Level 1, Unit 4, Section 1 – 1.2 Circuit Components

1.2.1 — Resistors: Selection & Identification

Resistors limit current, divide voltage, and set time constants in electronic control circuits. While you will rarely replace individual resistors in the field, understanding how they work and how to read their markings is essential for interpreting control board schematics and identifying faulty components during board-level troubleshooting.

Factors When Selecting Resistors

Factor	Description	Typical HVAC/R Context
Resistance value (Ω)	Determines current limiting and voltage division. Must match circuit requirements precisely.	Sensor biasing networks; timing RC circuits on control boards
Power rating (W)	Maximum power dissipation without overheating. Underrated resistors discolour, crack, or open.	¼ W and ½ W for low-voltage control circuits; larger for power supplies
Tolerance	Allowable variation from stated value (±1 %, ±5 %, ±10 %). Tighter tolerance = higher cost.	±5 % for general control; ±1 % for precision sensor calibration
Temperature coefficient	Rate of resistance change with temperature (ppm/°C). Low coefficient needed for stable measurements.	Thermistors use high temperature coefficient intentionally for temperature sensing

Resistor Identification Codes

Fixed resistors are marked with their value using colour bands or alphanumeric codes. Knowing these codes allows a technician to verify a replacement component before installation.

Four-Band Colour Code

Bands 1 & 2: First and second significant digits
Band 3 (multiplier): Power of 10 to multiply by
Band 4 (tolerance): Gold = ±5 %, Silver = ±10 %
Colour sequence: Black(0) Brown(1) Red(2) Orange(3) Yellow(4) Green(5) Blue(6) Violet(7) Grey(8) White(9)
Example: Brown–Black–Red–Gold = 1, 0, ×100 = 1 000 Ω ±5 %

SMD Numeric Code (Surface-Mount)

Three-digit code: first two digits = significant figures; third digit = multiplier (number of zeros)
Example: “472” = 47 × 10² = 4 700 Ω (4.7 kΩ)
Four-digit code: three significant figures + multiplier
Example: “1002” = 100 × 10² = 10 000 Ω (10 kΩ)
“R” in the code denotes the decimal point: “4R7” = 4.7 Ω

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Special resistors: thermistors and NTC sensors

Many HVAC/R control boards use NTC thermistors (Negative Temperature Coefficient) as temperature sensors. Their resistance decreases as temperature rises — the opposite of most materials. When a temperature sensor reads a fault, measure the thermistor resistance at a known temperature and compare to the manufacturer’s resistance-temperature table before condemning the control board.

1.2.2 — Conductor Materials & Construction

The conductor is the path current travels. Selecting the right material, size, and construction for each application is one of the most important decisions in any electrical installation — it directly affects safety, reliability, and compliance with the Canadian Electrical Code (CEC).

Conductor Materials

Copper

Lowest resistance of common conductor metals
Excellent mechanical strength and flexibility
Most common material in HVAC/R equipment leads, control wiring, and branch circuits
Easier to terminate without special preparation
Higher cost and weight than aluminum, but preferred for most HVAC/R applications

Aluminum

Higher resistance per unit area than copper — requires a larger gauge for the same ampacity
Lighter and less expensive; used for large feeders and service entrance conductors
Prone to oxide layer formation at terminations, increasing resistance over time
Requires anti-oxidant compound and CU/AL-rated terminals
Not used inside equipment; limited to building wiring feeders

Solid vs. Stranded Construction

Solid Conductor

Single wire of the full cross-sectional area
Maintains shape after bending — easier to route through conduit
Common in branch circuit building wiring (12 AWG and 14 AWG NMD-90)
Not suitable where repeated flexing occurs

Stranded Conductor

Multiple small wires twisted together to form one conductor
Highly flexible — resists fatigue from repeated bending
Used for motor leads, equipment wiring, and flexible conduit connections
Slightly larger overall diameter than equivalent solid conductor

American Wire Gauge (AWG)

AWG is the standard sizing system for conductors in North America. The gauge number runs inversely to size — a smaller number means a larger, heavier conductor with higher ampacity and lower resistance per foot.

AWG	Approx. Diameter (mm)	Ampacity at 60°C (copper)	Common HVAC/R Use
18	1.02	7 A	Thermostat cable, low-voltage control wiring (24 VAC)
14	1.63	15 A	15 A branch circuits; small single-phase equipment
12	2.05	20 A	20 A branch circuits; typical residential condensing unit circuits
10	2.59	30 A	30 A circuits; larger single-phase A/C and heat pump units
8	3.26	40 A	Electric heating, larger single-phase compressors
6	4.11	55 A	Larger commercial single-phase and small three-phase loads
4	5.19	70 A	Three-phase commercial equipment, large air handlers

Ampacity values are for copper conductors in free air at 30 °C ambient, 60 °C insulation rating. Always apply CEC derating factors for conduit fill, high ambient temperature, and continuous loads.

1.2.3 — Conductor Ampacity Rating

Ampacity is the maximum continuous current a conductor can carry without exceeding the temperature rating of its insulation. The published ampacity tables in the Canadian Electrical Code assume standard conditions; real installations often require derating (reducing the allowable current) to account for adverse conditions.

Factors That Determine Ampacity

Conductor material: Copper has lower resistance and higher ampacity than aluminum of the same gauge.
Conductor size (AWG or mm²): Larger cross-sectional area carries more current with less heat generation per unit length.
Insulation type and temperature rating: 60 °C, 75 °C, and 90 °C ratings; higher-rated insulation permits higher ampacity at a given temperature.
Ambient temperature: Tables assume 30 °C ambient. In hot equipment rooms or rooftop installations where ambient may reach 40–50 °C, ampacity must be reduced.
Number of current-carrying conductors in a raceway or cable: Grouped conductors cannot dissipate heat as easily; ampacity is derated when more than three conductors share a conduit.
Continuous load factor: The CEC requires conductors supplying continuous loads (operating ≥3 hours) to be sized at 125 % of the load current.

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Worked Example — Ampacity Derating for Conduit Fill

A rooftop unit requires three 10 AWG copper THHN conductors (90 °C rated, table ampacity = 40 A) run in the same conduit alongside three other current-carrying conductors from a different circuit — six conductors total.

CEC Table 5C derating factor for 7–9 conductors in a raceway: 70 % (for 4–6 conductors: 80 %).

Derated Ampacity

40 A × 0.80 = 32 A (6 conductors in raceway)

The rooftop unit has a minimum circuit ampacity (MCA) of 28 A. With a derated ampacity of 32 A, the 10 AWG conductor still meets requirements. If the MCA were 35 A, a larger conductor (8 AWG, table ampacity 55 A → derated 44 A) would be required.

Lesson: Always check conduit fill when multiple circuits share the same raceway. This is a common oversight on HVAC/R rooftop installations where conduit from several units converges at the service disconnect.

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Motor conductor sizing rule

For HVAC/R motor branch circuits, the conductor must be sized for at least 125 % of the motor’s FLA (CEC Rule 28-106). This is separate from the overcurrent device rating. Example: a compressor with 18 A FLA requires conductors rated for at least 18 × 1.25 = 22.5 A — select 10 AWG (30 A at 60 °C), not 12 AWG.

1.2.4 — Wire Insulating Materials

Insulation serves two purposes: it prevents current from taking unintended paths (shock, short circuit) and it protects the conductor from its environment (moisture, oil, heat, and mechanical damage). Selecting the correct insulation type is as important as selecting the correct conductor size.

Key Properties of Insulating Materials

Dielectric strength: Resistance to electrical breakdown under high voltage. Measured in volts per millimetre (V/mm); must exceed the circuit voltage by a large safety margin.
Temperature rating: Maximum continuous operating temperature. Using a conductor rated below the actual operating temperature degrades insulation and creates a fire hazard.
Moisture and chemical resistance: Critical in refrigeration plant rooms, outdoor condensing unit wiring, and areas exposed to refrigerant oil or cleaning chemicals.
Flexibility: Required for motor leads, equipment wiring, and any location where conductors are flexed during installation or service.
Mechanical (abrasion) resistance: Conductors routed through sheet metal knockouts, conduit fittings, or equipment enclosures need durable insulation that resists cutting.

Common Wire Types in HVAC/R

Wire Type / Designation	Temp. Rating	Characteristics	Typical HVAC/R Application
TW	60 °C	Thermoplastic insulation; moisture-resistant; basic building wire	Branch circuit wiring in dry locations; older installations
THW	75 °C	Thermoplastic, heat and moisture resistant; higher ampacity than TW at same gauge	Conduit wiring in damp or wet locations; equipment feeders
THHN / THWN	90 °C (dry) / 75 °C (wet)	Nylon outer jacket over PVC insulation; thin, flexible, high temperature rating; very common	Standard choice for conduit wiring in new HVAC/R installations
XHHW	90 °C (dry & wet)	Cross-linked polyethylene (XLPE) insulation; excellent moisture, heat, and chemical resistance	Outdoor and wet-location feeders; industrial refrigeration wiring
Thermostat cable (NMT / CL2)	60 °C	Multi-conductor (typically 2–8 conductors, 18 AWG); colour-coded individually; PVC jacket	24 VAC control circuits: thermostat, zone valves, heat anticipators
Equipment lead / appliance wire	105 °C and above	Flexible, heat-resistant (often fibreglass or silicone insulated); rated for high-temperature environments	Factory wiring inside equipment near compressors, heater elements, and motor terminals
FEP / Silicone high-temp	200 °C+	Fluoropolymer or silicone rubber; extreme temperature resistance; chemically inert	Wiring adjacent to electric defrost heaters, high-temperature furnace controls

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Worked Example — Insulation Selection for a Rooftop Unit

A 5-ton rooftop unit (208/230 V, 3-phase) is being wired with conductors run in rigid conduit from the rooftop disconnect to the unit. The conduit is exposed on the roof surface where summer ambient temperatures can reach 45 °C, and rain enters the conduit hub during storms.

Wet location: THWN or XHHW rated; eliminates standard TW or THHN (dry rating only in wet).
High ambient (45 °C): Applies a temperature derating factor to 75 °C-rated wire. XHHW at 90 °C wet has no derating until 40 °C — a slight derating applies but retains more margin.
Selection: XHHW-2 (90 °C wet) is the preferred choice for this application — it handles both moisture and elevated ambient temperature without a significant ampacity penalty.

The equipment leads inside the unit (near the compressor and heater) remain the factory-installed high-temperature appliance wire, which is not replaced in the field.

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Correct insulation = correct ampacity table column

CEC ampacity tables list values for 60 °C, 75 °C, and 90 °C rated conductors. A 10 AWG copper conductor has a 60 °C ampacity of 30 A, a 75 °C ampacity of 35 A, and a 90 °C ampacity of 40 A. Always use the column matching the actual insulation temperature rating of the installed conductor — not the highest column available.