11.4 Motor Disconnects, Controllers & Multimotor Feeder Calculations
Key Takeaways
- Under NEC 430.110, motor disconnecting means must have an ampere rating of not less than 115% of the motor Table FLC and must possess a horsepower (HP) rating not less than the motor rating.
- Under NEC 430.102, a disconnecting means must be located in sight from the motor controller AND in sight from the motor location and driven machinery (visible and not more than 50 feet away).
- Multimotor feeder conductors under NEC 430.24 must have an allowable ampacity of at least 125% of the largest motor Table FLC plus 100% of the sum of all other motor Table FLCs (+ 125% of continuous non-motor loads).
- Multimotor feeder short-circuit protection under NEC 430.62 is sized by taking the largest branch protective device rating of any motor in the group and adding the sum of the Table FLCs of all other motors.
- CRITICAL CODE RULE: Unlike branch circuits, NEC 430.62 strictly PROHIBITS rounding up when sizing feeder short-circuit protective devices; you must round DOWN to the next lower standard rating in NEC 240.6(A).
11.4 Motor Disconnects, Controllers & Multimotor Feeder Calculations
In industrial and commercial facilities, multiple motors are frequently powered from a central distribution switchboard or Motor Control Center (MCC). Designing these multi-load installations requires synthesizing multiple sections of NEC Article 430, including disconnecting means rules (Part IX), motor controller requirements (Part VII), multimotor feeder conductor sizing (Part II), and feeder short-circuit protection (Part X).
This section walks through the complete end-to-end engineering methodology required to size every element of a multimotor installation, emphasizing high-yield licensing exam topics such as the "in sight from" rule, horsepower-rated disconnect criteria, and the critical feeder overcurrent protection round-down rule.
1. Motor Disconnecting Means (NEC Article 430 Part IX)
A motor disconnecting means provides a physical point of electrical isolation to ensure that electricians and maintenance personnel can safely service the motor, controller, and driven machinery without risk of unexpected re-energization.
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| MOTOR DISCONNECT SIZING & LOCATION RULES |
| |
| [RATING REQUIREMENTS (NEC 430.110)] |
| * Ampere Rating: Minimum 115% of Table FLC (NEC 430.110(A)). |
| * Horsepower Rating: Must have a marked HP rating not less than the motor |
| horsepower rating (NEC 430.109). |
| * Single Disconnect for Motor + Other Loads: Sized for 115% motor FLC + |
| 100% of non-continuous loads + 125% of continuous non-motor loads. |
| |
| [LOCATION REQUIREMENTS (NEC 430.102)] |
| * Must be IN SIGHT FROM the motor controller (430.102(A)). |
| * Must be IN SIGHT FROM the motor location & machinery (430.102(B)). |
| |
| [DEFINITION OF "IN SIGHT FROM" (ARTICLE 100)] |
| * The equipment must be VISIBLE AND NOT MORE THAN 50 FEET (15 m) AWAY. |
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Exceptions to the Motor Disconnect Rule (NEC 430.102(B) Exception):
A separate disconnect is not required at the motor location if:
- Locating the disconnect adjacent to the motor is impracticable or introduces additional hazards (e.g., inside explosive atmospheres, submerged pumps, clean rooms).
- In industrial facilities with written safety procedures and qualified personnel, the controller disconnect is individually lockable in the open position in accordance with NEC 110.25 (the locking mechanism must remain in place on the switch whether the padlock is installed or removed; portable lock-out hasps do not satisfy 110.25).
2. Motor Controllers (NEC Article 430 Part VII)
A motor controller is any switch or device normally used to start and stop a motor by making and breaking the motor circuit current.
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| MOTOR CONTROLLER CATEGORIES |
| |
| [1. MANUAL MOTOR STARTERS] |
| * Manually operated toggle or pushbutton switch with built-in thermal |
| overload elements (common for small fractional & 1-5 HP equipment). |
| |
| [2. MAGNETIC MOTOR STARTERS (ACROSS-THE-LINE / FULL-VOLTAGE)] |
| * Electromagnetic contactor paired with an overload relay assembly. |
| * Controlled remotely via start/stop pushbuttons, PLCs, or sensors. |
| |
| [3. VARIABLE FREQUENCY DRIVES (VFDs) / INVERTERS (PART X)] |
| * Converts AC to DC, then synthesizes variable frequency/voltage AC. |
| * Sizing: VFD input conductors sized at 125% of VFD rated input current |
| (NEC 430.122). Overload protection is often integrated into VFD logic. |
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Controller Horsepower Ratings (NEC 430.83):
Each controller must have a horsepower rating not lower than the horsepower rating of the motor it controls, except:
- Stationary Motors <= 1/8 HP: General-use snap switches or branch-circuit breakers permitted.
- Stationary Motors <= 2 HP (300V or less): General-use AC snap switch rated for at least twice the motor Table FLC.
3. Multimotor Feeder Conductor Sizing (NEC 430.24)
When a single feeder supplies two or more motors, the feeder conductors must be sized to supply the starting surge of the largest motor while simultaneously carrying the full-load operating current of all remaining motors.
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| MULTIMOTOR FEEDER CONDUCTOR FORMULA (430.24) |
| |
| I_feeder = (1.25 x I_largest) + Sum(I_other motors) + (1.25 x I_cont) + I_noncont |
| |
| - I_largest: Table FLC of the single largest motor in the group |
| - Sum(I_other motors): 100% of Table FLC of all other motors |
| - I_cont: 125% of any continuous non-motor loads |
| - I_noncont: 100% of any non-continuous non-motor loads |
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[!NOTE] Determining the "Largest Motor": The "largest motor" is defined strictly by the highest Table FLC, NOT necessarily the highest marked horsepower. If two or more motors have identical highest FLC ratings, only one of those motors is multiplied by 125%; the other identical motor(s) are added at 100%.
4. Multimotor Feeder Short-Circuit Protection (NEC 430.62)
NEC 430.62(A) dictates that a feeder supplying a specific group of motors must be protected by an overcurrent device with a rating or setting not greater than the largest branch-circuit short-circuit and ground-fault protective device rating of any motor in the group plus the sum of the full-load currents (Table FLC) of all other motors supplied by the feeder.
Feeder OCPD Rating <= Largest Branch Protective Device + Sum(FLC of Remaining Motors)
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| CRITICAL EXAM TRAP: NO ROUNDING UP ON FEEDERS! |
| |
| * For Branch Circuits (430.52): If the math gives 148.5 A, you ROUND UP |
| to 150 A under Exception 1. |
| |
| * For Feeders (430.62): If the calculated maximum is 223 A, you |
| STRICTLY CANNOT ROUND UP TO 225 A! |
| You MUST round DOWN to the next lower standard rating (200 A). |
| |
| NEC 430.62 explicitly establishes a MAXIMUM CEILING. Sizing above this |
| calculated value is a direct Code violation! |
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5. Complete End-to-End Industrial Multimotor Feeder Problem
Let us execute a complete, real-world engineering calculation for an industrial motor control center (MCC) subfeeder.
The Problem Specification:
A 460-volt, 3-phase, 60 Hz feeder supplies three continuous-duty squirrel-cage induction motors (Design B, 75°C terminals):
- Motor 1: 50 HP (Table 430.250 FLC = 65.0 A)
- Motor 2: 25 HP (Table 430.250 FLC = 34.0 A)
- Motor 3: 10 HP (Table 430.250 FLC = 14.0 A)
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| MULTIMOTOR CALCULATION STEP-BY-STEP SOLUTION |
| |
| ======================================================================= |
| STEP 1: SIZE INDIVIDUAL BRANCH CIRCUITS |
| ======================================================================= |
| Motor 1 (50 HP, 65.0 A): |
| - Conductor (430.22): 65.0 A x 1.25 = 81.25 A -> 4 AWG Cu (75°C = 85 A) |
| - Inverse-Time CB (430.52): 65.0 A x 2.50 = 162.5 A -> Round UP = 175 A |
| - Time-Delay Fuse (430.52): 65.0 A x 1.75 = 113.75 A -> Round UP = 125 A |
| - Disconnect (430.110): 65.0 A x 1.15 = 74.75 A -> 50 HP rated switch |
| |
| Motor 2 (25 HP, 34.0 A): |
| - Conductor (430.22): 34.0 A x 1.25 = 42.5 A -> 8 AWG Cu (75°C = 50 A) |
| - Inverse-Time CB (430.52): 34.0 A x 2.50 = 85.0 A -> Round UP = 90 A |
| - Time-Delay Fuse (430.52): 34.0 A x 1.75 = 59.5 A -> Round UP = 60 A |
| - Disconnect (430.110): 34.0 A x 1.15 = 39.1 A -> 25 HP rated switch |
| |
| Motor 3 (10 HP, 14.0 A): |
| - Conductor (430.22): 14.0 A x 1.25 = 17.5 A -> 12 AWG Cu (75°C = 25 A) |
| - Inverse-Time CB (430.52): 14.0 A x 2.50 = 35.0 A -> Standard = 35 A |
| - Time-Delay Fuse (430.52): 14.0 A x 1.75 = 24.5 A -> Round UP = 25 A |
| - Disconnect (430.110): 14.0 A x 1.15 = 16.1 A -> 10 HP rated switch |
| |
| ======================================================================= |
| STEP 2: SIZE MULTIMOTOR FEEDER CONDUCTORS (NEC 430.24) |
| ======================================================================= |
| - Identify Largest Motor: Motor 1 (50 HP, Table FLC = 65.0 A). |
| - Feeder Ampacity = (65.0 A x 1.25) + 34.0 A + 14.0 A |
| = 81.25 A + 34.0 A + 14.0 A = 129.25 Amperes. |
| - Table 310.16 (75°C Copper): |
| * 1 AWG Copper = 130 Amperes (130 A >= 129.25 A). |
| -> Feeder Conductor Size: 1 AWG Copper (or 1/0 AWG for margin). |
| |
| ======================================================================= |
| STEP 3: SIZE FEEDER SHORT-CIRCUIT PROTECTION (NEC 430.62) |
| ======================================================================= |
| [OPTION A: FEEDER INVERSE-TIME CIRCUIT BREAKER] |
| - Largest Motor Branch Breaker = 175 A (from Motor 1). |
| - Add FLC of Other Motors = 175 A + 34.0 A + 14.0 A = 223.0 A Max. |
| - Apply Round-Down Rule: Next lower standard breaker in 240.6(A) <= 223 A |
| -> Maximum Feeder Breaker = 200 Amperes! (225 A is a Code Violation) |
| |
| [OPTION B: FEEDER DUAL-ELEMENT TIME-DELAY FUSES] |
| - Largest Motor Branch Fuse = 125 A (from Motor 1). |
| - Add FLC of Other Motors = 125 A + 34.0 A + 14.0 A = 173.0 A Max. |
| - Apply Round-Down Rule: Standard fuse size in 240.6(A) <= 173 A |
| -> Maximum Feeder Fuse = 150 Amperes! (175 A is a Code Violation) |
| |
| ======================================================================= |
| STEP 4: SIZE MAIN FEEDER DISCONNECTING MEANS (NEC 430.110(C)) |
| ======================================================================= |
| - Minimum Ampere Rating = (65.0 A x 1.15) + 34.0 A + 14.0 A |
| = 74.75 A + 34.0 A + 14.0 A = 122.75 Amperes. |
| - Horsepower Rating: Equivalent locked-rotor current sum -> 100 HP switch.|
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6. Summary Comparison: Sizing Feeder vs. Branch Circuit Elements
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| FEEDER VS. BRANCH CIRCUIT MASTER COMPARISON |
| |
| Circuit Dimension | Branch Circuit (430.22/52) | Feeder (430.24/62) |
| :--------------------- | :------------------------- | :------------------ |
| **Conductor Sizing** | 125% of single motor FLC | 125% largest FLC + |
| | | 100% sum of others |
| **Short-Circuit OCPD** | Table 430.52 % of FLC | Largest branch OCPD |
| | | + sum of other FLCs |
| **Rounding Rule** | **ROUND UP** to next | **STRICTLY ROUND |
| | standard rating (240.6(A)) | **DOWN** to lower |
| **Disconnect Rating** | Min 115% FLC + HP rated | Min 115% sum FLCs |
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What is the NEC Article 100 definition for equipment required to be 'in sight from' or 'within sight of' other electrical apparatus under NEC 430.102?
What is the minimum feeder conductor ampacity required under NEC 430.24 for three 230V, 3-phase motors with Table 430.250 full-load currents of 80 A (30 HP), 54 A (20 HP), and 28 A (10 HP)?
A 460V 3-phase feeder supplies three motors. The branch-circuit inverse-time circuit breaker ratings calculated for the three motors are 90 A (largest), 50 A, and 35 A. The Table 430.250 full-load currents are 34 A (for the largest motor), 21 A, and 14 A. What is the maximum rating permitted for the feeder inverse-time circuit breaker under NEC 430.62?