Guide to Low Voltage Distribution Systems

In an earlier article, we outlined common industrial voltages used in the US. Now let's look at how these electrical systems are set up. 

First let's define what a low voltage distribution system is.

What is a low voltage distribution system?

A low voltage system is the final stage of the electric grid. It is the infrastructure that moves electric power inside buildings and dwellings. These systems rely on transformers to step utility voltages down to usable levels. The most common voltages found in such systems in the United States include 120, 240, 208, and 480 volts. The voltage level will depend on the type of service needed.

After the step-down transformer, electric power goes to a main service panel. The service panel distributes electric power throughout the building or house. Low voltage electrical services are either single-phase or three-phase.

Single-Phase Distribution Systems

240/120 (Split Phase)

When you see 240/120 listed on a transformer nameplate, it indicates a single 240 volt winding with a center tap. This is the low voltage configuration you’ll find on most single-phase padmount transformers. Here’s how it works…

You get the full 240 volt rating of the winding between X1 and X3 as shown above. Because of the X2 center tap which splits the coil in half, the 240/120 setup is often called split phase. This split phase setup allows you to measure 120 volts between the line terminals and X2 (neutral). This winding type is perfect for serving single phase systems with 240 volt and 120 volt loads.

120/240

When you see 120/240 marked on a transformer nameplate, it denotes two separate 120 volt coils. The two coils are set up to connect in either series or parallel.

Series vs Parallel connections

In series, the coil voltages add to equal one total 240 volt winding. The point where the coils connect is the neutral point.  The series connection mimics the split phase 240/120 setup introduced earlier. In parallel, the voltage across both coils remains 120 volts.

Most indoor single phase applications include both 240 volt and 120 volt loads. As a result, the series connection is more common. The parallel connection is more frequent with polemount transformers like the one pictured below.

This image shows a single polemount with 120/240 windings in parallel. This configuration is the standard setup for three-phase polemount banks. When connected with two other identical units, the result is a 208Y/120 power supply.

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Three-Phase Distribution Systems

Larger commercial and industrial buildings usually have three-phase distribution systems. The advantage of three-phase power is its larger loading capacity. It can also supply both single-phase and three-phase loads. 

Three wire system (3W)

Ungrounded systems will have a three wire supply. In these systems, there is no grounded conductor (or neutral). The transformer winding supplying power has no intentional connection to earth ground.

Four wire system (4W)

A four wire system contains three ungrounded phase conductors and one neutral conductor. The bare copper safety ground is not counted as a current-carrying conductor. In a four wire system, the neutral conductor (fourth wire) bonds to earth ground. This system is the most common in newer facilities supplied with three-phase power.

240V Delta

The three-phase delta system is less common today. In this system, the line voltage and coil voltage are the same. Each phase measures 240 volts across each respective coil. This setup can supply 240 volt three-phase and single-phase power as shown below. 

The two-pole breaker on the left supplies power to single phase loads. The three-pole breaker on the right supplies power to three-phase loads.

240V Delta with 120V Center Tap

Many 240 volt Delta windings have a center tap on one of the three coils. This provides an option for a limited 120 volt output. This center tap is often called a lighting tap. It supplies single-phase power for small lighting and receptacle loads. The coil with the center tap acts identical to the 240/120 single-phase scenario. The measurement between the center tap and adjacent phases is 120 volts. The measurement between the center tap and the opposite phase yields 208 volts. The phase that measures 208 volts from the center tap goes by many names. Examples include high leg, wild leg, stinger leg, ect. The high leg is very common in older delta supply systems in the US. This configuration works best for three-phase installations with minimal 120 volt needs. The amount of single-phase 120 volt loading should not exceed 5 - 10% of the supply transformer kVA.

240V Corner Grounded Delta

In some cases, one phase of a delta winding may be solidly connected to earth ground. One of three normally ungrounded conductors will have a solid connection to earth. The grounded phase must remain consistent throughout the system.

208 Y/ 120

208 Y/ 120 systems supply both 120 volt single-phase and 208 volt three-phase loads. This configuration is common for commercial establishments such as apartment buildings. In this setup, three-phase power runs larger equipment. The 120 volt supply serves single-phase loads such as lights, receptacles, and appliances. This system works best for buildings with larger 120 volt loads. The common neutral point allows balanced loading (unlike the high leg delta system).

480 Y/ 277

Most industrial and large commercial buildings use a four wire 480 Y/ 277 system. The 480 volts available from this system supplies both single-phase and three-phase needs. Single-phase lighting loads make use of the available 277 volts.

How to measure voltage in your system

Measuring line to line

The voltage between the ungrounded conductors in a system is the line voltage. Another way to describe this would be the phase to phase voltage. To determine the phase to phase voltage, test the voltage between A, B, and C phases at the main panel. This equates to measuring the voltage between H1, H2, and H3 or X1, X2, and X3 on a transformer. In the picture below, the line voltage is 208 volts.

Measuring line to neutral

The voltage between the ungrounded and grounded conductor(s) in a system is the line to neutral voltage. In the picture above, the line to neutral voltage is 120 volts. At the main service panel, you measure between the neutral bus and the A, B, or C phase terminals.  

Transformer wiring diagrams show the neutral point of the windings. On three-phase transformers, the high voltage neutral is marked H0. The low voltage neutral is marked X0. You will not find an H0 or X0 on single-phase transformers. The easiest way to identify the neutral point is by referring to the nameplate wiring diagram. 

In three-phase, testing line to neutral measures one of three windings. In single-phase, testing line to neutral measures half of one winding.

Measuring line to ground

Line to ground measurements are sometimes misleading. A predictable measurement requires an intentional bond to earth ground. This bond is made at the source transformer neutral or first service disconnect. Redundant or faulty neutral-ground bonds will often obscure voltage readings. In a solidly grounded system, the line to ground and line to neutral voltages are the same.

The picture below shows an example of a solidly grounded system. The neutral point of the source transformer bonds to earth ground. The common bond causes the line to ground and line to neutral measurements to share the same value. In this example, the line to ground voltage is 120 volts.

Voltage readings line to ground vary for ungrounded systems. Examples of ungrounded systems include delta and ungrounded wye as mentioned earlier. In such cases, measurements may vary widely.

Delta Windings Grounded

240 Delta w/ 120 Center Tap Grounded

The center tap of a delta winding can be grounded. When this is done, line to ground and line to “neutral” (center tap) values will match. The diagram below shows the result of grounding the center tap of a delta winding. Line to ground values mimic the high leg delta system described earlier. The center tap is normally marked on the transformer nameplate by an X4 or X6 terminal.

Pro Tip: Distinguishing Between 208 Y/ 120 and 240 Delta High Leg Systems

Sometimes the high leg delta system is mistaken for 208 Y/ 120. This is usually when only line to ground voltages are recorded at the service panel. The 208 volt reading across the center tap and high leg can be misleading. Remember, a 208 Y/ 120 system will measure the same voltage line to neutral between all phases. You can avoid this problem by measuring the line to line voltage (240 volts) at your panel (instead of line to ground). If you measure line to line, you’ll do fine. 

Corner Grounded Delta

In a corner grounded system, the grounded phase will measure 0 volts to ground. The other two phases will measure the line voltage to ground. Corner grounding is not the same thing as bonding a 120V center tap to ground (as mentioned above). With corner grounding, there is no fourth conductor landed at an X4 or X6 terminal. Instead, one of the normally ungrounded phase conductors is solidly grounded.

Conclusion

Maddox has the right transformer for your type of electrical distribution system. For transformer quotes or questions, please fill out the form below. Our technical sales representatives are ready to help get your project online.