8.2 Motor Feeder & Branch-Circuit Conductor Sizing

8.2 Motor Feeder & Branch-Circuit Conductor Sizing

Motor circuits present unique electrical demands that differentiate them from general lighting and appliance branch circuits. Motors draw massive starting inrush currents—typically five to eight times normal operating current—and subject conductors to continuous thermal stress under mechanical loading. Consequently, NEC Article 430 mandates specialized rules for conductor sizing, terminal temperature evaluations, and disconnecting means.

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Branch-Circuit Conductor Sizing for a Single Motor (NEC 430.22)

Under NEC 430.22, conductors supplying a single motor used in a continuous-duty application must have an allowable ampacity of not less than $125\%$ of the motor full-load current (FLC) determined from the applicable NEC table (Table 430.248 for single-phase, or Table 430.250 for three-phase):

$I_{\text{conductor}} \ge 1.25 \times I_{\text{FLC (Table)}}$

Why 125% Ampacity?

Continuous-duty motors are classified as continuous loads under Article 100 (loads where the maximum current is expected to continue for 3 hours or more). Sizing conductors to $125\%$ prevents thermal accumulation inside raceways and enclosures, dissipates internal $I^2R$ resistive heating, and guarantees that terminal connections at the motor controller and disconnect switch remain below their listed operating temperature limits.

Noncontinuous and Duty-Cycle Motors (NEC Table 430.22(E))

If a motor is not rated for continuous duty, the conductor sizing multiplier is determined from NEC Table 430.22(E) based on duty classification:

• Short-Time Duty (operating for 5, 15, 30, or 60 minutes): Multipliers range from $110\%$ to $200\%$.
• Intermittent Duty (alternate intervals of load and no-load): Multipliers range from $130\%$ to $150\%$.
• Periodic Duty (regular repetition of load cycles): Multipliers range from $130\%$ to $200\%$.
• Varying Duty (loads vary widely over time): Conductor ampacity must be at least $140\%$ to $200\%$.

In standard commercial electrical construction, the vast majority of motors (fans, pumps, chillers, air compressors) are continuous-duty, requiring the universal $125\%$ multiplier.

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Terminal Temperature Ratings & Ampacity Derating (NEC 110.14(C))

When selecting conductors from NEC Table 310.16, electricians must navigate the termination temperature provisions of NEC 110.14(C):

1. Equipment Rated 100 Amperes or Less (or 14 AWG through 1 AWG): Conductors must be selected based on the $60^\circ\text{C}$ column of Table 310.16, unless the equipment terminals are listed and marked for $75^\circ\text{C}$. In modern commercial construction, virtually all motor controllers, disconnect switches, and circuit breakers carry dual $75^\circ\text{C}$ or $60^\circ/75^\circ\text{C}$ listings, permitting selection from the $75^\circ\text{C}$ column.
2. Equipment Rated Over 100 Amperes: Conductors are permitted to be selected based on the $75^\circ\text{C}$ column of Table 310.16.
3. Using 90°C Insulated Conductors (THHN / XHHW-2): Even when conductors have a $90^\circ\text{C}$ insulation rating (such as THHN or THWN-2), the final ampacity landed on equipment terminals cannot exceed the terminal rating (typically $75^\circ\text{C}$). However, the $90^\circ\text{C}$ rating is permitted as the starting point for applying ambient temperature correction factors (Table 310.15(B)(1)) and conductor bundling adjustment factors (Table 310.15(C)(1) for more than three current-carrying conductors in a raceway).

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Sizing Feeder Conductors for Multiple Motors (NEC 430.24)

When a single feeder raceway or busway supplies two or more motors, diversity dictates that not all motors will pull locked-rotor starting current simultaneously. However, the largest motor may start while all other motors are already operating at full continuous load.

Therefore, under NEC 430.24, conductors supplying several motors must have an allowable ampacity not less than $125\%$ of the highest rated motor full-load current (FLC) plus the sum of the full-load currents of all other motors in the group:

$I_{\text{feeder}} \ge (1.25 \times I_{\text{largest FLC}}) + \sum I_{\text{other FLCs}}$

Critical Application Rules for NEC 430.24:

• Determining the Largest Motor: The "largest motor" is strictly the motor with the highest full-load current (FLC) from NEC Tables 430.248 or 430.250, not necessarily the motor with the highest nameplate horsepower rating.
• Identical Highest Ratings: If two or more motors have identical highest full-load currents, only one is multiplied by $125\%$; all other motors are added at $100\%$.

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Combining Motor Loads with Other Loads on a Feeder (NEC 220.50)

In commercial distribution systems, a distribution panel feeder frequently supplies a mix of motor loads, continuous general lighting, and noncontinuous convenience receptacle circuits. Under NEC 220.50 and NEC 430.24, the feeder conductor ampacity is determined using the combined formula:

$I_{\text{total feeder}} \ge (1.25 \times I_{\text{largest motor FLC}}) + \sum I_{\text{other motor FLCs}} + (1.25 \times I_{\text{continuous non-motor}}) + \sum I_{\text{noncontinuous non-motor}}$

This calculation ensures that every continuous load is backed by $125\%$ conductor thermal capacity while preventing compounding of safety factors on noncontinuous loads.

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Motor Disconnecting Means: Location & Ratings (NEC Part IX)

NEC Article 430 Part IX establishes rigid safety rules ensuring electricians and maintenance personnel can physically de-energize and lock out a motor to prevent accidental startup during mechanical servicing:

1. Location Requirements (NEC 430.102)

• In Sight From Controller: A disconnecting means must be located in sight from the motor controller location (NEC 430.102(A)).
• In Sight From Motor Location: A disconnecting means must also be located in sight from the motor location and the driven machinery (NEC 430.102(B)).
• Definition of "In Sight From" (Article 100): The specified equipment must be visible and not more than 50 feet (15 meters) distant from the other equipment.

• Lockable Disconnect Exception (NEC 430.102(B)(2) Ex.): The disconnect adjacent to the motor is permitted to be omitted if the disconnect located at the controller can be locked in the open position in accordance with NEC 110.25 (the locking provision must remain in place with or without the padlock installed), and either:
  1. Locating the disconnect near the motor is impracticable or introduces greater hazards (e.g., inside hazardous chemical vapor zones), or
  2. The installation is in an industrial facility with written safety procedures and qualified maintenance personnel.

2. Disconnect Type (NEC 430.109)

The disconnecting means must be one of the following listed types:

• A listed horsepower-rated motor-circuit switch.
• A listed molded case circuit breaker.
• A listed molded case switch (non-automatic circuit interrupter).
• A listed manual motor controller marked "Suitable as Motor Disconnect" installed on the load side of branch-circuit protection.
• For motors of $\frac{1}{8}\text{ HP}$ or less: General-use snap switch.

3. Disconnect Ampere & Horsepower Ratings (NEC 430.110)

Under NEC 430.110(A), the disconnecting means for a single motor circuit must satisfy two distinct engineering ratings:

1. Ampere Rating: The switch or breaker must have an continuous ampere rating of at least $115\%$ of the motor full-load current (FLC) from Table 430.248 or 430.250: $\text{Disconnect Minimum Amperes} \ge 1.15 \times I_{\text{FLC (Table)}}$
2. Horsepower Rating: The switch must carry a nominal horsepower rating equal to or greater than the motor nameplate horsepower rating.

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Step-by-Step Worked Calculation: Commercial HVAC Installation

To see how these principles coalesce into real-world practice, let us engineer the branch circuits, disconnect switches, and common feeder for a commercial mechanical equipment room served by a 460-volt, 3-phase, 60 Hz distribution panelboard.

Installation Specifications:

• Motor 1 (Water-Cooled Chiller): 50 HP, 460V, 3-phase, continuous-duty squirrel-cage induction motor.
• Motor 2 (Chilled Water Pump): 15 HP, 460V, 3-phase, continuous-duty squirrel-cage induction motor.
• Motor 3 (Condenser Water Pump): 10 HP, 460V, 3-phase, continuous-duty squirrel-cage induction motor.
• Motor 4 (Mechanical Room Exhaust Fan): 3 HP, 460V, 3-phase, continuous-duty squirrel-cage induction motor.
• Wiring Method: Single raceway (EMT) supplying copper THHN/THWN-2 conductors landed on equipment terminals rated for $75^\circ\text{C}$.

460V / 3-Phase Main Distribution Feeder
    │
    ├─── [Feeder OCPD] ──── (Feeder Conductors: Sized per 430.24)
    │
    ▼ Distribution Panelboard
    ├─── [Branch 1] ──> Disconnect ──> Controller ──> 50 HP Chiller (FLC = 65A)
    ├─── [Branch 2] ──> Disconnect ──> Controller ──> 15 HP Chilled Pump (FLC = 21A)
    ├─── [Branch 3] ──> Disconnect ──> Controller ──> 10 HP Condenser Pump (FLC = 14A)
    └─── [Branch 4] ──> Disconnect ──> Controller ──> 3 HP Exhaust Fan (FLC = 4.8A)

Step 1: Look Up Table FLCs (NEC Table 430.250)

Per NEC 430.6(A)(1), we look up the full-load currents from NEC Table 430.250 for three-phase AC motors at 460 volts:

• Motor 1 (50 HP): $\text{FLC} = 65\text{ Amperes}$
• Motor 2 (15 HP): $\text{FLC} = 21\text{ Amperes}$
• Motor 3 (10 HP): $\text{FLC} = 14\text{ Amperes}$
• Motor 4 (3 HP): $\text{FLC} = 4.8\text{ Amperes}$

Step 2: Sizing Individual Branch-Circuit Conductors (NEC 430.22)

Apply the $125\%$ multiplier to each Table FLC and select conductors from the $75^\circ\text{C}$ column of NEC Table 310.16 (Copper THHN/THWN-2):

1. Motor 1 (50 HP, FLC = 65 A): $I_{\text{conductor}} \ge 1.25 \times 65\text{ A} = 81.25\text{ Amperes}$

   • Table 310.16 ($75^\circ\text{C}$ Cu): 4 AWG is rated 85 Amperes ($85\text{ A} \ge 81.25\text{ A}$).
   • Select: 4 AWG Copper THHN/THWN-2.

2. Motor 2 (15 HP, FLC = 21 A): $I_{\text{conductor}} \ge 1.25 \times 21\text{ A} = 26.25\text{ Amperes}$

   • Table 310.16 ($75^\circ\text{C}$ Cu): 10 AWG is rated 35 Amperes ($35\text{ A} \ge 26.25\text{ A}$).
   • (Note: 240.4(G) permits 10 AWG to carry its full 35A 75°C ampacity in motor branch circuits).
   • Select: 10 AWG Copper THHN/THWN-2.

3. Motor 3 (10 HP, FLC = 14 A): $I_{\text{conductor}} \ge 1.25 \times 14\text{ A} = 17.5\text{ Amperes}$

   • Table 310.16 ($75^\circ\text{C}$ Cu): 14 AWG is rated 20 Amperes ($20\text{ A} \ge 17.5\text{ A}$).
   • (Engineering best practice on commercial projects often specifies a minimum of 12 AWG for mechanical strength, rated 25A at 75°C; however, code minimum is 14 AWG).
   • Select: 14 AWG (or 12 AWG) Copper THHN/THWN-2.

4. Motor 4 (3 HP, FLC = 4.8 A): $I_{\text{conductor}} \ge 1.25 \times 4.8\text{ A} = 6.0\text{ Amperes}$

   • Table 310.16 ($75^\circ\text{C}$ Cu): 14 AWG is rated 20 Amperes ($20\text{ A} \ge 6.0\text{ A}$).
   • Select: 14 AWG Copper THHN/THWN-2.

Step 3: Sizing Disconnecting Means (NEC 430.110)

Each motor must have a disconnect switch within sight (visible and within 50 feet) satisfying minimum ampere ($115\%\text{ FLC}$) and horsepower ratings:

1. Motor 1 (50 HP): Minimum Amperes $= 1.15 \times 65\text{ A} = 74.75\text{ A}$. Standard switch size is 100 Ampere, 50 HP-rated switch.
2. Motor 2 (15 HP): Minimum Amperes $= 1.15 \times 21\text{ A} = 24.15\text{ A}$. Standard switch size is 30 Ampere, 15 HP-rated switch.
3. Motor 3 (10 HP): Minimum Amperes $= 1.15 \times 14\text{ A} = 16.1\text{ A}$. Standard switch size is 30 Ampere, 10 HP-rated switch.
4. Motor 4 (3 HP): Minimum Amperes $= 1.15 \times 4.8\text{ A} = 5.52\text{ A}$. Standard switch size is 30 Ampere, 3 HP-rated switch (or listed manual motor controller).

Step 4: Sizing Feeder Conductors Supplying All Four Motors (NEC 430.24)

Under NEC 430.24, the feeder ampacity must equal $125\%$ of the highest rated motor FLC plus the sum of all other motor FLCs:

• Highest rated motor: Motor 1 (50 HP, $\text{FLC} = 65\text{ A}$)
• Other motor FLCs: $21\text{ A} + 14\text{ A} + 4.8\text{ A} = 39.8\text{ A}$

$I_{\text{feeder}} \ge (1.25 \times 65\text{ A}) + 21\text{ A} + 14\text{ A} + 4.8\text{ A}$ $I_{\text{feeder}} \ge 81.25\text{ A} + 39.8\text{ A} = 121.05\text{ Amperes}$

Now, select the feeder conductor from NEC Table 310.16 ($75^\circ\text{C}$ Column, Copper):

• 2 AWG Copper is rated 115 Amperes ($115\text{ A} < 121.05\text{ A}$ — too small).
• 1 AWG Copper is rated 130 Amperes ($130\text{ A} \ge 121.05\text{ A}$ — compliant).
• Select: 1 AWG Copper THHN/THWN-2 conductors for the feeder.

Step 5: Feeder Sizing with Mixed Continuous & Noncontinuous Loads

Suppose the feeder also supplies:

• Continuous mechanical room fluorescent/LED lighting: $16\text{ Amperes}$
• Noncontinuous convenience utility receptacles: $10\text{ Amperes}$

Applying NEC 220.50: $I_{\text{feeder total}} = 121.05\text{ A (Motors)} + (1.25 \times 16\text{ A continuous}) + 10\text{ A noncontinuous}$ $I_{\text{feeder total}} = 121.05\text{ A} + 20\text{ A} + 10\text{ A} = 151.05\text{ Amperes}$

Evaluating Table 310.16 ($75^\circ\text{C}$ Cu):

• 1/0 AWG Copper is rated 150 Amperes ($150\text{ A} < 151.05\text{ A}$ — slightly undersized).
• 2/0 AWG Copper is rated 175 Amperes ($175\text{ A} \ge 151.05\text{ A}$ — compliant).
• Select: 2/0 AWG Copper THHN/THWN-2 conductors.