Equipment Design

Electric Motor Sizing

Size electric motors for pumps, compressors, fans, and conveyors per NEMA MG-1 standards. Calculate FLA, select frame sizes, apply derating factors, and determine wire and breaker sizing per NEC 430.

Launch Calculator Check Derating

Service factor

1.15 NEMA std

Standard motors can sustain 15% continuous overload.

NEMA Premium

93-96% efficient

Premium efficiency reduces energy cost 2-5% vs standard.

NEC 430 wire sizing

125% of FLA

Continuous duty conductors sized at 125% of motor FLA.

Contents

Use this guide when you need to:

  • Select NEMA standard motor size for driven equipment.
  • Calculate full load amps and starting current.
  • Apply altitude and temperature derating factors.
  • Size conductors and overcurrent protection per NEC.

1. Overview & Key Concepts

Electric motors convert electrical energy to mechanical shaft power and are the primary drivers for pumps, compressors, fans, and other rotating equipment in pipeline and gas processing facilities. Proper motor sizing ensures reliable operation, energy efficiency, and code compliance.

Essential Parameters

Parameter Symbol Units Definition
Horsepower HP HP, kW Rated shaft output power
Full Load Amps FLA Amperes Current at rated load and voltage
Locked Rotor Amps LRA Amperes Starting current (6-8x FLA)
Efficiency η % Shaft power / electrical input power
Power Factor PF - Real power / apparent power
Service Factor SF - Allowable continuous overload ratio
Synchronous Speed Ns RPM 120 × f / P (f = frequency, P = poles)

Motor Speed and Pole Count

Poles Sync Speed (60 Hz) Sync Speed (50 Hz) Typical Full Load Speed
2 3600 RPM 3000 RPM 3500-3560 RPM
4 1800 RPM 1500 RPM 1750-1780 RPM
6 1200 RPM 1000 RPM 1160-1185 RPM
8 900 RPM 750 RPM 870-890 RPM
10 720 RPM 600 RPM 690-715 RPM
Slip: Induction motors always run slightly below synchronous speed. The difference is called slip, typically 2-5% at full load. Slip increases with load. Actual full-load speed is stamped on the motor nameplate.

2. Motor Sizing Procedure

Motor sizing follows a systematic procedure: determine load requirements, apply service and derating factors, select standard NEMA motor size, then verify electrical parameters.

Step-by-Step Procedure

  1. Determine required shaft power (BHP) from driven equipment calculations
  2. Apply service factor: Required HP = BHP × SF
  3. Apply derating factors for altitude and temperature
  4. Select NEMA standard motor size (next size ≥ required HP)
  5. Calculate FLA and verify electrical system adequacy
  6. Size conductors and protection per NEC Article 430

NEMA Standard Motor Sizes

Range Standard HP Sizes
Fractional to 5 HP 1, 1.5, 2, 3, 5
7.5 to 30 HP 7.5, 10, 15, 20, 25, 30
40 to 100 HP 40, 50, 60, 75, 100
125 to 500 HP 125, 150, 200, 250, 300, 350, 400, 450, 500

FLA Calculation (3-Phase Motors)

Full Load Amps: FLA = (HP × 746) / (√3 × V × η × PF) Where: HP = Rated motor horsepower 746 = Watts per HP conversion factor V = Line-to-line voltage (volts) η = Motor efficiency (decimal, e.g. 0.93) PF = Power factor (decimal, e.g. 0.85) √3 = 1.732 (three-phase factor)

Input Power

Electrical Input Power: kW = (HP × 0.746) / η kVA = kW / PF Three-phase apparent power: kVA = (√3 × V × FLA) / 1000

Example Calculation

Given:

  • Pump BHP = 42 HP
  • Service factor = 1.15
  • Voltage = 460V, 60 Hz
  • NEMA Premium efficiency
  • Power factor = 0.85

Calculate:

Required HP = 42 × 1.15 = 48.3 HP
Selected motor: 50 HP (next NEMA standard size)
Motor efficiency = 94.5% (NEMA Premium, 50 HP)
FLA = (50 × 746) / (1.732 × 460 × 0.945 × 0.85) = 58.3 A
Input power = (50 × 0.746) / 0.945 = 39.5 kW

3. Efficiency Classes

Motor efficiency directly impacts operating cost. Federal regulations (DOE 10 CFR 431) and NEMA standards define minimum efficiency levels for general purpose motors.

NEMA Efficiency Classes

Class Description Typical Range
Standard Pre-EPAct minimum efficiency 78-95%
Energy Efficient EPAct 1992 minimum (now DOE baseline) 82-95.4%
NEMA Premium Highest standard efficiency class 85.5-96.2%

NEMA Premium Efficiency Values (4-Pole, 1800 RPM)

Motor HP NEMA Premium (%) Energy Efficient (%) Standard (%)
185.582.578.0
589.587.585.5
1091.789.588.5
2593.692.491.0
5094.593.092.4
10095.494.193.0
20096.295.094.5
50096.295.495.0
Energy Savings Example: Upgrading a 100 HP motor from standard (93.0%) to NEMA Premium (95.4%) efficiency saves approximately 1,900 kWh per 1,000 operating hours. At $0.08/kWh running 8,000 hrs/yr, this saves about $1,200 per year with a typical payback period of 1-3 years.

Efficiency at Partial Load

Motor efficiency varies with load. Most motors reach peak efficiency at 75-100% of rated load. At light loads (below 50%), efficiency drops significantly, and power factor decreases even more.

Load (%) Efficiency (typical) Power Factor (typical)
100%93-95%0.85-0.88
75%93-95%0.80-0.85
50%90-93%0.70-0.78
25%82-88%0.50-0.60

Avoid Oversizing: Significantly oversized motors operate at low load factor, resulting in poor efficiency and low power factor. A motor loaded at 25% may have a power factor below 0.55, increasing reactive power charges and transformer loading.

4. Derating Factors

NEMA MG-1 rates motors for operation at or below 3,300 ft (1,000 m) elevation and 104°F (40°C) ambient temperature. Operation outside these conditions requires derating the motor output.

Altitude Derating

At higher altitudes, air density decreases, reducing the motor's cooling capability. Above 3,300 ft, derate the motor output by approximately 1% per 330 ft of additional elevation.

Altitude Derating: Derating (%) = (Altitude - 3,300) / 330 × 1% Example: At 6,600 ft elevation: Derating = (6,600 - 3,300) / 330 = 10% derating A 100 HP motor can only deliver 90 HP continuously.

Temperature Derating

When ambient temperature exceeds 40°C (104°F), the allowable temperature rise decreases, requiring motor output derating.

Ambient Temp (°C) Ambient Temp (°F) Derating Factor Effective HP (100 HP Motor)
401041.00100 HP
451130.9797 HP
501220.9393 HP
551310.8888 HP
601400.8383 HP
651490.7878 HP
701580.7272 HP
Insulation Class: Motors with Class F insulation (155°C) and Class B temperature rise (80°C) have a built-in thermal margin. This is the most common industrial configuration and provides some tolerance for occasional high ambient conditions.

Combined Derating

When both altitude and temperature derating apply, the factors are additive. A motor at 6,600 ft and 55°C ambient would need approximately 22% total derating (10% altitude + 12% temperature).

5. Electrical Sizing (NEC 430)

NEC Article 430 governs motor circuit conductors, overcurrent protection, and controller sizing. Key requirements include conductor sizing at 125% of FLA and branch circuit protection limits.

Conductor Sizing (NEC 430.22)

Minimum conductor ampacity: Ampacity ≥ 1.25 × FLA For a 50 HP motor at 460V with FLA = 58 A: Required ampacity ≥ 1.25 × 58 = 72.5 A Select: 4 AWG copper (85 A @ 75°C)

Wire Size Reference (Copper, 75°C THWN)

Wire Size Ampacity (A) Typical Motor HP (460V)
14 AWG201-2 HP
12 AWG253 HP
10 AWG355 HP
8 AWG507.5-10 HP
6 AWG6515 HP
4 AWG8520-25 HP
3 AWG10030 HP
2 AWG11540 HP
1 AWG13050 HP
1/0 AWG15060 HP
2/0 AWG17575 HP
3/0 AWG200100 HP
4/0 AWG230125 HP
250 MCM255150 HP
350 MCM310200 HP
500 MCM380250-300 HP

Branch Circuit Protection (NEC 430.52)

Motor branch circuit short-circuit and ground-fault protection limits depend on the type of protective device:

Protection Type Maximum % of FLA Example (58 A FLA)
Inverse-time circuit breaker 250% 145 A → use 150 A breaker
Instantaneous-trip breaker 800% (Design B) 464 A
Dual-element fuse 175% 101.5 A → use 110 A fuse
Non-time-delay fuse 300% 174 A → use 175 A fuse

Motor overload protection is separate from branch circuit protection. NEC 430.32 requires overload relays set at 115% of motor nameplate FLA (or 125% for motors with 1.15 SF). Overload relays protect the motor; branch circuit devices protect the conductors.

6. Motor Selection Guide

Enclosure Types

Type Description Application
ODP Open Drip Proof - allows air circulation, prevents dripping liquids Clean indoor environments
TEFC Totally Enclosed Fan Cooled - sealed, external fan cooling Outdoor, dusty, wet environments (most common)
TENV Totally Enclosed Non-Ventilated - sealed, no fan Small motors, clean environments, inverter duty
XPRF Explosion Proof - Class I, Div 1/2 Hazardous (classified) locations per NEC 500/505

NEMA Design Letters

Design Starting Torque Starting Current Application
Design A Normal (100-200% FL) High (no limit) Fans, blowers, machine tools
Design B Normal (100-200% FL) Limited (6-8x FLA) Most common. Pumps, fans, general purpose
Design C High (200-250% FL) Limited (6-8x FLA) Compressors, conveyors, crushers
Design D Very High (275%+ FL) Limited Punch presses, hoists, high-inertia loads

IEEE 841 - Severe Duty Motors

For petroleum and chemical industry applications, IEEE 841 specifies additional requirements beyond NEMA MG-1:

  • Enclosure: TEFC required (minimum)
  • Insulation: Class F insulation with Class B temperature rise
  • Bearings: Anti-friction bearings with minimum L10 life of 100,000 hours
  • Shaft: Carbon steel, minimum 1045
  • Frame: Cast iron or steel, no aluminum
  • Service Factor: 1.15 minimum
  • Efficiency: NEMA Premium minimum

Motor Selection by Equipment Type

Equipment NEMA Design Starting Torque Need Notes
Centrifugal Pump Design B Low (20-40% FL) Variable torque load, square law
Reciprocating Compressor Design C High (150-200% FL) Requires unloaded start or VFD
Centrifugal Fan Design B Low (20-40% FL) Variable torque, consider VFD
Conveyor Design C High (breakaway torque) Loaded start condition common
Screw Compressor Design B Moderate (50-75% FL) Slide valve unloads for start

Medium Voltage Motors (>600V)

Motors above 200-250 HP are often specified at medium voltage (2,300V or 4,160V) to reduce conductor cost and voltage drop:

  • 2,300V: Common for 200-500 HP motors
  • 4,160V: Common for 300-5,000+ HP motors
  • Advantages: Lower current, smaller conductors, reduced I²R losses
  • Requirements: Medium voltage switchgear, VTs, CTs, protective relaying
  • Starting: May require reduced voltage starting (autotransformer, VFD, soft starter)

Variable Frequency Drives (VFDs)

VFDs are increasingly used with motors for energy savings on variable-torque loads:

  • Best applications: Centrifugal pumps and fans with varying flow requirements
  • Energy savings: Power varies as speed cubed (affinity laws). Reducing speed by 20% saves ~50% power
  • Motor requirements: Inverter-duty rated motor recommended (Class F insulation minimum)
  • Caution: VFDs produce harmonic distortion; may require output filters for long cable runs

Common Mistakes to Avoid

  • Selecting motor too large (causes low efficiency and poor power factor at partial load)
  • Forgetting altitude and temperature derating for remote or hot locations
  • Using FLA from NEC tables instead of motor nameplate for overload sizing
  • Ignoring starting torque requirements for loaded-start equipment
  • Not verifying motor thermal capability for frequent starts (jogging duty)
  • Specifying ODP enclosure for outdoor or dusty midstream locations
  • Forgetting to coordinate motor protection with upstream overcurrent devices

Key Standards & References

  • NEMA MG-1 — Motors and Generators (frame sizes, efficiency, derating)
  • NEC Article 430 — Motors, Motor Circuits, and Controllers
  • IEEE 841 — Severe Duty Motors for Petroleum, Chemical, and Gas Industry
  • DOE 10 CFR 431 — Energy Conservation Standards for Electric Motors
  • API 541 — Form-Wound Squirrel Cage Induction Motors (large motors)
  • API 546 — Brushless Synchronous Machines (synchronous motors)
  • API 547 — General Purpose Form-Wound Squirrel Cage Induction Motors
  • NFPA 70 — National Electrical Code (NEC)

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