Electrical working drawings
Every electrician, in every branch of electrical work, will need to consult and understand the information presented on electrical drawings to locate the various outlets, the routing of circuits, the location and size of panelboards, and other similar electrical details.
A complete set of working drawings for the average electrical system in industrial, commercial, and large residential projects will usually consist of the following:
- Plot plan showing the location of the building on the property and all outside electrical wiring, including the service entrance. This plan is drawn to scale with the exception of the various electrical symbols, which must be enlarged to be readable.
- Floor plans showing the walls and partitions for each floor level. The physical locations of all wiring and outlets are shown for lighting, power, signal and communication, special electrical systems, and related electrical equipment.
Again, the building partitions are drawn to scale, as are such electrical items as fluorescent lighting fixtures, panelboards, and switchgear. The locations of other electrical outlets and similar components are only approximated on the drawings because they have to be exaggerated to be readable.
- Power-riser diagrams to show the service-entrance and panelboard components.
- Control wiring schematic and single-line diagrams.
- Schedules, notes, and large-scale details on construction drawings.
To be able to “read” electrical as well as other types of drawings, one must become familiar with the meaning of symbols, lines, and abbreviations used on the drawings and learn how to interpret the message conveyed by the drawings.
Let’s start with 10 simple questions. Answers are at the bottom, and please don’t peek :)
How many branch circuits are shown in Figure 1?
How many duplex receptacles are installed outside the building in the drawing in Figure 1?
To comply with the NEC, what device must be used on all three outside duplex receptacles shown on the drawing in Figure 1?
- Double-pole circuit breaker
- A ground-fault protector (ground-fault circuit interrupter)
- A single-pole mercury switch
- A 40-amp circuit breaker
In the power-riser diagram in Figure 2, what does “C/T cab.” stand for?
- Central telephone cabinet
- Current transformer cabinet
- Control terminal cabinet
- Central termination cabinet
Which of the following best describes where the type and size of overcurrent protection may be found?
- Floor plans
- Lighting-fixture schedule
- Panelboard schedule
- Cross-sectional views
What wire size and how many conductors are specified for the feeder circuit supplying the rooftop unit No. 2 in the drawing in Figure 2?
- Three No. 1 AWG
- Three No. 10 AWG
- Four No. 2 AWG
- Two No. 4 AWG
How many sections of busway are specified in the drawing in Figure 2?
If each of the Type C lighting fixtures in Figure 3 have a total lamp and ballast load of 200 volt-amperes, what is the total connected load of circuit A-1 (in volt-amperes)?
- 1600 volt-amperes
- 1700 volt-amperes
- 1800 volt-amperes
- 1900 volt-amperes
What is one good reason that circuit lines are drawn curved rather than straight?
- This is how conduit is installed in buildings
- To enable the drafters to route the lines around partitions
- So as not to confuse the circuit lines with building lines
- Curved lines are easier to draw on CAD systems than straight lines
What do arrowheads placed on circuit lines mean?
- Designates the room in which the circuit is installed
- Home runs to the designated panel
- The direction of current flow
- Designates that the circuit is to be controlled by a wall switch
NEC articles mentioned in answers you can observe in a free draft version of NEC 2014 //
Two circuits serve the kitchen, 1 serves the master bedroom, 1 serves both bedroom #2 and bedroom #3, 1 serves the living room/vestibule, and 1 supplies the GFCI circuit which supplies protection to the carport receptacle, both front and rear receptacles, NEC Section 210.8(A). One circuit supplies the GFCI receptacle in the bathroom, NEC Section 210.11(C)(3).
Three outside duplex receptacles are shown by symbol on the floor-plan drawing and also described in the Note.
A ground-fault protector is indicated by notes on the drawing in Figure 1. See Section 210.8(A)(3) of the NEC.
The service conduit and conductors enter the C/T cabinet where current transformers (CTs) are used in conjunction with the electric meter for metering the amount of power used.
The panelboard schedule on drawings usually indicates the type and size of overcurrent protection.
The note adjacent to the feeder indicates three No. 10 AWG conductors.
There are seven sections shown in the power-riser diagram in Figure 2.
Since there are eight fixtures fed by this circuit, 8 × 200 = 1600 volt-amperes.
When circuit lines are drawn straight, they are sometimes confused with the building lines. Thus, Answer C is one good reason for drawing curved circuit lines.
Arrowheads are used to indicate a home-run to a panelboard.
Reference // Electrician’s exam preparation guide (Buy hardcopy from Amazon)