NATE (North American Technician Excellence): HVAC Electrical & Controls

HVAC Electrical & Controls

Audience: working professional / trade certification candidate

HVAC electrical competency covers Ohm's Law and circuit analysis, motor and control components, capacitors and transformers, schematic voltage tracing, and NEC safety basics — the foundation of every refrigeration, heating, and air conditioning system diagnosis.

Key Points

• Ohm's Law triangle: V = I × R; I = V ÷ R; R = V ÷ I. Watt's Law: P = V × I.
• Series circuit: current is the same everywhere; voltage drops add up to source voltage.
• Parallel circuit: voltage is the same across each branch; total resistance is LESS than the smallest resistor.
• Motor FLC tables (NEC) are used for conductor sizing; nameplate current is used for overload sizing.
• Capacitors are rated in microfarads (MFD) and volts; always discharge before testing with ohmmeter.
• Run capacitor replacement: same MFD (±10%) and equal or higher voltage rating.
• Transformer ratio: Vsecondary ÷ Vprimary = Nsecondary ÷ Nprimary.
• Voltage across a CLOSED (good) contact = 0V; voltage across an OPEN (bad) contact = source voltage.
• Voltage across a good fuse = 0V; voltage across a blown fuse = source voltage.
• NEC color codes: black = hot (line 1), red = hot (line 2), white = neutral, green/bare = EGC.
• GFCI trips at ~5 milliamps ground fault — required in bathrooms, kitchens, outdoors, garages.
• ECM (Electronically Commutated Motor) is the most energy-efficient motor type in HVAC.
• 1 horsepower = 746 watts.
• Maximum allowable supply voltage variation: ±10% (NEC/utility standard); ±5% per most HVAC equipment specs.

Why It Matters

Every HVAC service call eventually becomes an electrical problem — a failed contactor, a weak capacitor, a blown fuse, or a miswired control board. Technicians who can trace voltage through a schematic and apply Ohm's Law diagnose in minutes what others chase for hours. NATE exam writers build schematic-based troubleshooting scenarios specifically because this skill separates certified technicians from parts-changers.

Terms To Remember

Resistance (R)

Opposition to current flow; measured in ohms (Ω).

Impedance (Z)

Opposition to AC current flow; includes resistance AND reactance.

Inductive Reactance (XL)

Opposition to AC from a coil/winding; EMF opposes supply voltage.

Current (I)

Flow of electrons in a circuit; measured in amperes.

Voltage (V or E)

Electrical pressure that drives current through a circuit.

Capacitance

Ability to store electrical charge; rated in microfarads (MFD) and volts.

Conductor

Material that readily releases electrons (copper, silver, gold).

Semiconductor

Material with partial conductivity (silicon, germanium); used in controls/electronics.

ECM Motor

Electronically Commutated Motor; 3-phase DC; most energy-efficient HVAC motor.

Contactor

High-current switching device; rated by amperage through contacts.

Line Starter

Contactor with built-in overload protection; used to control motors.

Current Relay

Disconnects start winding on single-phase compressor motor after startup.

PTC Thermistor

Positive Temperature Coefficient — resistance INCREASES as temperature rises.

NTC Thermistor

Negative Temperature Coefficient — resistance DECREASES as temperature rises.

Transducer

Converts a pressure signal into an electrical signal.

Thermocouple

Generates a small DC current when heated; used as a flame sensor on furnaces.

GFCI

Ground Fault Circuit Interrupter; opens circuit when ~5 mA leak to ground detected.

DDC (Direct Digital Control)

Electronic control system for HVAC equipment operation and monitoring.

High Leg (Stinger Leg)

Phase on 3-phase open delta with ~208V to ground; identified by orange color code.

Synchronous Speed

Motor speed = (120 × Hz) ÷ number of poles; 4-pole 60 Hz = 1,800 RPM.

Step Process Formula

Two Core Skills: Ohm's/Watt's Law Calculations + Schematic Voltage Tracing — Side-by-Side

Part A OHMS Watts Law

Part A — Ohm's Law & Watt's Law Calculations

Formulas

OHMS Law

Voltage: V = I × R

Current: I = V ÷ R

Resistance: R = V ÷ I

Watts Law

Power 1: P = V × I

Power 2: P = I² × R

Power 3: P = V² ÷ R

Transformer Ratio: V1 ÷ V2 = N1 ÷ N2 (primary turns ÷ secondary turns)

Synchronous Speed: RPM = (120 × Hz) ÷ number of poles

Parallel Resistance Two: Rt = (R1 × R2) ÷ (R1 + R2) — two resistors only

Parallel Resistance General: 1/Rt = 1/R1 + 1/R2 + 1/R3 + ... — three or more

 

Worked Examples:

Three defrost heaters, each 15 Ω, wired in parallel to 120V. Total watts?

Steps:

1. Step 1 — Total parallel resistance: 1/Rt = 1/15 + 1/15 + 1/15 = 3/15 → Rt = 5 Ω.
2. Step 2 — Total current: I = V ÷ R = 120 ÷ 5 = 24A.
3. Step 3 — Total watts: P = V × I = 120 × 24 = 2,880W.
4. Shortcut: Each heater draws 120² ÷ 15 = 960W. Three heaters = 960 × 3 = 2,880W.

Answer: 2,880 watts

Transformer: 1,000 primary turns, 2,000 secondary turns, 120V input. Output voltage?

Steps:

1. Step 1 — Turns ratio: N1/N2 = 1,000/2,000 = 0.5.
2. Step 2 — V2 = V1 × (N2 ÷ N1) = 120 × (2,000 ÷ 1,000) = 120 × 2 = 240V.
3. Rule: More secondary turns = step-UP transformer. Fewer turns = step-DOWN.

Answer: 240V output (step-up transformer)

How much resistance limits a 120V circuit to 2A?

Steps:

1. R = V ÷ I = 120 ÷ 2 = 60 Ω.

Answer: 60 ohms

Synchronous speed of a 4-pole, 60 Hz motor?

Steps:

1. RPM = (120 × 60) ÷ 4 = 7,200 ÷ 4 = 1,800 RPM.

Answer: 1,800 RPM

Parallel circuit: R1=30Ω, R2=15Ω, R3=10Ω. Current through R2 = 4A. Source voltage?

Steps:

1. Step 1 — Voltage across R2: V = I × R = 4 × 15 = 60V.
2. Step 2 — In a parallel circuit, voltage is the same across every branch.
3. Source voltage = 60V.

Answer: 60 volts

 

Series Vs Parallel Summary

Series

Current: Same through every component

Voltage: Drops add up to equal source voltage

Resistance: Rt = R1 + R2 + R3 + ...

Failure Mode: One open component breaks the entire circuit (one bulb out → all go out)

Parallel

Current: Divides between branches

Voltage: Same across every branch

Resistance: Always LESS than the smallest individual resistor

Failure Mode: One open branch does not affect other branches

Part B Schematic Voltage Tracing

Part B — Schematic Voltage Tracing (Energized vs. De-energized Logic)

Fundamental Rules

• Rule 1 — Good closed contact: 0V across it. Current is flowing freely — no voltage drop.
• Rule 2 — Open contact (or blown fuse): source voltage appears across it. The break = the voltage drop.
• Rule 3 — Energized coil: voltage across it = coil voltage (e.g., 24V or 240V). Current is flowing.
• Rule 4 — De-energized load: source voltage appears across it if circuit is broken upstream.
• Rule 5 — Good fuse in a live circuit: 0V across it.
• Rule 6 — Blown fuse: source voltage across it.
• Rule 7 — Transformer primary: always reads supply voltage (e.g., 240V) when powered.
• Rule 8 — Transformer secondary: reads output voltage (e.g., 24V) when primary is energized.

Typical 240v Ac System Voltages

L1 to L2

Measurement Point: L1 to L2

Expected Voltage: 240V

L1 or L2 to ground

Measurement Point: L1 or L2 to ground

Expected Voltage: 120V each

Across transformer primary (240V system)

Measurement Point: Across transformer primary (240V system)

Expected Voltage: 240V

Across transformer secondary (24V control)

Measurement Point: Across transformer secondary (24V control)

Expected Voltage: 24V

Across R to G on thermostat (control circuit powered)

Measurement Point: Across R to G on thermostat (control circuit powered)

Expected Voltage: 24V

Across a closed/good contactor contact

Measurement Point: Across a closed/good contactor contact

Expected Voltage: 0V

Across an open/bad contactor contact

Measurement Point: Across an open/bad contactor contact

Expected Voltage: 240V

Across a good fuse (circuit live)

Measurement Point: Across a good fuse (circuit live)

Expected Voltage: 0V

Across a blown fuse (circuit live)

Measurement Point: Across a blown fuse (circuit live)

Expected Voltage: 240V

Across a running compressor motor

Measurement Point: Across a running compressor motor

Expected Voltage: 240V

Terminal Y to input side of contactor coil (cooling call active)

Measurement Point: Terminal Y to input side of contactor coil (cooling call active)

Expected Voltage: 0V (same potential — no drop)

Terminal Y to output side of contactor coil (coil energized)

Measurement Point: Terminal Y to output side of contactor coil (coil energized)

Expected Voltage: 24V (coil voltage drop)

Troubleshooting Scenarios

Thermostat contacts closed. Compressor and condenser fan short cycling. Blower NOT running.

Cause: Faulty blower motor or blower relay (coil on blower relay line open).

Same as above. Moving thermostat fan switch from AUTO to FAN ON makes blower run.

Cause: Faulty thermostat AUTO/FAN ON switch — G terminal circuit open in AUTO mode.

Thermostat contacts closed. Compressor and condenser fan NOT running. Blower IS running.

Cause: HP switch, LP switch, overload, or oil pressure switch open (protective device in compressor contactor coil circuit).

Fuse in L2 is blown on a 240V system. Voltage readings?

Cause: L1 to ground = 120V (L1 still live). L2 to ground = 0V (L2 dead). Across blown fuse = 120V.

Voltmeter reads 0V across contactor L1–T1 with motor running at full load.

Cause: GOOD — contacts are clean and closed. 0V across a closed contact = normal operation.

Crankcase heater operation — when does it run?

Cause: Crankcase heater runs when the compressor is OFF. Wired to energize on the de-energized state of the contactor.

High Leg Delta Note

Description: On a 3-phase open delta system, one phase (the 'high leg' or 'stinger leg') reads approximately 208V to ground while the other two phases read 120V to ground.

Identification: High leg is color-coded orange per NEC.

Why It Matters: Connecting 120V equipment to the high leg causes immediate damage — always verify before wiring.

Motors And Controls

Motors & Motor Control Components — Quick Reference

Motor Types

Split-Phase Motor

Type: Split-Phase Motor

Start Device: Centrifugal switch — drops start winding out at ~75% synchronous speed

Torque: Low starting torque

Application: Small fans, pumps

Capacitor-Start (CSIR)

Type: Capacitor-Start (CSIR)

Start Device: Current relay — disconnects start capacitor after motor starts

Torque: High starting torque, normal running torque

Application: Compressors, hard-start applications

Capacitor-Start, Capacitor-Run (CSCR)

Type: Capacitor-Start, Capacitor-Run (CSCR)

Start Device: Start capacitor + current relay; run capacitor stays in circuit

Torque: High starting AND running torque

Application: Large compressors, demanding loads

ECM (Electronically Commutated Motor)

Type: ECM (Electronically Commutated Motor)

Start Device: Built-in variable speed controller

Torque: Variable — adjusts to maintain proper airflow

Application: Air handlers, high-efficiency furnaces — most energy-efficient HVAC motor

3-Phase Induction Motor

Type: 3-Phase Induction Motor

Start Device: None needed — self-starting

Torque: High, consistent

Direction Change: Swap any two of three supply wires (T1, T2, T3)

Single Phase Motor Winding Resistance Rule

Common To Run: Lowest resistance

Common To Start: Medium resistance

Run To Start: Highest resistance (sum of both windings)

Grounded Indication: Any terminal to motor frame reads measurable resistance (should be infinite/OL)

Control Components

Contactor

Component: Contactor

Function: Switches high-current loads (compressor, condenser fan); rated by contact amperage.

Line Starter

Component: Line Starter

Function: Contactor + thermal overload relay; used to control motors; overloads are manually reset.

Relay

Component: Relay

Function: Low-current switching device; controls inductive loads up to ~25A.

Current Relay

Component: Current Relay

Function: Senses starting current spike to disconnect start winding; used on single-phase compressors.

Thermal Overload

Component: Thermal Overload

Function: Protects motor from sustained overcurrent/overheating; manually reset after trip.

Solenoid Valve Coil

Component: Solenoid Valve Coil

Function: Operates on electromagnetism; energizing coil opens or closes valve.

Capacitors And Transformers

Capacitors & Transformers — Exam Targets

Capacitor Rules

• Rated in microfarads (MFD) and volts — both must be specified.
• Run capacitor replacement: MFD within ±10% of original; voltage rating equal to or HIGHER.
• Acceptable swap example: 35 MFD @ 440VAC → 37.5 MFD @ 440VAC is acceptable.
• Always discharge before testing: use a 15,000–20,000 ohm, 2–5 watt resistor across terminals.
• Ohmmeter test: needle deflects toward zero (charges up) then returns to high/infinite reading = GOOD.
• Capacitors are always wired IN SERIES with the start winding.
• Capacitor analyzer reads actual capacitance in MFD — more accurate than ohmmeter.

Transformer Rules

• Rated in VA at the secondary (not watts).
• Step-up transformer: more secondary turns → higher output voltage.
• Step-down transformer: fewer secondary turns → lower output voltage.
• Residential HVAC control transformers: 240V primary → 24V secondary.
• Open coil on ohmmeter: reads infinite (OL) — no continuity.
• Shorted coil on ohmmeter: reads near zero ohms — very low resistance.

NEC Basics For HVAC

NEC Basics Tested on HVAC Exams

Color Codes

Black

Color: Black

Meaning: Hot conductor (Line 1) in AC circuits

Red

Color: Red

Meaning: Hot conductor (Line 2) in 240V circuits

White or Gray

Color: White or Gray

Meaning: Neutral (grounded) conductor

Green or Bare

Color: Green or Bare

Meaning: Equipment grounding conductor (EGC / safety ground)

Orange

Color: Orange

Meaning: High leg (stinger leg) on 3-phase open delta systems

Key NEC Rules

• GFCI required in: bathrooms, kitchens (within 6 ft of sink), outdoors, garages, basements, near pools/spas.
• GFCI trips at approximately 5 milliamps of ground fault current.
• NEC purpose: minimum standards for safe electrical installation — not a design manual.
• Circuit breakers and fuses: primary purpose is protection from overcurrent (short circuit / overload).
• Ohmmeter uses its own internal battery — NEVER connect to a live circuit or charged capacitor.
• Maximum allowable supply voltage variation for most HVAC equipment: ±10% of rated voltage.

Common Confusions

• Students confuse voltage across a good contact with voltage across a bad contact because both are measured the same way — a CLOSED (good) contact reads 0V across it (current flows freely, no drop); an OPEN (bad) contact reads full source voltage across it (the break is where the voltage appears).
• Students confuse run capacitor MFD tolerance with voltage tolerance because both have replacement rules — MFD must be within ±10% of the original rating; voltage rating must be equal to OR HIGHER than the original (never lower); exceeding MFD by too much causes overheating.
• Students confuse PTC with NTC thermistors because both change resistance with temperature — PTC (Positive) resistance INCREASES with heat (like a self-resetting fuse); NTC (Negative) resistance DECREASES with heat (common in temperature sensors).
• Students confuse the purpose of a line starter with a contactor because both switch motors — a contactor switches the load with no built-in overload protection; a line starter is a contactor WITH an integrated thermal overload relay; only a line starter provides motor overload protection.

Quick Questions

Question 1

Question: A voltmeter placed across a contactor's L1–T1 contacts reads 240V with the system energized. What does this indicate?

Answer: The contacts are OPEN (bad/failed). In a live circuit, full source voltage appears across an open contact because it represents the entire voltage drop point. Good closed contacts read 0V.

Question 2

Question: A run capacitor is rated 35 MFD @ 440VAC. Which replacement is acceptable: (A) 30 MFD @ 440VAC, (B) 37.5 MFD @ 440VAC, (C) 35 MFD @ 370VAC?

Answer: B — 37.5 MFD @ 440VAC. It is within ±10% of 35 MFD (range: 31.5–38.5 MFD) and the voltage rating matches. Option A (30 MFD) is more than 10% below. Option C has a lower voltage rating (370V < 440V) — never use a lower voltage rating.

Question 3

Question: On a single-phase compressor motor, an ohmmeter reads: C-to-R = 3Ω, C-to-S = 5Ω, R-to-S = 8Ω, C-to-ground = 12Ω. What does this indicate?

Answer: Grounded condition. The winding resistance values are normal (R-to-S = C-to-R + C-to-S = 8Ω ✓). However, any terminal to motor frame (ground) should read infinite (OL). A reading of 12Ω to ground indicates a winding is grounded — motor must be replaced.

Exam Answer Frame

Style: 5-mark

Question: A technician is troubleshooting a 240V split system. The thermostat contacts are closed. The contactor coil is de-energized. The compressor and condenser fan are not running. The blower motor IS running. Describe a systematic voltage-tracing approach to diagnose the fault.

Model Answer: The compressor contactor coil is de-energized despite a cooling call — the fault is in the 24V control circuit between the thermostat Y terminal and the contactor coil. Systematic approach: Step 1 — Confirm 240V across transformer primary (L1 to L2). Step 2 — Confirm 24V across transformer secondary (R to C on control board). Step 3 — Confirm 24V at thermostat R terminal. Step 4 — With thermostat calling for cooling, confirm 24V at Y terminal. Step 5 — Trace from Y through each series-connected safety control (high-pressure switch, low-pressure switch, thermal overload, oil pressure switch) toward the contactor coil. The component that shows 24V across it is OPEN — that is the fault. A good closed safety switch reads 0V across it; a tripped or open safety reads 24V. Replace or reset the faulted safety device and retest.