28.1 Wiring Single-Phase & Three-Phase Equipment

28.1 Wiring Single-Phase & Three-Phase Equipment

Trade Area D (Equipment Component Installation) is 14 percent of both Class A and Class B. After condensers, compressors, ducts, piping, and the control parts in Chapter 27, the outline asks you to wire single-phase equipment, wire three-phase equipment, install single-phase equipment, and install three-phase equipment. The 2026 Air Conditioning CBT books that carry this cluster are Refrigeration & Air Conditioning Technology, 9th Edition (2021), and Florida Building Code — Mechanical, 2023 (including Section 301.10, which sends mechanical electrical work to the adopted NFPA 70). F.S. 489.105 is the license fence that decides which of those wires an air-conditioning contractor may actually pull. Chapter 27.3 already selected transformers, capacitors, contactors, and the safety disconnect. This section is the circuit: Ohm's law, nameplate currents, single-phase versus three-phase, rotation, and when to call an electrician.

Quick Answer: V = I × R. Read RLA on hermetic compressors, FLA on fans and pumps, LRA at locked rotor. Land the circuit at or above MCA and protect at or below MOCP. Class A and Class B may work the line or load of a dedicated existing single-phase disconnect, repair a dedicated HVAC circuit with a breaker lock, and run low-voltage controls. Swap any two leads to reverse a three-phase motor. New three-phase building power is an electrical contractor.

F.S. 489.105 — the electrical fence before a lug is landed

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

1. Replace, disconnect, or reconnect power wiring on the line or load side of a dedicated existing electrical disconnect switch on single-phase electrical systems.
2. Repair or replace power wiring, disconnects, breakers, or fuses for dedicated HVAC circuits with proper use of a circuit-breaker lock.
3. Install, disconnect, and reconnect low-voltage heating, ventilating, and air-conditioning control wiring.

The same subsections then exclude electrical power wiring as a building trade. Unlimited Class A tonnage does not create unlimited electrical authority. The existing-disconnect grant is single-phase only. A 208/230/460-volt three-phase reconnect at a rooftop disconnect, a new feeder from switchgear, and a shared lighting homerun used as “the HVAC circuit” are the usual failures. The breaker lock is the statutory tool that keeps a dedicated HVAC circuit dedicated while you replace a burned whip, a fused disconnect, or the HVAC breaker itself. OSHA lockout/tagout (Chapter 33) still applies; the CILB card is not a substitute for LOTO.

FBC Mechanical 301.10 still wants the electrical installation to match the listing and the adopted electrical code. HVAC reads the nameplate, lands what 489.105 allows, and subcontracts the rest. Class B’s 25-ton / 500,000 Btu cap (F.S. 489.105(3)(g)) is a separate fence: a 20-ton 460-volt RTU can be inside Class B tons and still be outside Class B electrical scope for the three-phase feeder.

Ohm's law, power, and the currents on the nameplate

Ohm's law relates voltage (V, volts), current (I, amperes), and resistance (R, ohms):

(V = I \times R \qquad I = V / R \qquad R = V / I)

Power in watts:

• Single-phase: (P = V \times I) (true power also multiplies power factor; resistive heat strips are near PF = 1).
• Three-phase: (P = 1.73 \times V \times I \times \text{PF}) (1.73 is (\sqrt{3})).
• Also: (P = I^{2}R) and (P = V^{2}/R).
• Mechanical horsepower on a motor nameplate is output. Electrical input watts ÷ 746 is not the same number unless efficiency is 100 percent.

Worked single-phase. A 240-volt heat strip draws 20 A. Resistance is (240 / 20 = 12) ohms. Power is (240 \times 20 = 4{,}800) W, which is 4.8 kW (about 16,400 Btu/h at 3,413 Btu/h per kW). If the measured current is 24 A on the same 240-volt circuit, something is wrong — shorted element, wrong voltage, or a failed limit — not “extra capacity.”

Worked three-phase. A 460-volt three-phase blower draws 10 A at PF 0.85:

(P = 1.73 \times 460 \times 10 \times 0.85 \approx 6{,}764\ \text{W} \approx 6.8\ \text{kW})

That is input. Do not stamp it on a bid as “9 horsepower” (6,764 / 746 ≈ 9) unless you have also applied efficiency.

Nameplate current words are not interchangeable:

A hermetic that shows RLA 16.7 / LRA 79 / MCA 21 / MOCP 35 is a 16.7-amp running listing, not a 16.7-amp fuse. LRA 79 is why a clamp meter “spikes” on start and why MOCP is larger than MCA. Typical listing math (UL / nameplate method, also the RAT story) puts MCA near 125 percent of compressor RLA plus other FLA. You still use the printed MCA and MOCP; you do not invent a breaker from a forum “175 percent” rule that exceeds the stencil.

Running amps well above RLA with normal suction and head is a motor or capacitor problem (Chapter 27.3). Running amps well below RLA with a hot, short-cycling compressor can be an open start winding, a failed start device, or a single-phased three-phase machine — not a “light load bonus.”

Wiring and installing single-phase equipment

Florida residential and light-commercial splits are usually 240-volt single-phase: two ungrounded legs (L1, L2), 240 V between them, and an equipment grounding conductor. A neutral appears when the cabinet also has 120-volt loads (some crankcase heaters, some transformers). Do not land a 240-volt compressor across one hot and a neutral. Do not use the ground as a neutral.

The equipment disconnect is listed, lockable, and within sight of the unit (Chapter 27.3). On a changeout, Class A or Class B may work line and load of that dedicated existing single-phase disconnect. Replace a burned whip (often liquidtight flexible conduit with strain relief), a fused pullout, or pitted lugs. Put the circuit-breaker lock on the dedicated HVAC breaker while the cover is off. If the outdoor unit is hanging on a shared 20-amp lighting circuit, that is not a dedicated HVAC circuit — stop and get the circuit made dedicated (electrical contractor if it is new building power).

Compressor terminals on a single-phase hermetic are C (common), S (start), R (run). Identify them with an ohmmeter on an isolated compressor:

• Highest resistance is S to R (start winding plus run winding).
• C to S is the start winding (higher than C to R).
• C to R is the run winding (lowest).
• Check: ((C\text{–}S) + (C\text{–}R) = (S\text{–}R)).

A run capacitor sits between S and R (HERM to S, C to the capacitor common — dual cans also feed the fan). A start capacitor and start relay are intermittent (Chapter 27.3). Ground the cabinet to the green screw. Tape or listed connectors on low-voltage R, C, Y, G, W/O/B stay inside the HVAC grant.

Install the single-phase machine as Chapter 20 taught (pad, wind, clearances) and then land power: MCA circuit, MOCP device, within-sight disconnect, tight lugs, drip loop, strain relief, and a whip that is not a chewed NM cable lying in a puddle. Dual-voltage 115/230 motors must be jumpered for the actual site voltage — a 115-volt connection on 230 V is a one-time event.

Wiring and installing three-phase equipment — and when you subcontract

Three-phase HVAC in Florida is commonly 208 V, 230/240 V, or 460/480 V, three ungrounded conductors plus ground (a neutral only if 120/277-volt loads live in the same cabinet). Line-to-line voltage is what the motor nameplate must match. A 208-volt motor on 230 V runs hot; a 460-volt motor on 208 V will not start.

Dual-voltage three-phase motors (often 230/460, nine leads) reconnect wye or parallel for low voltage and series for high voltage. Use the connection diagram on the motor, not a memory of “T4 to T5.” Wrong reconnect is a smoked winding, not a “high-voltage option.”

Rotation. A three-phase motor reverses if you swap any two of the three power leads. That is the exam sentence. Scroll compressors and some pumps are rotation-sensitive: reverse rotation makes noise, low oil pressure, and almost no capacity. Check rotation momentarily before leaving a new three-phase blower or compressor in auto. Swapping one lead does nothing useful. Swapping leads on single-phase does not reverse a PSC compressor; reverse a belt-driven single-phase fan by changing the mechanical drive or a reversing switch the listing provides, not by swapping L1 and L2 on a run-cap motor that is not built to reverse.

Single-phasing is the loss of one of the three legs. The motor may keep turning on the remaining phases while those windings overheat. Overloads in each phase (Chapter 27.4) and a phase monitor (Section 28.3) are the protections. Oversizing heaters “so it stops tripping” is how you buy a rewind.

Install a three-phase RTU, chiller, or tower fan as a machine inside Class A/B tonnage, but treat power wiring as the fence: specify MCA/MOCP, confirm the electrical contractor’s feeder and disconnect, land low-voltage controls yourself, bump rotation, and set the phase monitor. Do not pull a new 460-volt feeder from the switchgear on an HVAC ticket. Do not treat the single-phase existing-disconnect grant as a three-phase grant. Class B still owes this theory on a 40-ton 460-volt plant; Class B does not contract that plant as one system.

Florida HVAC scenario

Palm Coast Comfort, certified Class B in Volusia County, change-outs a 3-ton 240-volt single-phase split. The pad disconnect is dedicated and within sight. The helper may work line and load of that disconnect, replace the whip, lock the HVAC breaker, land 24-volt thermostat cable, and read the new nameplate: RLA 14.1, LRA 72, MCA 18.4, MOCP 30. The existing 25-amp breaker is between MCA and MOCP — legal if the conductor still meets MCA. A clamp meter that shows 70 A for a few cycles on start is LRA, not a short. The owner also wants a 20-ton 460-volt three-phase RTU on the strip center (inside the 25-ton Class B cap). The HVAC shop may set the cabinet, ducts, condensate, and controls, and must know that swapping two leads reverses the supply fan — but the new three-phase feeder and disconnect belong to a licensed electrical contractor. If the helper “just lands T1-T2-T3” on a nine-lead 230/460 motor using the 230-volt diagram on a 460-volt site, the motor dies on the first call. If the same drawings showed a 30-ton RTU, Class B would stop at the bid table (F.S. 489.105(3)(g)) even before the electrician is called.

Traps: (1) Treating RLA as FLA or as the fuse size. (2) Circuit below MCA or breaker above MOCP. (3) Using the ground as a neutral. (4) Calling LRA a short. (5) Pulling a new 460-volt feeder on an HVAC ticket. (6) Expecting L1-L2 swap to reverse a single-phase compressor. (7) Class B on a 30-ton three-phase plant because “we understand rotation.”