27.3 Transformers, Capacitors, Contactors, Relays & Safety Disconnects

27.3 Transformers, Capacitors, Contactors, Relays & Safety Disconnects

This Trade D cluster is the electrical hardware on the HVAC machine: transformers, capacitors, contactors, current relays, and safety disconnects. Chapter 28 will wire single- and three-phase equipment and motor controllers as a system. This section is selecting and installing the parts without stepping outside F.S. 489.105. The 2026 CBT support is Refrigeration & Air Conditioning Technology, 9th Edition (2021). OSHA 29 CFR 1926 still wants lockout/tagout before a cover comes off (Chapter 33). The CILB card is not a substitute for LOTO.

Quick Answer: Size 24-volt control transformers in VA (VA = 24 × amps). Run capacitors stay in; start capacitors come out after a second. Never short a capacitor to test it. Contactors switch the load; current and potential starting relays are not interchangeable. The disconnect is within sight, lockable, and on a dedicated HVAC circuit with a breaker lock. New three-phase feeders belong to an electrical contractor.

F.S. 489.105 — the electrical fence, then the parts

Class A (489.105(3)(f)) and Class B ((3)(g)) may:

• Replace, disconnect, or reconnect power wiring on the line or load side of a dedicated existing electrical disconnect on single-phase systems.
• Repair or replace power wiring, disconnects, breakers, or fuses for dedicated HVAC circuits with a circuit-breaker lock.
• Install, disconnect, and reconnect low-voltage HVAC control wiring.

They may not treat electrical power wiring as a new building trade. A new feeder from the switchgear to a new 460-volt RTU is an electrical contractor. HVAC still reads MCA (minimum circuit ampacity) and MOCP (maximum overcurrent protection) on the nameplate so the specified breaker matches the listing, still lands control wiring, and still puts a lock on the dedicated HVAC breaker when working that circuit. Pneumatic control piping remains Class A only (Section 27.2).

The safety disconnect at the equipment is a listed, lockable disconnect within sight of the unit (you can see it and get to it while you work — the listing and mechanical/electrical install rules; do not invent a foot count the open-book list does not print). It is not a light switch in a locked electrical room on another floor. For a changeout on an existing single-phase condensing unit, the HVAC contractor may work line and load of that dedicated existing disconnect. For a dedicated HVAC circuit, the contractor may repair or replace the disconnect, breaker, or fuse with a breaker lock. That lock is how the statute keeps the circuit HVAC-dedicated instead of a shared lighting homerun.

Transformers — 24 volts, VA, and Class 2

A control transformer steps 120, 208, or 240-volt primary down to a 24-volt secondary that feeds thermostats, contactors’ coils, and electronic boards. It is a Class 2 control source when it is listed and protected as Class 2. Size it in volt-amperes (VA):

(\text{VA} = V \times I = 24 \times I_{\text{secondary}})

Worked VA example. A contactor coil and electronic board draw 1.5 A at 24 V:

(24 \times 1.5 = 36\ \text{VA})

A 40 VA transformer still has a little headroom. Add two 8 VA zone-damper actuators (16 VA). Total is 52 VA. A 40 VA leftover from the old furnace will run hot, droop below 24 V, and chatter the contactor on a 95°F Tampa roof. Step to 75 VA (or the manufacturer’s accessory transformer). Undersize and the secondary voltage collapses when the coil pulls in (inrush). Oversize is usually harmless electrically but is not a license to load the transformer past its nameplate VA with extra dampers, a UV ballast, and a Wi-Fi module you found in the van.

Identify R (secondary hot) and C (secondary common). Many boards fuse the R or C leg; a blown 3-A fuse is a shorted thermostat cable or a failed actuator, not a ‘weak transformer’ you replace three times. Primary taps must match actual site voltage: a 240-volt primary on 208-volt power puts a low secondary on every coil; a 208 tap on 240 overvolts boards.

Capacitors — run versus start, and never short them to test

A capacitor stores charge and shifts current out of phase with voltage so a single-phase motor can produce a rotating field.

Run capacitors are continuous-duty, oil-filled (or equivalent), relatively low microfarad, and stay in the circuit whenever the motor runs. A dual run capacitor in one can is labeled HERM, FAN, and C (common): HERM to the compressor run winding, FAN to the condenser fan, C to the run-capacitor common. Start capacitors are intermittent-duty, higher microfarad, often in a plastic can, and are in the circuit only a second or two while the motor starts. A potential (voltage) relay or a current relay (below) drops the start capacitor out. Leaving a start capacitor in the run circuit cooks it and the start winding.

Never test a capacitor by shorting the terminals. Shorting a charged capacitor dumps the energy in a spark, can explode the can, can weld a screwdriver, and tells you nothing about microfarads. Discharge through a resistor if you must discharge, then measure with a capacitor tester or an meter that reads µF. Typical run-capacitor tolerance is about ±6 to ±10 percent of nameplate (use the listing; ±10 percent is the common field band). A 45 µF HERM section that measures 35 µF is failed — high compressor amps, hot run winding, hard start. A 5 µF FAN section that measures 5.1 µF is fine.

Worked dual-can read. Nameplate 45/5 µF, 370/440 V. HERM band at ±10 percent is 40.5–49.5 µF. FAN band is 4.5–5.5 µF. Replace the can if either section is out of band, the can is bulged, or the top is leaking oil. Match VAC rating at or above the old can; a 370 V can is not a free upgrade path onto a 440 V listing.

Contactors, current relays, and starting relays

A contactor is a line-voltage switch closed by a coil (often 24 V, sometimes 120 or 240 V). Compressor, electric-heat, and condenser-fan loads belong on a contactor (or a magnetic starter — Section 27.4) sized for the FLA and LRA. Pitted points, a burned 24-volt coil, and a welded contact that will not drop are install/replace items. The coil sits downstream of the safeties in Section 27.1: HP, LP, oil failure, freeze, and overflow all open the coil circuit. Jumping the coil to ‘see if the compressor runs’ bypasses every safety at once.

Relays switch lighter loads: fans, reversing valves, accessories. A current starting relay (series, often on small hermetics) sits in series with the run winding. High start current closes a normally-open contact that puts the start winding (and often a start capacitor) in the circuit; as run current falls, the relay drops out. A potential (voltage) starting relay sits across the start winding. As the motor approaches speed, back-EMF rises and the relay opens its normally-closed contact, dropping the start capacitor. Do not interchange them. A current relay on a potential-relay motor (or the reverse) leaves the start winding energized or never starts it.

Coil voltage must match the transformer or line you actually have. A 24-volt contactor coil on 120 V is a one-time event. Auxiliary contacts on the contactor can prove the compressor is commanded on to a DDC point; they are not a substitute for a current sensor if you need to know the motor is really drawing amps.

Florida HVAC scenario

Sunrise Comfort, certified Class B in Hillsborough, change-outs a 3-ton single-phase split. The existing dedicated disconnect is on the pad, within sight. The helper may work line and load of that disconnect, replace a burned contactor, replace a 45/5 dual run capacitor (tested with a µF meter, not a screwdriver short), and replace a 40 VA transformer that now feeds a communicating stat plus two dampers — except 40 VA is too small, so the ticket must include a 75 VA transformer. The helper puts a lock on the HVAC breaker while the cover is off (OSHA LOTO plus the statutory breaker lock). The owner also wants a new 200-amp three-phase panel and a 60-foot feeder for a future 20-ton RTU. That feeder is not Class B HVAC work. The 20-ton RTU itself would be inside the 25-ton cap, but the new three-phase building power still belongs to an electrical contractor. Shorting the old capacitor ‘to hear the pop’ is how the helper earns a burn and a failed exam item in the same afternoon.

Traps: (1) Shorting capacitors. (2) 40 VA transformers on 75 VA of load. (3) Swapping current and potential start relays. (4) Jumping a contactor coil around safeties. (5) Treating a shared lighting breaker as a dedicated HVAC circuit. (6) Pulling a new 460-volt feeder on an HVAC ticket. (7) Wrong primary tap (208 vs 240).