Before any electrician pulls a wire or mounts a panel, someone has drawn the job on paper. Electrical drawings tell you where the power comes from, what equipment is installed, and which breaker feeds which room. For a student, they look like a secret code of lines, circles and tables. This post explains the three drawings you will meet most often.
What Is a One-Line Diagram?
A one-line diagram, also called a single-line diagram, shows the path of electrical power from the utility to the loads, using one line to stand for all three phases. It shows the equipment and how it is connected, not every wire. Reading it from left to right, you follow the power: utility service, meter, main disconnect, panel, and finally the loads.
Every symbol is one piece of equipment, and the numbers beside it are its rating. Two overlapping circles are a transformer, and the number beside it is its size in kVA. A rectangle is a panelboard, and the number beside it is its amp rating. The voltage is written under the line, for example 480Y/277 V.
Common Symbols You Must Know
Transformer: two overlapping circles. The kVA rating and the primary and secondary voltages are written beside it.
Circuit breaker: a rectangle or a switch symbol with a trip rating, such as 225 A.
Panelboard: a rectangle with the panel name inside, such as LP-1, and its voltage and amp rating.
There is also a legend or symbol key on every drawing set. If a symbol is unfamiliar, check the legend first.
How to Read a Floor Plan
The floor plan shows where the equipment and outlets go in the building. It is drawn on top of the architectural plan. Receptacles are small circles on the wall, light fixtures are circles on the ceiling, and switches are the letter S.
On a floor plan, a solid line between two symbols is usually a conduit or cable run. The number beside it gives the wire size and the circuit number, which tells you which breaker feeds that outlet. A receptacle marked "7" means circuit 7 in the panel schedule.
The Panel Schedule
The panel schedule lists every circuit in a panel. It tells you exactly what each breaker feeds. A typical schedule has columns for the circuit number, the load, the breaker size, and the load in volt-amperes (VA).
Circuit numbers are arranged in a fixed pattern: odd numbers run down the left side of the panel and even numbers down the right side. So circuits 1 and 2 are the top two breakers, one on each side.
Diagram: A simple one-line diagram, read from left to right. The utility feeds a 150 kVA transformer, then a 400-amp main disconnect, then panel LP-1 at 208Y/120 V.
Reading a Drawing Set in Order
A drawing set is read in a fixed order. Start with the one-line diagram: where the power comes from, the voltage at each level, and the size of each panel. Then read the floor plans to see where the outlets and lights are. Then read the panel schedules to find the breaker for each circuit.
Finally, check the notes and the title block. The notes tell you the installation rules for that job. The title block, at the bottom right, gives the project name, drawing number, scale and revision date. Always work from the latest revision: an old drawing can show equipment that has already been changed.
Diagram: A sample panel schedule for panel LP-1. Odd circuits run down the left, even down the right. Each row gives the load, the breaker size and the load in VA.
Key Takeaways
Electrical drawings are read from the power source to the loads. The one-line diagram shows the whole system with one line for all three phases, and every symbol is one piece of equipment with its rating beside it. The floor plan shows where the outlets and lights go, and the circuit number links to the panel schedule. The panel schedule lists every circuit and its breaker size. Read the one-line first, then the floor plans, then the schedules.
Objective Questions
1. What does a one-line diagram show?
a) Every single wire in the building
b) The path of power from the utility to the loads, using one line to stand for all three phases
c) Only the lighting circuits
d) The architectural layout of the rooms
Answer: (b) The path of power from the utility to the loads, using one line to stand for all three phases
Explanation: A one-line diagram shows the equipment and how it is connected, using one line to stand for all three phases, from the utility service to the loads.
2. On a one-line diagram, what does the symbol of two overlapping circles mean?
a) A motor
b) A circuit breaker
c) A transformer
d) A disconnect switch
Answer: (c) A transformer
Explanation: Two overlapping circles are the symbol for a transformer, and the kVA rating and voltages are written beside it.
3. On a floor plan, how do you find which breaker feeds a receptacle marked "7"?
a) Count seven breakers from the top
b) Look at circuit 7 in the panel schedule
c) Measure the distance from the panel
d) Check the title block
Answer: (b) Look at circuit 7 in the panel schedule
Explanation: The circuit number on the floor plan links to the same circuit number in the panel schedule, which gives the breaker size and load.
4. In a panel schedule, how are the circuit numbers arranged?
a) In random order
b) Odd numbers down the left side and even numbers down the right side
c) Even numbers on the left and odd numbers on the right
d) In order of load size
Answer: (b) Odd numbers down the left side and even numbers down the right side
Explanation: Circuit numbers run in a fixed pattern: odd numbers down the left side of the panel and even numbers down the right side.
5. Where on a drawing sheet do you find the project name, drawing number, revision date and scale?
a) In the legend
b) In the panel schedule
c) In the title block at the bottom right of the sheet
d) On the floor plan
Answer: (c) In the title block at the bottom right of the sheet
Explanation: The title block, at the bottom right of the sheet, gives the project name, drawing number, scale, revision date and who drew it, so you can confirm you are using the latest revision.
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