⚙ Recommended NEMA Size
Selection Breakdown
Full Calculation Table
| Value | Basis | Result |
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How to Use the Motor Starter Calculator
NEMA starter sizing only needs these two values. Pick the supply voltage from the motor's nameplate.
These values are used only for the overload relay setting, which is separate from NEMA starter size selection.
The calculator returns the minimum NEMA starter size and the recommended overload relay setting range.
Sizing a Motor Starter for HVAC Equipment
A motor starter has two separate jobs, and each one gets sized differently. The contactor switches the motor on and off and has to survive the inrush current every time it starts. The overload relay protects the motor from sustained overcurrent and has to be set close to the motor's actual running current. Mixing up these two sizing methods is one of the most common mistakes in motor control work.
NEMA Starter Sizing: Horsepower and Voltage Only
NEMA designed its starter sizing system to be simple on purpose. Every NEMA size, from 00 up through larger sizes, has a published maximum horsepower rating at each standard voltage: 200V, 230V, 460V, and 575V. A NEMA Size 1 starter, for example, handles up to 7.5 HP at 200V or 230V and up to 10 HP at 460V or 575V. You don't need the motor's exact full load amps to pick the NEMA size, just the horsepower and voltage, which makes NEMA starters convenient for projects where the exact motor isn't selected until later in the job.
Why Voltage Changes the Answer
The same horsepower motor draws less current at a higher voltage, so a 10 HP motor needs a bigger NEMA size at 230V than it does at 460V. This is the same principle behind why three-phase circuits draw less current than single-phase circuits for the same power, covered in more detail in the three phase calculator.
Overload Relay Sizing: A Completely Different Calculation
Once the NEMA size is picked, the overload relay setting comes from a completely separate calculation based on the motor's actual nameplate full load amps. NEC Article 430.32 sets the standard: motors with a marked service factor of 1.15 or higher, or a marked temperature rise of 40 degrees Celsius or less, get an overload setting of 125% of FLA. Motors without those markings use a more conservative 115% of FLA. Always use the real nameplate FLA for this step rather than a horsepower-based estimate, since two motors with the same horsepower can have noticeably different actual current draw.
NEMA vs. IEC Starters
IEC starters, more common outside North America but increasingly used in Canada too, are sized by current rating rather than a simple horsepower lookup table, which means they need more precise selection but tend to be smaller and less expensive than an equivalent NEMA starter. NEMA starters trade some of that compactness for a wider operating margin and interchangeable heater elements, which makes field maintenance simpler. This calculator focuses on NEMA sizing since it remains the dominant standard for HVAC motor control work in Canada and the US.
Next Steps
Once you've confirmed the starter size and overload setting, check the breaker size calculator for the upstream branch circuit protection, and the wire size calculator to confirm the conductor feeding the starter is sized correctly for the motor's MCA.
Frequently Asked Questions
NEMA starter selection only needs two things: the motor's horsepower and its supply voltage. Each NEMA size, from 00 up through 6 and beyond, has a maximum horsepower rating at each standard voltage (200V, 230V, 460V, and 575V). Pick the smallest NEMA size whose maximum horsepower at your voltage is equal to or greater than the motor's horsepower, then confirm the overload relay separately using the motor's actual nameplate full load amps.
Overload relay sizing is separate from NEMA starter sizing and is based entirely on the motor's nameplate full load amps, not its horsepower rating. Per NEC Article 430.32, motors with a marked service factor of 1.15 or higher, or a marked temperature rise of 40 degrees Celsius or less, use an overload setting of 125% of FLA. Motors without those markings use 115% of FLA. Always use the actual nameplate FLA for this calculation rather than a horsepower-based estimate.
Yes. Using a larger NEMA starter than the minimum required is acceptable and gives extra margin for inrush current and a longer service life. The overload relay still needs to be set based on the motor's actual nameplate FLA regardless of which starter size is installed, since the overload protects the motor, not the starter.
A motor draws less current at a higher voltage for the same horsepower output, so NEMA's sizing table reflects that by allowing a smaller starter size to handle more horsepower as voltage increases. A 10 HP motor at 460V fits a NEMA Size 1 starter, while the same 10 HP motor at 230V typically needs a NEMA Size 2, since the 230V version draws roughly twice the current of the 460V version.
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