Part 18(b)—Electrical Systems and Control, Part 2
By Ed Butts, PE, CPI
We provided an overview of the basic groups of electrical load management systems and a brief review of national electrical codes in the last installment of The Water Works. We will expand on this introduction this month and discuss common voltages and power supplies used in the United States and internationally.
Step 5. Electrical Application and Design Techniques
I have been privileged to instruct various groups of pump installers and water works personnel on basic electrical design throughout my career. From the beginning, I have always stressed that in my opinion there are three elements to engineering and design in the real world:
- What nature will allow you to do (i.e., the theoretical element)
- What regulatory agencies will allow you to do (i.e., the code element)
- What you or the client really want to do (i.e., the practical element).
We usually don’t have much control over elements 1 and 2, so it is up to us to make the most of what we can out of element 3. After all, this is where our reputations and paychecks are made or buried, where our customers learn to appreciate our talents or try to find someone else, and where we can perhaps be innovative or simply content to go along with the pack.
A good, forward-thinking designer will always strive to understand and learn from their past mistakes as well as their successes when embarking on a new or untried concept. Additionally, there is no single field of engineering growing faster and with more technological improvements than electrical and electronics engineering.
Here is just one example. Variable speed drives were fairly limited to the largest and most complex municipal and industrial pumping applications 20-25 years ago, but we are now using them in domestic and irrigation applications in increasing numbers today.
The list of examples could go on and is only limited by your imagination and perhaps your client’s budget. This is where the final step of electrical design comes in: the application and design techniques.
As an example, you often get to decide if you should design a feeder or branch circuit with a 3% or 5% voltage drop, or if you should use a VFD and electronic soft-starter, or simply another less expensive type of reduced voltage starter on the next job.
Although there are too many variables involved with local customs, code restrictions, and personal preferences in electrical application and design for me to cite my personal method of system designing, I can pass on a few general tips.
However, please heed a few words of caution first. Although I commonly use the following two tips in my design work, I warn you to apply them with prudent caution and make sure you are not violating any local code or safety requirement and that all conditions related to the tip apply and are effective before you attempt to use them.
Tip No. 1
The majority of new domestic and many irrigation and commercial/industrial water systems use submersible pumps and motors as the unit of choice. Because of this, it is vital for a system designer to be intimately familiar with them.
You need to know all there is to know about submersible motors—voltage tolerances, maximum and service factor amp values, motor cooling criteria and techniques, differences between manufacturers, motor code letters, fusing and circuit breaker sizing and differences, and maximum wire length considerations.
You should eventually become so knowledgeable about these you can tell someone off the top of your head the full load current and minimum wire size for a 1 hp submersible motor.
Tip No. 2
Another controversial topic in electrical design is the allowable or permitted voltage drop in a feeder or branch circuit conductor. This is one element of design that is usually at the total discretion of the system designer, providing he initially satisfies a few critical conditions (although some jurisdictions do limit the permitted voltage drop).
The NEC specifies the minimum size of conductor, based on the ampacity of the circuit. As previously stated, for a 1 hp, 230-volt, one-phase motor, we would need to use the NEC published value for the design of our branch circuit conductors (or 8 amps) although NEC Article 430 requires each branch circuit conductor to be designed to provide a minimum of 125% of the full load current—in this case, 8 amps × 1.25 =10 amps.
Referring to NEC Table 310.16, a no. 14-gauge, copper conductor with a total of no more than three current-carrying no. 14-gauge conductors grouped together in a common race-way or individual conductors directly buried is capable of 15 amps, easily satisfying the first requirement.
Once the NEC requirement is met, the designer can technically design the voltage drop for 3%, 5%, or even 10%. Thus, the question then becomes “Which value should I use?” Voltage drop of a circuit is addressed in the NEC through use of a Fine Print Note.
To ensure the proper level of performance under all conditions, the Fine Print Note in the NEC recommends limiting this to a conservative value of the maximum voltage drop to 3% in a circuit, while most motor manufacturers require the total voltage drop to be at or less than 5%.
There are even certain reasons you can design a circuit with up to a 10% voltage drop, based on the following rationale. Most single-phase electrical systems now supply a nominal power source of 120/240 volts. Since most single-phase motors are designed to operate with a nominal supply of 230 volts, this difference of 10 volts provides the designer with a built-in voltage safety factor of approximately 4% before he even starts the design.
This same concept often applies to three-phase electrical systems, such as a 230- or 460-volt motor on a 240- or 480-volt power supply, for example. In addition, since most motors will easily tolerate up to a 5% voltage drop while running, this means you could conceivably design a drop cable or offset wire for up to 9% to 10% voltage drop, right?
Although competitive situations may frequently tempt a system designer to undersize wire in the hopes of saving those few dollars needed to secure the job, he should resist the temptation for the following three reasons.
Reason 1. Most pumps and motors used in current water system designs are the submersible type. These motors are built with specific and limited design characteristics. Most notable is the fact the motor is always located at the end of a run of possibly thousands of feet of wire. Since the voltage drop in a circuit is cumulative, the longer the run, the higher the overall voltage drop. All functions of an electric motor are inherently tied to other functions. In other words, if you vary the voltage up or down, it will show up in other motor conditions such as starting torque, starting or running amperage, or motor efficiency.
Let’s consider a circuit for a single-phase AC power system where a 120-volt, 60 Hz, AC voltage source (120 VAC) is delivering 2 amps of power to a purely resistive load.
In this example, the current to the load would be 5 amps RMS, and the power dissipated at the load would be 600 watts or 120 VAC × 5 amps. Because this load is purely resistive (i.e., without reactance), the current is in phase with the voltage and the calculations will be the same as that obtained in an equivalent DC circuit. This means that power is always being dissipated by the resistive load and never returned to the source as it is with reactive loads (refer to Figure 1).
If the source were a mechanical generator, it would take 600 watts worth of mechanical energy or around three-fourths of a horsepower to turn the shaft. A resistive circuit is measured in ohms. On the other hand, a reactive load includes motors (inductance) and fluorescent light fixtures (capacitance) and is a load carried by an alternating current generating system in which the current and voltage are 90° out of phase (for a purely reactive load) measured in volt-amperes or kilovoltamperes.
For two quarters of each cycle, the product of voltage and current is positive, but for the other two quarters the product is negative. This of course indicates that on average exactly as much energy flows into the load as flows back out; thus, there is no net energy flow over each half cycle and only reactive power flows. This also means there is no net transfer of energy to the load, but electrical power does flow along the wires and returns by flowing in reverse along the same wires.
The current required for this reactive power flow dissipates energy in the line resistance even if the ideal load device consumes no energy itself. Practical loads have resistance as well as inductance or capacitance, so both active and reactive powers will flow to normal loads.
A reactive circuit is measured in impedance, which is a combined function of the offsetting values of capacitance (in farads or C) and induction (in henrys or L) along with basic circuit resistance with the final result also measured in ohms.
A circuit with both capacitive and inductive influence is referred to as an LC circuit. Reactance is the opposition to a change of electric voltage or current due to the inductance or capacitance present in a circuit. If the circuit contains inductors, then the reactance is inductive. Conversely, if the circuit contains capacitors, then the reactance is said to be capacitive.
Reactance is not constant and varies according to the frequency of the input frequency. Purely inductive circuits supply reactive power with the current waveform lagging the voltage waveform by 90° (see Figure 2), while purely capacitive circuits absorb reactive power with the current waveform leading the voltage waveform by 90° (see Figure 3).
The result of this is that equivalized capacitive and inductive circuit elements tend to cancel each other out. A reactive circuit in which the level of capacitance and induction cancels each other so that only circuit resistance remains is said to be a resonant or tuned circuit.
Unlike a resistive circuit, the impedance is a function of the applied frequency in a reactive load. As we change the frequency in hertz, capacitor and inductor changes its impedance to the frequency. In the case of voltage drop, too much loss of voltage at the motor, particularly during starting, will affect the motor’s torque, possibly to the point the motor cannot accelerate adequately to overcome the inertia of the pump impeller stack and opposing water head.
In this case, the motor may stall and rapidly burn out. Always remember that starting current is generally five to seven times the full load current value of a motor. A 1 hp submersible motor with a full-load amperage of 8.2 has a locked rotor, also known as stuck motor, starting inrush current of approximately 42 amps.
Disregarding power factor and other complex power issues, an instantaneous current value of 42 amps on a motor circuit already designed to allow 10% of voltage drop at 8 amps will incur an approximate voltage drop of around 120 volts, approximately 50% at the motor.
To maintain an adequate level of starting torque, most motor manufacturers recommend a maximum voltage drop during startup of 30%. Obviously, 50% is considerably more than 30%, and in this example, the motor may not even start. Conversely, by limiting the total voltage drop to 5% based on an electric power supply of 230 volts, the designer will ensure the total voltage drop at the motor during starting will not exceed 58 volts, or roughly 25%, ensuring there will be adequate voltage to start the motor and get it running.
As far as the difference between the 230-volt motor operating voltage and the 240-volt voltage provided from utilities, a smart designer will not use this buffer in the design. Rather, he will retain it as an additional safety factor to protect against possible voltage drop from the utility’s transformer or transmission/ feeder lines due to a high electrical load consumption on the system (common during summer months), weakened supply transformer, bad or weak connections, or within the facility itself.
In addition, the extra voltage will help stabilize the starting voltage and torque at the motor, which can be critical with sandy wells or systems with long offset runs or high operating heads.
To summarize: When calculating the voltage drop of an electrical circuit, I suggest you use the value of the nominal voltage supply (115 or 230 volts), verify that each conductor has a minimum size to handle the full load motor current with a 125% NEC and safety factor (not service factor current), and design the total combined length of the well drop cable and any offset wiring for a maximum of 5% voltage drop.
Reason 2. I have also observed numerous (too many) instances where customers decided to upsize their well pump to realize additional capacity, or the well had to be deepened due to a loss of water from nearby well use and draft.
In either case, a larger pump and motor horsepower will almost always be required. If the offset or well drop cable size was compromised during the initial installation, there will obviously be no latitude towards the use of the existing cable for a larger motor.
This can be hard to explain to a client after using their system for only a year or so following the initial installation. In many of these instances, I have resorted to using a boost transformer at the wellhead to increase the motor voltage back to permissible levels. Although increased heat will result in the conductors, this is generally allowed as long as the conductor size complies with the 125% ampacity requirement.
Reason 3. Trying to squeeze more electrical power through a given area of conductor will obviously result in higher heat in the wire. Higher heat in an electrical conductor results in lower efficiency, greater risk of overload and fire, and shorter life of the insulation.
Although the added heat may not be injurious, the effect from day-to-day usage can be cumulative upon the system, and in certain cases, the extra power costs to the customer from the added heat losses in the conductors can amount to more than the difference between the initial cost of using larger or smaller wire.
If you work with electrical design of any kind, always hold one thought paramount in your thinking: Heat kills electrical equipment. Thus, the greater the level of heat any electrical device must endure, the shorter the life it will have.
U.S. and International Voltages and Power Supplies
Alternating current (AC) voltages and power supplies vary throughout the world and in different countries. However, there are still two fundamental types of frequency and phases in use for all AC electrical systems. The following information summarizes this similarity and can all be classified by the following properties:
- Frequency: 50 Hertz or 60 Hertz (Hz)
- Number of phases: single-phase (1ɸ) or three-phase (3ɸ)
- Number of wires: 2, 3, or 4 (not counting the grounding wire)
- Neutral present:
–Wye (Y or W) connected systems possess a neutral
–Delta (Δ or D) connected systems typically do not use a neutral
- Voltage classes: (RE: ANSI C84.1-2016)
–Low Voltage: 1000VAC or less
–Medium Voltage: greater than 1000VAC and less than 100 kilovolts (kV)
–High Voltage: greater than 100 kV and equal to or less than 230 kV
–Extra-High Voltage: greater than 230 kV but less than 1000 kV
–Ultra-High Voltage: equal to or greater than 1000 kV
The graphics in Figure 4 illustrate the connections for the most common AC electrical systems in use. Each one is shown as the voltage with AC power, or VAC for short, and will be described in greater detail below.
Wye-Single Phase Secondary (Figure 4a): Also known as an Edison system, split-phase or center-tapped neutral. This is the most common residential service in North America for single-phase (1ɸ) loads. Line 1 to neutral or Line 2 to neutral are used to power 120VAC lighting and plug loads. Line 1 to Line 2 is used to power 240VAC-single phase loads such as a water heater, electric range, or most water pumps.
Wye-Three Phase Secondary (Figure 4b): The most common commercial building and industrial electrical service in North America is 120/208VAC Wye, which is used to power 120VAC plug loads, lighting, and smaller HVAC systems. This connection type is also used to power the majority of three-phase water well and booster pumps. In larger facilities, the voltage is 277/480VAC and used to power single-phase 277VAC lighting and larger HVAC loads. In western Canada, 347/600VAC is common.
Delta Secondary (Figure 4c): Used primarily in industrial facilities to provide power for three-phase motor loads and in utility power distribution applications. Nominal service voltages of 240VAC, 400VAC, 480VAC, 575VAC, 600VAC, and higher are typical.
Corner Grounded Delta Secondary (Figure 4d): Used to reduce wiring costs using a service cable with only two insulated conductors rather than the three insulated conductors used in a conventional three-phase service.
High Leg Delta Secondary (Figure 4e): Also known as a wild-leg delta system. This connection is used in older manufacturing facilities and some irrigation pump services with mostly three-phase motor loads and some 120VAC single-phase lighting and plug loads (like the Delta secondary but with a centertap on one of the transformer windings to create a neutral for 120 VAC single-phase loads). Motors are connected to Phases A, B, and C, while single-phase loads are connected to either Phase A or C and to neutral. Phase B, the high or wild leg, is not used for 1ɸ loads as the voltage to neutral is 208VAC.
The information in Table 1 outlines the available power supplies used in most of the world.
In addition to connection types, many people simply reference electrical systems as two-, three-, or four-wire. Figure 5 illustrates the most common electrical systems in two-, three-, and fourwire configurations to coincide with Figure 4 and the information shown in Table 1 for single- and three-phase systems.
This wraps up this edition of The Water Works. In the next column, planned for April, we will continue this series on electrical system design for pumping systems with an overview of the four groups of electrical management equipment and devices and motor controllers.
Until next time, keep them pumping!
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at firstname.lastname@example.org.