Electrical DesignElectrical WiringHow ToMotorsProtection

Part 5 – Sizing Motor Branch Circuit Conductors and Cables

Electrical Technology
0 6 minutes read

How to Determine the Correct Wire Size for Motor Branch Circuit?

Sizing Motor Branch Circuit Conductors

Sizing the conductors for motor branch circuit is selected at 125% of the motor FLC [430.22(A)]. The ampacity of branch circuit conductors serving a continuous-duty single motor must not be less than 125% of the motor’s full load current (FLC), as determined by NEC 430.6(A)(1) and 430.22(A) to (G).

The size of conductors for a motor branch circuit is determined using NEC Table 310.16, based on the FLC. The FLC values for different types of motors are listed in NEC Tables 430.147, 430.148, 430.149, and 430.150.

Each table applies to a specific type of motor:

  • Table 430.147: DC motors
  • Table 430.148: Single-phase AC motors
  • Table 430.149: Two-phase AC motors
  • Table 430.150: Three-phase AC motors

Sizing Motor Branch Circuit Conductors and Cables

The ampacity of conductors used for motors must be determined using the values of full load current given in related tables, not the current rating values printed on the motor nameplate.

NEC Article 430, Part II provides guidelines for sizing branch circuit conductors and ampacity without overheating the motors. The following steps shows how to determine the right wire size for motor circuits.

Step 1: Determine the Motor FLC

First, determine the motor’s load type (e.g., continuous, non-continuous, or special-purpose application). Also, choose the conductor material (copper or aluminum) and the type of insulation.

The full load current (FLC) of a motor is found in Table 430.247 (for DC motors), Table 430.248 (for single-phase motors), and Table 430.250 (for three-phase motors).

Warning: Do not use the motor nameplate current for conductor sizing; instead, refer to the associated NEC tables to find the correct FLC of motors.

Step 2: Apply the NEC Sizing Factor (125%)

NEC 430.22(A) states that the branch circuit conductor must be sized at 125% of the motor’s FLC.

The formula to calculate the minimum ampacity of the conductor:

Iconductor = FLC × 1.25

Step 3: Select the Proper Conductor Size

Use NEC Table 310.16 to select the appropriate wire size based on the ampacity calculated in Step 2.

Step 4: Apply Correction Factors (If applicable)

Apply correction factors based on NEC Article 310 if necessary.

For instance:

  • Ambient Temperature Correction:

Use Table 310.15(B)(1)(1) and Table 310.15(B)(1)(2) for temperature adjustments at 30°C (86°F) and 40°C (104°F).

  • Insulation Type and Rating:

Refer to Table 310.4(1) to ensure compatibility with the selected conductor type (e.g., THHN, THW, XHHW).

  • Adjustment Factors for Conductors in Raceway:

Apply Table 310.15(C)(1) when more than three current-carrying conductors are in a single raceway.

  • Equipment Terminal Connection Requirements:

Per NEC 110.14(C)(1):

Use the 60°C ampacity rating for equipment circuits rated at ≤ 100A or those marked for 14 AWG through 1 AWG conductors.

Use the 75°C ampacity rating for equipment circuits rated at > 100A or those marked for conductors larger than 1 AWG.

  • Voltage Drop Considerations:

Maintain a voltage drop ≤ 3% for single-phase circuits and ≤ 5% for three-phase circuits over long distances.

  • Size EGC:

Size the equipment grounding conductor (EGC) based on Article 250.122 and Table 250.122

  • Correct Wire Size

Finally, use NEC Table 310.16 to determine the correct conductor size.

Example: 1 – Sizing Conductors for 1-Phase Motor

Determine the appropriate branch circuit conductor size for a 5 HP, 230V, single-phase motor.

Solution:

1. Find the Motor FLC

The full load current for a 5 HP, 230V single-phase motor is determined using Table 430.248

FLC = 28 A

2. Apply 125% Rule for Continuous Load

According to 430.6(A)(1), the ampacity of conductors supplying a continuous-duty motor must not less than 125% of the motor full-load current (FLC) rating:

Iconductor = FLC × 1.25

Iconductor = 28A × 1.25 = 35A

3. Select the Wire Size

Select the wire size for the branch circuit serving a single-phase motor from NEC Table 310.16. According to the table, the correct wire size is #8 AWG (Cu), which has an ampacity of 40A at 60°C. Alternatively, you may use the following conductors at different temperature ratings as a branch circuit for a 5 HP, 230V, single-phase motor:

  • #8 AWG Copper (TW/UF) at 60°C (140 °F)
  • #10 AWG Copper (THWN) at 75°C (167 °F)
  • #10 AWG Copper (THHN) at 90°C (194 °F)

Example: 2 – Sizing Conductors for 3-Phase Motor

Determine the correct size of THWN conductors for a 50 HP, 460V, three-phase motor in a 104°F (40°C) ambient temperature.

Solution:

1: Determine Full-Load Current (FLC)

The full load current of three phase mores are determined by Table 430.250. Based on the table data, for a 25 HP, 460V three-phase motor, the full load current:

FLC = 65 A

2: Apply 125% Rule

Similarly to example 1, the continuous load applications shall be rated at 125%. By apply the NEC 125% rule:

Iconductor = 65A × 1.25 = 81.25 A

3: Apply Correction Factor for Ambient Temp.

From the Table 310.15(B)(1)(1), the ambient temperature correction factor based on 30°C (86°F), the correction factor for THWN at 104°F (40°C) is .88.

81.25A × 0.88

Corrected Ampacity = 71.5A

4: Select Conductor Size

Based on the calculated values, refer to the Table 310.16 to select the wire size for 3-phase motor branch circuit conductors.

  • #4 AWG Copper (THWN) at 75°C (167 °F)
  • #6 AWG Copper (THWN) at 90°C (194 °F)

Example: 3 – Wire Sizing for DC Motor

What size THHN for branch circuit can be used for a 5 HP DC motor with a 75°C (167°F) conductor temperature rating.

1 – Find the Full Load Current

The FLC for a DC motor is calculated using the power formula or using the Table 230.47:

From the table data, the FLC for 5 HP, 240V:

FLC = 20A

Alternatively, the FLA of a DC motor using power formula

Full Load Current of DC Motor

Assuming efficiency (η)= 85% (typical for DC motors):

DC Motor FLA Calculations

*We will stick to the NEC Table 230.47 value which is 20A.

2 – Apply 125% Rule for Continuous Load

Iconductor = 20 × 1.25 = 25A

3 – Select the Wire Size

From NEC Table 310.16, #10 AWG (Cu) THHN has an ampacity of 35A, which is sufficient to handle the 25A of current at 75°C (167 °F). Hence, the answer is = #10 AWG Copper (THHN) conductor.

Series Overview: Motor Circuit Calculations

Resources & Tutorials:

Wire Sizing Guides

Tags
Electrical Technology
0 6 minutes read

Electrical Technology

All about Electrical and Electronic Engineering & Technology. Join us on WhatsApp at Electrical Technology Official Channel, to receive the latest content, articles, and updates. You can also like and follow our social media networks below, or subscribe with your email to receive premium engineering articles in your mailbox.

Leave a Reply

Your email address will not be published. Required fields are marked *

Back to top button