Use 10/2 wire for your air conditioner — not 10/3. AC condensers run on pure 240V power and don't have any 120V components, so they don't need the neutral conductor that 10/3 provides. Using 10/3 works electrically but wastes $10–$15 per run on an unused wire, and the extra conductor makes the cable stiffer and harder to pull through walls.
The confusion arises because people associate "240V" with "three-wire cable." But 240V circuits come in two flavors: those that need a neutral (dryers, ranges) and those that don't (AC units, water heaters). Your AC condenser is in the second category.
10/2 vs. 10/3: The Key Difference
| Feature | 10/2 NM-B | 10/3 NM-B |
|---|---|---|
| Insulated conductors | 2 (black + white) | 3 (black + red + white) |
| Ground wire | 1 bare copper | 1 bare copper |
| Total wires | 3 | 4 |
| Used for | 240V-only loads (no neutral) | 240V loads with 120V components |
| Common applications | AC, water heater, well pump | Electric dryer, some sub-panels |
| Cost per foot (2026) | $0.65–$1.00 | $0.85–$1.30 |
| Cable diameter | ~0.47 inches | ~0.55 inches |
| Weight per 100 ft | ~18 lbs | ~24 lbs |
What's Inside Each Cable
10/2 NM-B contains:
- 1 black wire (Hot 1 — Line 1)
- 1 white wire (Hot 2 — Line 2, re-identified with black tape)
- 1 bare copper wire (Equipment ground)
10/3 NM-B contains:
- 1 black wire (Hot 1 — Line 1)
- 1 red wire (Hot 2 — Line 2)
- 1 white wire (Neutral)
- 1 bare copper wire (Equipment ground)
When using 10/2 for 240V circuits: The white wire serves as the second hot conductor, NOT as a neutral. NEC 200.7(C)(1) requires you to re-identify this white wire with black or red tape, paint, or other permanent marking at both ends to indicate it's being used as a hot conductor.
Why AC Units Don't Need a Neutral
An AC condenser is a pure 240V load. Here's what's inside:
- Compressor motor — runs on 240V (between the two hot legs)
- Condenser fan motor — runs on 240V (between the two hot legs)
- Contactor — a relay that connects/disconnects power, operates on 24V from the thermostat transformer
- Capacitor — stores energy for motor starting, operates on 240V
None of these components need 120V. There's no clock, no light, no 120V motor. The only voltages present are 240V (power) and 24V (control from the thermostat cable).
Compare this to an electric dryer:
- Heating element — runs on 240V ✓
- Drum motor — runs on 120V (needs neutral)
- Timer motor — runs on 120V (needs neutral)
- Drum light — runs on 120V (needs neutral)
The dryer needs both 240V AND 120V, which is why it requires a neutral conductor and uses 10/3 cable.
When to Use 10/2 (No Neutral)
| Equipment | Cable | Why No Neutral? |
|---|---|---|
| Central AC condenser | 10/2 NM-B | Pure 240V motors, no 120V components |
| Heat pump condenser | 10/2 NM-B | Same as AC — pure 240V |
| Electric tank water heater | 10/2 NM-B | Heating elements are 240V only |
| Well pump | 10/2 NM-B | 240V motor only |
| Baseboard heater | 12/2 or 10/2 NM-B | Resistance heating, 240V only |
| Mini-split outdoor unit | 12/2 or 14/2 NM-B | 240V or 120V, but no neutral needed either way |
When to Use 10/3 (Neutral Required)
| Equipment | Cable | Why Neutral? |
|---|---|---|
| Electric dryer | 10/3 NM-B | 120V motor, timer, light |
| Some electric ranges (30A) | 10/3 NM-B | 120V controls, light |
| Sub-panel feeder | 10/3 NM-B | Sub-panel needs neutral for 120V circuits |
| Some hot tubs | 10/3 NM-B | 120V pump, 120V lights, 240V heater |
Never substitute 10/2 where 10/3 is required. If the installation calls for a neutral (white wire to neutral bus), using 10/2 means you'd be using the ground wire as a neutral — which was banned by NEC in 1996 for new installations. This is dangerous because a broken neutral puts the equipment chassis at 120V.
The Thermostat Wire Is Separate
A common confusion: people think the AC condenser needs a neutral for the thermostat connection. It doesn't. The thermostat communicates with the condenser through a separate low-voltage cable (typically 18/2 or 18/4 thermostat wire) that carries 24V AC from the indoor air handler's transformer.
The power circuit (10/2 NM-B, 240V, 30A) and the control circuit (18-gauge thermostat wire, 24V) are completely independent.
| Circuit | Cable | Voltage | Purpose |
|---|---|---|---|
| Power | 10/2 NM-B | 240V | Runs compressor and fan motors |
| Control | 18/2 or 18/4 thermostat wire | 24V AC | On/off signal from thermostat |
Cost Comparison: 10/2 vs. 10/3
For a typical 50-foot AC condenser run:
| Cable Type | Cost per Foot | Total (50 ft) | Notes |
|---|---|---|---|
| 10/2 NM-B | $0.65–$1.00 | $32.50–$50.00 | Correct choice for AC |
| 10/3 NM-B | $0.85–$1.30 | $42.50–$65.00 | Unnecessary for AC |
| Difference | $0.20–$0.30/ft | $10–$15 | Wasted on unused neutral |
The cost difference is modest on a single run, but if you're wiring multiple circuits, it adds up. More importantly, 10/3 is physically harder to work with — the extra conductor makes the cable ~17% thicker and ~33% heavier, making it harder to pull through tight spaces.
Wiring an AC Condenser: Step-by-Step
Materials for a Typical 3-Ton AC Installation
| Item | Qty | Cost |
|---|---|---|
| 10/2 NM-B cable (interior run) | 40 ft | $26–$40 |
| 30A double-pole breaker | 1 | $8–$20 |
| Non-fused disconnect (60A) | 1 | $15–$30 |
| Flexible metal conduit ("whip") with 10 AWG THHN | 6 ft | $15–$25 |
| Cable clamps, staples, connectors | — | $10–$15 |
| Total materials | — | $74–$130 |
Wiring Steps
Step 1: Install 30A double-pole breaker in main panel.
Step 2: Run 10/2 NM-B from panel to disconnect location. Use appropriate cable clamps and staple every 4.5 feet and within 12 inches of boxes (NEC 334.30).
Step 3: At the disconnect box, connect:
- Black wire → Line 1 terminal
- White wire (re-identified with black tape) → Line 2 terminal
- Bare copper → Ground terminal/bar
Step 4: Run flexible metal conduit with 10 AWG THHN from disconnect to condenser whip connection.
Step 5: At the condenser, connect:
- Black THHN → L1 terminal
- Red THHN → L2 terminal
- Green THHN → Ground terminal
Step 6: Verify connections match the unit's wiring diagram (usually on the access panel cover).
Pro tip: Re-identify the white wire. When using 10/2 for a 240V circuit, wrap black electrical tape around the white wire at both ends (panel and disconnect). This tells the next electrician that the white wire is being used as a hot conductor, not a neutral. Failing to re-identify is technically a code violation and creates confusion.
Real-World Examples
Example 1: New AC Installation in Existing Home
Scenario: Installing a Carrier 24ACC636 (3-ton, 16 SEER2) condenser. Nameplate: MCA 19.0A, MOP 30A. Panel is in the basement, condenser location is 45 feet away through an exterior wall.
Correct wiring:
- Cable: 10/2 NM-B (NOT 10/3 — no neutral needed)
- Breaker: 30A double-pole
- Disconnect: 60A non-fused pull-out
- Whip: 6 ft flexible conduit with 10 AWG THHN
Why 10/2: The condenser has no 120V components. The compressor motor and fan motor both run on 240V. The thermostat connection is a separate 24V control circuit that uses its own low-voltage wire.
Example 2: Homeowner Bought 10/3 by Mistake
Scenario: DIYer purchased 10/3 NM-B instead of 10/2 for their AC installation.
Can they still use it? Yes — 10/3 works fine electrically. The installation would be:
- Black wire → Hot 1
- Red wire → Hot 2
- White wire → Capped off (not connected to anything)
- Bare copper → Ground
The white neutral conductor simply goes unused. It's not dangerous, just wasteful. If the cable is already purchased and the homeowner doesn't want to return it, using 10/3 for an AC circuit is code-compliant — there's no rule against having an unused conductor.
Example 3: Converting Dryer Circuit to AC Circuit
Scenario: Homeowner is relocating their laundry and wants to repurpose the old 10/3 dryer circuit for a new mini-split outdoor unit.
Is this allowed? Yes, with modifications:
- Change the 30A receptacle (NEMA 14-30R) to a blank cover with a junction box, or run the cable directly to a disconnect
- Cap off the white neutral wire — it's not needed for the mini-split
- Verify the mini-split's MCA is under 30A (most are under 25A)
The 10/3 cable is oversized for most mini-splits (which typically need only 12 AWG or 14 AWG), but using larger wire is always safe and code-compliant.
Example 4: Heat Pump with Electric Backup Heat
Scenario: Installing a heat pump system with 10kW strip heat in the air handler. Does this change the wire requirement for the outdoor unit?
No — the outdoor unit still uses 10/2. The outdoor heat pump condenser is identical to an AC condenser — pure 240V, no neutral needed.
However, the indoor air handler with strip heat is a separate circuit. Strip heat is also pure 240V (resistance heating elements), so that circuit also uses 10/2 or 8/2 depending on the kW rating — typically:
- 5kW strip heat → 10/2 NM-B, 30A breaker
- 10kW strip heat → 8/2 or 6/2, 50–60A breaker
- 15kW strip heat → 6/2 or larger, 60–80A breaker
Neither the condenser nor the air handler with strip heat requires a neutral.
What About Mini-Splits?
Mini-split systems vary more than central AC systems. Most mini-splits need only 10/2, 12/2, or even 14/2 depending on capacity, but always check the installation manual.
| Mini-Split Size | Typical MCA | Typical MOP | Wire | Cable |
|---|---|---|---|---|
| 9,000 BTU (0.75 ton) | 5–8A | 15A | 14 AWG | 14/2 NM-B |
| 12,000 BTU (1 ton) | 7–10A | 15–20A | 14–12 AWG | 14/2 or 12/2 NM-B |
| 18,000 BTU (1.5 ton) | 11–15A | 20A | 12 AWG | 12/2 NM-B |
| 24,000 BTU (2 ton) | 14–18A | 25–30A | 12–10 AWG | 12/2 or 10/2 NM-B |
| 36,000 BTU (3 ton) | 19–24A | 30A | 10 AWG | 10/2 NM-B |
None of these sizes require a neutral — 10/3 is overkill for any mini-split outdoor unit.
Some mini-splits use 120V instead of 240V. Many 9,000–12,000 BTU units are available in 120V versions for easier installation. These still don't need a neutral — they use a standard 15A or 20A, 120V dedicated circuit wired with 14/2 or 12/2 NM-B (hot, neutral, ground). The neutral in this case is a true neutral because the circuit is 120V, not 240V.
Key Takeaways
- Use 10/2 NM-B for air conditioners — AC condensers don't need a neutral conductor
- 10/3 is unnecessary (overkill) — you're paying $10–$15 extra for an unused wire
- Re-identify the white wire when using 10/2 for 240V — wrap black tape on both ends
- Dryers and ranges need 10/3 — they have 120V components (motors, lights)
- The thermostat wire is separate — 24V control circuit, not part of the 240V power circuit
- Heat pumps also use 10/2 — same as AC, pure 240V outdoor unit
- If you already bought 10/3, use it — just cap off the white neutral wire
- Mini-splits vary — check the installation manual, but most use 12/2 or 14/2, never 10/3
Frequently Asked Questions
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