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Sizing Motors FLC, HP, Voltage, Breaker Size and Wire Size

Electrical Technology
7 9 minutes read

How to Size Motor HP, Input Voltages, Full Load Current (FLC), Breaker Size, And Copper Wire Size for Single Phase & 3-Phase Motors

Motor load circuits are significantly different from ordinary circuits such as resistive load and lighting points. This is because motors draw 6–8 times their normal running current (known as inrush current) during startup. For this reason, three-phase motors require contactors and starters to ensure safe operation. Consequently, overcurrent protective devices (OCPDs) for motor circuits are sized differently than those for general loads. To address this, the NEC covers the requirements for motor circuits, controllers and associated calculations in a comprehensive Article 430.

The following tabulated charts provide reference values and quantities related to electric motors. In addition, solved examples and calculations are provide for both single-phase and three-phase motors.

For example, you can determine the conductor size in AWG, and breaker size in amperes for single-phase and three-phase motors. In addition, you can easily size motor circuits based on horsepower (HP) or kilowatts (kW), input rated voltage, and full-load current (FLC) in amperes.

How to Size Motors FLC, HP, Voltage, Breaker Size and Wire Size

Before proceeding, it is important to understand the difference between Full-Load Current (FLC) and Full-Load Amperes (FLA):

  • FLC: A standardized current value based on motor horsepower (HP), as specified in NEC Tables 430.247 through 430.250 and in UL 508A (Table 50.1 for AC motors and Table 50.2 for DC motors).
  • FLA: The actual rated current of the motor, typically printed on the motor nameplate. For this reason, the NEC refers to it as the nameplate value. In the IEC 60364 standard, the rated current (FLA) is represented as Ib, while the nameplate current is represented as IN.

Good to Know: 

FLC is used to determine the proper size of protective devices (e.g., fuses and circuit breakers) for branch-circuit conductor and short-circuit and ground-fault protection for motors, while FLA is used to determine the appropriate setting for motor overload protection.

Below are some important formulas for key quantities and terms associated with single-phase and three-phase motors.

Motor Shaft Power in kW

Shaft Power in kW = Input Power in kW × Efficiency in %

Where:

  • Shaft power = Output mechanical power available at motor’s shaft.
  • Input Power = Electrical input power in kW × efficiency in % or kilo-volt-amperes (kVA)

Based on the relation between input and output power:

Shaft Power of Single-Phase Motor

Motor Shaft Power (kW) = (V × A × P.F * η) ÷ 1000

Shaft Power of Three-Phase Motor

Motor Shaft Power (kW) = (√3 × V × A × P.F * η) ÷ 1000

Where

  • V = Line Voltage
  • A = Line Current (Rated or Full Load Current)
  • P.F = Power Factor (Cos ϕ)
  • ɳ = Motor efficiency

Calculation of Full Load Current

The Full-Load Current (FLC) in amperes for DC motors as well as single-phase and three-phase motors can typically be obtained directly from the NEC tables. However, in the absence of tabulated data, the FLC can also be calculated using the standard formulas for 1-phase and 3-phase motors.

Example 1 – FLC of 1-Phase Motor:

Let’s calculate the FLC of a 1.5 kW (2 HP) single-phase motor operating at 240V, with a power factor (P.F.) of 0.75 and 70% efficiency.

FLC in Amp = Power in kW × 1000 / (V × P.F × ɳ)

Lets calculate the value of FLC for 1.5kW (2HP) single phase motor operated at 240V single phase having P.F 0.75 and 70% efficiency

FLC in Amp = (1.5 × 1000) ÷ (240 × 0.75 × 0.7)

= 1,500 ÷ 126

= 11.9 Amps.

The calculated FLC is approximately 12A, which closely matches the tabulated value of FLC given in Table 1 for 1.5kW (2HP) single phase motor operated at 240V  given in Table 1 for single-phase motors.

Example 2 – FLC of 3-Phase Motor:

Now, consider a 30 kW (40 HP) motor operating on a 415 V, three-phase supply, with a power factor (P.F.) of 0.8 and 80% efficiency.

FLC in Amp = Power in kW × 1000 / (√3 × V × P.F × ɳ)

For instance, let’s calculate the FLC of a 30kW (40HP) motor operated at 415V three-phase supply having P.F 0.8 and efficiency 80%.

FLC in Amp = (30 × 1000) ÷ (√3 × 415 × 0.8 × 0.8)

= 30,000 ÷ 460)

= 65.21 Amps.

The calculated FLC is approximately 66A, which is consistent with the tabulated value provided in Table 2 for 30kW (40HP) operating at 425V three-phase motors.

Required Wire Size in AWG for Motors

Now, let’s find suitable size of wire in AWG for single phase or three phase motors based on FLC.

Example:

What is the appropriate wire in AWG for a 7½ HP (5.6 kW) single phase motor operated at 240V?

Solution:

The value of full load current (FLC) for 7½ HP (5.6 kW) 1-phase motor is 40-amps (Table 1 – Based on NEC Table 430.248). NEC 430.22(A) and 430.6(A)(1) requires the branch circuit conductors must be sized at 125% of the continuous-duty motor’s full load current (FLC).

Amps = FLC × 1.25

= 40 × 1.25

= 40 × 1.25

= 50 Amps

Using the table 1 (based on NEC Table 310.16), the suitable wire size for 7½ HP (5.6 kW) single phase motor operated at 240V is #6 AWG (Cu), which has an ampacity of 55A at 60°C.

Required Wire Size of Breakers & Fuse

The protective devices for short circuit and ground fault protection is sized based on NEC Table 430.52(C)(1) in accordance with NEC Table 310.16. Let’s have a look:

Example:

What is the correct size of inverse time circuit breaker to protect a 20HP (15 kW) three phase motor operated at 460-480V?

Solution:

The full load current of 20HP (15 kW), 3-phase motor operated at 460V is 27-amp (Table 2 – based on NEC table 430.250).

Inverse Time Breaker

The size of breaker for motors shall be rated at 250% of the FLC using NEC Table 430.52(C)(1).

Cricut Braker Rating = 27A × 2.5 = 85A

= 67.5

The next standard available size of breaker is 70-amp. Hence, you can use 70-amp inverse time breaker to protect a 20HP (15 kW), 3-phase motor operated at 460V from short circuit and ground faults.

Good to Know: Circuit breakers and fuses are designed to protect only against short circuits and ground faults. They do not provide protection against motor overloads. For overload protection, you must use a properly sized overload relay in conjunction with the motor starter.

Tables

Table – 1 for Single-Phase Motors

Single-Phase Motors HP, FLC, Voltage, Breaker Size in Amp & Wire Size in AWG
HP kW 115V – 120V 230V – 240V
FLC in Amp Breaker Size (A) Wire Size in AWG FLC in Amp Breaker Size (A) Wire Size in AWG
⅙ 0.124 4.4 15 14* 2.2 10 – 15* 14*
¼ 0.186 5.8 15 14* 2.9 10 – 15* 14*
⅓ 0.250 7.2 20 14* 3.6 10 – 15* 14*
½ 0.373 9.8 25 14* 4.9 15 14*
¾ 0.560 13.8 35 12 6.9 20 14*
1 0.746 16 40 12 8.0 20 14*
1½ 1.119 20 50 10 10 25 14
2 1.50 24 60 10 12 30 14
3 2.24 34 90 6 17 45 10
5 3.73 56 150 4 28 70 8
7½ 5.60 80 200 1 40 100 6
10 7.46 100 250 1/0** 50 125 4
  • Table is Based on NEC Table 430.248 & Table 50.1 – UL 508A for FLC, Table 430.52(C)(1) for Breakers & Table 310.16 for Wire Size.
  • Use Proper Ambient Temperature Correction Factor using 310.15(B)(1)(1).
  • 1.25 Safety Factor is used from with FLC for Sizing Branch Circuit Conductor – NEC 430.6(A)(1).
  • 2.5 Factor is Applied from Table 430.52(C)(1) for Sizing Inverse Time Breakers.
  • *Minimum Required Size by NEC.
  • ** Wire Size in AWG or kcmil – Based on 60°C (140°F)

Table – 2 for Three-Phase Motors

Three-Phase Motors HP, FLC, Voltage, Breaker Size in Amp & Wire Size in AWG
HP kW 230V – 240V 400V – 415V 440V – 480V
FLC (A) Breaker Size (A) Wire Size (AWG) FLC (A) Breaker Size (A) Wire Size (AWG) FLC (A) Breaker Size (A) Wire Size (AWG)
½ 0.373 2.2 10 – 15* 14* 1.3 10 – 15* 14* 1.1 10 – 15* 14*
¾ 0.560 3.2 10 – 15* 14* 1.8 10 – 15* 14* 1.6 10 – 15* 14*
1 0.746 4.2 15 14* 2.3 10 – 15* 14* 2.1 10 – 15* 14*
1½ 1.119 6 15 14* 3.3 10 – 15* 14* 3 10 – 15* 14*
2 1.50 6.8 20 14* 4.3 15 14* 3.4 10 – 15* 14*
3 2.24 9.6 25 14* 6.1 20 14* 4.8 15 14*
5 3.73 15.2 40 12 9.7 25 14* 7.6 15 14*
7½ 5.60 22 55 10 14 35 12 11 30 14
10 7.46 28 70 8 18 45 10 14 35 12
15 11.12 43 110 6 27 70 8 21 55 10
20 15 54 150 4 34 90 6 27 70 8
25 18.7 68 175 3 44 110 6 34 90 6
30 22.4 80 200 1 51 150 4 40 100 6
40 30 104 300 2/0 66 175 3 52 150 4
50 37.3 130 350 3/0 83 225 1 65 175 3
60 44.8 154 400 4/0 103 300 2/0 77 200 1
75 56 192 500 300** 128 350 3/0 96 250 1/0
100 75 248 700 500** 165 450 250** 124 350 3/0
  • Table is Based on NEC Table 430.250 & Table 50.1 – UL 508A for FLC, Table 430.52(C)(1) for Breakers & Table 310.16 for Wire Size.
  • Use Proper Ambient Temperature Correction Factor using 310.15(B)(1)(1).
  • 1.25 Safety Factor is used from with FLC for Sizing Branch Circuit Conductor – NEC 430.6(A)(1).
  • 2.5 Factor is Applied from Table 430.52(C)(1) for Sizing Inverse Time Breakers.
  • *Minimum Required Size by NEC.
  • ** Wire Size in AWG or kcmil – Based on 60°C (140°F)

If you would like to download them, the tables below are provided in image format for reference.

Table 1 – Click image or open in a new tab to enlarge

Single-Phase Motors HP, FLC, Voltage, Breaker Size & Wire Size

Table 2 – Click image or open in a new tab to enlarge

Three-Phase Motors HP, FLC, Voltage, Breaker Size & Wire Size

Related Motor Circuit Calculations

Resources & Tutorials:

Wire Sizing Guides

 

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Electrical Technology
7 9 minutes read

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7 Comments

  2. Above 7.5HP motors may up-to 400Hp What would be the cable size(in sqmm), full load current and CB, pls give the easiest calculation..

    Reply

    1. Above 7.5HP motors may up-to 400Hp What would be the cable size(in sqmm), full load current and CB, pls give the easiest calculation..

      Reply

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