Reading a Wiring Diagram: Legend, Ladder, and Test Points

The short version

Two diagrams describe the same unit, and you read each one for a different question. The connection diagram, which many manufacturers and technicians also call a pictorial or a wiring diagram, shows where every wire physically lands. The schematic, drawn as a ladder, shows what the circuit does when a call comes in. Reading either one from memory is the trap, because symbols, component letters and line styles vary between manufacturers, and the legend on the diagram in front of you controls how that diagram reads.

A rung is one path across that ladder, and the customer pays for a misread rung twice. The no-heat call is still a no-heat call, because the switch that opened the circuit is still open, and a control board or a contactor that was working goes on the invoice.

Work the legend, then the ladder, then the test points. The legend gives the abbreviations, the symbols and the line styles. The ladder gives the path from one side of the supply to the other, through every switch in series with the load you care about. Those two together give you the terminals worth putting a probe on.

The two diagrams on the unit

Both drawings commonly come with residential equipment, printed on the same label or on facing pages of the installation instructions, and what a given manufacturer supplies varies. They are two different drawings with two different jobs, and the one you want depends on which question you are answering.

AC Service Tech’s training material separates them in a line each. “The Schematic Diagram is a ladder type diagram that shows how the system works,” and “The Connection Diagram shows how the system is wired.” The legend belongs to both, and AC Service Tech’s own sequence for a student is to review all three before installing any wiring.

The textbook Modern Refrigeration and Air Conditioning divides them by the task in front of you: “In general, ladder diagrams are more useful for troubleshooting electrical circuits, while pictorial diagrams are used to locate devices within the unit and properly wire the unit.” The same page states that pictorial diagrams “specify wiring color and terminal locations on devices” and that they “are useful for physically locating terminals and for tracing wires from terminal to terminal.”

The schematic leaves out the physical layout by design. Contracting Business, in its basic electricity material for technicians, states the intention plainly: “The intention of the overall schematic is to show how the circuit functions, not how it actually looks.” Two components an inch apart in the cabinet can sit on rungs at opposite ends of the ladder, and one relay with a coil and two sets of contacts appears in three places on the same drawing.

What you need to know	Which drawing	What to read on it
Where a component sits in the cabinet	Connection diagram	Component positions relative to each other, which on some drawings match their positions in the equipment
Which terminal a wire lands on	Connection diagram	Terminal designations and the wire color code
What a symbol or an abbreviation means	Legend	The symbol column and the component code block
Whether a wire is factory or field	Legend	The line style key
Which switches must close before a load runs	Schematic	Every switching device in series with the load, and which branches are in parallel with each other
Whether two crossing lines connect	Schematic	The junction convention that drawing uses, a dot at the crossing or a loop jumping over it
Which contacts a coil moves	Schematic	The designation shared by the coil and the contacts
What the sequence does after a call	Schematic and the sequence of operation	The rungs fed by the thermostat terminals, plus the written sequence for what a control board does inside its own case

Use the diagram attached to the unit rather than one you found online for a similar model number. Goodman prints the reason on its GMS9 and GCS9 furnace diagram: “Wiring is subject to change, always refer to the wiring diagram on the unit for the most up-to-date wiring.” Wiring can change within a model family, so start from the drawing on the unit and check that what it shows matches the board actually installed. That label is often adhered inside an equipment access panel, and MEP Academy’s instruction is to look on an access panel, on an electrical cover, or in a pocket inside the unit.

Start at the legend

The legend is what makes the rest of the drawing readable, and it is where to start. AC Service Tech states its job in one line: “The Legend shows the abbreviation and symbol meanings.” Gary McCreadie, writing for HVAC Know It All, gives the same instruction as a correction to a common habit, listing “Assuming all manufacturers use the same symbols” among the mistakes and answering it with “Always check the legend first.” MEP Academy’s guidance is the same: “There should be a legend that identifies what each symbol represents.”

Two organizations publish the symbol standards behind the drawings, and neither one settles every drawing. Modern Refrigeration and Air Conditioning states that “The international body that standardizes electrical symbols is the International Electrotechnical Commission (IEC),” and that “In the United States, electrical drawing standards and symbols specified by the National Electrical Manufacturers Association (NEMA) are often used in electrical diagrams.” The two publish differently. The IEC puts its symbols in one place, the IEC 60617 database, which holds more than 1500 graphical symbols for use in electrotechnical diagrams. NEMA is a standards developer with more than 800 standards to its name rather than a single symbol sheet.

The textbook page includes two qualifications worth holding onto: “In some instances, the electrical symbols specified by NEMA differ from those specified by IEC,” and “the electrical symbols used in diagrams may vary slightly from one manufacturer to another.”

Line style is the second thing to read in the legend. SkillCat’s training material gives the common convention in two sentences: “A solid line represents factory wiring on a diagram,” and “A dashed line represents field wiring on diagrams.” Factory wiring is what the manufacturer connected before the unit shipped.

Field wiring is what the installer connects on the job, which on a split system means the line voltage supply and its equipment ground, the control wire between the two units, and the thermostat wire. Read the full set of field connections off the drawing in front of you, because a unit can have more of them than those four: Goodman’s split system condensing unit diagram gives EQUIPMENT GROUND as a connection the installer makes, with the note “USE COPPER CONDUCTORS ONLY.”

Do not carry that convention from one brand to the next without checking, because SkillCat’s own instruction is that “The legend will have a definition for each type of wiring in a circuit.” Some diagrams use more than two line styles, splitting field wiring into a line voltage style and a low voltage style, and some mark the difference by line weight instead.

How a schematic marks a connection at a crossing is itself a convention, and there is more than one. Contracting Business gives the common one: “If two wires are connected internally, the connection usually is shown as a dot (a solid, black circle).” On a drawing that marks its junctions that way, a crossing with no dot is one wire passing over the other. The same article shows the alternative, where “crossover wires are shown with half circles or loops that ‘jump’ over other wires,” and on that kind of drawing, lines that meet without a jump can be connected even though no dot says so. Work out which convention the sheet uses before you read a crossing, because that detail determines whether you are looking at a shared node or at two unrelated circuits that happen to overlap on paper.

Most diagrams mark the wire colors on the lines themselves, such as BK or BLK for black, and SkillCat’s instruction is that some legends define those color symbols in a block of their own. Most diagrams also carry a short component code block. The component block gives the letters used for parts, and those letters are not consistent between manufacturers. Read the colors and the component letters off the drawing in your hand rather than off the last unit you worked on.

How a ladder diagram is laid out

A ladder diagram is drawn between two vertical lines that represent the two sides of the supply. MEP Academy describes the arrangement: “line voltage L1 and L2 are shown as vertical lines, and all the electrical components shown on horizontal lines.” L1 and L2 are the labels for the two sides of the supply. Those two verticals are the rails. Every load the unit switches sits on a path between them. What a control board decides inside its own case is not drawn as a rung, and the manufacturer’s written sequence of operation is where that lives.

Modern Refrigeration and Air Conditioning gives the rest of the geometry: “Rungs are horizontal lines that connect the rails. Each rung represents one parallel path in the circuit.” Reading a rung is reading one complete circuit from one rail to the other. The rungs above and below it are separate circuits fed from the same two rails, so a component failing open stops its own rung and often leaves the others running.

Two things break that separation. Several rungs share the supply, a fuse and the transformer, so an open in any of those takes all of them down at once. A coil sitting on one rung also does its switching elsewhere, and the same textbook states it plainly: “Some loads, such as coils and solenoids, close or open contacts when energized. These contacts are shown on a different rung of the ladder diagram.”

Position on the rung means something. The same textbook states that “Switching devices are generally located on the left side of a rung, and a rung can include multiple switching devices,” and that “Generally, each rung includes only one load and the load is shown as the right-most symbol on the rung.” McCreadie states the same order from the other direction: “Loads usually sit at the end of a circuit.”

A load is the component that consumes the power. Contracting Business defines it as a device that consumes power and converts it to some other form of energy, such as motion or heat. McCreadie’s list is the practical one: “Loads include motors, compressors, contactors, relay coils, and light bulbs,” and “Loads perform work and draw amperage.” On a contactor or a relay it is the coil that is the load, while the contacts it moves are switching devices. A switch has the other job, and Contracting Business defines it as a device that interrupts power to the load. McCreadie’s definitions cover both of its states: “An open switch is a switch that does not allow power to flow from one side to the other,” while “A closed switch allows that same power to flow through it.”

One load per rung is what makes a voltage reading on that rung easy to interpret. When every switch on that rung is closed, the load has the full voltage between the rails across it, less the small amount the closed switches and connections drop. Nothing else on the path divides the voltage, because the other loads are on their own rungs. The textbook states the condition both ways: “If any switching device on a rung is open, the load device is de-energized. When all switching devices on the rung are closed, the circuit is completed and the load device is energized.” That second half stays true while the supply, the conductors, the connections and the load itself are intact, which is what the measurements are for.

That arrangement gives you two readings with different meanings. A load sitting at rail voltage and doing nothing points at the load itself. A load with little or no voltage across it means its two terminals are sitting at close to the same potential, and the reason is elsewhere in its path, which is a switch, a safety, a connection, or the supply on either side of it.

Measure each of its terminals back to a rail to find out which side lost the voltage. Those readings are energized work, and 29 CFR 1910.334(c) leaves testing on an energized circuit to a qualified person using a meter and leads rated for that circuit.

One common arrangement divides the sheet by voltage, and Contracting Business describes it: “Normally, the high-voltage section is placed at the top of the diagram, and the low-voltage section is placed at the bottom of the diagram.” The control transformer is drawn as a load on a line voltage rung, and its secondary becomes the pair of rails the control rungs run between. The same article makes the transformer the landmark: “An easy way to determine the different voltages in this type of schematic is to look for the transformer. It normally is the ‘dividing line’ for voltage changes.” Read the rail labels to confirm what you should expect rather than going by position on the page alone.

Contacts, coils, and the de-energized rule

Every switch and contact on a schematic is drawn in one particular condition, and that condition is a reference rather than a description of the running unit. MEP Academy states the rule: “The contacts are shown on the diagram as they would be normally without any electrical power being supplied.” Interplay Learning’s version applies to relays specifically: “Relay contacts are depicted as if the relay coil is de-energized.” Interplay splits the rest of the switches by what they are for: “Operating controls such as relays and thermostats are usually depicted in the open position,” while “Safety controls (overloads, high pressure/low pressure, limit switches) are usually depicted in the closed position.” A closed safety on a running unit is therefore sitting in exactly the position the drawing gives it.

Normally open and normally closed describe that drawn condition rather than what is usual. MEP Academy’s example makes the point: “You can see that all of our example contacts are what is called normally open, meaning with no power they are set to be open.” A normally closed limit switch is drawn closed on the diagram of a furnace whose limit has tripped, so the symbol is not a prediction of what your meter will read. It is the reference position you compare your reading against.

A relay appears in at least two places on a ladder. The coil is a load, so it sits at the right end of its own rung. The contacts that the coil moves are switches, so they sit to the left of whatever load they control, on rungs that can be a long way from the coil. The designation shared between the coil and its contacts is the link, and on some diagrams the rung numbers printed beside the coil give the locations of those contacts. Interplay Learning’s material covers that marking: “Underlined numbers on the right side will denote contacts that are normally closed.”

Reading the rung rather than the component name is the skill this whole exercise is for. Writing on HVAC School about lockout relays, Bryan Orr describes an arrangement most technicians would not guess from the part’s name: “The lockout relay coil is a high-resistance coil wired in series with the contactor coil but wired in parallel with the safety switches.”

Per Orr, the closed safeties carry the current around that high-resistance coil, so almost no voltage appears across it. With the control circuit energized and a call in, a safety opening leaves the lockout coil and the contactor coil as the only path, in series, and in Orr’s worked example the lockout coil then has 21.6 volts across it while the contactor coil has 2.38 volts.

The lockout coil energizes on that 21.6 volts, and its normally closed contacts open to keep the contactor from picking back up. You get none of that from the part’s name, and all of it from reading which components on the rung are in series and which are in parallel.

Line voltage and low voltage on one page

A residential split system’s diagram commonly draws two electrical circuits on the same sheet. The line voltage side runs the loads that do the work, meaning the compressor, the condenser fan motor and the indoor blower motor. The low voltage side is the 24 volt control circuit that the transformer supplies, and it runs the thermostat, the safety switches and the coils of the relays and the contactor. The transformer is the component that separates the two, stepping the 208/230 volt single phase supply down to roughly 24 volts for the controls.

The contactor is where the two circuits meet on the page. Its coil is a load on a 24 volt rung, energized through the thermostat’s cooling call and whatever safeties are in that path. Its contacts are switches on the line voltage rungs that feed the compressor and the condenser fan motor. One designation appears in both places at two different voltages, and a technician who does not connect them reads the compressor rung as though nothing controls it.

On many residential diagrams the safety switches sit in series with the contactor coil, so any one of them opening drops the coil out and the compressor stops. Orr’s lockout relay description depends on exactly that arrangement. When you are trying to work out why a contactor will not pull in, the rung the coil sits on is the list of everything drawn that can keep voltage off it. A failed coil, a mechanical bind, or something spliced in that the drawing never had is still yours to check separately.

Reading a diagram is a paper exercise, and the work it leads to is not. The Occupational Safety and Health Administration (OSHA) has a rule for that work. Under 29 CFR 1910.333(a)(1), “Live parts to which an employee may be exposed shall be deenergized before the employee works on or near them,” and the regulation allows exceptions where the employer can demonstrate that deenergizing introduces additional or increased hazards or is infeasible.

Kill power at every source that feeds the unit, then lock and tag each disconnect you opened. Paragraph (b) of the same rule sets out what that takes. The circuits and equipment worked on “shall be disconnected from all electric energy sources,” and one cabinet can have more than one feed. Goodman prints the warning on its condensing unit and heat pump literature: “DISCONNECT ALL POWER BEFORE SERVICING OR INSTALLING THIS UNIT. MULTIPLE POWER SOURCES MAY BE PRESENT.”

Then “A lock and a tag shall be placed on each disconnecting means used to deenergize circuits and equipment on which work is to be performed,” except in the two cases the rule states, which are a tag used without a lock where tagging gives protection equivalent to a lock, and a lock used without a tag under the three conditions the rule lists. Parts that have been deenergized without being locked out or tagged “shall be treated as energized parts.”

Proving the circuit dead belongs to a qualified person, because “A qualified person shall use test equipment to test the circuit elements and electrical parts of equipment to which employees will be exposed and shall verify that the circuit elements and equipment parts are deenergized.”

Check the meter on a known live source before that test and again after it. NFPA 70E is the National Fire Protection Association standard for electrical safety in the workplace. Fluke states that safety standards like NFPA 70E require that live-dead-live sequence, and the second check is what catches a meter that failed partway through. Do your lifting and landing of wires with the circuit dead. The 24 volt terminals sit inside a cabinet that also has line voltage terminals in it.

Voltage measurements on a running system are a separate decision, taken under your employer’s safe work practices. Under 29 CFR 1910.334(c), “Only qualified persons may perform testing work on electric circuits or equipment,” the leads and probes are “visually inspected for external defects and damage before the equipment is used,” and the instrument and its accessories “shall be rated for the circuits and equipment to which they will be connected.” Open only the cover the measurement needs and put back what you removed. Reading the diagram first is what keeps the number of energized measurements small, because you go in knowing which two terminals to measure across.

Turning the diagram into test points

Use this when a load does not run and you need to know whether the fault is in the load, in a switch feeding it, or in the supply. It replaces a guess about which part failed with a short list of terminals worth measuring, in an order that costs you nothing to follow. The steps that put a probe on a live terminal are energized work, and 29 CFR 1910.334(c) leaves testing work on an energized circuit to a qualified person using a meter and leads rated for that circuit.

Find the load on the schematic and read its designation out of the legend, so you know which physical component you are chasing.
Read its rung from the load back to each rail and list every switching device in series with it, in the order they appear.
Cross to the connection diagram for the terminal each wire lands on, and, where that drawing places its components positionally, for where each device sits in the cabinet.
With the system calling for the function you are testing, measure across the load’s two terminals and compare what you read against the rail voltage for that part of the ladder.
Where the load has full voltage across it and does nothing, suspect the load, and confirm it with the test that belongs to that component, such as a resistance measurement on a coil, taken with the power off and any capacitor in the circuit discharged, or a current measurement on a motor.
Where the load has no voltage across it, measure across each switching device on the rung in turn, because a closed switch drops close to zero volts and, in a circuit that is complete except for one open device, that device has most of the rail voltage across it.

The open switch is the one with the voltage across it, as long as it is the only one open, and the diagram gave you the list of candidates before you opened the panel.

Two limits are worth stating. The diagram shows the unit as the factory built it and as the drawing revision covers it, which is the reason Goodman prints the note about wiring being subject to change. Accessories added later, a zone panel, a condensate overflow switch spliced into the control circuit, or a previous repair, may not appear on it at all. When the wiring in the box and the drawing disagree, the equipment runs on the wiring, so correct your reading from what is actually landed rather than assuming the drawing is current.

Common mistakes

Read the legend before you read the diagram, because symbol sets vary between manufacturers and the legend is what gives the meanings for the sheet in your hand.
Use the diagram attached to the equipment rather than one found online for a similar model number, since Goodman states on its own furnace diagram that wiring is subject to change and that the diagram on the unit is the current one.
Trace a load’s rung all the way to both rails and list every switch in series with it before you put a probe anywhere.
Read a relay or contactor contact in the position it takes with its coil de-energized, and read a thermostat, a limit or a pressure switch in the normal position its own drawing gives it, since Interplay Learning states that operating controls are usually drawn open and safety controls usually drawn closed.
Match a relay’s coil to its contacts by the designation they share, because the coil sits on its own rung and the contacts it moves can be several rungs away.
Kill power at every source that feeds the unit, lock and tag each disconnect you opened, and have the parts you will be exposed to tested dead before lifting or landing wires, because OSHA requires the circuits worked on to be disconnected from all electric energy sources, generally requires a lock and a tag on each disconnecting means used, subject to the exceptions the rule states, and treats parts that are deenergized without being locked or tagged as energized.

Don’t

Don’t read a symbol from memory of another brand’s drawing, because Modern Refrigeration and Air Conditioning states that NEMA and IEC symbols differ in some instances and that symbols vary slightly between manufacturers.
Don’t assume a dashed line means low voltage, because the common convention SkillCat gives is that dashed means field wiring, and the legend on that diagram gives a definition for each style it uses.
Don’t rule a crossing connected or unconnected until you know which junction convention the drawing uses, because one kind marks its junctions with a dot and another jumps its crossovers with a loop.
Don’t expect a normally closed switch drawn closed to measure closed, because the symbol gives the device’s normal unactuated position and a tripped safety reads open on that same symbol.
Don’t condemn a component before you have found it on the schematic and confirmed which switches are in series with it, because a load that does nothing is as often the symptom of an open elsewhere in its path as it is a failed load.
Don’t identify a wire by color alone, because a diagram’s color code describes the wiring as the factory built it and a previous repair may have used whatever was on the truck.

Frequently asked questions

What is the difference between a schematic and a connection diagram?

You read them for different questions about the same unit. AC Service Tech states that “The Schematic Diagram is a ladder type diagram that shows how the system works” and that “The Connection Diagram shows how the system is wired.” Modern Refrigeration and Air Conditioning divides them the same way by task, calling ladder diagrams more useful for troubleshooting circuits and pictorial diagrams the ones used to locate devices in the unit and wire it correctly. Use the schematic to work out which switch or load is at fault, and the connection diagram to find that component and its terminals in the cabinet.

What do dashed lines mean on an HVAC wiring diagram?

On most diagrams a dashed line represents field wiring, meaning the connections the installer makes on the job, while a solid line represents the factory wiring done before the unit shipped. That is the convention SkillCat’s training material gives. It is not universal, and SkillCat’s own instruction is that the legend will have a definition for each type of wiring in the circuit, so check the line style key before you rely on it. Some diagrams use separate styles for line voltage field wiring and low voltage field wiring.

Why are the contacts drawn open on a diagram for a unit that runs fine?

Because the drawing generally shows a contact in its normal unactuated condition rather than the condition it is in while the unit runs. MEP Academy states that contacts are shown as they would be normally with no electrical power being supplied, and Interplay Learning states that relay contacts are depicted as if the relay coil is de-energized. Interplay states that operating controls are usually drawn open and safety controls usually drawn closed, so a closed limit on a running furnace is sitting where the drawing put it. Normally open and normally closed are labels for that drawn condition rather than descriptions of what happens most of the time. A normally open contact closes when its coil is energized, and it is drawn open no matter how often the unit runs.

Where do I find the wiring diagram for the equipment?

The diagram is normally attached to the unit itself, often adhered inside an access panel, and MEP Academy’s instruction is to look on an access panel, on an electrical cover, or in a pocket inside the unit. A wiring diagram also appears in the installation instructions, and it is not necessarily the same revision as the one on the unit. Use the one on the unit. Goodman’s GMS9 and GCS9 furnace diagram states it directly: “Wiring is subject to change, always refer to the wiring diagram on the unit for the most up-to-date wiring.” A diagram found online for a similar model number may cover a different control board.

How do I use a wiring diagram to find which switch is open?

Find the load that will not run on the schematic, then read its rung and list every switching device in series with it. The measuring is energized work, so it belongs to a qualified person with a meter and leads rated for the circuit. With the system calling, measure across the load first. Full rail voltage across a load that does nothing points at the load, and no voltage across it means both of its terminals sit at close to the same potential, so measure each one back to a rail to find the side that lost it. Then measure across each switching device in that path, because a closed switch drops close to zero volts and, where the circuit is complete except for one open device, that device has most of the rail voltage across it.

Glossary

AC Service Tech: a practitioner training organization whose electrical training material is cited here.
Coil: the winding in a relay, contactor or solenoid that becomes an electromagnet when energized, drawn as a load on a ladder diagram.
Component code block: the part of a legend that gives the letters used on that diagram for each part.
Connection diagram: the drawing that shows how the unit is wired, with components and terminals in their physical positions. Many manufacturers and technicians also call it a pictorial or a wiring diagram, while HVAC School lists point to point or connection diagrams and pictorial diagrams as separate types.
Contactor: an electrically operated power switch. On conventional residential equipment a 24 volt coil closes its contacts to feed the compressor and condenser fan motor, and contactor coils are made for many other voltages.
Contracting Business: a trade publication whose basic electricity material for technicians is cited here.
De-energized: the condition of a circuit or coil with no power applied, which is the condition a relay or contactor contact on a schematic is drawn in.
Equipment ground: the grounding conductor landed with the line voltage supply, which a wiring diagram gives as a connection the installer makes.
Factory wiring: the wiring the manufacturer connected before the unit shipped, commonly drawn as a solid line.
Field wiring: the wiring the installer connects on the job, commonly drawn as a dashed line.
HVAC Know It All: a practitioner reference organization whose article on reading wiring diagrams is cited here.
HVAC School: a practitioner training and reference organization cited here.
IEC: the International Electrotechnical Commission, the international body that standardizes electrical symbols.
IEC 60617: the IEC’s database of graphical symbols for use in electrotechnical diagrams, holding more than 1500 of them.
Interplay Learning: a trades training company whose material on HVAC diagrams is cited here.
L1 and L2: the labels for the two sides of the supply, drawn as the two vertical rails of a ladder diagram.
Ladder diagram: a schematic drawn as rungs between two vertical rails, each rung a path across the rails that can itself hold series and parallel branches.
Legend: the block on a diagram that gives the meanings of its symbols, abbreviations and line styles.
Line voltage: the supply voltage feeding the equipment, as distinct from the 24 volt control circuit.
Live-dead-live: checking a meter on a known live source, testing the circuit for absence of voltage, then checking the meter on a known live source again.
Load: a component that consumes power and converts it to motion, heat or magnetism, such as a motor, a compressor or a coil.
Low voltage: on conventional residential equipment, the nominal 24 volt control circuit the transformer supplies. Control circuits are built at other voltages too, and a low voltage circuit shares its cabinet with line voltage parts.
MEP Academy: an engineering training organization whose guidance on reading wiring diagrams is cited here.
Modern Refrigeration and Air Conditioning: the Goodheart-Willcox textbook cited here for diagram conventions.
NEMA: the National Electrical Manufacturers Association, whose electrical drawing standards and symbols are often used in the United States.
NFPA 70E: the National Fire Protection Association standard for electrical safety in the workplace, cited here for the live-dead-live check.
Normally closed: a contact drawn closed in the de-energized position, which opens when its coil is energized.
Normally open: a contact drawn open in the de-energized position, which closes when its coil is energized.
OSHA: the Occupational Safety and Health Administration, which sets the deenergizing requirement cited here.
Pictorial diagram: a drawing that shows components and wiring in an approximately physical arrangement, which many manufacturers and technicians also call a connection or a wiring diagram, though HVAC School keeps pictorial and connection diagrams as separate types.
Rail: one of the two vertical lines on a ladder diagram, representing one side of the supply.
Rung: a horizontal line between the rails, representing one path across them.
Schematic: the drawing that shows how the circuit functions rather than how the unit physically looks.
SkillCat: a trades training company whose material on wiring symbols is cited here.
Switching device: any contact or switch that opens and closes a path, typically drawn to the left of the load on a rung.
Transformer: the component that steps line voltage down to roughly 24 volts for the control circuit.

Drafted with AI assistance and reviewed by the author.

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