PAUL HAY Capital Projects

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Electricity: Definition, Units, Sources, Alternating Current, Codes & Installation

Author:          Paul Hay

e-mail:            paul.hay@phcjam.com

profile:           www.linkedin.com/in/phcjam

1.0  DEFINITION OF ELECTRICITY

1.1_      Electricity is the flow of electrons that results from the creation of a greater positive charge at one end of a conductor than exists at the other end: it exists naturally as lightning, other static discharges, or galvanic action which results in corrosion.

1.2       An Electrical Circuit requires an uninterrupted conductive path from a power source to an electrical load and another from the load back to the power source, thus closing the loop from the power source to the load:

1.2.1   A conductor is a metal wire, typically copper, that can conduct electricity from a power source to a load, and vice versa;

1.2.2   A load is a device that uses electricity to operate;

1.2.3   Ground is the conductor between the circuit and the earth.

1.3       Electricity can be either Direct Current [DC] or Alternating Current [AC]:

1.3.1   Direct Current [DC] flows in one direction, from positive to negative poles of a power source;

1.3.2   Alternating Current [AC] reverses direction of its flow at regular intervals: standard current from a local electrical utility changes direction 50 cycles per second (60 Hz within the U.S.A.).

2.0   UNITS OF ELECTRICITY

2.1       Voltage [V] a measure of electromotive force between two wires on a circuit:

2.1.1   It is measured in volts [V] with use of a voltmeter; and

2.1.2   It is analogous to pressure in an hydraulic or pneumatic system.

2.2       Wattage [W] is the power of an electrical load:

2.2.1   It is measured in watts [W] with use of a wattmeter;

2.2.2   Watt-hours is a measure of a Watt of power consumed during an hour.

2.3    Current or Amperage [I] is the number of electrons flowing through a given point on a conductor:

2.3.1   Amperage is measured in amperes with use of an ammeter; and

2.3.2   Amperage can be calculated by dividing the total watts of an electrical load by its voltage.

i.e.       I =        W/V                                                                                                         [2.1]

2.4      Resistance [R] is a measure of a conductor=s ability to conduct electricity: it is measured in Ohms [Ω] with use of an ohmmeter.

2.5       Ohm's Law states that voltage is the product of current and resistance:

i.e.       V =      I x R                                                                                                         [2.2]

2.6       Power Factor [PF] is the ratio of true power to apparent power (0<PF<1):

2.6.1   Current through resistors have PF = 1;

2.6.2   Power is quoted in Volt-ampere if PF is not used.

3.0   SOURCES OF ELECTRICITY

3.1       DC power is generated from batteries, DC generators, or produced by rectifiers from AC:

3.1.1   Primary batteries are typically not rechargeable; and

3.1.2   Secondary batteries (e.g. lead-acid types) can be recharged repeatedly;

3.1.3   Elevator controls and telephone equipment use DC power.

3.2    AC power is generated by an AC generator (called an alternator) or produced from an inverter:

3.2.1   Electrical energy has to be used as soon as it is generated because it cannot be stored;

3.2.2   Electricity used in buildings are generated at a power plant and transmitted to the site at high voltage through insulated transmission lines.

4.0   TRANSFORMERS    

4.1       A transformer is a device used to increase or decrease AC voltage:

4.1.1   It is comprised of primary and secondary windings about an iron core;

4.1.2   An efficient transformer wastes approximately 2% of power flowing through it.

4.2     Step-up transformers increase the output voltage (e.g. they increase voltage from a power plant to transmission lines);

4.3      Step-down transformers decrease the output voltage (e.g. they reduce voltage from transmission lines at sub-stations or at transmission poles).

5.0   AC PHASES

5.1       AC systems operate in phases:

5.1.1   Residences typically use single-phase systems;

5.1.2   Commercial and Industrial buildings typically use single or 3-phase systems.

5.2       Single phase power consists of voltage and current that vary at the same frequency.

5.3       3-phase power is equivalent to three connected single-phase systems:

5.3.1   Utility companies produce only 3-phase power;

5.3.2   Single-phase output can be derived from 3-phase input, but not the converse.

Figure 1: Temporary Job-Site Power

[source - JLC Field Guide to residential construction, vol. 2]

6.0   CODES

6.1       The National Electrical Code [NEC] is the U.S. electrical standard, which is updated every three years.

6.2       Most local codes are based on the NEC standard: local authorities having code jurisdiction include governments and utility companies.

6.3       The Underwriters Laboratory [UL] certifies that devices submitted for testing have passed standardized safety tests.

7.0   AC CONNECTIONS

7.1       Single-phase connections use 2 - 3 wires:

7.1.1   Either 110V or 220V supply can be obtained from 3-wires;

7.1.2   220V is provided for electrical water heaters, stoves and air-conditioners;

7.1.3   One lead from the centre tap of the transformer is the neutral wire in a 3-wire system;

7.1.4   The neutral wire must be grounded.

7.2       3-phase connections use 3 - 4 wires:

7.2.1   3-phase transformers have 3 No. single-phase secondary windings connected;

7.2.2   Connections are either delta or star/wye format.

7.2.3   Delta connections are usually made when load is mainly power:

7.2.3.1            3 No. terminals connected together provide 220V or 440V output.

7.2.3.2            One of the secondary windings may be centre-tapped and grounded for a 4-wire system providing both single and 3-phase power.

7.2.4   Star/Wye connections are used when load is mainly lighting:

7.2.4.1            One lead of each secondary winding is connected in common and grounded;

7.2.4.2            Voltage from line-to-line is 1.73 times the line-to-neutral voltage.

8.0   SERVICES

8.1      Service Drop refers to overhead service conductors that carry power from poles, or terminals of pole-mounted transformer, to the building.

8.2       Service-drop conductors are spliced to service-entrance conductors outside the building.

8.3       A Pothead is a water-tight weather head through which the service entrance conductors pass to bring power into the building.

8.4       Service Equipment include billing meters, circuit breakers and accessories located near the service entrance.

Figure 2: Main Distribution Panel

[source - JLC Field Guide to residential construction, vol. 2]

8.5       A panel box or switchboard is the termination of the service equipment:

8.5.1   A Panel box is an enclosed cabinet containing circuit breakers which serve as a distribution point of branch circuits to a floor or part of a floor.

8.5.2   A Switchboard is used in large installations:

8.5.2.1            It is located in special rooms large enough to permit services access around the equipment;

8.5.2.2            Equipment include transformers, circuit breakers and distribution panels;

8.5.3   Where loads exceed 250 kVA, independent feeders and check metering need to be provided for (a) lighting and receptacles, (b) VAC systems and equipment, (c) service water heaters, elevators, and other equipment over 20 kW, and (d) computer rooms with special power requirements.

8.6      Each live wire is connected to a circuit breaker and the ground cable connected to an electrode driven into the earth:

8.6.1   A Circuit Breaker disconnects electrical current automatically when an overload occurs and must be reset manually;

8.6.2   Breaker size is specified to match the capacity of the conductor.

8.7       There are four residential branch circuit loads:

8.7.1   Lighting and General-use circuit loads involve current flowing to most lights and wall receptacles: NEC recommends 30 watts/m² of finished floor area;

8.7.2   Small-appliance circuit loads involve current flowing to fixed small appliances (e.g. in Kitchen/Dining room and Laundry) not to be served by general-use circuits:

8.7.2.1            NEC requires a minimum of two 1,500W circuits in the Kitchen for dishwasher, microwave, etc.;

8.7.2.2            NEC requires a 1,500W circuit in Laundry for small appliance (excluding dryer).

8.7.3   Dedicated circuit loads involve one circuit dedicated for each major appliance (e.g. refrigerator, electric water heater, etc.

8.7.4   Ventilation and Air-Conditioning [VAC] loads involves the cooling system, fans, etc.

8.8       Derated Load is the computed load of an electrical circuit or group of circuits after it has been adjusted for loads which will not be used at the same time:

8.8.1   VAC loads are excepted from the general loads;

8.8.2   It assumes all lights and appliances will not be used concurrently;

8.8.3   For residences, NEC requires that the derated load be calculated as the first 10,000 W of general load and 40% of remaining general load.

8.9       Emergency Power may be provided by either manual or automatic transfer:

8.9.1   The generator is sized to carry the required emergency load;

8.9.2   Generators require provision for its fuel storage, exhaust fumes, noise and vibration.

Figure 3: Distribution Sub-panel

[source - JLC Field Guide to residential construction, vol. 2]

9.0   WIRING

Amp	Metric Wire size (mm dia)		AWG	CONDUIT DIAMETER (mm)
	Solid	Stranded		1-wire	2-wire	3-wire	4-wire
15	1.6	-	14	12	12	12	12
20	2.0	-	12	12	12	12	12
30	2.6	-	10	12	12	20	20
40	3.3	-	8	12	20	20	20
60	4.1	4.7	6	12	20	25	25
70	5.2	5.9	4	12	25	25	30

Table 1: Wire & Conduit Sizes

 [source - Architectural Handbook]

9.1       Wires are sized to carry more current than load:

9.1.1   Long-period loads should be limited to 80% of the conductor load rating;

9.1.2   Voltage drop due to wire resistance should not exceed -5% to minimize dimming of lights when additional loads are applied;

9.1.3   Loads should be less than two-thirds of rated conductor load rating to allow for growth.

9.2       Conduits may be installed to receive insulated electrical wires, or wires may be surface-mounted with straps, or "flex" may be used.

9.3       All connections (e.g. switches, receptacles, lights and junctions) must be made within code-approved boxes.

9.4       Conduits for electrical power should be independent of conduits used for transmission of voice or data.

10.0 LIGHTING

10.1    PF = 1.0 for incandescent lamps.

10.2    0.85<PF<0.95 for fluorescent lamps.

10.3    Switches interrupt hot leads to control lamps:

10.3.1 Multiple-way switches are used when a room has more than one entrance.

10.3.2 Dimmers or rheostats are variable resistors in a lighting circuit that control the current in order to dim the lamps. 

11.0 MOTORS

11.1    Small load motors use single-phase AC.

11.2    Elevators use DC motors with adjustable speed control.

11.3    A contactor is a magnetically operated switch.

11.4    A starter is a contactor used to control a motor and includes overload relays or thermal cutout to ensure that motors do not overheat.

12.0 RECEPTACLES

12.1    A Receptacle is a wall outlet to provide power for small electrical devices.

12.2    3-wire receptacles are required for new buildings.

12.3    Receptacles should be located at intervals no greater than 3.66 m apart.

Figure 4: NEC requirements for location of receptacles

[source - JLC Field Guide to residential construction, vol. 2]

Figure 5: Electrical Symbols 

[source - JLC Field Guide to residential construction, vol. 2]

                                           _______________________________

FURTHER READING

Architectural Handbook, Alfred M. Kemper, John Wiley & Sons Inc., USA, 1979;

            Mechanical and Electrical Equipment for Buildings, 8^th edition, Benjamin Stein, John S. Reynolds, John Wiley & Sons Inc., USA, 1992

Construction Materials & Processes, Don G. Watson, McGrawHill Book Co., USA, 1978;

            Ramsey/Sleeper Architectural Graphic Standards, AIA, Robert T. Packard (ed), John Wiley & Sons Inc., USA, 1981;

JLC Field Guide to Residential Construction: A Manual of Best Practices, Vol. 2, Manley Wood LLC, USA, 2006.