How to Size a NEMA Motor Starter

Motor starter sizing: direct answer #

Choose a motor starter from the manufacturer's horsepower table for the motor HP, voltage, phase and duty, then verify the starter's continuous-current rating, overload-relay range, utilization category, enclosure and short-circuit rating. A NEMA size is a catalog selection—not simply motor full-load current multiplied by 125%.

For example, Schneider lists a NEMA Size 3 starter rated 90 A and 50 HP at 460 V AC, three phase for the cited configuration. That is useful evidence for a 50 HP selection, but the exact coil, enclosure, overload unit and SCCR still depend on the chosen product.

Best for: screening a NEMA across-the-line starter from current manufacturer tables. Not suitable for: final overload settings, branch protection or an IEC contactor selection without project-specific code and product data. Browse the Protection Calculator Hub for the downstream fault and protection checks.

NEMA starter size screening chart #

The following horsepower values are a quick reading of typical Eaton Freedom Series open-starter tables. Confirm the current catalog and exact product before specification.

These are horsepower application ratings under the manufacturer's stated conditions. They are not interchangeable with IEC AC-3 amp ratings and they do not establish the overload setting or branch-circuit protection.

Selection workflow #

1. Record the motor and application #

Collect motor nameplate HP, voltage, phase, frequency, full-load current, service factor and duty. Also record starts per hour, acceleration time, plugging or reversing duty, ambient temperature and enclosure environment.

If the motor HP and voltage combination is absent from a manufacturer's table, do not extrapolate the NEMA size number. Use the supplier's application data.

2. Select the starter frame from the manufacturer table #

Match HP and voltage first, then confirm that the starter's continuous-current and duty ratings cover the application. NEMA starters usually provide standardized horsepower ratings; IEC contactors are commonly selected by utilization category and operational current. A size number in one system does not translate directly into the other.

3. Select and set the overload relay separately #

The overload relay protects against sustained motor overload; the contactor frame performs switching. Select a relay whose adjustable range contains the required setting and follow the motor nameplate, relay instructions and the electrical code edition enforced by the authority having jurisdiction.

Do not use the contactor's maximum amp rating as the overload setting. Conversely, selecting a larger contactor does not by itself improve motor overload protection.

4. Verify short-circuit protection and SCCR #

The starter must be applied with the branch-circuit protective device and combination ratings specified by the manufacturer. Compare the marked assembly SCCR with available fault current at the installation point.

Read what SCCR means, then use the SCCR Calculator for a screening comparison. This does not replace the manufacturer's combination tables or a fault study.

5. Verify controls and installation details #

Confirm coil voltage and frequency, control transformer requirements, auxiliary contacts, disconnect arrangement, enclosure type, ambient derating and whether the starter must reverse, jog or operate at unusually high frequency.

Worked selection: 50 HP at 460 V #

Given: 50 HP, 460 V, three-phase motor with non-reversing across-the-line starting.

1. A current Schneider Type S product page lists NEMA Size 3, 90 A, 50 HP at 460 V.
2. Confirm the actual motor nameplate and starting duty fit the selected product's application limits.
3. Choose an overload relay range that contains the required setting; set it using the applicable instructions and code rules.
4. Select the manufacturer-approved branch protective device or combination rating.
5. Verify that the resulting starter or panel SCCR is at least the available fault current.
6. Select the correct coil and enclosure for the control supply and location.

The example identifies a plausible frame; it is not a complete bill of material.

When to consider a soft starter or VFD #

A NEMA across-the-line starter may be unsuitable when the supply cannot tolerate the starting current, the process requires controlled acceleration, or mechanical shock is unacceptable. Use measured load requirements and manufacturer application guidance to compare across-the-line, reduced-voltage, soft-starter and VFD options. The motor starting current guide explains the transient and protection questions.

Common mistakes #

1. Calculating NEMA size as FLC × 125%: catalog HP and duty tables determine the frame; protection settings are separate.
2. Using nameplate current for every code calculation: different code rules may call for table current or nameplate data for different purposes.
3. Ignoring starts per hour and acceleration time: contact life and thermal duty can govern.
4. Assuming breaker kAIC equals starter SCCR: interrupting and withstand ratings are different.
5. Mixing NEMA size and IEC utilization ratings: verify the selected manufacturer's system directly.

Technical sources #

• Eaton Freedom Series NEMA contactors and starters catalog — horsepower tables and product-selection data.
• Schneider Electric NEMA Size 3 starter example — 50 HP at 460 V, 90 A product configuration.

FAQ #

Next step #

Use the motor nameplate and current manufacturer catalog to select the frame, then complete overload, branch protection and SCCR checks as separate documented decisions.