Before you can read a wiring diagram, measure a circuit, or diagnose an electrical fault,
you need a precise vocabulary. This lesson establishes the core terms, quantities,
units, and symbols that appear throughout every electrical job in 313A/313D work.
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1.1.1 — Core Electrical Terms
A refrigeration and air conditioning mechanic must be fluent in common electrical
terminology to read documentation, communicate with other trades, and troubleshoot
systems. The seven foundational quantities below appear on every equipment nameplate,
wiring diagram, and test instrument you will use.
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Electric Charge
A property of matter that causes electrical effects. Electrons carry a negative charge; protons carry a positive charge. The movement of charge constitutes electric current.
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Voltage (Potential Difference)
The electrical “pressure” that pushes electrons through a circuit. Measured in volts (V). HVAC/R systems use 24 V (control), 120 V, 208 V, 240 V, 277 V, or 600 V depending on the load and jurisdiction.
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Current
The rate of flow of electric charge. Measured in amperes (A). Motor nameplates state full-load amps (FLA) and locked-rotor amps (LRA), which are critical for sizing conductors and protection devices.
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Resistance
The opposition to current flow. Measured in ohms (Ω). All conductors, loads, and wiring introduce resistance that converts electrical energy into heat or other useful energy forms.
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Power
The rate at which electrical energy is consumed or converted. Measured in watts (W). Power determines equipment efficiency, circuit loading, and conductor sizing requirements.
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Impedance
Total opposition to AC current, combining resistance and reactance. Measured in ohms (Ω). Relevant when working with inductive loads such as motors and transformers, which store and release energy in their magnetic fields.
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Frequency
The number of AC cycles per second. Measured in hertz (Hz). Canadian utility power is 60 Hz. Equipment, controls, and variable-frequency drives must be matched to system frequency.
1.1.2 — Circuit & Component Terms
Beyond the basic quantities, every HVAC/R technician must understand the names and
roles of the components that make up a circuit. These terms appear daily on wiring
diagrams, service manuals, and equipment specifications.
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Source
Any device that supplies electrical energy: a utility service, a transformer secondary, or a control transformer. The source establishes the voltage that drives current through the circuit.
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Load
Any device that consumes electrical energy and converts it to another form: a motor, heater, solenoid valve, electronic control board, or light. The load is what the circuit is designed to power.
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Conductor
A material that allows current flow with low resistance — copper and aluminium in building wiring. Conductors are sized by ampacity: the maximum continuous current they can carry safely.
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Insulator
A material that resists current flow: PVC, rubber, glass. Insulators separate live conductors from each other, from equipment enclosures, and from people, preventing shock and short circuits.
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Switch
Opens or closes a circuit to control current flow — manually (disconnect, toggle) or automatically (thermostat, pressure switch, float switch, flow switch). Every control circuit is built from a series of switches.
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Relay / Contactor
An electrically operated switch: a coil (electromagnet) pulls in contacts to control one or more circuits. Contactors switch high-power loads (compressors, fan motors); relays switch lower-power control circuits.
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Overload / Over-Current Device
A protective device — fuse, circuit breaker, or thermal overload relay — that interrupts current when it exceeds a safe level, protecting conductors and equipment from heat damage.
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Transformer
Transfers electrical energy between circuits by electromagnetic induction, stepping voltage up or down. In HVAC/R: steps 120 V or 208/240 V down to 24 VAC for thermostats and contactor coils.
1.1.3 — Alternating Current & Direct Current in HVAC/R
Electron Flow vs. Conventional Current
In metallic conductors, current is carried by electrons moving from regions of
negative potential to positive potential.
However, electrical diagrams and calculations use conventional current,
defined as flowing from positive to negative — the opposite direction.
For practical HVAC/R work you will draw and read schematics using conventional current.
Understanding actual electron movement is useful conceptually but does not change how
circuits are wired or measured.
Direct Current (DC)
Constant polarity and magnitude of voltage
Current flows continuously in one direction
Requires correct polarity connection
Used in: electronic control boards, VFDs (internal bus), ECMs, rectified low-voltage supplies
Voltage periodically reverses polarity in a sinusoidal waveform
60 Hz in Canada — 60 complete cycles per second
No polarity concern for most resistive and motor loads
Used in: compressors, fan motors, heaters, large solenoid valves
24 VAC control circuits produced by control transformers for thermostats and contactor coils
Single-Phase AC
One alternating voltage; two current-carrying conductors plus equipment ground
Common in residential and light-commercial HVAC/R
Typical voltages: 120 V / 240 V split-phase; 208 V from three-phase panel
Motor starting requires a run capacitor (and often a start capacitor) to create the phase shift needed for rotation
Three-Phase AC
Three voltages equal in magnitude, 120° apart in phase
Standard for commercial and industrial compressors and large fan motors
Typical voltages: 208 V / 460 V / 575 V
Self-starting — no run capacitor required; phase rotation must be verified to prevent reverse rotation
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AC behaviour in inductive circuits
Motors and transformers are inductive loads. At start-up they draw inrush current
(up to 6–8× FLA for compressors) before reaching steady state.
Inductive loads also introduce a power factor lag between voltage and current,
meaning the apparent power (kVA) is higher than the true power (kW). Both effects
must be considered when sizing conductors, protection devices, and transformers.
1.1.4 — Ohm’s Law & Electrical Power
Three relationships govern almost every electrical calculation a field technician
needs to make: Ohm’s Law links voltage, current, and resistance; the power
formula adds watts; and the two combined cover every unknown on a nameplate or circuit.
Ohm’s Law
Ohm’s Law — Three Forms
V = I × R I = V ÷ R R = V ÷ I
V = voltage (volts) | I = current (amperes) | R = resistance (ohms Ω)
Knowing any two of the three quantities allows you to calculate the third. This is used constantly when:
Calculating expected current draw from a known resistance and supply voltage
Checking conductor resistance against expected voltage drop
Estimating heater element resistance from measured current and known voltage
Diagnosing a high-resistance connection by measuring an unexpected voltage drop
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Worked Example — Defrost Heater Current
A defrost heater element is rated at 240 V.
Measured resistance of the element: 48 Ω.
I = V ÷ R = 240 ÷ 48 = 5 A
The technician clamps the heater circuit and reads 5.1 A — within normal
tolerance. A reading of 0 A would indicate an open element; a reading significantly
above 5 A would indicate a partial short or incorrect supply voltage.
Electrical Power
Power Formula — Three Forms
P = V × I P = I² × R P = V² ÷ R
P = power (watts W) | V = voltage (V) | I = current (A) | R = resistance (Ω)
These relationships are used to:
Estimate the load placed on a circuit or panelboard
Verify that measured current is consistent with the nameplate wattage
Size conductors and over-current protective devices correctly
Calculate energy consumption (kWh = kW × hours of operation)
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Worked Example — Sizing for a 3 kW Electric Heater
An electric strip heater is rated at 3 kW (3 000 W) at 240 V.
What current does it draw, and what is the minimum circuit breaker size?
I = P ÷ V = 3 000 ÷ 240 = 12.5 A
The Canadian Electrical Code requires the overcurrent device for a continuous electric
heater load to be sized at 125 % of the load current:
12.5 × 1.25 = 15.6 A → use a 20 A breaker (next standard size up)
The conductor must also be rated for at least 15.6 A continuous — a 12 AWG copper
conductor (rated 20 A at 60 °C) is the typical selection.
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The Ohm’s Law / Power Triangle
Cover the quantity you want to find; the remaining two show the operation.
For the V–I–R triangle: cover V → I×R; cover I → V÷R; cover R → V÷I.
For the P–V–I triangle: cover P → V×I; cover V → P÷I; cover I → P÷V.
These two triangles cover every basic electrical calculation needed in 313A/313D field work.
kW (kilowatt, motor and heater ratings); MW (megawatt, large plant)
Frequency
Hertz
Hz
60 Hz standard in Canada; VFDs output 0–120 Hz typical range
Energy
Watt-hour
Wh / kWh
kWh on electricity bills; MWh for large facilities; BTU conversion: 1 kWh = 3 412 BTU
Capacitance
Farad
F
µF (microfarad) for run and start capacitors; rated ±6 % (run) or ±20 % (start)
Common Schematic Symbols
Electrical diagrams use standardised symbols to represent devices and connections.
Minor variations exist between standards (IEC, NEMA, Canadian Standards), but the
symbols below are consistent across most HVAC/R equipment documentation.
Device
Symbol Description
Notes
Conductor (wire)
Solid line
Crossing wires with a dot = connected; crossing without a dot = not connected
Normally open (NO) contact
Two parallel lines with a gap between them
Open at rest; closes when the coil or actuator energises
Normally closed (NC) contact
Two parallel lines connected by a diagonal line
Closed at rest; opens when the coil or actuator energises
Relay / contactor coil
Circle or rectangle labelled with the device designation
When the coil is energised, all associated contacts change state simultaneously
Transformer
Two coils facing each other, often with core lines between them
Primary coil on left; secondary on right. Voltage ratio = turns ratio
Motor
Circle with the letter M inside
Three terminals shown for three-phase; two for single-phase
Fuse
Rectangle or S-curve in the line
Interrupts the circuit irreversibly on overcurrent; must be replaced after operation
Circuit breaker
Rectangle with a dot and arc
Interrupts and can be reset; provides both overcurrent and short-circuit protection
Ground (earth)
Three horizontal lines decreasing in length downward
Equipment grounding conductor (green/green-yellow) connects metallic enclosures to ground
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Diagram literacy is a core skill
A technician who cannot read and trace a wiring diagram is limited to replacing
parts at random. Section 1.4 (Electrical Wiring Diagrams) builds directly on these
symbols and teaches systematic circuit-tracing technique. Keep this symbol table as
a reference as you work through the section.