For a standard 5 HP, 240V single-phase air compressor motor (28A Full Load Current), use 8 AWG copper THHN wire and a 60A dual-pole breaker. This assumes copper conductors, 75°C terminations, 30°C ambient temperature, and installation in EMT conduit.
The Motor Sizing Paradox: Why Wire and Breaker Sizes Diverge
If you apply standard residential branch circuit logic to a motor, you will cause nuisance tripping on every startup. A 5 HP motor drawing 28A at full load might seem like it only needs a 35A or 40A breaker. However, when an induction motor starts, it draws Locked Rotor Amps (LRA)—often 600% to 800% of its Full Load Current (FLC). For our 28A motor, that inrush current can momentarily spike to over 160A.
If you install a 40A standard thermal-magnetic breaker, the magnetic trip latch will interpret that 160A inrush as a dead short and instantly kill the power before the motor reaches operating speed. To solve this, NEC Article 430 intentionally separates wire sizing from breaker sizing. The wire is sized to handle the continuous running heat (125% of FLC), while the breaker is sized large enough to swallow the starting inrush without tripping, relying on the downstream overload relay to protect the motor windings from sustained overcurrent.
Baseline Assumptions and Specifications
Electrical sizing is entirely dependent on environmental and material variables. The 8 AWG / 60A recommendation above is based on the following strict baseline assumptions. If your installation deviates from these, you must recalculate.
- Conductor Material: Copper (Stranded THHN/THWN-2)
- Temperature Column: 75°C (Standard for most modern breakers and motor terminals, per NEC 110.14(C))
- Ambient Temperature: 30°C (86°F) or lower
- Raceway Type: EMT (Electrical Metallic Tubing) or PVC conduit in free air
- Motor Specs: 5 HP, 240V, 1-Phase, 60Hz, Standard Design B
- NEC Table 310.16 Reference: 8 AWG Copper at 75°C = 50 Amps
Step-by-Step Decision Path
Use this decision tree to verify the sizing for your specific motor nameplate. Never rely solely on horsepower ratings; always use the exact amperage printed on the motor data plate.
| Step | Action & Formula | 5HP Compressor Example |
|---|---|---|
| 1. Find FLC | Read Full Load Current from nameplate (or NEC Table 430.248 if nameplate is missing). | Nameplate reads 28A. |
| 2. Size the Wire | Multiply FLC by 1.25 (125%). This is your minimum wire ampacity. | 28A × 1.25 = 35A minimum. |
| 3. Select AWG | Pick wire from the 75°C column of NEC Table 310.16 that meets or exceeds Step 2. | 10 AWG is 35A, but we select 8 AWG (50A) for voltage drop margin. |
| 4. Size the Breaker | Multiply FLC by 2.5 (250% max for single-phase inverse time breakers per NEC 430.52). | 28A × 2.5 = 70A maximum. |
| 5. Final Pick | Select the nearest standard breaker size (NEC 240.6) that does not exceed Step 4. | Standard sizes: 60A or 70A. We pick 60A to provide tighter short-circuit protection while still clearing the LRA inrush. |
Voltage Drop and Run Length Adjustments
Sizing for ampacity keeps the wire from melting; sizing for voltage drop keeps the motor from overheating. Motors are highly sensitive to low voltage. A 10% drop in voltage can cause a 19% drop in starting torque, causing the motor to stall and trip the overload relay.
Let us run a voltage drop check for our 8 AWG copper wire on a 75-foot run from the panel to the compressor disconnect.
K (Copper) = 12.9 | I (Current) = 28A | D (Distance) = 75 ft | CM (Circular Mils for 8 AWG) = 16,510
Plugging in the numbers: VD = (2 × 12.9 × 28 × 75) / 16,510 = 3.28 Volts.
Percentage Drop: (3.28 / 240) × 100 = 1.36%.
This is well under the NEC recommended 3% maximum for branch circuits. However, if your compressor is located in a detached garage 200 feet away, the drop jumps to 8.7V (3.6%). At that distance, 8 AWG is no longer acceptable. You must bump the wire to 6 AWG copper to bring the drop back under 3%, even though the 60A breaker remains the same.
What Changes the Answer? Edge Cases and Derating
The 8 AWG / 60A baseline falls apart if you ignore installation environment factors. Here is what forces a redesign:
- Conductor Bundling: If you pull more than three current-carrying conductors through the same conduit (e.g., wiring a 3-phase motor alongside a 120V control circuit), NEC Table 310.15(C)(1) requires derating. Four to six conductors require an 80% derating factor. Your 8 AWG wire (50A) derates to 40A, which is still above the 35A minimum, but leaves zero margin for voltage drop.
- High Ambient Temperatures: If the conduit runs through an unconditioned attic in a southern climate where ambient temperatures exceed 113°F (45°C), you must apply the temperature correction factors from the bottom of NEC Table 310.16. At 45°C, the 90°C THHN insulation is derated to 87% of its base rating.
- Aluminum Conductors: Never treat aluminum and copper interchangeably. If you must use aluminum wire (e.g., SER cable for a large outdoor disconnect), you must use the aluminum column in Table 310.16 and ensure your breaker and motor terminals are explicitly rated for aluminum (marked AL or CU/AL). For a 35A minimum, you would need 6 AWG aluminum, not 8 AWG. Furthermore, aluminum requires anti-oxidant paste and specific torque values to prevent thermal runaway at the lugs.
When an Engineer or AHJ Must Confirm
While the NEC Article 430 rules cover 95% of standard shop and residential air compressors, certain scenarios require professional engineering or explicit Authority Having Jurisdiction (AHJ) approval:
- Variable Frequency Drives (VFDs): If you are feeding a VFD rather than a standard across-the-line contactor, the input current contains high-frequency harmonics. Standard thermal-magnetic breakers may nuisance-trip due to harmonic heating. VFD manufacturers like WEG or Allen-Bradley often specify HACR (Heating, Air Conditioning, and Refrigeration) breakers or even semiconductor fuses for input protection.
- Fire Pump Motors: Governed by NEC Article 695, fire pumps have entirely different rules. The breaker must be sized large enough that it will never trip, even under locked-rotor conditions, because running a fire pump to destruction is preferable to losing water pressure during a fire. Standard Article 430 math does not apply here.
- Motors Over 10 HP: For large 3-phase industrial motors, the fault current calculations and short-circuit bracing requirements of the panelboard itself must be verified by an electrical engineer to ensure the breaker's AIC (Ampere Interrupting Capacity) rating is sufficient.
For your standard 5 HP, 240V single-phase compressor, stick to the calculated baseline: pull 8 AWG copper THHN, terminate on a 60A dual-pole breaker, and verify your motor starter's overload dial is set exactly to the nameplate FLC. This provides a code-compliant, physically robust circuit that will survive years of high-inrush startups without nuisance tripping.
