Wire Sizing for Motors: NEC Article 430 Rules & Ampacity Tables

Wire sizing for motors is the process of selecting a conductor gauge that can safely carry 125% of a motor’s full-load ampacity (FLA) while accommodating the massive, brief inrush current of startup without nuisance tripping. Unlike standard lighting or receptacle circuits, sizing conductors for inductive loads fundamentally changes how you select both the wire and the overcurrent protection, effectively decoupling the breaker rating from the wire’s continuous ampacity limits. The most common mistake DIYers and junior apprentices make is attempting to size the wire to match the breaker, or relying on the motor’s nameplate amperage rather than the National Electrical Code (NEC) reference tables.

The Core Rule: Why Motor Wire Sizing Defies Standard Logic

In a standard branch circuit governed by NEC Article 210, the rule is simple: the wire ampacity must equal or exceed the breaker rating. A 20A breaker requires at least 12 AWG copper wire. If you try to apply this logic to a motor circuit, your breaker will trip every time the motor starts.

Motors draw a massive spike of inrush current—often 600% to 800% of their running current—for the first few seconds of startup. To handle this, NEC Article 430 flips the standard protection model. The circuit breaker is sized to tolerate the starting inrush current, while the wire is sized only for the running continuous current.

Think of the wire as a highway and the breaker as a toll booth. The highway (wire) only needs to be wide enough to handle the steady daily commute (running FLA plus a 25% safety margin). The toll booth (breaker), however, must be wide enough to let a massive, once-a-day parade (inrush current) through without shutting down the entire road.

Because the motor’s internal thermal overloads (or the overload relay in the starter) are specifically calibrated to protect the motor and the wire from slow, continuous overheating, the branch-circuit breaker is freed up to act solely as a short-circuit and ground-fault protector. This is why it is perfectly legal—and common—to see a 40A breaker protecting a 12 AWG wire on a motor circuit.

NEC Article 430 Sizing Table: Full-Load Amps to Wire Gauge

When sizing conductors, you must use the NEC tables (430.247 through 430.250) to find the Full-Load Amps (FLA), rather than the specific nameplate rating on the motor casing. The nameplate is used for sizing the overload heaters, but the code tables ensure standardization for wire and breaker sizing across different motor efficiencies and manufacturers.

The table below maps common motor sizes to their NEC table FLA, the required 125% wire ampacity target, and the minimum copper wire gauge based on the 75°C column of NEC Table 310.16. We use the 75°C column because, per NEC 110.14(C), most motor termination lugs and disconnect switches are rated for 75°C, even if you pull 90°C THHN wire.

*Note: While 14 AWG meets the strict ampacity math for the 5 HP 3-phase motor, voltage drop and physical fragility usually dictate upsizing to 12 AWG or 10 AWG in real-world installations.

Worked Example: Sizing Conductors for a 5 HP Air Compressor

Let’s walk through a real jobsite scenario. You are wiring a new 5 HP, 230V, 3-phase air compressor in a woodshop. The run from the subpanel to the motor disconnect is 85 feet. The motor nameplate states an FLA of 14.5A.

1. Find the Code FLA: Ignore the 14.5A nameplate for wire sizing. Look up 5 HP at 230V, 3-phase in NEC Table 430.250. The value is 15.2A.
2. Apply the 125% Multiplier: Multiply 15.2A by 1.25. Your target wire ampacity is 19.0A.
3. Select the Wire Gauge: Check NEC Table 310.16 (75°C column). 14 AWG copper is rated for 20A. Technically, 14 AWG satisfies the code requirement for ampacity.
4. Calculate Voltage Drop (The Reality Check): Here is where bench theory meets jobsite reality. A 3-phase voltage drop calculation over 85 feet with 14 AWG copper yields roughly a 4.2% voltage drop during running, and a massive dip during startup. NEC recommends keeping voltage drop under 3% for branch circuits. Furthermore, the mechanical vibration of an air compressor will fatigue thin 14 AWG strands at the terminal lugs over time.
5. The Final Decision: Upsize to 10 AWG THHN copper. This drops the voltage loss to under 1.5%, provides a robust physical connection, and leaves headroom for future ambient temperature derating in a hot shop.

For the breaker sizing (governed by NEC 430.52), you multiply the 15.2A FLA by 250% for an inverse-time breaker, yielding 38A. The next standard breaker size up is 40A. You will install a 40A breaker protecting 10 AWG wire. For standard circuits, this would be a severe fire hazard and an immediate code violation; for a motor circuit with proper overload protection, it is exactly what the NFPA National Electrical Code requires.

Where You Meet This in Practice (and Common Confusions)

You will encounter motor wire sizing rules anywhere inductive loads with high starting torque are installed. Common residential and commercial applications include:

• HVAC Condensers: Typically 2 to 5 HP, 240V single-phase. These often use dual capacitors to manage the starting torque, but the wire sizing rules remain identical.
• Well Pumps: Submersible 2-wire or 3-wire pumps (usually 1 to 3 HP, 230V). Because these are buried deep underground, the wire must be sized not just for the motor FLA, but to compensate for the extreme voltage drop down the well casing.
• Pool Pumps: Often 1.5 to 3 HP. Modern variable-speed pool pumps use internal VFDs (Variable Frequency Drives), which changes the inrush profile, but the supply conductors must still be sized to the maximum rated input current on the drive's nameplate.

What People Commonly Confuse

The most dangerous confusion in motor wiring is treating the branch-circuit breaker as the primary overload protector. In a standard circuit, the breaker stops the wire from melting if you plug in too many space heaters. In a motor circuit, the breaker is practically blind to a 10% continuous overload; it will happily let 17A flow through a 15A-rated motor until the windings melt. The overload relay inside the motor starter is what actually protects the motor from burning out. If you bypass or improperly size the overload heaters, the wire and motor are entirely unprotected against slow thermal destruction, regardless of how perfectly you sized the conductors.

For a deeper understanding of how motor protection coordinates with conductor sizing, Fluke’s guide on measuring motor current provides excellent field-testing methodologies to verify your installed loads match your calculations.

Frequently Asked Questions