Condensing Unit Circuit, Disconnect and Whip Field Card
Why this matters
On a changeout, the new condensing unit's nameplate usually asks a little more of the circuit than the old one did. Most of the time the existing breaker and wire still qualify, and the tech swaps the disconnect and whip and moves on. Sometimes they do not, and a unit hung on a circuit that no longer meets its minimum circuit ampacity runs fine for a summer and then cooks its conductors' insulation in the attic. This card is the HVAC tech's check before the old unit comes off: read two numbers, compare them with what is in the wall, and know which side of the line the job falls on.
The full wiring procedure, from panel to unit, is the electrician's and is covered in the Electrical library's How to Wire an AC Condenser Disconnect and Whip. Running or upsizing a branch circuit is routed to the electrician, never done on an HVAC ticket.
Safety before any of it
The disconnect's line side stays live when its handle is off. Before touching the disconnect, the whip or the unit's contactor terminals:
- Open the branch-circuit breaker at the panel, lock and tag it.
- Prove your tester on a known live source, test the disconnect's line-side terminals phase to phase and each phase to ground, then prove the tester on the live source again. A meter with a blown fuse reads a believable zero; the second proving catches it. This is the before-and-after verification in NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024), with electrical work practices under 29 CFR 1910.333(b)(2) in general industry and 29 CFR 1926.417 on construction work.
- Only then open the disconnect cover and work.
Restoring is the reverse and gets its own check: disconnect handle off, remove your lock and tag, breaker on, read line-side voltage at the disconnect, close the disconnect cover and turn it on, read voltage at the unit's contactor, then close the unit's panels, let it start and clamp its running amps against the nameplate RLA. Stand to the side of the disconnect, not square in front of it, when you close it.
The layout the code asks for
PANEL OUTSIDE WALL, NEXT TO UNIT
+------------+
| 2P breaker |--- L1 L2 G branch circuit -+
| | |
| | +-----v------+
| 1P 20A |--- H N G ---+ | DISCONNECT |
| (GFCI) | | | line side |
+------------+ | | stays hot |
| +-----+------+
+-----v----+ | whip
| 120 V | | L1 L2 G
| WR recep | +-----v------+
+----------+ | CONDENSING |
within 25 ft, | UNIT |
same level +------------+
Disconnect within sight. NEC 440.14 (2023 edition, as adopted by your jurisdiction) requires a disconnecting means for the air-conditioning equipment located within sight from and readily accessible from the equipment. "Within sight" is defined in NEC Article 100 as visible and not more than 50 feet away. The disconnect may be mounted on the unit, but 440.14 does not allow it on panels designed to give access to the equipment or where it hides the nameplate.
Service receptacle within 25 feet. NEC 210.63 (2023 edition, as adopted) requires a 125-volt, single-phase, 15- or 20-ampere receptacle outlet within 25 feet of HVAC equipment, at an accessible location on the same level, and it may not be connected to the load side of the equipment's disconnect. That last clause is why the receptacle sits on its own 120-volt circuit in the diagram: a tech who opens the disconnect to work still needs power for a drill or a vacuum pump.
GFCI on outdoor outlets. The 2020 and 2023 editions extended GFCI protection at 210.8(F) to outdoor outlets at dwellings, and how that reaches a hardwired condensing unit has been handled with exceptions, delayed effective dates and local amendments. Do not decide it from memory on a changeout; check what your jurisdiction adopted and what the inspector enforces.
Working space. Inspectors commonly apply the working space rules of NEC 110.26 in front of the disconnect. A new unit set closer to a fence or a meter than the old one can take that space away.
Two numbers on the nameplate
MCA, minimum circuit ampacity. The smallest ampacity the branch-circuit conductors may have. The maker has already applied the 125 percent factor for the largest motor plus the other loads, so the conductor is sized to MCA directly, not to MCA times anything. See related: Condensing Unit Nameplate Read for Install and Service.
MOCP, maximum overcurrent protection. The largest fuse or breaker allowed (NEC 440.22, as adopted). Read the exact wording: "Max fuse or HACR breaker" allows either, and "Max fuse" alone means the protection must be fuses, which a fusible disconnect provides. A breaker may be smaller than MOCP as long as it holds the compressor's start; one sized close to MCA can nuisance-trip on a locked-rotor start.
Why a small wire on a big breaker is legal here. A 12 AWG conductor is normally limited to 20-ampere protection by NEC 240.4(D). For air-conditioning equipment, 240.4(G) sends you to Article 440 instead, which allows protection up to the nameplate MOCP. A 30-ampere breaker on 12 AWG can be correct for a condensing unit and wrong for anything else on the same panel. Do not "fix" it on a unit that calls for it, and do not copy it to a circuit that does not.
Ampacity comes from the wiring method, not the wire size alone. NEC Table 310.16 gives copper 12 AWG 20 amperes in the 60 C column and 25 amperes in the 75 C column. Which column you may use depends on the conductor's insulation and on the lowest temperature rating of the terminations at each end. NM cable (Romex-style nonmetallic sheathed cable) is held to the 60 C column by 334.80 regardless of the conductors inside it.
Worked example: a 2.5-ton to 3-ton changeout
The old unit's plate: MCA 17.5 A, MOCP 30 A. The new 3-ton plate: MCA 21.0 A, MOCP 35 A, "Max fuse or HACR breaker". In the wall: a 30-ampere two-pole breaker. The conductor depends on the house, so here are two houses with the same plates.
House A: 12-2 NM cable from the panel to the disconnect.
- Conductor: 12 AWG copper in NM, 60 C column, 20 A.
- Against MCA: 20 A is less than 21.0 A. Fails.
- Breaker: 30 A is at or under MOCP 35 A. Passes, but it does not matter; the conductor failed.
- Result: the circuit has to be replaced with a larger conductor (10 AWG copper in NM is 30 A in the 60 C column, which clears 21.0). That is a new branch circuit and goes to the electrician before the new unit is energized.
House B: 12 AWG THHN in EMT from the panel to the disconnect, terminations rated 75 C at both ends.
- Conductor: 12 AWG copper, 75 C column, 25 A.
- Against MCA: 25 A is at least 21.0 A. Passes, with 4 A to spare (25.0 minus 21.0).
- Breaker: 30 A is at or under MOCP 35 A. Passes.
- Result: the circuit stays. The tech replaces the disconnect and whip like for like.
The plates did not change between houses; the wiring method did. That is why the check starts in the attic or the basement, at the cable jacket, before the quote goes out.
What the whip has to be. The whip conductors are part of the branch circuit, so their ampacity must also clear MCA. Outdoors, conductors inside a raceway are in a wet location (NEC 300.9) and must be wet-rated, typically THWN-2. Liquidtight flexible nonmetallic conduit is not a grounding path, so pull an equipment grounding conductor in it. Whether a liquidtight flexible metal whip may serve as the ground depends on its length, trade size and fittings under NEC 250.118, and pulling a green conductor removes the question.
What the HVAC tech replaces, and what goes to the electrician
| Work | Typically on the HVAC ticket | Goes to the electrician |
|---|---|---|
| Disconnect, same rating, same location | Yes, where your state or locality licenses HVAC for it | Where it does not |
| Whip from disconnect to unit | Yes, sized to the new MCA | |
| Breaker swap within MOCP, same conductor that clears MCA | Depends on licensing; many shops route it | Often |
| Conductor that fails new MCA | Always | |
| New circuit, or a circuit moved to a new location | Always | |
| Missing service receptacle | Usually | |
| Disconnect relocated outside sight of the unit | Always |
Licensing is not uniform. Some states let a licensed HVAC contractor wire equipment from the disconnect onward; some extend that to the disconnect itself; panel work and branch circuits are electrical work in nearly every jurisdiction that licenses trades. Your shop's own policy, written down, should say where the line is, so the tech on site does not decide it under pressure.
What changes the answer
A unit with a variable-speed or inverter compressor. Inverter units often list a lower MCA than a fixed-speed unit of the same capacity and can have different breaker requirements; some makers specify a breaker type. Read the plate and the manual, never the old unit's numbers.
A longer conductor run. NEC sizes for heat, not voltage drop. A long run to a detached pad can need a larger conductor than MCA alone requires for the unit to start reliably on a hot afternoon. That is the electrician's call, but noticing a 150-foot run on the walk-through is yours.
A different panel temperature rating. Older breakers and panel lugs rated 60 C hold the conductor to the 60 C column even when the wire is THHN.
How to verify before the old unit comes off
- New plate MCA written on the quote next to the existing conductor's ampacity and the column it came from.
- New plate MOCP and its wording next to the existing breaker rating.
- Disconnect location within sight (visible, not more than 50 feet) and readily accessible after the new pad is set.
- Service receptacle within 25 feet on the same level, on its own circuit.
- Anything that fails routed to the electrician, with the reason on the ticket.
References
- NFPA 70, National Electrical Code (2023 edition as adopted by your jurisdiction): Article 100 definition of within sight, 210.8, 210.63, 240.4, 250.118, 300.9, 310.16, 334.80, 440.14, 440.22, 110.26.
- NFPA 70E (current edition), Article 120; 29 CFR 1910.333 and 29 CFR 1926.417.
- See related: Condensing Unit Nameplate Read for Install and Service; Electrical library, How to Wire an AC Condenser Disconnect and Whip.
- Condensing unit nameplate and installation manual.