reliably. Even a whole lot of extra copper wire can cost less than a few hours' worth of lost data.
Upsize the Wire
The size of electrical wire in branch circuits is governed by the NEC and by whatever additional provisions to the Code are made by local jurisdictions. Those provisions are in place for one reason: safety. The Code specifies wires in sizes that will avoid overheating under conditions of maximum load, i.e., below the level at which the protective circuit breaker trips. However, it is possible in some cases for wires to be large enough from a safety standpoint, yet be small enough so as to create a drop in voltage that could be detrimental to sensitive equipment.
Home wiring for 15-A branch circuits is minimally sized at AWG 14, but many contractors are now installing wire of the next larger size, AWG 12, in new homes, the smaller the gage number, the larger the wire. The cost differential is small, and a portion of that incremental cost will be recovered over time since the heavier wire is more energy efficient, wasting less electrical energy in the form of resistive (I 2 R) heating.
If there is one thing you can do to ensure good power quality for your home office equipment, it is to make certain that the equipment (along with the entire electrical system) is properly grounded. Good grounding is essential for several reasons: first, it provides safety for the equipment operator in the event of a fault or short circuit-that's why the Code requires it. From a power quality standpoint, however, good grounding provides all connected equipment with a common electrical reference point. That's important in any event, and it's particularly important if the equipment is networked. Ideally, the ground reference voltage should be invariant and as close to zero volts as can practically be attained.
The electrical resistance of the grounding circuit should be as low as possible. The NEC cites a maximum ground resistance of 25 ohms for rod, pipe or plate electrodes. This is usually a copper-clad steel rod driven into the earth near the service entrance. If ground resistance exceeds this value, the Code requires that a second electrode be installed, but once that has been done, no upper limit on resistance is specified. Thus, even a grounding system that exhibits several hundred ohms resistance to earth would satisfy this provision of the code.
The IEEE and computer manufacturers generally agree that ground resistance for electrical systems containing sensitive electronic equipment should be lower than the level cited in the Code, and values of five ohms or less are often recommended. The lowest value attainable (without resorting to exotic techniques) depends largely on soil conditions, and emplacement of several electrodes may be sufficient, especially if 10-ft (3-m) rather than the more common 8-ft (2.4-m) electrodes are installed. A low-resistance grounding connection can also be made at metallic well casings.
If a home has a metal water service line 10 feet (3 m) or more in length however, it must be electrically tied in, or "bonded," to the grounding electrode system. However, connection to underground metal water pipes, once widely accepted as providing a reliable low-resistance ground, may no longer offer assured protection since many water service lines are now made from nonconductive plastics. The best grounding technique, used increasingly in commercial installations, is a copper ground ring, extending under the ground horizontally at least 20 feet (6 m) and made of bare copper wire not smaller than AWG 2. It might pay to ask your electrician to measure your electrical system's ground resistance using certified equipment.
Inside the home, make certain that all circuits contain a separate ground wire, even if wiring is strung inside metallic conduit. The ground wire is either bare copper or identifiable by its green insulation, and it should be properly connected to the third (round) socket in wall outlets, to metallic junction boxes, and to metallic conduit, if installed. The green grounding wire should also lead, uninterruptedly and without connection to a neutral (white wire) conductor, directly to the ground-neutral bus at the service entrance panel.
We're using more electricity in our homes every year, but we're also using electricity in ways that didn't even exist 20 years ago. Whether in new homes or old, electrical wiring systems must take these new uses into account. If yours is or will be an "electronic" home, protect your sensitive equipment by installing a wiring system that provides adequate power quality. It won't cost much, and your equipment will reward you with higher reliability for years to come.
Also in this Issue:
- Power Quality in Your "Electronic" Home