The power grid systems in the United States differ fundamentally from those in Europe. Anyone wishing to export an electrically powered device to the United States must therefore consider not only plugs and outlets, but also different line voltages and power grids that are designed entirely differently.
Types of Electric Power Grids in the United States
First, an important note: The U.S. power grid has evolved over more than 100 years, for a long time without significant regulatory intervention by the government. As a result of this, and due to the country’s enormous size and federal structure, there are still a large number of different grid configurations and connections in use today.
Although there have been increased efforts in recent decades to standardize networks in this area as well, local peculiarities and deviations from the standard are still to be expected—especially in rural areas.
Many of the points discussed in this article also apply to other English-speaking countries. For example, single-phase three-wire systems are also common in the United Kingdom, though they operate at 230 V. In the following, however, we will focus exclusively on the most common types of electrical power systems in the United States and Canada.
Mesh Networks and Stitch Networks: Medium-Voltage Networks in the United States
In Europe, power grids are generally designed as meshed networks, meaning there are numerous interconnection points within the grid. This makes it easier to compensate for outages and maintain a constant voltage. In the U.S., particularly in the sparsely populated states of the western interior, such grids are often uneconomical, which is why radial grids are frequently found there. These are usually laid out in a star configuration with a grounded neutral point.
Although star networks are less expensive to set up than mesh networks, they are not only significantly less redundant —and thus less fault-tolerant—than mesh networks, but also considerably more susceptible to large voltage drops. The supply to end users also differs in key respects from the connections we are accustomed to.
Single-phase three-wire system
The single-phase three-wire system (split-phase electric power)—sometimes incorrectly referred to as a two-phase power system—is the standard power supply for residential households and small businesses in the United States. Three-phase alternating current (“three-phase power”), which has been standard in Europe for many decades, is generally not available to these consumers, but only to large commercial customers with correspondingly high consumption.
In North American single-phase three-wire systems, the voltage between the two phase conductors is typically 240 volts, with the phases shifted by 180 degrees relative to each other. The voltage between the neutral conductor and one of the two phase conductors is generally approximately 120 volts. It should be noted, however, that some local power grids also operate at 120 V, 240 V, 277 V, or 480 V. The frequency is usually 60 Hz.
Special Case: Single-Wire Earth Return
In rural areas of Canada in particular, a particularly cost-effective variant of the single-phase three-wire system is used: the Single-Wire Earth Return (SWER) system. While the low-voltage side is structured the same way as the single-phase three-wire systems described above, the medium-voltage grid consists of only one electrical conductor—usually an overhead line—and the ground serves as the return conductor. As a rule, each end-user connection has its own pole-mounted transformer that converts the electricity.
In addition to frequent grounding issues, the main drawback of this system is the lack of a rotating field, which means that high-power machines—such as induction motors—cannot be connected directly to the power grid. A variable-frequency drive can remedy this problem. Another disadvantage is the large voltage fluctuations. These are caused by the extensive reach of the power grids and sometimes require the use of special regulating transformers.
Ungrounded Electrical Systems for Industrial Use
While residential service connections are generally grounded, this is often not the case for commercial customers. In particular, in factories and other large production facilities, it is common for the electrical system to be installed without grounding. This offers the advantage that the system does not shut down immediately upon the occurrence of a single insulation fault, thereby preventing unplanned interruptions in operations. However, the lack of grounding must be taken into account when designing the protective devices in the control panel.
Voltage in U.S. power grids
In addition to the grid architecture, we need to consider another factor that often causes confusion: voltage. European users and manufacturers are accustomed to the voltage in an electrical grid always being referred to as operating voltage —from the power plant to the consumer. In North America, however, a distinction is made between the voltage in the utility grid (service voltage) and the voltage in the consumer grid (utilization voltage). According to the National Electrical Code (NEC), the voltage difference between these two values must not exceed 5 percent.
With regard to the tolerances within which voltage is allowed to fluctuate, a distinction is made in the U.S. between two zones, one of which is called Range A, within which only minor voltage fluctuations occur. Range B, on the other hand, refers to a range in which more significant voltage fluctuations occur that cannot be tolerated on a permanent basis. However, the exact limits of these tolerances and the precise point at which Range B begins are not defined; rather, they depend on the sensitivity of the respective loads.
What does all this mean for exports to the U.S.?
The question German plant and machinery manufacturers are now asking themselves is probably: How do the various types of power grids affect my export business?
At the site of operation, different voltages and greater voltage drops than in Europe are to be expected. However, a frequency of 60 Hz can be assumed with reasonable certainty.
If a three-phase power supply is desired or required in the low-voltage grid, the easiest way to implement it is with a three-phase extension, known in the U.S. as a Delta 4-Wire or High Leg Delta. To achieve this, a third conductor (the high leg) in a delta configuration is powered via the two existing conductors of the single-phase three-wire system.
If, during the installation of a system, it becomes apparent that it cannot be operated with the voltage available on site, a voltage or input transformer can provide a solution. Such solutions are used primarily for mass-produced equipment, but also when a system needs to be relocated within the United States.
Exporters should always be aware that, at their facility’s location of all places, there may be a local power grid whose voltage, for example, does not correspond to the values discussed above. To avoid unpleasant surprises, it is therefore strongly recommended to discuss the local conditions in advance with the on-site customer or a qualified specialist appointed by the customer .
