Level 1, Unit 4, Section 1 – 1.3 Circuits, Panels & Protection

1.3.1 — Types of Electrical Circuits

HVAC/R systems use all three circuit configurations — series, parallel, and combination — often within the same unit. Recognising which configuration a portion of a circuit uses determines both how it behaves under normal conditions and how you approach troubleshooting.

Series Circuits

In a series circuit, components are connected end-to-end so that the same current flows through every component. There is only one path for current.

Total resistance: R_total = R₁ + R₂ + R₃ + … (always greater than any individual value)
Current: Identical through every component — I_total = I₁ = I₂ = I₃
Voltage: The supply voltage divides across each component in proportion to its resistance (V = I × R)
Open-circuit failure: If any component opens, the entire circuit de-energises — a key design feature for safety chains

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HVAC/R Application — Compressor Safety Chain

A rooftop unit places four safety devices in series with the compressor contactor coil (24 V control circuit):

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Low-Pressure Switch

Opens on low suction; prevents compressor damage from loss of charge

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High-Pressure Switch

Opens on high discharge; protects against blocked condenser or overcharge

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Thermal Overload

Opens on compressor over-current; protects motor windings from heat

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COIL

Contactor Coil

Energises only when all three switches remain closed

Any single safety opening breaks the series chain and de-energises the coil, stopping the compressor. When diagnosing a “compressor won’t start” complaint, measure 24 V across each device in sequence; the first one showing full voltage across it (with the coil de-energised) is the open contact.

Parallel Circuits

In a parallel circuit, each component connects across the same two voltage points, so every branch has the same voltage applied to it.

Voltage: Identical across every branch — V_total = V₁ = V₂ = V₃
Current: Total current is the sum of all branch currents — I_total = I₁ + I₂ + I₃
Total resistance: Always less than the smallest individual resistance: 1/R_total = 1/R₁ + 1/R₂ + …
Open-branch failure: Only the opened branch loses power; other branches remain energised

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Worked Example — Three Parallel Fan Motors

A condenser section has three fan motors connected in parallel across 240 V. Each motor draws 3 A at full load.

Total current drawn by the condenser fan circuit

I_total = 3 + 3 + 3 = 9 A

If fan motor 2 fails open (winding burned out), motors 1 and 3 continue to run. Total current drops to 6 A. The condenser section now has reduced airflow — head pressure rises, capacity drops, and compressor amps increase. This is a common diagnostic pattern: high head pressure with one fan not turning.

Circuit check: Measure 240 V at the terminals of the stopped fan motor. Voltage present but motor not running = open winding or failed capacitor, not a wiring problem.

Series-Parallel (Combination) Circuits

Most real HVAC/R control circuits are combinations: safety devices in series feeding parallel branches that power multiple loads simultaneously.

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Troubleshooting combination circuits

Identify which portion of the circuit is series (safeties, switches) and which is parallel (loads: coils, lamps, relays). A fault in the series portion knocks out all parallel branches. A fault in one parallel branch leaves other branches unaffected. This mental model immediately narrows where to look.

1.3.2 — Distribution Panels & Wiring Configurations

The distribution panel (load centre or panelboard) is the point where utility power is divided into branch circuits that supply individual loads. HVAC/R technicians must be able to identify single-phase and three-phase panels, read the conductor arrangement, and safely work with both.

Feature	Single-Phase Panel	Three-Phase Panel
Ungrounded conductors	2 (Line 1 and Line 2, each 120 V to neutral)	3 (Phase A, B, C — 120° apart)
Grounded (neutral) conductor	1 (required for 120 V loads)	1 (in 4-wire wye systems); absent in 3-wire delta systems
Typical voltages (Canada)	120/240 V split-phase; 120/208 V from 3-phase panel	208Y/120 V (wye); 600/347 V (wye); 480 V (delta)
Breaker arrangement	Single-pole (120 V) and double-pole (240 V) breakers	Single-pole, double-pole, and triple-pole breakers; bus bars on 3 phases
Common HVAC/R loads	Residential split systems, small heat pumps, ERVs, electric baseboard	Commercial RTUs, chillers, large compressors, cooling towers, large AHUs
Motor starting	Requires run/start capacitors for phase shift	Self-starting; phase rotation must be verified

Single-Phase Panel — Conductor Arrangement

A typical residential split-phase service has two ungrounded (hot) conductors at 120 V each to neutral, and 240 V between them. The panelboard bus bars alternate between Line 1 and Line 2, so adjacent breaker slots draw from opposite hot legs:

Single-pole breaker (120 V circuit): connects one hot leg to the load, neutral completes the circuit
Double-pole breaker (240 V circuit): bridges both hot legs; no neutral needed for purely 240 V loads (e.g., condensing unit motors)
Double-pole breaker with neutral (120/240 V): used for dryers, ranges — not typical in HVAC/R
Equipment grounding conductor (bare or green): runs with every circuit to bond metallic enclosures to ground

Three-Phase Panel — Phase Balance

Loads in a three-phase panelboard should be distributed evenly across all three phases. Unbalanced loading causes one phase to carry more current, increasing neutral current (in wye systems), reducing motor efficiency, and potentially tripping overloads.

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High-leg delta (wild-leg) panels

Some older Canadian commercial installations use a 120/240 V delta panel with a centre-tapped transformer. The third phase (high leg, often identified with an orange wire) sits at approximately 208 V to neutral — not 120 V. Connecting a 120 V device to this leg will destroy it. Always measure phase-to-neutral voltage on all three phases before connecting any 120 V load in an unfamiliar panel.

1.3.3 — Over-Current & Overload Protection Devices

Protection devices interrupt current before it causes fire or equipment damage. HVAC/R systems typically use two layers of protection: short-circuit / ground-fault protection (fast, for wiring) and overload protection (slower, tuned to the thermal tolerance of the motor or compressor).

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Fuse

A sacrificial element that melts and opens the circuit when current exceeds its rating. Fast-acting fuses respond almost instantly; time-delay (slow-blow) fuses tolerate brief motor inrush without opening but still clear hard faults. Must be replaced after every operation — never bypassed or upsized.

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Circuit Breaker

A resettable overcurrent device that opens via a bi-metal thermal strip (overload) and/or an electromagnetic trip (short circuit). Rated by amperes, voltage, and interrupting capacity (kAIC). Can be reset after a trip once the fault is cleared — but a breaker that trips repeatedly must be investigated, not simply reset again.

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Thermal Overload Relay

Part of a motor starter assembly. A heater element warms a bi-metal strip proportional to motor current; the strip trips open contacts in the control circuit when sustained overcurrent is detected. Trip class (e.g., Class 10, 20, 30) defines how quickly it responds — matched to the motor’s thermal characteristics.

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Internal Motor Overload

A thermostat or PTC device embedded in the motor windings or compressor shell. Opens when winding temperature becomes excessive, regardless of current. Hermetic compressors and PSC fan motors commonly use internal overloads. Requires the motor to cool before the overload resets — automatic or manual reset depending on design.

Fuse Types & Motor Circuit Application

Fuse Type	CEC Class	Time-Delay?	Motor Inrush Tolerance	Typical HVAC/R Use
Time-delay (dual-element)	RK1, RK5	Yes	High — withstands 500 % FLA for up to 10 s	Motor branch circuit protection (compressors, fan motors); preferred for HVAC/R
Fast-acting	K1, K5	No	Low — trips on inrush; not suitable for motors	Control circuit and electronic equipment protection
Current-limiting (Class J, CC)	J, CC	Usually time-delay	High; also limits let-through energy during faults	Motor circuits where high fault current protection is required

1.3.4 — Protection Device Selection & Coordination

Selecting the right protection device requires balancing two opposing requirements: the device must be large enough to allow normal operation (including motor inrush) without nuisance tripping, yet small enough to open promptly under a true fault condition.

CEC Motor Circuit Protection Rules (Rule 28-200)

Short-circuit / ground-fault protection (fuse or breaker): Maximum rating = 175 % of motor FLA for time-delay fuses; 250 % for inverse-time breakers; 150 % for non-time-delay fuses — subject to manufacturer maximums
Overload protection: Set at 115–125 % of motor FLA (depending on service factor and temperature rise class)
Conductor sizing: Minimum 125 % of motor FLA (CEC Rule 28-106)
Coordination: The overload must trip before the short-circuit device for motor overcurrent; the short-circuit device must clear hard faults faster than the overload can handle

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Worked Example — Compressor Circuit Protection Selection

A single-phase hermetic compressor has the following nameplate data: 240 V / 18 A FLA / 108 A LRA / 1.15 SF. The equipment data tag specifies a Maximum Overcurrent Protection (MOP) of 35 A and a Minimum Circuit Ampacity (MCA) of 22.5 A.

Step 1 — Conductor size (125 % of FLA)

18 A × 1.25 = 22.5 A → select 10 AWG copper (30 A @ 60°C)

Step 2 — Short-circuit protection (time-delay fuse, max 175 % FLA)

18 A × 1.75 = 31.5 A → next standard size = 35 A (within MOP tag limit)

Step 3 — Overload relay (115 % of FLA, SF ≥ 1.15)

18 A × 1.15 = 20.7 A → set overload to 20–21 A, or use internal overload in the hermetic

Result: 10 AWG conductor / 35 A time-delay fuse / 21 A overload. The fuse allows the 108 A locked-rotor inrush during start-up (lasting <0.5 s) without tripping, while the overload catches sustained overcurrent before the motor windings overheat.

Replacing Protection Devices

Match type and rating exactly: A fast-acting fuse replacing a time-delay fuse will nuisance-trip every start; an oversized fuse provides inadequate protection
Never replace a blown fuse without finding the cause: A second fuse will blow immediately if the fault persists — diagnose first
Circuit breakers that trip repeatedly: Investigate the cause (overload, ground fault, wiring problem) before resetting; a breaker that trips on reset has a hard fault on the circuit
Overload relay heater elements: Sized to the motor FLA; if the motor is replaced with one of a different FLA, the heater element must be changed to match
Interrupting rating (kAIC): The replacement device’s interrupting capacity must equal or exceed the available fault current at the panel — check with the electrical contractor if unknown

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Never bypass or defeat a protection device

Bypassing a fuse, jumper-wiring across a tripped overload, or wedging a breaker in the ON position eliminates the protection the device was designed to provide. This is a violation of the Canadian Electrical Code and creates a serious fire and shock hazard. If a protection device keeps operating, the fault must be found and corrected — not the protection removed.