Motor Starter Size Calculator — Free Online Calculator
Size motor starters, contactors, and overload relays based on motor HP, voltage, and full load amps.
How to Use This Calculator
Enter motor HP, voltage, and phase.
The Formula Explained
FLA estimated from HP rating with typical efficiency and power factor. Overload = FLA × 115% per NEC 430.32. NEMA starter size based on HP and voltage ranges.
Motor Starters: The Control Interface
Motor starters are the workhorses of industrial electrical control. They start, stop, and protect motors in everything from conveyor belts to HVAC systems to large pumps. At their core, a motor starter consists of three components: a contactor that switches power to the motor, an overload relay that protects against sustained overcurrent, and a control circuit (typically a start/stop pushbutton with holding contacts). Selection depends on motor horsepower, voltage, operating environment, and control strategy.
The industry recognizes two main standards: NEMA (common in North America) and IEC (common internationally). NEMA starters are sized by horsepower categories and tend to be robust, oversized, and expensive. IEC starters are sized by current and tend to be compact, economical, and require more precise selection. A NEMA Size 1 starter costs 200-400 USD and handles 10 HP at 460V; an IEC equivalent (contactor + overload) costs 80-150 USD for the same duty. For new industrial installations, IEC is increasingly common; for retrofits and replacements, NEMA often wins due to universal dimensional standards.
Worked Example: 15 HP Pump Motor Starter
A 15 HP three-phase 460V pump motor with FLA 21 amps (NEC Table 430.250). Motor starter selection: NEMA Size 2 (rated 25 HP at 460V, covers our 15 HP with margin).
Overload relay setting: NEC 430.32 requires overload at 115-125% of motor FLA depending on service factor. For a motor with service factor 1.15: OL set to 125% × 21 = 26.25A. Standard thermal overload heater elements: select the one with trip current closest to 26A. Most thermal overload relays are field-adjustable within a range (20-30A range covers 26A).
Branch circuit protection (NEC 430.52): 250% of FLA for thermal-magnetic breaker = 21 × 2.5 = 52.5A, round up to 60A breaker. Conductor: 8 AWG copper at 125% of 21 = 26.25A, so 10 AWG (30A) is adequate but 8 AWG (40A) is more conservative.
Control circuit: 120V control transformer (usually built into the starter or separate) with fused control. Start/stop pushbuttons wired through overload auxiliary contacts — if the OL trips, both the holding circuit and the contactor coil drop out.
Worked Example: 100 HP Motor with Soft Starter
A 100 HP three-phase 460V motor for a large fan (wear-sensitive to inrush). Direct-on-line starting would draw 100 × 7 = 700% of 124A FLA = 868A inrush for several seconds, causing voltage sag and mechanical stress.
Solution: soft starter that ramps voltage from 30% to 100% over 10-30 seconds. Starting current is limited to about 300% of FLA = 372A, and torque builds smoothly rather than hammering the motor and load. Soft starter rating: 100 HP at 460V, model-specific.
Cost comparison: NEMA Size 4 starter (100 HP rated) about 1,500 USD. Autotransformer reduced-voltage starter about 3,500 USD. Soft starter about 2,500 USD. VFD about 8,000-12,000 USD but adds speed control and energy savings for variable loads. For fixed-speed large motors, soft starters are the typical compromise. For variable-speed applications (most fans and pumps), VFDs pay back through energy savings in 2-4 years.
Five Motor Starter Mistakes
1. Using an oversized contactor without overload. A contactor alone does not protect the motor. Overload relay is required by NEC 430.32 for any motor over 1 HP.
2. Setting overload too high to prevent nuisance trips. If the overload trips, something is wrong — the motor is genuinely overloaded, the current is higher than expected, or the cooling is inadequate. Fix the underlying problem; do not just set OL higher. NEC maximum is 125-140% of FLA depending on conditions (430.32).
3. Not providing thermal overload with each start/stop. Magnetic contactors do not provide overload protection. Even the simplest motor control circuit needs thermal overload in the motor's phase conductors.
4. Using NEMA Size 0 for 5+ HP motors. NEMA Size 0 is rated for only 3 HP at 230V. Larger motors need Size 1 or bigger. Size tables are non-negotiable for NEMA ratings.
5. Ignoring the control transformer sizing. Control circuits need a dedicated control transformer sized for the coil VA plus auxiliary loads (pilot lights, auxiliary contacts). Typical: 100 VA for small circuits, 200-500 VA for complex controls.
NEMA Motor Starter Size Table
Size 00: 1.5 HP at 230V, 2 HP at 460V (single- or three-phase).
Size 0: 3 HP at 230V, 5 HP at 460V three-phase.
Size 1: 7.5 HP at 230V, 10 HP at 460V three-phase.
Size 2: 15 HP at 230V, 25 HP at 460V three-phase.
Size 3: 30 HP at 230V, 50 HP at 460V three-phase.
Size 4: 50 HP at 230V, 100 HP at 460V three-phase.
Size 5: 100 HP at 230V, 200 HP at 460V three-phase.
Size 6: 200 HP at 230V, 400 HP at 460V three-phase.
Size 7: 300 HP at 230V, 600 HP at 460V three-phase.
Control voltages: typically 120V control circuit stepped down from line voltage through a control transformer. Some starters use 24VDC PLC control for integration with automation systems.
NEC Motor Circuit References
NEC 430.24 — Feeder conductor sizing for multiple motors. NEC 430.32 — Motor overload protection sizing (115-125% of FLA). NEC 430.52 — Branch circuit short-circuit and ground-fault protection (multiplier of FLA depending on device type). NEC 430.82 — Motor controller requirements. NEC 430.102 — Motor disconnect requirements (within sight of the motor in most cases).
UL 508 — Standard for industrial control equipment including motor starters. NEMA ICS 2 — Standards publication for motor controllers. NFPA 79 — Electrical standard for industrial machinery, covering control wiring and protection. These standards are referenced in commercial and industrial project specifications and must be followed for UL-listed equipment.
Motor starter sizing: NEMA size, overload heaters, and the difference between OL and SCP
A motor starter is a contactor sized for motor starting current plus an overload relay sized for thermal protection at running current. The NEMA size scale (00, 0, 1, 2, 3, 4, 5) maps to motor hp and voltage. Modern IEC starters are smaller and continuously sized; NEMA dominates US industrial practice.
The formula and what it does
NEMA Size 00 covers 1.5 hp at 230 V or 2 hp at 460 V. Size 1: 7.5 hp at 230 V, 10 hp at 460 V. Size 2: 15 hp at 230 V, 25 hp at 460 V. Size 3: 30 hp at 230 V, 50 hp at 460 V. Each step roughly doubles capacity. Overload heater (or electronic OL) sized per NEC 430.32 at 115% (SF 1.0) or 125% (SF above 1.15) of motor FLA.
Worked example
Scenario: 25 hp motor on 460 V 3-phase, service factor 1.15, FLA 32 A per nameplate.
NEMA size: Size 2 (covers 25 hp at 460 V). Overload heater at 125 percent: 32 x 1.25 = 40 A trip. Pick standard heater closest to 40 A. Short-circuit branch protection per NEC 430.52: inverse-time breaker at 250 percent of FLA = 80 A standard size. Feeder conductor per 430.6 and Table 430.250: 25 hp at 460 V = 34 A, at 125% = 42.5 A, so 8 AWG copper.
Code references and standards
NEC 430.32 overload protection: 115 percent for SF 1.0 motors, 125 percent for SF above 1.15. Set the OL heater accordingly.
NEC 430.52 short-circuit branch-circuit protection: 175-300 percent of FLA depending on device type.
NEC 430.6 motor branch-circuit FLA from tables 430.247-250, not nameplate.
Common mistakes to avoid
undefinedFrequently asked questions
What is the difference between IEC and NEMA starters?
NEMA is sized by hp and is over-sized for typical applications, giving long life. IEC is sized continuously by FLA and is smaller and cheaper. Industrial practice in US is mostly NEMA; Europe is IEC.
Can a VFD replace a motor starter?
Yes, and it adds speed control. VFDs include the starting and overload functions internally. NEC 430.122 sizes conductors at 125 percent of VFD input rating.
Do soft starters require an overload relay?
Yes, unless the soft starter has integrated motor protection. NEC 430.6(C) accepts integrated motor protection per UL 60947 listings.
How does this differ for single-phase motors?
Same concepts. Single-phase motors usually have built-in thermal overload protection in addition to (or replacing) external OL relay. Starters are smaller and often integrated into the motor wiring.
What is reduced-voltage starting?
Wye-delta, autotransformer, or soft-starter methods that limit starting inrush. Common on large motors (50+ hp) where direct-on-line start would dim lights or trip upstream breakers.
How do I select OL trip class?
Class 10 trips fast (good for general motors), Class 20 medium, Class 30 slow (compressors, pumps with long startup). Class 10 is the default.