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Three-Phase Service Basics


Marc
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The 3 phase configuration is used in most utility power transmission and distribution applications. The only other configuration in current use is DC (like the voltage from your car battery). Several decades ago, a few 2 phase systems were in use but have since been replaced by 3 phase. The primary attraction of the 3 phase configuration is that it delivers more power per length of conductor. It is the most economical use of copper, aluminum or whatever material is chosen as conductor. It also is an ideal choice for powering induction motors. 3 phase induction motors have a much higher starting torque than any capacitor-start single phase motor. You will notice that 3 phase AC condenser units do not have nor need start capacitors.

The simplest way to explain 3 phase is to visualize 3 portable power plants alongside each other that are of identical manufacture. Each one is a single phase source of AC power. AC means alternating current, which means that the current alternates in direction from positive to negative. Most gasoline powered portable power plants run at 3,600 RPM which is 60 revolutions per second. The frequency of electrical power used in the United States is also 60 cycles per second. Now, visualize that we expose the shaft of the rotor on each generator and we bolt an arrow onto it so that the arrow rotates about its center just as the rotor does. When the power plant is running at 3,600 rpm, the arrow is also spinning along with the rotor, and the vertical distance from the tip of the arrow to the shaft of the generator goes from a positive number when it’s above the shaft to a negative number when it is below the shaft. Let’s now rotate the position of the arrow on the shaft so that the arrow is pointed straight up when the voltage on the generator leads is at its positive maximum at that point in time. Mathematics will show that the vertical distance from the tip of the arrow to the shaft of the generator varies in exactly the same fashion as the voltage on the power plant. It’s called a sinusoidal waveform and all countries that use AC power also use this same waveform. None of us have eyes that can follow the arrow tip at 3,600 RPM but if we could, the vertical distance to the shaft is what the voltage available from any household outlet would be proportional to.

Now let’s add a sprocket to the each generator shaft. What we have now is 3 power plants alongside each other with an arrow and a sprocket attached to the shaft of each generator. Let’s number them #1, # 2 and #3 from left to right. Adjust the position of the generator shaft on power plant #1 by pulling on the engine crank until the arrow points straight up. Do the same for #2 and #3 except that the arrow should point 120 degrees past #1 and the arrow on #3 should point 240 degrees past #1. Now let’s link the 3 power plants together by installing a common bicycle chain on all 3 sprockets. Start it up and if the leads from the three power plants are properly connected to each other, you will have a 3 phase power plant. Apart from the mechanics, it would work.

There are different ways to connect the generator leads, depending on what type of 3 phase configuration you want. See the attached drawings for the two most common.

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A 3rd configuration, called ‘open delta’ has only 2 transformers instead of 3 but functions identically as a regular delta configuration except that the available power is much reduced from that of the transformer nameplates. This is concern for the utility and not the home inspector. Open delta is often used where most of the electrical loads for a particular application are single phase loads with only a few three phase loads which are usually electric furnaces or AC condenser units.

The home inspector can identify a 3 phase panel by the presence of three ungrounded bus bars instead of two. 3 phase breakers may or may not be present. The third bus bar also may or may not be connected. This happens sometimes when the electrical service to a residence has been demoted from 3 phase to single phase. If the inspector encounters this, he should be concerned with whether or not the size of the original 3 phase panel remains adequate for the loads present. The power available from a 3 phase panel with a given ampacity served by single phase power is much less than that of the same panel (same ampacity) served by 3 phase. Available currents remain unchanged.

The inspector should report the characteristics of the service supplied by the utility, whether it is 3 phase balanced neutral, 3 phase unbalanced neutral or single phase. He should also report the available voltages.

It is vitally important that the inspector report on an unbalanced 3 phase installation that the voltage on one certain leg of the panel board (identified by orange markings on the main service conductor) is 208V, not 120V. It IS possible to install a single pole breaker on the 208 bulbar. What’s important is that the load on this breaker be a 208V load, not 120V and that the breaker is rated for 208V service.

Marc

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Marc,

The term, "unbalanced neutral configuration" threw me a bit. Are you referring to a 4-wire delta connected system? Such a connection does provide for a "wild leg" that, by NEC rules, must be placed on the "B" phase in the panel and marked orange where accessible. Such transformer connections (4-wire delta) do exist where small single-phase loads are pulled from the panel. From my experience, and I'm no genius by any means, to truly balance an electrical system that provides virtually no current flow on the neutral is nearly impossible if I remember correctly. Thoughts?

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The term, "unbalanced neutral configuration" threw me a bit. Are you referring to a 4-wire delta connected system?

Yes. You can refer to an unbalanced neutral as a 4-wire delta as long as you make it clear that the neutral is provided by a center tap on one of the 240V transformers. Because this neutral is a longer 'graphical' distance to one line than it is to the other two, the voltage to neutral on that line is higher, 208V and can be a hazard if not properly identified as codes require. It can certainly be used if you have a load for it.

From my experience, and I'm no genius by any means, to truly balance an electrical system that provides virtually no current flow on the neutral is nearly impossible if I remember correctly. Thoughts?

Practically impossible, yes. 'Balanced neutral' means that the utility neutral is electrically equidistant to each of the three lines. The unbalanced delta or '4 wire delta' uses a transformer center tap so it is equidistance to two of the lines but not the third one. That's why it's called unbalanced delta. I should have pointed this out a little more clearly. Thanks.

I'm not a genius either, just happen to be educated on this topic.

Those were good questions.

Marc

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