Your FREE NATE Flashcards 2026 – 250+ Cards

V = I × R (voltage = current × resistance).

I = V ÷ R.

R = V ÷ I.

P = V × I (watts = volts × amps).

P = V² ÷ R.

P = I² × R.

I = P ÷ V = 1500 ÷ 120 = 12.5 amps.

P = V² ÷ R = 120² ÷ 60 = 14,400 ÷ 60 = 240 watts.

The same at every point in the circuit.

Resistances add (R total = R1 + R2 + …).

Voltage divides across the loads (three equal loads on 120 V drop 40 V each).

The same across each branch.

Current divides among the branches.

Less than the smallest individual branch resistance.

Volts / amperes / ohms / watts.

The number of complete AC cycles per second; U.S. line frequency is 60 Hz.

AC reverses direction periodically (line power); DC flows one direction (batteries, ECM internals).

Single-phase = one waveform (residential 120/240 V); three-phase = three offset waveforms (commercial, smoother).

Three offset waveforms give smoother, more efficient motor operation.

Run cap stays in the circuit (efficiency/torque); start cap gives a brief starting boost then drops out.

Microfarads (µF or MFD).

They can hold a dangerous charge after power is off — discharge them safely before handling.

Contactor switches large continuous loads (compressor/fan); relay switches smaller control loads.

Steps voltage up or down; HVAC controls commonly run on 24 V from a step-down transformer.

The secondary voltage is stepped down lower than the primary.

Volt-amperes — its maximum power-handling capacity.

Permanent split-capacitor — a single-phase induction motor that uses a run capacitor.

Electronically commutated — a brushless DC motor, variable-speed and high-efficiency, holds constant airflow.

ECM is far more efficient and variable-speed; PSC is fixed-speed and lower efficiency.

To open the circuit when current exceeds a safe level, protecting the wiring.

A short circuit or a grounded fault.

A hot conductor contacting a grounded surface or neutral — a low-resistance path that lets current rise sharply.

Resistance — and must be used on a de-energized circuit.

To verify the very high resistance of winding insulation to ground (insulation quality).

Near mid-scale.

Voltage, resistance (ohms), and current (amps).

It reads motor current without breaking the circuit, by sensing the magnetic field around a conductor.

120 / 208 / 240 / 480 V (also 277 V).

Ground-fault protection for people; it trips at about 4–6 mA of imbalance, far below a breaker's rating.

Low-voltage control circuits (thermostat, contactor coil, control board).

It has failed and must be replaced.

A weak or failed run/start capacitor.

Poor connection or a stuck-closed circuit on outdoor condensing units.

A wiring/schematic shows the circuit logic; a pictorial shows the physical layout of components.

Senses indoor temperature and signals the system (often via 24 V) to heat or cool.

A device that energizes heating elements in stages to limit inrush current.

The high current a motor draws at the instant of starting, before it turns.

The current a motor draws at its rated running load.

It is near locked-rotor current; a failed capacitor or seized bearing can cause it.

A single-phase compressor or motor.

A relay that drops the start capacitor out of the circuit once the motor is up to speed.

A start capacitor and relay added to boost a struggling compressor's starting torque.

An ohmmeter applies its own voltage; measuring a live circuit damages the meter and gives false readings.

Checking for a complete (low-resistance) path; used to find open windings, fuses, and switches.

Infinite (OL) — there is no complete path.

Low resistance from a winding to the casing (ground) — a failed motor.

A loose/corroded connection adds resistance, dropping voltage and generating heat.

Inrush is the brief high current at start; running current is the steady draw once up to speed.

It is about 26% of the NATE Core exam — more than any other KATE area.

Basic Electricity (~26%) and Safety (~18%).

A direct short or a grounded fault is drawing excessive current.

To isolate energy, lock/tag it so it can't be re-energized, and verify de-energization before service.

Verifying the absence of voltage with a meter before touching the equipment.

29 CFR 1910.147 (control of hazardous energy).

Electrical, stored refrigerant pressure, capacitor charge, rotating parts, thermal.

Ordinary combustibles — wood, paper, cloth, trash.

Flammable liquids and gases — solvents, oils, gasoline, propane.

Energized electrical equipment — uses a nonconductive agent.

Combustible metals — magnesium, sodium, titanium.

Cooking oils and fats (commercial kitchens).

Class C — the only safe choice on energized equipment (an ABC unit qualifies).

Classes A, B, and C — the typical service-van choice.

Water conducts electricity — a shock hazard. De-energize first or use Class C.

Safety glasses or goggles (liquid refrigerant can cause frostbite/eye injury).

Hearing protection — earplugs or earmuffs.

Place the base 1 foot out for every 4 feet of ladder height.

About 4 feet out from the wall.

At least 3 feet above the edge for safe access.

Federal certification required to handle refrigerants.

Small appliances.

High-pressure systems (split systems, heat pumps).

Low-pressure systems (chillers).

Covers Type I, II, and III.

No — it is illegal to knowingly vent refrigerant; it must be recovered.

Oxygen combined with refrigerant oil can ignite explosively; use dry nitrogen.

Energized, until a meter proves otherwise.

A dangerous release of energy from an electrical fault; covered by NFPA 70E electrical-safety practices.

Colorless, odorless gas from incomplete combustion; binds to hemoglobin and starves the body of oxygen.

Respiratory protection.

It can hold a lethal charge even with power off.

Refrigerant can displace oxygen; ensure ventilation and monitor before entry.

Eye protection (shaded if needed), gloves, and ventilation for fumes.

To prevent oxidation (scale) inside the tubing that can foul the system.

Frostbite — wear gloves and eye protection.

Ensure safe access and fall protection; extend the ladder ≥3 ft above the edge.

De-energize, lock/tag, and verify absence of voltage with a meter.

A document describing a chemical/refrigerant's hazards, handling, and first aid.

≈ 68–75°F.

≈ 30–50% (best-practice guidance, not a fixed ASHRAE clause).

≈ 400 CFM per ton (band 350–450).

≈ 1,200 CFM (3 × 400).

More dehumidification (latent removal) — good in humid climates.

More sensible cooling — good in dry climates.

Qs = 1.08 × CFM × ΔT (°F).

Qt = 4.5 × CFM × Δh (enthalpy, BTU/lb).

Ql = 0.68 × CFM × ΔW (humidity ratio, grains/lb).

Total = sensible + latent (Qt = Qs + Ql).

Not enough warm air over the coil lets the refrigerant temperature drop below freezing.

Brief runs don't keep the evaporator running long enough to remove much moisture.

Supply + return static pressure; a typical residential target is ≈ 0.5 in. w.c.

Warmer air can hold more moisture, so relative humidity is lower.

Its surface dropped to or below the air's dew point, so moisture condenses.

Senses indoor humidity and switches a humidifier on/off to hold a set level.

The pressure difference between the higher-pressure supply and lower-pressure return.

When there's no convenient return-side connection (its own fan moves the air).

Provides a wetted surface so warm air picks up moisture by evaporation.

In the direction of airflow — toward the blower.

Restricts airflow → lower capacity and possible coil freeze.

Captures finer particles but adds airflow resistance — don't over-restrict the blower.

It preserves the design airflow the system was sized for.

Mold, mildew, condensation, and material damage.

Dry air, static electricity, and cracking/shrinking wood.

The air filter — a neglected filter is the most common cause.

Minimum Efficiency Reporting Value — a rating of a filter's particle-capture efficiency.

An adjustable plate that controls airflow in a duct (e.g., balancing or zoning).

Adjusting dampers/registers so each room gets its designed share of airflow.

A register has a damper to control airflow; a grille is a fixed-louver cover.

The air chamber on the supply or return side of the air handler that distributes air.

High return static pressure, reduced airflow, and strained equipment.

Ordinary air temperature read by a standard thermometer.

Temperature from a wetted-wick thermometer; evaporation cools it, so it's lower than dry-bulb.

The temperature at which air saturates and water vapor begins to condense.

Dry-bulb ≥ wet-bulb ≥ dew point for the same air.

Equal — dry-bulb = wet-bulb = dew point.

Water vapor in the air vs the most it could hold at that temperature, as a percent.

Evaporation removes heat until the air can no longer absorb more moisture.

An instrument whose only job is to measure the moisture (humidity) of air.

A chart relating dry-bulb, wet-bulb, dew point, RH, humidity ratio, and enthalpy; any two fix the state.

Its electrical resistance changes predictably as its temperature changes.

32°F.

The thermometer is reading 4° high and should be corrected.

So the sensing portion fully reaches the temperature being measured.

Low surface emissivity reflects surrounding radiation instead of emitting its own.

Moving air carries away evaporated moisture so evaporation can cool the wet bulb.

The dry-bulb drop from return to supply air — a quick operational check.

Picking up unconditioned surrounding air instead of the duct air.

The difference between dry-bulb and wet-bulb temperature; larger = drier air.

1 pound of water (humidity ratio is measured in grains per pound of dry air).

Yes — any two known properties locate a single point on the chart.

Relative humidity and dew point (from dry-bulb and wet-bulb readings).

Evaporating water absorbs latent heat from the wick, lowering its temperature.

RH and dew point from measured dry-bulb and wet-bulb temperatures.

The air is fully saturated (100% relative humidity).

Grains per pound of dry air is a convenient small unit for water vapor (7,000 grains = 1 lb).

Total heat content of the air — basis for total-heat (4.5 × CFM × Δh) calculations.

Red = high side, blue = low-side compound, yellow = center charging/recovery hose.

Both pressure above atmospheric and vacuum below it.

Each refrigerant has a different pressure-temperature relationship.

Confirming a deep vacuum (target ≤500 microns) that a compound gauge can't resolve.

It pegs near 29–30 in. Hg and can't resolve the small changes near a deep vacuum.

To boil off moisture and remove air and non-condensable gases that harm performance.

500 microns or below.

Evacuates a system to a deep vacuum to remove air and moisture before charging.

Recover = remove refrigerant; recycle = clean on-site; reclaim = restore to virgin spec at a facility.

So liquid can expand safely without dangerously high pressure as temperature rises (never overfill).

Gas pressure and air static pressure, in inches of water column.

Dry-bulb and wet-bulb temperature, to determine humidity.

Expanding one tube end so a matching tube slips inside for a brazed joint — no coupling needed.

It produces a smooth-radius bend with no added joints to leak or restrict flow.

It cleans the joint and prevents oxidation so the filler metal flows and bonds.

When refrigerant vapor reaches its sensor.

Dye circulates and glows at the leak under a UV lamp.

Dry nitrogen under pressure — bubbles form at the leak.

The sound of gas escaping under pressure (good in large/noisy systems).

Reading a motor's amp draw without breaking the circuit.

Differential (static) pressure across a duct or filter, showing airflow resistance.

Watch for a pressure rise to confirm the system is dry and leak-free.

A valve on the system used to connect gauges, charge, or isolate refrigerant during service.

Weighing in an exact refrigerant charge for accuracy.

Making clean, square cuts in copper tubing without crushing it.

Removing burrs from a cut tube end so flow isn't restricted and joints seat properly.

It forms a 45° cone on a tube end for a mechanical flare fitting.

Flue-gas O₂, CO, CO₂, and stack temperature to assess combustion safety and efficiency.

Compressor → condenser → metering device → evaporator → back to the compressor.

The evaporator — low-pressure refrigerant boils and absorbs heat from the indoor air.

The condenser (high side) — hot vapor releases heat to the outdoor air and condenses to liquid.

It is the pump — it raises the pressure and temperature of low-pressure vapor and circulates refrigerant.

Drops pressure and meters refrigerant flow into the evaporator; splits the high and low sides.

No — it moves heat. Heat is absorbed at the evaporator and rejected at the condenser.

The compressor and the metering device.

Sensible heat changes temperature; latent heat changes state (phase) with no temperature change.

Degrees a refrigerant vapor is above its saturation (boiling) temperature once it is 100% vapor.

Degrees a refrigerant liquid is below its saturation (condensing) temperature.

Condensing saturation temperature − measured liquid-line temperature (e.g., 105° − 95° = 10°).

It confirms only vapor (never damaging liquid) returns to the compressor.

Conduction (contact), convection (fluid motion), radiation (electromagnetic waves).

From warmer to cooler.

12,000 BTU/hr.

1 BTU per pound per °F.

≈ 970 BTU/lb (at 212°F).

≈ 144 BTU/lb (at 32°F).

British Thermal Unit — heat to raise 1 lb of water by 1°F.

The temperature at which a refrigerant changes state at a given pressure; liquid and vapor coexist.

The total heat content of a substance (sensible + latent), in BTU per pound.

A refrigerant's saturation (boiling) temperature drops as its pressure drops.

A warm copper line heating a fitting it touches.

Warm air rising off a furnace; a blower moving conditioned air.

The sun warming a roof; an infrared heater warming objects.

At constant temperature, halving a gas's pressure doubles its volume (P and V are inversely related).

The gas laws are based on absolute zero; using °F/°C directly gives wrong results.

Total pressure of a gas mixture equals the sum of each gas's partial pressure.

A solid changing directly to a gas without becoming liquid (e.g., dry ice).

Solid → liquid → gas.

CH₄ + 2 O₂ → CO₂ + 2 H₂O + heat (a clean blue flame).

Carbon monoxide (CO), soot, and unburned fuel — a yellow, lazy flame.

O₂ (excess air), CO₂ (completeness), CO (safety), and stack temperature (efficiency).

Annual Fuel Utilization Efficiency — the % of a furnace's fuel energy that becomes usable heat over a year.

≈ 80% (today's DOE minimum).

≈ 90–98.5% by reclaiming latent heat from flue gases.

Flue-gas water vapor condenses, producing condensate that must drain away.

≈ 1,000 BTU per cubic foot.

≈ 10:1 by volume (with some excess air in practice).

Burning with exactly the theoretical amount of air for complete combustion.

It binds to hemoglobin far more readily than oxygen, starving the body of oxygen.

Can leak combustion products (including CO) into the conditioned air stream.

A thermostatic expansion valve modulates refrigerant flow to maintain target superheat.

A simple metering device with no moving parts; flow varies with pressure difference.

A small amount of refrigerant that vaporizes as pressure drops at the metering device.

A refrigeration system with a reversing valve so it can heat or cool by moving heat either direction.

Switches a heat pump between heating and cooling by reversing refrigerant flow.

Low-pressure superheated vapor returning to the compressor.

High-pressure subcooled liquid leaving the condenser.

Refrigerant boils (changes state) while absorbing heat at nearly constant temperature.

Refrigerant condenses (changes state) while rejecting heat at nearly constant temperature.

To confirm the refrigerant is fully vaporized before it reaches the compressor.

The vertical wall framing members in wood-frame construction.

The horizontal framing members that support floors and ceilings.

The sloped framing members that form the roof.

Through the open bay between studs, away from key structural members.

Running down the center of the house, parallel to the ridge.

The overhead drawing used to locate equipment and runs.

The unobstructed distance a structural member crosses without support.

The branch duct run that feeds a register off the main trunk.

Otherwise conditioned air leaks out and unwanted heat transfers in, wasting energy.

Leave it undisturbed and have it assessed — disturbing it releases fibers.

Stop; do not dry-grind or saw it; follow abatement requirements.

A horizontal member spanning an opening (door/window) to carry loads above.

A material's resistance to heat flow — higher R = better insulation.

U = 1 ÷ R; a lower U-value (higher R-value) insulates better.

It offers little traction, making slips and falls likely — a fall hazard.

Yes — combined R-value is the sum of the layers.

A material that limits moisture migration through walls/floors to prevent condensation.

To prevent air leakage and maintain the building envelope.

The underside of an overhang; often used to route ducts and provide intake ventilation.

Attics get very hot/cold, so high R-value limits heat gain/loss from the conditioned air.