Where: V = voltage (volts), I = current (amps), R = resistance (ohms), P = power (watts)
Example: A 240V compressor draws 15A → P = 240 × 15 = 3,600 watts = 3.6 kW
Series: Same current flows through all. One open = entire circuit dead. Rtotal = R1 + R2 + …
Parallel: Same voltage across all branches. One branch can open without stopping others.
1/Rtotal = 1/R1 + 1/R2 + … (total resistance is always less than the smallest branch)
In HVAC: Control circuits are series. Line voltage loads (compressor, blower, condenser fan) are parallel.
Turns ratio: Vprimary / Vsecondary = Nprimary / Nsecondary
Residential control transformer: 240V primary → 24V secondary (common: 40VA, 75VA, 100VA)
Control circuit fuse: Typically 3A fuse on 24V secondary. Blown fuse = no 24V = no thermostat call response.
| kVA Rating | Max Load at 24V |
|---|---|
| 20 VA | 0.83 A |
| 40 VA | 1.67 A |
| 75 VA | 3.1 A |
| 100 VA | 4.2 A |
| Type | Full Name | How It Starts | Where Used |
|---|---|---|---|
| PSC | Permanent Split Capacitor | Run capacitor only — always in circuit | Condenser fans, most blower motors |
| CSIR | Capacitor Start, Induction Run | Start capacitor + centrifugal switch, disconnects at ~75% speed | Single-phase compressors, pumps |
| CSCR | Capacitor Start, Capacitor Run | Start cap + run cap; start cap removed by switch after start | Hard-start compressors |
| ECM | Electronically Commutated Motor | Brushless DC motor driven by internal inverter — no capacitor | Variable-speed blowers (high efficiency) |
| Shaded Pole | Shaded Pole | Shading coil creates phase shift — no capacitor | Small evaporator fans, low-torque loads |
Test with a capacitor meter (capacitance function). A capacitor is good if the measured microfarad (MFD/µF) value is within ±6% of the rated value printed on the label.
| Rated MFD | Acceptable Range (±6%) |
|---|---|
| 5 MFD | 4.7 – 5.3 MFD |
| 10 MFD | 9.4 – 10.6 MFD |
| 30 MFD | 28.2 – 31.8 MFD |
| 45 MFD | 42.3 – 47.7 MFD |
Danger: Discharge capacitors before testing or handling. A charged run capacitor can deliver a dangerous shock even with power off.
A ladder diagram has two vertical rails (L1 and L2, or R and C for 24V). Each horizontal rung is one control function. Current flows LEFT to RIGHT from the power rail through contacts and loads.
| Symbol | Meaning | State When Coil De-Energized |
|---|---|---|
| NO contact (normally open) | Open at rest — closes when relay/contactor coil energizes | Open → no current |
| NC contact (normally closed) | Closed at rest — opens when relay/contactor coil energizes | Closed → current flows |
| Coil circle | Relay or contactor coil — energizes when rung path is complete | — |
| Safety switch (open) | Limit switch, pressure switch, or thermostat — interrupts circuit | Depends on type |
Trace a cooling call: Thermostat Y closes → 24V energizes contactor coil → NO contacts of contactor close → 240V to compressor and condenser fan.
| Symptom | Likely Cause | Quick Test |
|---|---|---|
| System does not start at all | Blown fuse/tripped breaker, failed transformer, open thermostat | Check supply voltage, then 24V at transformer secondary terminals |
| Compressor hums, will not start | Bad start or run capacitor, low voltage, locked rotor | Measure capacitor MFD; measure voltage at compressor terminals |
| Blower runs, compressor does not | Contactor not pulling in, low-pressure switch open, start delay timer | Measure voltage at contactor coil (should be 24V when Y is closed) |
| System runs, trips breaker | High amp draw — bad capacitor, dirty coil, refrigerant overcharge | Clamp-meter amps at compressor leads vs. nameplate RLA |
| Control transformer fuse blows repeatedly | Short circuit in 24V control wiring or shorted coil | Disconnect each 24V load (contactor, reversing valve, etc.) one at a time until fuse holds |
| No cooling but blower and compressor run | Reversing valve stuck (heat pump), low refrigerant, bad TXV | Check supply and return air temps, compressor suction and discharge pressures |
| Rule | What It Means |
|---|---|
| Use nameplate MCA for conductor sizing | MCA (Minimum Circuit Ampacity) is on the equipment label — size the branch circuit wire to this value, not just the compressor FLA |
| Use nameplate MOCP for breaker/fuse | MOCP (Maximum Overcurrent Protection) is on the label — never exceed this for the branch circuit OCPD |
| Disconnect within sight | A disconnect switch must be within sight (visible, ≤ 50 ft) of outdoor A/C equipment. Must be lockable in the open position. |
| Nameplate wins over NEC tables | For HVAC equipment, follow the nameplate. It already accounts for locked-rotor current and compressor motor characteristics. Art. 440 modifies Art. 430. |