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ELECTRICAL SYSTEMS & FORMULAS

ELECTRICAL SYSTEMS & FORMULAS Previously, the discussion covered the introduction of electrical design, from the generation of the power through its distribution to a residential consumer.

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ELECTRICAL SYSTEMS & FORMULAS

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  1. ELECTRICAL SYSTEMS & FORMULAS Previously, the discussion covered the introduction of electrical design, from the generation of the power through its distribution to a residential consumer. The basics of service of 240 maximum volts provided the necessary requirements of electrical service to operate such things as lights, convenience outlets, small motors such as the furnace fan, and larger devices found in a residence such as water heaters, automatic clothes dryers, cooking ranges, and air conditioning compressors. Rarely do residences have such equipment that have power requirements that exceed the basic single phase 240 volt system. So much for residences, but they constitute a small part of the service requirements for our present day life style.

  2. Commercial applications require the big stuff. Such as a motor that will propel a loaded elevator car upward to 100 stories at a speed of 1000 feet per minute. Or consider the electrical requirements of a mechanical system that provides constant environmental comfort to a building the size of Sears tower in Chicago. But then consider the grocery store where you buy your food. While the system is not large enough to supply a city the size of Lubbock, the facility does require sufficient energy to store frozen food for several days at a time, or to maintain a sufficient level of lighting over 20,000 square feet of merchandise. All the while providing environmental comfort to the facility for its customers. The same system that works for a residence is not sufficient for general commercial applications – or even meet the power needs to sustain this school classroom. So a system of electrical power is available to service the bigger stuff.

  3. Actually two types of service are available, and they come in two sizes; And considering size, realize that in electrical service for buildings there are single phase systems and three phase systems. Two phase electrical service is not utilized in buildings. It is easy to think of three basic types of outlets, and three formulas associated with them - - - and to think that well, formula 1, outlet type one is single phase. formula 2, outlet type two is two phase. formula 3, outlet type three is three phase. THAT IS ABSOLUTELY NOT THE CASE !

  4. Consider the two types of electrical service. The system spoken of previously is called a “DELTA” system. You may remember the very elementary example of the electrical generator where three coils of wire are arranged to form a tube, with a magnetized shaft that turns inside the tube to create electrical flux. You also may remember the cylindrical container spoken of on the electrical pole in your alley, that is used to reduce the 20,000 or so volts down to 240 volts for use in a simple residence. In a delta system, if you remove the lid of the transformer and see what is inside, you will find three coil windings illustrated by the following diagram.

  5. There are 3 wire coil windings inside, connected in the form the form of a triangle, as the Greek letter DELTA, A wire extends from each connection point to the place that requires service. Since there must be a neutral, or ground wire, it is simply connected to one of the coils, which then extends with the other three phase wires. Voltage between any two phase wires is 240. Voltage between any phase wire and the neutral is 120. But connection of all three phase wires and the neutral will result in 415.69 volts.

  6. The other type is called a “WYE” system, also called because of the arrangement of the coils in the transformer. Remove the lid and find three wire coils connected as in the following diagram. The electrical service in both the wye and the delta configuration, each which contain 3 phase wires and a neutral, are THREE PHASE SYSTEMS. ALL THREE PHASE SYSTEMS HAVE THREE PHASE WIRES AND A NEUTRAL. Anything less is not three phase. That is why the 240 volt service to the residence with 3 wires, two phase and a neutral, is a SINGLE PHASE SYSTEM.

  7. There are 3 wire coil windings inside, connected in the form the form of the letter “Y” hence the name. A wire extends from each connection point to the place that requires service. The neutral, or ground wire is connected to the center, common coil connection, which then extends with the other three phase wires. Voltage between any two phase wires is 208. Voltage between any phase wire and the neutral is 120. But connection of all three phase wires and the neutral will result in 360 volts.

  8. So now consider the formulas: the first two are from the previous discussion. Formula One: one phase wire (poles) , one neutral wire. single phase amperes = watts / volts here volts is always 120. Formula Two: two phase wires (poles) , one neutral wire single phase amperes = watts / volts here volts is 240 for delta, and 208 for wye. Formula Three: three phase wires (poles), one neutral wire three phase amperes = watts / √3 x volts here volts is 240 for delta and 208 for wye.

  9. In considering formula three, the bottom amounts to the square root of 3, which equals 1.732, multiplied by the appropriate voltage. 1.732 x 240 = 415.69 1.732 x 208 = 360 So in the case of Delta: amperes = watts / 415.69 And for wye: amperes = watts / 360 For the designer, a building will have either a wye system or a delta system, not both. The designer chooses, based on the need and what is available.

  10. So, what is all this good for? Remember the design of an electrical system requires the connection of all the necessary lighting, convenience outlets, and special devices required for the building function, and in a manner that is in compliance with all applicable building codes, specifically the National Electric Code. So what is involved? All the things that are between the electrical service wires and the outlets . . . Namely the proper size of circuit the size of wires required to carry the amperage the size of conduit (pipe) in which the wires are placed the size of the circuit breaker required for safety.

  11. Since electrical current is transferred from service to devices by wires, and in commercial applications, wire must be placed inside a conduit or pipe, then data must be available to determine the requirements. In your packet is a chart that shows electrical wire, electrical conduit, and requirements for ground wires;

  12. Also in your packet is a page with two charts: SINGLE PHASE MOTORS THREE PHASE MOTORS Realize that standard voltage has not been the same designation back through electrical history. Many charts that have been carried over time after time did not change with the modernization of generated voltage – especially the delta system, which is the one Thomas Edison used in the 1800’s. Often you will see 110v, or 115v, which is now 120v. You will also see 210v, 215v, or 230v, which is really 240v. The motor charts are two that show discrepancy from what you have been previously told, but are still useful in determining the amperage of motors.

  13. So why can’t you simply use the formulas to determine the amperage of motors, especially since all of you know that one horsepower is rated at 746 watts ? ? You remember you were told in the first introduction to ignore p.f. which is the abbreviation for “power factor.” Power factor deals with the inefficiency of electrical gadgets. Motors fit into this category. In the windings of the wires is a great deal of static trash – the armature of a motor is heavy, and takes a certain amount of power to get it started turning, and a certain amount of power to keep it turning. Power factor is taken into account in the motor charts. If it were not, look at a single phase, 1 horsepower motor and see the amperage is 8. So amps = watts/volts x p.f. p.f. = 746 / 230 x 8 = .40 which means a 1 hp motor is about 60% efficient.

  14. CHARTS TO DETERMINE THE AMPACITY OF MOTORS

  15. The motors chart is useful when items of specification rate motors in horsepower and phase, rather than in running amps. Amperes for all outlets must be known, because wire size and breaker size is based on their ampere carrying capacity. Designing circuitry for lighting and convenience outlets remains simple, since the NEC specifically states that these circuits be limited to 20 amperes. There are, however, lighting circuits that are exempt from that limit because of their separation from common public access. These include systems for parking lot and street lighting, lighting in sports arenas, and specialty type lighting for display and theatrics. And in most cases, these types of lighting circuits are not limited to 120 volts.

  16. EXAMPLE PROBLEMS involving electrical connections. ONE: 120/240 v delta • 1. 6 lighting circuits, each with connected load of 1800 watts, single pole. For each circuit find: • Quantity of wires_________________ • Amperage of each circuit__________ • Circuit breaker size in amps________ • Size of wires (awg)_______________ • 2. One electric range outlet, 2 pole, with total load of 9,600 watts. Find: • Amperage of circuit_______________ • Quantity of wires required__________ • Size of wires required_____________ • Circuit breaker size in amps________

  17. 3. Two pole outlet for water heater with heat element load of 27 amperes. Find: • Amperage of circuit_______________ • Quantity of wires required__________ • Size of wires required_____________ • Circuit breaker size in amps________ • 4. A single phase 2 pole attic fan motor rated at 2 horsepower. Find: • Amperage of circuit_______________ • Size of wires ____________________ • Quantity of wires required__________ • Circuit breaker size in amps________

  18. PROBLEM B • A commercial building has a 120/240 volt, 3 phase, delta electrical system with the following outlets: • 1. A 3 pole A/C compressor rated at 33 amperes. Find: • Amperage for the circuit___________ • Quantity of wires required__________ • Size of wires required_____________ • Circuit breaker size in amps________ • Size of conduit required___________ • 2. A 2 pole electric heater element with a connected load of 5600 watts. Find: • Amperage for the circuit___________ • Quantity of wires required__________ • Size of wires required_____________ • Circuit breaker size in amps________ • Size of conduit required___________

  19. 3. If the total load for the electrical panel from all connected circuits amounts to 86,000 watts, find: • Amperage for the panel____________ • Quantity of wires required__________ • Size of wires required_____________ • Circuit breaker size in amps________ • Size of conduit required___________ • Size of ground wire_______________

  20. END OF DAY 14

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