Do You Know How to Read Basic Electrical Plans?

By: Bambi Turner

The reflected ceiling plan is like a floor plan to show you what's happening on the ceiling. It's the perfect place to look when laying out light fixtures or other components that will be installed on the ceiling.

No set of electrical plans is complete without the accompanying set of spec books. For all the information you need as an electrician, check out Division 26, which is based on the widely-used Constructions Specification Institute system and covers all things electrical.

This symbol is used on electrical plans to indicate a recessed fluorescent light fixture. If the fixture is meant to be surface-mounted rather than recessed into the ceiling, it is shown without the diagonal line passing from corner to corner.

A panelboard, or breaker panel, is the hub of a home's electrical service. It's where the power that has been fed into the building is divided among various circuits. This symbol indicates a surface-mounted panelboard when it is unfilled, or a flush-mounted panelboard when it is colored in.

When you see this symbol, you should install a single 120-volt receptacle. If the symbol shows two lines passing through the circle, the intention is for the installer to use a duplex receptacle instead.

Indicated on electrical plans via a box with the letter "T" inside, a transformer is an important piece of equipment for any home or building. This device increases or decreases the main power coming from the electrical grid, transforming it into a safe voltage for everyday use.

The letters WP in the electrical notes indicate that the equipment in question should be rated as weatherproof. For instance, a weatherproof outlet comes with corrosion-resistant hangers, screws and contacts to hold up in outdoor use.

A ground fault circuit interrupter, or GFCI, is a special safety feature used on electrical outlets and other equipment. Required in almost all outdoor settings, and many indoor ones -- like bathrooms -- the GFCI feature stops the flow of power in a fraction of a second when a fault is detected.

Telecommunications fixtures are indicated by tiny triangles on electrical plans. On larger projects, they may be included on a separate Telecommunications drawing. Blank triangles indicate data ports, while blackened triangles indicate phone ports.

A junction box is a metal or plastic casing that is used to keep wire connections protected. These elements are indicated on electrical plans by a circle with the letter "J" inside.

The letters AFF are commonly seen on building and electrical plans. When you see 48" AFF next to the symbol for a light switch, it means the switch should be installed 48 inches above finished floor.

Plot plans show the electrician what is going to happen outside of the structure. This includes things like wiring locations and where equipment such as the transformer should be installed.

The letter "S" on an electrical drawings simply stands for a single pole switch. A number after the S, such as S4, means that the installer should use a four-way switch.

The number next to a wire on an electrical plan helps you match the wire to the correct breaker. These numbers, and the paths the wires will take, are shown on wiring diagrams.

The letters "OS" on an electrical plan stand for occupancy sensors. They may be listed in notes, or shown contained within a circle indicating where the sensor should be placed.

The letter "S" on its own indicates a standard electrical switch. When the letter "D" is placed next to it, it indicates that the switch should be equipped with a dimmer function.

A rectangular symbol split diagonally and shown on electrical drawings always indicates a fluorescent fixture. When half of the fixture is shaded and half is not, that means the fixture should be tied to emergency power. This means that the fixture will be powered by a backup supply if the main power goes out.

A small square with a solid black circle inside indicates a doorbell pushbutton. Because this device relies on electricity, it is shown on the electrical plans so that it will be wired correctly.

Circles shown on the reflected ceiling plan indicates round light fixtures called downlights. When there is an "R" inside the circle, that means the fixture should be recessed into the ceiling.

Circuit breakers are represented by various incarnations of this symbol. These breakers are designed to automatically stop the flow of current if a problem, such as a fault, is detected.

This symbol showing a circle with a triangle inside is used to represent a piece of equipment. Letters placed next to the symbol reveal what type of equipment will be used. Ref, for example, stands for refrigerator.

Electrical engineers and designers use symbols to not only indicate what type of equipment should be installed, but also where it should be placed. This symbol is always used for an outlet in the floor, which will usually be covered to keep dirt out and prevent the outlet from becoming a trip hazard.

Branch circuits are used to divide and transfer power coming from the panel into various fixtures and equipment throughout a home or building. When the branch is shown as a continuous line on the drawings, that means it is meant to be concealed within the ceiling or wall.

Branch circuits represented by a line broken into alternating long and short dashes should be concealed in the floor. These branch circuits are usually placed under access flooring or some form of panel so they can be accessed at a later time.

Smoke detectors are often hard-wired in, which eliminates issues with dead batteries. This means they must be wired by the electrician so they will function properly. Most are indicated on drawings using either a circle or square with the letters SD inside.

The letter "T" enclosed in a circle is the standard symbol for a thermostat. To properly install this device, electricians must also reference wiring diagrams, which may be found on a separate details page.

Many electrical outlets are designed for general use. Those with a special function may require extra amps or other modifications, and are indicated on the drawings by the letters "SP."

A telecommunications main grounding busbar, or TMGB, is a critical device to help protect computer, phone and data equipment. This equipment is too sensitive to simply use standard grounding techniques, so a TMGB is added to protect the equipment in case of a fault or power surge.

The National Electric Code provides the minimum standards used in the electrical industry. Many electrical plans include NEC notes and references, which all electricians should be familiar with.

The letters UPS stand for uninterruptible power supply. This is a battery=powered backup used to supply continuous power in case of an outage or other issue.,

A branch circuit shown as a dashed line on the plans is meant to be left exposed. That could mean installing it in conduit, without placing it in the wall, ceiling or floor.

Air handling units are generally supplied and installed by the mechanical contractor, but the electrician also plays a role in finalizing the process. This includes adding specialty wiring and running the wiring to the device, though the scope may be further expanded on some projects.

An outlet or other fixture shown with the letters "EP" beside it on the drawings should be rated as Explosion-Proof. This means it is designed so that a potential fault or spark inside the fixture won't ignite surrounding substances like vapors, gas or chemicals.

Electrical grounding is critical for safety, and is represented on electrical plans by the letter "G" -- "UNG" stands for ungrounded. Grounding is accomplished by techniques like using a second neutral wire when pulling wires, or by using that third prong you see on things like an extension cord.

An outlet annotated with a letter "B" on the plans is meant to be blank. This means it should be wired for future use, but won't be used immediately and typically requires only a blank cover.

A standard conduit contains two wires -- one that is live and one neutral wire for grounding. Circuits shown on the drawings with a number indicate that the installer should use that quantity of wires.

This symbol means that a wire should be turned up. The same symbol in which the circle is filled in means that the wire is meant to be turned down toward the ground.

Amps measure current. A standard residential circuit measures 15 to 20 amps, which means it can accommodate 1800 to 2400 watts before it flips.

As door hardware becomes more advanced, the installation of this hardware requires a joint effort between the hardware installer and an electrician. Look out for hardware symbols like DS, ES and M, which stand for door switch, electric strike and mag lock.

This symbol represents a security camera. The number of cameras shown on the symbol generally equates to the number and pattern that should be installed.

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About This Quiz

Trying to build a house without blueprints is like trying to build a giant puzzle without looking at the picture as a guide. While a good contractor might be able to puzzle out things like walls and floors, it's impossible to install a complete electrical system without a well-prepared set of plans. 

Of course, these plans won't do you much good if you don't know how to read them. Designed by electrical engineers or pro electrical companies, these plans reveal everything from how the main source of power should enter the building, to the path wires should take throughout the structure, to the location of each and every light fixture, outlet, switch and piece of equipment. 

Electrical plans also reveal how electrical components will interact with other systems in the home. This includes where special power sources are needed, how air handling units and HVAC equipment will be powered, how fire alarms and smoke detectors will receive power and so much more required for a complete, safe and comfortable dwelling. 

Think you can interpret the various symbols and sayings on a standard electrical plan or blueprint? Prove your basic blueprint reading knowledge with this quiz!

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