Part 6 – Sizing Motor Feeder Conductors and Cables
How to Determine the Correct Wire Size for Feeder Supplying Multiple Motors?
In previous sections on motor circuit calculations, we determined conductor sizing, overcurrent protection, and overload protection with solved examples for a single-motor circuit. In this section, we will determine the sizing of a feeder circuit supplying multiple motors on the same phase.
Sizing Feeder Conductor Supplying Multiple Motors
NEC 430.53 permits two or more motors on the same branch circuit in accordance with 430.53(A) through (D). According to 430.25, the ampacity of conductors supplying multi-motor and other load circuits must not be less than the marked ampacity on the equipment nameplate, as specified in 430.7(D). If the nameplate rating is unavailable, the conductor ampacity must be determined based on 430.24.
According to the 430.24, the ampacity of conductors supplying multiple motors must not be less than:
- 125% of the full-load current (FLC) of the highest-rated motor (430.6(A), 430.17 & 430.24).
- The sum of the FLCs of all other motors on the same phase (430.6(A)).
- 100% of non-continuous loads and 125% of continuous non-motor loads on the feeder.
According to 430.26, If all motors do not operate simultaneously (all motor running at a time), feeder demand factors may be applied to reduce the calculated ampacity per NEC 430.24. However, the conductors must have sufficient ampacity to handle the maximum load based on the size and number of motors.
Example 1 – Sizing Feeder Conductors for Single Phase Motors
Determine the required feeder conductor size (THHN) with a 75°C terminal rating for a system supplying two 230V single-phase motors: one rated at 2 HP and the other at 10 HP.
Solution:
1: Determine FLC of Each Motor
Based on Table 430.248, the full-load current in amperes for single-phase AC motors at 230V are as follow:
- FLC of 2 HP Motor = 12A
- FLC of 10 HP Motor = 50A
2: Determine the Total Ampacity
The ampacity of feeder circuit supplying multiple motors can be calculated in accordance with 430.24(A). Since the largest motor is 50A based on FLC;
50A × 1.25 = 62.5A.
Now, sum app with the FLC of other motor(s).
62.5A + 12A = 74.5A.
2: Determine the Size of Feeder Conductors
The suitable wire size can be selected from the Table 310.16. For a feeder supplying 74.5A of current with a 75°C terminal rating, the right size of feeder conductor is #4AWG cupper or #6AWG aluminum (THHN).
Example 2 – Sizing Feeder Conductors for Multi-phase Motors
What is the required size of feeder conductor (THHN) at 75°C terminal rating supplying the following multiphase motors?
- Three 1.5 HP, 115V, 1-Phase motors
- Three 7.5 HP, 208V, 1-Phase motors
- One 40 HP, 460V, 3-Phase squirrel cage or wound rotor motor
Solution:
1 – Find the Full Load Current of Each Motor
The full load current (FLC) can be determined using Table 430.248 and 430.150 for single-phase and three-phase AC motors respectively.
- FLC of 1.5 HP, 115V, 1-Φ motor = 20A
- FLC of 7.5 HP, 208V, 1-Φ motor = 44A
- FLC of 40 HP, 460V, 3-Φ squirrel cage or wound rotor motor = 52A
2 – Calculate the Total Ampacity
As previously stated, the ampacity of conductors supplying multiple motors must not be less than 125% of the full-load current (FLC) of the highest-rated motor (per NEC 430.6(A) and 430.24). Additionally, the FLC values of all other motors connected to the same phase must be added to the FLC of highest-rated motor multiplied by 1.25 (per NEC 430.6(A)).
Thus, the total ampacity is calculated as follows:
(52A×1.25) + 44A + 44A + 20A = 173A.
3: Size The Feeder Conductors
The size of feeder conductor supplying 173A of multiple motors circuits with a 75°C terminal rating, Table 310.16 suggests (THHN) conductors for the feeder:
- #2/0AWG cupper
- #4/0 AWG aluminum
When sizing a feeder conductor supplying multiple motors, only the motors on the same phase should be considered for additions of FLCs. This is why our calculations include four motors instead of all seven. The same rule applies when sizing protection for feeder conductors, as they must be protected against motor overload and overcurrent caused by short circuits, and ground faults (Discussed in the next part).
Series Overview: Motor Circuit Calculations
- Part 1 – Motor Load Circuits: NEC Terms and Basic Terminologies
- Part 2 – NEC Requirements for Motor Circuits
- Part 3 – Understanding NEMA Motor Nameplate Data
- Part 4 – Calculating Locked Rotor Current (LRC) for Motors
- Part 5 – Sizing Motor Branch Circuit Conductors
- Part 6 – Sizing Motor Feeder Conductors … (You are Here)
- Part 7 – Sizing Motor Overcurrent Protection and OCPD’s Devices
- Part 8 – Sizing Motor Feeder Protection
- Part 9 – Sizing Motor Overload Protection
- Part 10 – Overcurrent Protection for Motor Control Circuits
- Part 11 – Sizing Disconnecting Means for Motor & Controller
- Part 12 – Sizing Motor Starter & Contactor – NEMA – NEC
- Part 13 – Sizing Direct Online (DOL) Starters/Contactors for Motors (*Bonus)
- Part 14 – Sizing Star-Delta Motor Starters/Contactors for Motors (*Bonus)
Resources & Tutorials:
- Cable Size Calculation for LT & HT Motors
- Three Phase Motor Power & Control Wiring Diagrams
- What is Motor Efficiency & How to improve it?
- Star Delta 3-phase Motor Automatic starter with Timer
- How to Run a Three-Phase Induction Motor on a Single-Phase Power Supply?
- How to Wire ST01 Timer with Relay & Contactor for 120V/240V Motors?
- How to Wire Twin Timer in Repeat Cycle & One-Shot Mode for 120V/240V Motors?
- How to Control a Single-Phase Motor from Multiple Locations?
- Automatic & Manual Control of Motor Using VFD & DOL Starter
- How to Reverse and Forward a Three-Phase Motor Using a VFD?
- How to Wire a VFD with Motor, PLC, Switches & External Devices
- Sequential Motor Control Circuit Using LOGO! V8 PLC
- Automatic Reverse Forward Motor Control Circuit Using Delta – DVP-14SS PLC
- Reverse Forward Motor Control Circuit Using PLC – ZEN Programming Relay
- Motor Protection – Types of Faults and Protection Devices
- Cables Feeder Protection – Faults Types, Causes & Differential Protection
Wire Sizing Guides
- How to Find the Proper Size of Wire & Cable In Metric & Imperial Systems
- How to Size a Branch Circuit Conductors with Protection?
- How to Size Feeder Conductors with Overcurrent Protection
- How to Size Service-Entrance Conductors and Feeder Cables?
- How to Size Equipment Grounding Conductor (EGC)?
- How to Size Grounding Electrode Conductor (GEC)?
- What is the Right Wire Size for 15A Breaker and Outlet?
- What is the Suitable Wire Size for 20A Breaker and Outlet?
- How Size a Circuit Breaker for Different Load Applications
