11.3 Motor Feeder Conductor Sizing & Feeder Overcurrent Protection (430.24 & 430.62)

Motor Feeder Conductor Sizing & Feeder Overcurrent Protection (NEC 430.24 & 430.62)

In industrial and commercial electrical distribution systems, a single feeder commonly supplies an entire motor control center (MCC) or a group of discrete motor branch circuits. Sizing motor feeders requires careful application of two core principles: preventing cumulative conductor overheating from multiple running motors, and providing feeder overcurrent protection large enough to permit the largest motor to start while remaining small enough to protect against major short circuits. On the Oklahoma Journeyman Electrician exam, multi-motor feeder calculations are heavily tested.

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1. Feeder Conductor Sizing for Multiple Motors (NEC 430.24)

Conductors supplying several motors must have an allowable ampacity not less than the sum of the full-load current (FLC) ratings of all the motors supplied, plus 25% of the highest-rated motor in the group:

+-----------------------------------------------------------------------------+
|                   NEC 430.24 MOTOR FEEDER SIZING FORMULA                    |
|                                                                             |
|   I_feeder >= (1.25 x I_FLC_highest) + Sum(I_FLC_all_other_motors)          |
|                                                                             |
|   WHERE:                                                                    |
|   - I_FLC_highest is the Table FLC of the single largest motor in group.    |
|   - Sum(I_FLC_all_other_motors) is the sum of 100% of all other motor FLCs. |
|   - All FLC values MUST come from NEC Tables 430.248 or 430.250!            |
+-----------------------------------------------------------------------------+

Identical Largest Motors Rule

Where two or more motors in the group have the identical highest full-load current rating, only one motor is considered the highest rated and multiplied by 125%. The other identical motor(s) are added at 100% of their Table FLC.

Mixed Loads: Motors Plus Other Loads (NEC 430.25)

Where a feeder supplies both motors and other electrical loads (such as continuous lighting, heaters, and general convenience receptacles), the minimum feeder ampacity is calculated as:

$I_{\text{feeder}} = (1.25 \times I_{\text{FLC, largest motor}}) + \sum(I_{\text{FLC, other motors}}) + (1.25 \times I_{\text{continuous loads}}) + (1.00 \times I_{\text{non-continuous loads}})$

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2. Feeder Short-Circuit & Ground-Fault Protection (NEC 430.62)

Feeder overcurrent protective devices (OCPDs) must be sized to allow the largest motor in the group to start without tripping the main feeder protective device, while all other motors are already running at full load. NEC 430.62(A) establishes the mandatory mathematical procedure:

+-----------------------------------------------------------------------------+
|                 NEC 430.62(A) FEEDER OCPD SIZING FORMULA                    |
|                                                                             |
|   Feeder OCPD Max = (Largest Branch OCPD) + Sum(I_FLC_all_other_motors)     |
|                                                                             |
|   WHERE:                                                                    |
|   - Largest Branch OCPD = The MAXIMUM permitted branch-circuit short-        |
|     circuit protective device rating for the LARGEST motor under Table       |
|     430.52 (including the Round-Up rule under 430.52(C)(1) Ex 1).           |
|   - Sum(I_FLC_all_other_motors) = The sum of 100% Table FLCs of all other   |
|     motors in the group.                                                    |
|                                                                             |
|   *** THE GOLDEN RULE OF MOTOR FEEDERS: ROUND DOWN! ***                     |
|   If the calculated sum does NOT match a standard rating in NEC 240.6(A),   |
|   you MUST select the NEXT LOWER standard ampere rating.                    |
|   Rounding UP is a SEVERE CODE VIOLATION under NEC 430.62(A)!               |
+-----------------------------------------------------------------------------+

+-----------------------------------------------------------------------------+
|          CONTRASTING ROUNDING RULES: BRANCH OCPD VS. FEEDER OCPD            |
|                                                                             |
|   BRANCH CIRCUIT OCPD (NEC 430.52(C)(1) Ex 1):                              |
|   - Round UP to the NEXT HIGHER standard rating in NEC 240.6(A).            |
|                                                                             |
|   FEEDER OCPD (NEC 430.62(A)):                                              |
|   - Round DOWN to the NEXT LOWER standard rating in NEC 240.6(A).           |
+-----------------------------------------------------------------------------+

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3. Complete Step-by-Step Multi-Motor Feeder Calculation

Comprehensive Scenario: An industrial feeder supplies a motor control center (MCC) operating at 460 Volts, 3-Phase, 3-Wire. The MCC supplies three continuous-duty squirrel-cage induction motors:

• Motor 1: 30 HP, 460V, 3-phase
• Motor 2: 15 HP, 460V, 3-phase
• Motor 3: 10 HP, 460V, 3-phase

Inverse-time circuit breakers are used throughout for both branch-circuit and feeder protection. All conductors are 75°C THHN Copper.

+-----------------------------------------------------------------------------+
|                   WORKED CALCULATION ARCHITECTURE DIAGRAM                   |
|                                                                             |
|   [ 460V 3-Phase Main Distribution Panel ]                                  |
|        |                                                                    |
|   [ Feeder Breaker: ??? A ]                                                 |
|        |                                                                    |
|   ===== Feeder Conductor (Ampacity >= 85A) =====> [ Motor Control Center ]  |
|                                                    |     |     |            |
|                        +---------------------------+     |     +-----+      |
|                        |                                 |           |      |
|                   [100A Breaker]                   [60A Breaker] [35A Breaker]|
|                        |                                 |           |      |
|                   Motor 1 (30 HP)                  Motor 2 (15 HP) Motor 3 (10 HP)
|                   FLC = 40.0 A                     FLC = 21.0 A  FLC = 14.0 A
+-----------------------------------------------------------------------------+

Step 1: Look Up Table Full-Load Currents (NEC Table 430.250)

• Motor 1 (30 HP, 460V): FLC = 40.0 Amperes (Highest rated motor)
• Motor 2 (15 HP, 460V): FLC = 21.0 Amperes
• Motor 3 (10 HP, 460V): FLC = 14.0 Amperes

Step 2: Calculate Minimum Feeder Conductor Ampacity (NEC 430.24)

$I_{\text{feeder}} = (1.25 \times 40.0\text{ A}) + 21.0\text{ A} + 14.0\text{ A}$ $I_{\text{feeder}} = 50.0\text{ A} + 21.0\text{ A} + 14.0\text{ A} = 85.0\text{ Amperes}$

• Conductor Selection (NEC Table 310.16, 75°C Cu):
  • #4 AWG THHN Cu has an allowable ampacity of 85 Amperes (exact match).
  • (If 90°C derating is needed, #3 AWG THHN Cu rated 100A at 75°C provides generous margin).

Step 3: Determine Individual Motor Branch Circuit Breakers (NEC Table 430.52)

Using inverse-time circuit breakers (250% multiplier):

• Motor 1 (30 HP): $40.0\text{ A} \times 2.50 = 100.0\text{ A} \rightarrow$ 100A Breaker (Exact standard rating in 240.6(A)).
• Motor 2 (15 HP): $21.0\text{ A} \times 2.50 = 52.5\text{ A} \rightarrow$ Round UP under 430.52(C)(1) Ex 1 to 60A Breaker.
• Motor 3 (10 HP): $14.0\text{ A} \times 2.50 = 35.0\text{ A} \rightarrow$ 35A Breaker (Exact standard rating in 240.6(A)).

Step 4: Calculate Maximum Feeder Circuit Breaker Rating (NEC 430.62(A))

$\text{Feeder OCPD Max} = (\text{Branch OCPD of Largest Motor}) + \sum(I_{\text{FLC of other motors}})$ $\text{Feeder OCPD Max} = 100.0\text{ A} + 21.0\text{ A} + 14.0\text{ A} = 135.0\text{ Amperes}$

• Select Standard Rating from NEC 240.6(A):
  • Standard ratings in this range are: 100A, 110A, 125A, 150A, 175A.
  • 135 Amperes does not correspond to a standard rating.
  • Applying the Mandatory Round-Down Rule: We must select the NEXT LOWER standard rating = 125 Amperes.
  • Exam Warning: Selecting 150 Amperes is a classic licensing examination failure trap!