The most common electrical drawings are wiring diagrams or wiring plans. In these drawings the electrical lines and symbols are superimposed on an architectural floor plan. The drawing scale must be large enough to permit symbols and line conventions to be drawn legibly. Floor plans for new homes, office buildings, and large industrial, commercial, or government projects are drawn by architectural drafters to a scale appropriate to the floor area of the building or project. The scales selected are typically from 1⁄4 in. equals 1 ft to 1 in. equals 1 ft, and depend on building size. The electrical designer marks the symbols for the electrical devices such as switches, receptacles, and luminaires and lines representing wires or relationships on a copy of the floor plan.
Some electrical design CAD software includes code for making changes in the architectural drawing if that becomes necessary. For example, it might be necessary to relocate a wall to provide enough space behind it to run cable bundles or permit deep electrical panels to be flush-mounted. The electrical designer might make those changes based on his or her knowledge of building construction and the space requirements for the cables or panels to be placed behind the walls.
above Figure is a one-sheet electrical wiring diagram for a two-bedroom private home. It contains many of the same elements that would be found on wiring diagrams for larger commercial or industrial buildings. This diagram contains a key of symbols and a list of branch circuit ratings to guide the electricians or installers in the field.
electrical wiring diagrams are important because they are required for obtaining work permits from local building inspectors and approval by the local power utility. The wiring diagram gives the building electrical inspector an overview of the scope of the work to be performed and later serves as a guide for the inspector during the work in progress and after the work is completed. In situations where the project involves new or updated connections to the power line, the local power utility must be informed and may ask for a copy of the wiring diagram.
The wiring diagram is also a major source of information for preparing lists of materials, and it also serves as a guide for sizing cable lengths and scheduling the installation of electrical devices and building services. A properly prepared wiring diagram should be comprehensible to those who are familiar with the symbols and conventions used in preparing it.
Because wiring diagrams are two-dimensional floor plan drawings, they do not include information about the heights of switches, receptacles, and luminaires above the floor. With the exception of ceiling fixtures, which are obvious from the diagram, the heights of receptacles and switches above the floor and their separation distances are dictated by the National Electrical Code® (NEC®)* and local building codes. In most cases the local codes will be keyed to the NEC, but they may be more specific about certain details based on local experience. For example, where soil conditions are typically dry a more elaborate grounding system might be required, or more provisions for protection against lightning strikes might be required in parts of the country where there is a high incidence of lightning. Any exceptions to the NEC requirements for device placement must be noted on the diagram. For example, switches and receptacles might be placed at more convenient heights for the handicapped occupants or those in wheelchairs
Some basic rules for the preparation of an acceptable electrical wiring diagrams are as follows.
- Draw or obtain a scaled architectural drawing of the floor area to be wired showing walls, doors, windows, plumbing pipes and fixtures, and heating and cooling ducts.
- Determine the floor area by multiplying the room length by width and then deduct any floor areas occupied by closets and storage areas. Indicate this figure on the diagram.
- Mark the location of switches, receptacles, luminaires, and permanent appliances such as ranges, microwave ovens, heat exchangers, and attic fans with standard electrical symbols.
- Draw in cable runs between wiring devices, indicating approved cables by type designation, wire gauge, insulation type, and branch circuit amperage. If conduit is used, size and location should be given.
- Identify the wattages for luminaires, permanent appliances such as ranges and airconditioning systems, building service equipment such as furnaces and hot water heaters, and the type and size of each electrical box.
ONE-LINE DIAGRAMS
One of the most important drawing types for the design of a new electrical system or modernizing an existing system is the one-line drawing. It uses single lines and standard symbols to show electrical wiring or busbars and component parts of an electric circuit or system of circuits. The one-line drawing differs from the wiring diagram in that it does not specify device (receptacle, switch, luminaire, etc.) locations or switch locations for controlling those devices.
The one-line diagram in below Figure gives an overview of a complete system and how it works. For example, a three-phase load requires three wires, and each wire has its own pole of a control switch and one overcurrent device. It is not necessary to repeat this information three times on the diagram; one line shows what happens to all three wires. General rules must be followed in preparing one-line diagrams. Compliance with these rules helps to ensure a complete, accurate, and easily interpreted electrical wiring diagrams .
- Indicate relative positions of components in a building or factory. For example, distinguish between those parts of the system that are inside or outside a building. This makes the drawing easier to interpret because components are properly located with respect to each other.
- Avoid duplication of lines symbols, figures, and letters. A one-line drawing is a precise form of technical communication, and every line, symbol, f igure, and letter has a definite meaning. Unnecessary duplication will make interpretation more difficult.
- Use standard electrical symbols for the more common wiring devices. The use of alternative or modified symbols for common wiring devices leads to confusion and detracts from the correct interpretation of symbols for unusual or special components.
- Allow for future expansion, either on the drawing or with explanatory notes.
- Include correct title data. Assign titles with care to be sure that they identify each component correctly, eliminating confusion with other components in the system.
- Include all pertinent technical information.
The following checklist will be helpful in avoiding the omission of important technical
information.
- Manufacturers’ designations and ratings of all machines and power transformers included in the project
- Ratios of current and voltage transformers, taps to be used on multiratio transformers, and connections of dual-ratio current transformers
- Connections of power transformer windings
- Circuit breaker ratings in volts and amperes, interrupting ratings, and type and number of trip coils on circuit breakers
- Switch and fuse ratings in volts and amperes
- Any special features of fuses (current limiting, dual element, etc.)
- Functions of relays
- Size and type of conductors
- Voltage, phase, and frequency of incoming circuits; indicate wye and delta systems, and show whether they are grounded or ungrounded
POWER RISER DIAGRAMS
Power riser diagrams are single-line diagrams showing electrical equipment and installations in elevation. below Figure is an example drawn for a combined office and warehouse. It shows all of the electrical equipment and the connecting lines for service entrance conductors and feeders. Notes identify equipment, the size of conduit necessary for each feeder, and the number, size, and type of conductors in each conduit.
ELECTRICAL SCHEMATIC DIAGRAMS
Electrical schematic drawings are usually prepared by equipment manufacturers to show the electrical connections that must be made by the electrician or installer. They are also used for testing, troubleshooting, and maintenance of the equipment. As an example of an electrical schematic diagram, below Figure shows an across-the-line starter for a three-phase motor powered from a three-phase, three-wire supply.
It can be seen from the diagram that the motor starting equipment is housed in two separate enclosures. This starter would normally be shipped by the manufacturer with the motor it will control. The contactors, overcurrent protective devices, transformer, and operating coil are in one enclosure, and the start/stop pushbuttons are in a separate enclosure so that they can be mounted some distance from the motor electrical wiring diagrams.
In this schematic each component is represented by a graphic symbol, and each wire is shown making individual connections between the devices. However, multiple wires could appear as one line on the drawing. As on this drawing, each wire is usually numbered to indicate where it enters the enclosure, and those numbers are repeated for the same wires connected inside the enclosure.
The three supply wires are identified as L1, L2, and L3; the motor terminals are designated T1, T2, and T3; and the normally open line contactors controlled by the magnetic starter coil C are designated as C1, C2, and C3. Each contactor has a pair of contacts that open or close for control of the motor.
The remote control station consists of the stop and start pushbuttons connected across lines L1 and L2 by the primary of an isolation control transformer. The transformer secondary in the control circuit is in series with the normally closed overload contactors (OC) and the magnetic starter coil (C). The stop button is also connected in series with the starter coil, and the start button is connected in parallel with the starter coil.
In this circuit, the control transformer isolates the control circuit and prevents it from responding to any ground faults that could cause the motor to start accidentally. The isolating transformer can have its primary winding identical to its secondary winding so that input voltage equals output voltage, or it can step the motor circuit voltage down to a lower level as an added safety measure for the control circuit.