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Motor Nameplate Decoder - Full Decode with NEC 430 Wire & Breaker Sizing

Decode NEMA motor nameplates and get wire gauge, conduit size, overload, and branch circuit breaker per NEC Article 430

Decode every field on an AC motor nameplate: horsepower, voltage, full-load amps (FLA), service factor, efficiency, frame size, enclosure type, insulation class, NEMA design letter, duty cycle, locked rotor code, and thermal protection. Then get NEC Article 430 wire sizing (125% of FLA), conduit fill, motor overload protection (115% or 125% of FLA based on service factor), and branch circuit short-circuit protection (breaker/fuse type and size). Covers single-phase and three-phase motors from 1/4 HP to 500 HP at 115V through 4160V.

Pro Tip: Never size motor wire from the nameplate FLA. NEC Table 430.248 (single phase) and 430.250 (three phase) list standard FLA values by HP and voltage that may differ from the nameplate. Use the NEC table values for conductor and overcurrent protection sizing, and use the actual nameplate FLA only for overload relay sizing. This distinction catches many electricians off guard and is a common code violation in the field.

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Motor Nameplate Decoder
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How It Works

  1. Enter Motor Horsepower and Voltage

    Input motor HP (or kW) and voltage rating from the nameplate. Select single-phase or three-phase. The calculator loads NEC Table 430.248 or 430.250 FLA values for conductor and breaker sizing. Common voltages: 115V, 208V, 230V, 460V, 575V for industrial motors.

  2. Enter Nameplate Details

    Input the remaining nameplate fields: full-load amps (FLA), RPM, service factor, efficiency, frame size, enclosure type (ODP, TEFC, TENV), insulation class (B, F, H), NEMA design letter (A, B, C, D), and locked rotor code letter.

  3. Review Decoded Information

    See a plain-language explanation of every nameplate field. Frame size decodes to shaft height, shaft diameter, and mounting dimensions. Insulation class shows maximum temperature rating. NEMA design letter indicates torque-speed characteristics and typical applications.

  4. Get NEC 430 Wire Sizing

    The calculator applies NEC Article 430 rules: conductor sized at 125% of NEC table FLA, overload protection at 115% (SF 1.15+) or 125% of nameplate FLA, and branch circuit protection per Table 430.52 for the selected overcurrent device type (inverse time breaker, dual element fuse, or instantaneous breaker).

  5. Select Conduit and Verify

    See recommended conduit size for the calculated wire gauge based on NEC Chapter 9 conduit fill tables. Verify that all values meet your local code amendments and the authority having jurisdiction (AHJ) requirements.

Built For

  • Electricians sizing wire, conduit, breakers, and overloads for new motor installations
  • Maintenance techs identifying replacement motor specifications from existing nameplate data
  • Plant engineers specifying motor requirements for new equipment purchases
  • Electrical inspectors verifying NEC 430 compliance for motor circuit installations
  • Apprentice electricians learning to read and interpret motor nameplates
  • Estimators calculating material quantities for motor circuit rough-in
  • Reliability engineers documenting motor specifications for maintenance records and CMMS

Features & Capabilities

Complete Nameplate Decoder

Explains every standard motor nameplate field in plain language: HP, voltage, FLA, RPM, service factor, efficiency, power factor, frame size, enclosure, insulation class, NEMA design, duty type, locked rotor code, and thermal protection. Includes the practical significance of each field for installation and operation.

NEC Article 430 Wire Sizing

Sizes branch circuit conductors at 125% of NEC Table 430.248/250 FLA per 430.22. Handles multi-motor feeders per 430.24 (largest motor at 125% + sum of others). Shows copper and aluminum wire options with THHN/THWN-2 insulation ratings. Conductor ampacity from NEC Table 310.16.

Overcurrent Protection Calculator

Calculates branch circuit short-circuit and ground-fault protection per NEC Table 430.52. Shows maximum breaker or fuse size for inverse time breakers (250% of FLA), dual element fuses (175%), and instantaneous breakers (800-1700% depending on motor design). Rounds to next standard device size per 430.52(C)(1).

Overload Protection Sizing

Sizes motor overload relays per NEC 430.32: 115% of nameplate FLA for motors with service factor 1.15 or higher, 125% of nameplate FLA for motors with SF less than 1.15 or temperature rise not exceeding 40 degrees C. Uses actual nameplate FLA, not NEC table values.

Frame Size Reference

Decodes NEMA frame size to physical dimensions: shaft height (frame number / 4 = inches from base to shaft center), shaft diameter, shaft extension length, and mounting bolt pattern. Covers standard frames from 42 through 449T, including C-face and D-flange variants.

Locked Rotor Code Letter

Decodes the NEMA locked rotor code letter (A through V) to locked rotor kVA per horsepower. Calculates actual locked rotor amps for the motor's HP and voltage rating. Essential for sizing motor starters, soft starters, and determining whether reduced-voltage starting is required.

Comparison

Motor Type NEC Wire Sizing Basis Overload Sizing Basis SCPD Max (Inv. Time) Typical Application
1-Phase, 115V Table 430.248 FLA x 125% Nameplate FLA x 115% 250% Table FLA Shop tools, compressors, HVAC residential
1-Phase, 230V Table 430.248 FLA x 125% Nameplate FLA x 115% 250% Table FLA Well pumps, large HVAC, farm equipment
3-Phase, 208V Table 430.250 FLA x 125% Nameplate FLA x 115% 250% Table FLA Commercial HVAC, small industrial
3-Phase, 460V Table 430.250 FLA x 125% Nameplate FLA x 115% 250% Table FLA Industrial pumps, fans, conveyors
3-Phase, 4160V Table 430.250 FLA x 125% Nameplate FLA x 115% 250% Table FLA Large process motors, utility pumps

Frequently Asked Questions

NEC tables 430.248 and 430.250 list standardized FLA values that represent typical motors at each HP and voltage rating. Individual motor nameplates may show higher or lower FLA depending on efficiency, power factor, and manufacturer design. NEC requires using table values (not nameplate FLA) for conductor and short-circuit protection sizing to ensure the circuit can handle any motor of that HP rating. Nameplate FLA is used only for overload protection sizing.
Service factor (SF) is a multiplier indicating how much continuous overload the motor can handle. A 10 HP motor with SF 1.15 can deliver up to 11.5 HP continuously without damage. For NEC purposes, a service factor of 1.15 or higher allows overload protection at 115% of nameplate FLA instead of 125%. Service factor does not affect conductor sizing, which is always based on 125% of NEC table FLA regardless of service factor.
The frame number divided by 4 gives the shaft centerline height in inches above the mounting base. A 256T frame has a shaft height of 256/4 = 64 inches. Wait - that would be too tall. Actually, for frame 256T, the shaft height is the first two digits divided by 4: 25.6/4 = 6.4 inches. The T suffix indicates a standardized shaft diameter and extension per NEMA MG-1. The C suffix indicates a C-face flange mount. Common frames for 1-25 HP motors range from 143T to 286T.
The locked rotor code letter (A through V) indicates the starting kVA per HP when the motor rotor is locked (stalled). It is used to calculate locked rotor current (starting inrush) for sizing motor starters, fuses, and circuit breakers. For example, code letter G (5.6-6.3 kVA/HP) on a 10 HP, 460V, 3-phase motor gives a locked rotor current of about 75-84 amps. Higher code letters mean higher starting current, which may require reduced-voltage starting methods.
ODP (Open Drip Proof) motors have ventilation openings that allow air to circulate through the windings for cooling. They are suitable for clean, dry indoor environments. TEFC (Totally Enclosed Fan Cooled) motors have a sealed enclosure with an external fan that blows air over the motor housing. TEFC motors handle dusty, wet, and outdoor environments. TEFC motors run hotter internally and may cost 15-30% more, but they last longer in harsh industrial environments.
Per NEC Table 430.52: for an inverse time circuit breaker, maximum size is 250% of NEC table FLA. For dual element time-delay fuses, maximum is 175%. For instantaneous breakers, maximum is 800% for Design B motors and 1100% for Design D motors. Round up to the next standard device size per 430.52(C)(1). For a 10 HP, 460V, 3-phase motor (NEC FLA = 14A): 14 x 2.5 = 35A, so a 35A or 40A inverse time breaker is acceptable.
Class F insulation (155 degrees C maximum winding temperature) is the most common for modern industrial motors and should be the minimum specification for replacement motors. Class H (180 degrees C) provides additional thermal margin for motors in high-ambient environments or with frequent starting. Class B (130 degrees C) is older technology and should be avoided for new installations. Specify "Class F insulation with Class B temperature rise" for the longest winding life.
NEC 430.102 requires a disconnecting means for every motor and its controller, located within sight of the motor (or capable of being locked in the open position). The disconnect must be rated for the motor load and voltage. For motors 1/8 HP and below, the branch circuit overcurrent device can serve as the disconnect. Common disconnect types include motor circuit switches (heavy-duty toggle), non-fusible disconnects, fusible disconnects, and manual motor starters for small motors.
Disclaimer: This tool provides NEC-based calculations for motor circuit design reference. Actual installations must comply with the current edition of the National Electrical Code (NEC/NFPA 70), local amendments, and the authority having jurisdiction (AHJ). Wire sizing, overcurrent protection, and conduit fill calculations are for reference only and do not replace engineering judgment or licensed electrical design. ToolGrit is not responsible for electrical installations or code compliance outcomes.

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