Tag: voltage

Keep in mind when reading ANY article about electrical theory or application is that it only scratches the surface of the topic. You can dedicate years of your life to understanding electrical theory and design the way many engineers do and still know just enough to be dangerous.  In HVAC we rarely need to have a DEEP understanding of electrical design but there are a few cases where a little understanding can go a long way to identifying issues before they cause trouble and that is the intent of this short article.


What is Three Phase Power?

Power is generated at the utility in three phases that are 120 degrees out of phase with one another at 60hz (Hertz). This simply means that 60 times per second each individual leg of power makes one peak and valley (a full circle), and all three of the phases together split the cycle into thirds (trisect).

This video is the best visual demonstration I have seen of three-phase power and how it works.

What Does an HVAC Tech Need to Know About Three Phase Power?

Three-phase motors don’t’ require a run capacitor because the 120-degree phase difference is ideal for efficiently spinning a motor so a “start” winding and phase shifting capacitor isn’t needed.

The biggest concern for the techs and installers with three phase is getting the phasing correct so that motors run in the correct direction. While this doesn’t matter for reciprocating compressors it is important for condenser fans and blowers and it is absolutely CRITICAL for scroll and screw compressors. Changing the direction of rotation is just as simple as swapping any two phases.

Keep in mind when installing replacement parts and equipment that if you keep the phases connected in the same way you will generally be in good shape. It is still a good practice to use a phase rotation indicator like the one above to confirm proper rotation. In most cases, clockwise phase rotation is what you are looking for but I’m sure there are exceptions to that. Alternatively, you can disconnect the compressor that could be damaged by improper phasing and start up the blower to see if it runs the correct direction before bringing the compressor(s) online. One caveat is that when motors use a VFD (variable frequency drive) the phase rotation will automatically correct making them an unreliable test in those cases.

Balancing Phases

Electricians are responsible for balancing the amperage of single-phase loads (both 120v single leg and 208v two leg loads typical on a wye three-phase system) both so the neutral doesn’t carry high amperage on the 120v loads and so the one leg of power doesn’t carry significantly more or less load than the other two. As the amperage load on a particular phase goes up, there is more opportunity for voltage drop depending on the size of the load, size of the transformer and service feeding the space and well as wire size and connection quality. This can become a challenge when there is a mix of single phase outlets, 208v appliances, and three-phase equipment.

Let’s say someone connects a bunch of space heaters on phase A, as well as a few smaller HVAC systems between phases A and B and almost nothing on phase C. If you have a large RTU that uses three-phases phase C will tend to have less load and therefore higher voltage while the load on phases A and B will fluctuate based on when the smaller systems and space heaters go on and off.

This can cause overheating of conductors and damage but it can also cause voltage imbalance which is a real cause for concern for an HVAC technician.

3 Phase Voltage Imbalance

Voltage imbalance is a motor killer. It causes poor motor performance and increased winding heat which leads to premature failure. In the case of HVAC blowers and compressors, this additional heat ends up in either the refrigerant or the air which must then be removed, further decreasing efficiency.

To test for three-phase imbalance always check from phase to phase not from phase to ground. You simply check the voltage from each of the three phases to one another and find the average (add all three and divide by three). Then compare the reading that furthest from the average and find the % of deviation. For most of you I know that sounds like a giant pain so we made this easy calculator for you.

The US Department of Energy recommends that the voltage imbalance be no more than 1% while other industry sources say up to 4% is acceptable. In general, you will want to make SURE the imbalance is below 4% and work to rectify anything over 1%.

What Can I Do About It?

You want to first look for the obvious. Melted wires, loose terminals and lugs, undersized wire, pitted contacts, poor disconnect fuse contact etc… Obviously, if you aren’t licensed or allowed to open a panel you won’t always be able to fully rectify the issue yourself but you can go a long way towards the diagnosis.

When checking voltage it is generally best to do it with the system running as close to the motor you are checking as possible. This is the actual voltage the motor is “seeing” and is what matters to the operation of the motor. You can then test back towards the distribution point, if you see a big increase in voltage as you test back towards the source you know you found a voltage drop and a cause or contributor to the issue.

From there the issues of amperage load imbalance in the panel, service size and utility issues must be considered once all the basics are covered. Most of all, if the imbalance is severe (over 4%) you don’t want to leave your motors running or you risk damage and expensive repairs.

— Bryan

Disclaimer: Ghost voltage is a term used by techs to explain a phenomenon where they measure voltage they don’t expect or when the voltage they see doesn’t do the work they expect. More advanced techs know how to use the Lo-Z (Low impedance) mode on their volt meter if it has one to help eliminate this. The vast majority of what techs call “ghost voltage” is just a circuit with high voltage drop under load and not stray inductance from other conductors. I write this first so that more experienced techs understand the context of this article. 


This article serves two purposes. First, it is an article for technicians who have heard of the dreaded “ghost” voltage but never understood why it happens. Second, for my own apprentices and techs who I stumped this morning with a diagnosis problem that involved “ghost” voltage that they were unable to diagnose.

If they read my tech tips they will get the answer… sneaky right?

 So what is meant by ghost voltage?

In some cases, you will be diagnosing an electrical issue, usually controls / low voltage issue. You will be measuring potential on a circuit and then when the circuit is connected to the load the voltage will disappear … like a “ghost”.

For example, you make be measuring 24v at a condensing unit on the “Y” contactor circuit when the conductor (wire) is disconnected, but as soon as you connect it to the contactor/control board the voltage “disappears” when measured across the load (across the contactor coil) or more simply from Y to C.

In other cases the voltage may not disappear completely, it may just drop way down, or in other cases the contactor may chatter, circuit board lights dim etc…

I have heard all of these situations called “ghost” Voltage, but they are actually just voltage drop and these symptoms are caused by additional resistance in the circuit OTHER than the designed load.

Quick Note: there are also “induced” voltages that can appear as ghost voltage due to conductors running in parallel with other current carrying conductors. This is more common in Commercial and industrial applications where many wires are bundled or in close proximity over long distances. These charges are usually small and often “disappear” under load.

Rarely do we want more than one electrical load (resistance point) in a single circuit. When this does occur it is usually undesigned and caused by of long wire lengths, improperly sized wire and poor connections.

Now to CLARIFY, when referring to a circuit we mean one complete path between electrically different points (say L1 and L2 in single phase 240 or 24v hot to 24v common on a control transformer). Some think of parallel circuits as a single circuit, but while they may share conductors they have an individual load path.

To cut to the chase, whenever wire is undersized, runs of wire are too long or the circuit contains poor connections there will be additional resistance introduced to the circuit. When there is more resistance added in places other than the load (in this case a contactor coil) there will be a voltage drop and therefore the voltage applied to the load will be decreased. When a wire isn’t connected to the load this drop will be invisible because the load isn’t in the circuit and therefore you are simply reading across the OTHER, unintended load (resistance) which will often be the full voltage depending upon the exact issue and when you are making the measurement.

In every complete and independent circuit, including a series circuit, the amperage is the same no matter where in the circuit you measure it. Before the load, between loads, after the loads… it doesn’t matter. The amperage is dictated by the total applied voltage and the resistance (or more accurately the impedance) of the entire circuit.

The voltage applied to each load is dependent on the resistance of the load in comparison to the total resistance of the circuit. In the example below, you can see that the amperage is the same on each load and is dictated to be 500 micro-amps because the total circuit ohms is 18,000.

The voltage drop of each load in series is equal to it’s percentage of the total circuit resistance. Since  load R1 is 16.5% of the total resistance in the circuit, the voltage drop across R1 is 1.5V because 1.5 is 16.5% (0.165) of 9V.

There are a few other factors that make the trouble with voltage drop worse. Let’s say you use an undersized wire to feed a lightbulb, an undersized wire means that the conductor has a lower ampacity (amp capacity) than it should have. Once the circuit is energized the wire will begin to heat up, as it heats up the molecules in the wire begin moving faster which increases the resistance of the wire. The greater the resistance of the wire the greater the voltage drop across the wire resulting in a hot, dangerous wire, increased voltage drop at the bulb, less light from the bulb and decreased circuit amperage (less total work being accomplished).

In the case of many loads including inductive (magnetic) loads like a compressor contactor, the resistance in the coil isn’t just resistance you can measure with the contactor de-energized. This resistance that is created within an electromagnet once it is energized is called “inductive reactance” and it is measured in ohms of impedance. In order for the contactor coil to properly engage it requires the correct applied voltage and without the properly applied voltage, the resistance of the coil remains low. The crudely drawn diagram below (I’m no artist) shows a contactor coil circuit with no issues and a 0.5 amp  current at 48 ohms

When you add in a 200 ohm “bad connection” or any other type of resistance, not only does it create huge voltage drop, it also drops the impedance of the contactor coil itself with the result being a very low applied voltage (3.13V) on the contactor coil with it connected and under load. Under these conditions, the contactor won’t try to pull in at all. Under less extreme conditions it may chatter or become noisy.

Now, this is a hypothetical situation, but you will notice that the poor connection is AFTER the contactor coil in what we call the common circuit in 24v controls. It doesn’t matter WHERE in the circuit resistance is added, whether before the switch (in this case a thermostat) in the line side or after the switch on the load side. It could even be in common or in the switch itself.

Anytime additional resistance is added to a circuit it results in voltage drop when the circuit is intact. When we disconnect wires to test voltage or test voltage with a circuit that has an open switch we can create confusion and observe “ghost” voltage. In reality it is simply extreme voltage drop caused by additional resistance in series with the load.

— Bryan

Have you ever noticed a blower motor rated for 120V draws about twice the amperage of  the same horsepower motor rated at 240V?

This is because motors are rated in Watts or Horsepower and according to Watts law Watts = Volts x Amps.

In order to keep the Wattage output the same at 120V, it draws twice as much current.

This is different than what happens when you drop the voltage of a motor below its rating.

Here is an experiment I did.

I took a regular 1/6 HP 208 – 230v condenser fan motor and tested it under normal conditions at my office and here is what I got


I then connected the common wire to neutral instead of L1 power which leads to approximately 120v applied and here is what I got.


By dropping the voltage by around 50% the amperage dropped slightly, the wattage went to less than half and the power factor also went in half and the motor slowed way down.

The motor slowing down is due to slip in the motor, meaning that the motor is running significantly slower than the speed it is designed for.

This means that not only is the motor running inefficiently, but it is also going to get hot because as the motor runs slower it has lower inductive reactance (the magnetic resistance in the windings). As the inductive reactance drops the windings have lower resistance and thus get hotter.

Even after all of this, the motor still consumes less than half the watts.

Rubber meets the road is that when a motor is designed for lower voltage it will draw more amperage to do the same work becasue it is designed to hit a wattage (horsepower) target at a designed voltage.

When you apply lower voltage you both decrease the work done as well as the efficiency and life of the motor because more of the energy goes to heat instead of mechanical work as the motor slips more and more. You also see higher power factor as the motor begins to slip resulting in even worse power efficiency.

This is one reason why voltage drop is a such an important thing to consider when sizing conductors and why 208-230V units are slightly derated or n both capacity and efficiency when installed on 208v.

Pay attention to Voltage, it can save a lot of money over time in both power efficiency and motor longevity.

— Bryan

Testo 760 Category IV Multimeter

I was standing at a booth at the HVAC Excellence Educators conference and an instructor walks up, grabs a meter and asks me “what’s the difference between a category 3 and a category 4 meter”?

Well, I really wasn’t sure other than that the category 4 is rated for more demanding conditions. So I did some research and dug into IEC 61010-1 and found that category 3 is rated for most uses OTHER than outdoor utility connections and category 4 meters are rated for all uses.

Courtesy of Fluke

There are also some voltage considerations and limitations to the different categories but the primary difference is not the regular duty but the high voltage transients. High voltage transients are often called “surges” or spikes and are most likely when working on outdoor transformers and distribution panels.

Rubber meets the road is that for HVAC use a category 3 meter is likely going to do the job but if you ever work in main panels, or outdoor transformers go for a cat 4 meter.

— Bryan

PS – Fluke has a great info sheet on this HERE

You can see more about the Testo 760 shown HERE

When testing a run capacitor many techs pull the leads off and use the capacitance setting on their meter to test the capacitor. On a system that is not running there isn’t anything wrong with this test, but when you are CONSTANTLY checking capacitors as a matter of regular testing and maintenance that extra step of pulling the connectors off can be time consuming and in these cases it is also totally unnecessary. Testing the capacitors UNDER LOAD (while running) is a great way to confirm that the capacitor is doing it’s job under real load conditions which is also more accurate than taking the reading with the unit off.

First, if you are used to doing capacitor checks during the “cleaning” stage of a PM you are going to need to change your practices and do your tests during the “testing” phase. These readings will be made at the same time you are taking other amperage and voltage readings during the run test.

This method is a practical method and is a composite of two different test practices combined –

  1. Read your Volt (EMF) and Amp (Current) readings like usual and note your readings.
  2. Measure the amperage of just the start wire (wiring connecting to the start winding), this will be the wire between your capacitor and the compressor. In the case of 4 wire motors it will usually be the brown wire NOT the brown with white stripe. Note your amperage on this wire..
  3. Measure the voltage between the two capacitor terminals, for the compressor that would be between HERM and C, for the cond fan motor that would be between FAN and C. Note the voltage readings
  4. Now take the amp reading you took on the start wire (wire from the capacitor) and multiply by 2,652 (some say 2650 but 2652 is slightly more accurate) then divide that total by the capacitor volts you measured.  the simple formula is Start Winding Amps X 2,652 ÷  Capacitor Voltage = Microfarads
  5. Read the nameplate MFD on the capacitors and compare to your actual readings. Most capacitors allow for a 6%+/- tolerance, if outside of that range then replacement of the capacitor may be recommended. Always double check your math before you quote a customer. We need to make sure we are accurate when advising a repair.
  6.  Repeat this process on all of the run capacitors and you will have assurance whether they are fully functional under load or not.
  7. Keep in mind that the capacitor installed may not be the CORRECT capacitor. The motor or compressor may have been replaced or someone may have put in the wrong size. When in doubt refer to the data plate or specs on the specfic motor or compressor.

If you need a visual, here are some good videos on the topic. Note that some will use 2650, some 2652 and some 2653. It all depends on how many decimals of pi they are using in their calculation but all of them will result in an accurate enough conclusion for our use.

At first doing it this way may take a few minutes longer but in the long run you will go quicker, have fewer mistakes (forgetting to put the terminals back), have a better understanding of how the equipment is operating and get a more accurate reading.

Once you replace a capacitor always recheck your readings to ensure the new capacitor reads correctly under load.

It is also a good practice to check Capacitors you have removed with your capacitance setting on your meter as a reference point.

While this method is good, it is only as good as your tools and your math. When in doubt, double check… and always be in doubt.

— Bryan

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