8.1 Motor Full-Load Current (FLA) & Conductor Sizing (NEC Article 430)

8.1 Motor Full-Load Current (FLA) & Conductor Sizing (NEC Article 430)

Exam Focus: NEC Table FLA vs. Nameplate FLA (430.6(A)(1)), Single motor branch-circuit conductor sizing at 125% (430.22), Multi-motor feeder conductor sizing (430.24), and terminal temperature ampacity selection (110.14(C)).

Motor circuits present unique electrical challenges due to high starting inrush currents (locked-rotor current) and sustained inductive mechanical loads. Consequently, NEC Article 430 establishes specialized rules that depart significantly from standard general branch-circuit sizing principles. Understanding how to navigate Article 430 tables and calculation rules is one of the highest-yield topics on the Michigan PSI Journeyman Electrician examination.

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NEC Table Full-Load Current (FLA) vs. Nameplate Current

One of the most frequent traps on the journeyman exam involves selecting the correct motor current value for circuit calculations. NEC 430.6(A)(1) mandates a strict division between when to use NEC Table values and when to use Motor Nameplate values.

+---------------------------------------------------------------------------------+
|                        NEC 430.6(A)(1) CURRENT SELECTION RULE                   |
+---------------------------------------------------------------------------------+
|  USE NEC TABLES (430.247, 430.248, 430.250) FOR:                                |
|    1. Branch-Circuit Conductor Sizing (430.22)                                  |
|    2. Feeder Conductor Sizing (430.24)                                          |
|    3. Short-Circuit & Ground-Fault Protection Sizing (430.52 & 430.62)           |
|    4. Disconnecting Means Ampacity Rating (430.110)                            |
|                                                                                 |
|  USE MOTOR NAMEPLATE RATING ONLY FOR:                                           |
|    1. Motor Overload Relay / Thermal Unit Sizing (430.32 & Part III)             |
|    2. Separate overload protection devices                                      |
+---------------------------------------------------------------------------------+

Mandatory NEC Tables:

• Table 430.247: Direct-Current Motors (FLA for 90V, 120V, 180V, 240V, 500V, 600V DC).
• Table 430.248: Single-Phase AC Motors (FLA for 115V, 200V, 208V, 230V AC).
• Table 430.250: Three-Phase AC Motors (FLA for 115V, 200V, 208V, 230V, 460V, 575V, 2300V AC).

Why Use NEC Tables Instead of Nameplate?

Motor nameplate ratings vary based on manufacturer efficiency and construction quality. If a motor fails and is replaced by a less efficient motor of the same horsepower, the replacement motor will draw more full-load current. Using NEC Table values ensures that branch-circuit conductors and conduit are sized conservatively for worst-case standard motor replacements.

[!WARNING] Exam Trap: A question may state: 'A 230V, single-phase, 5 HP motor has a nameplate rating of 24 Amperes. What is the minimum branch-circuit conductor ampacity?' Ignore the 24A nameplate rating! Turn to NEC Table 430.248, locate 5 HP at 230V (28 Amperes), and multiply 28 A by 125% = 35 Amperes.

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Single Motor Branch-Circuit Conductor Sizing (NEC 430.22)

Under NEC 430.22, conductors supplying a single motor used in a continuous-duty application shall have an ampacity of not less than 125% of the motor full-load current rating determined from the applicable NEC Table.

$\text{Minimum Conductor Ampacity} = \text{NEC Table FLA} \times 1.25$

Terminal Temperature Limitations (NEC 110.14(C))

Once the minimum required ampacity is calculated, select the conductor size from NEC Table 310.16 enforcing terminal temperature limits:

• Equipment rated 100 Amperes or less (or for #14 AWG through #1 AWG conductors) is generally restricted to the 60°C column rating, unless terminals are explicitly listed for 75°C.
• Equipment rated over 100 Amperes (or for conductors larger than #1 AWG) permits using the 75°C column rating.
• THHN insulation (90°C rated) is commonly used in practice, but its ampacity must be taken from the 60°C or 75°C column to match terminal ratings, unless performing derating calculations for ambient temperature or conduit bundle fill.

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Worked Calculation Example 1: Single Motor Branch Circuit

Question: A 10 HP, 230-Volt, single-phase continuous-duty AC induction motor is installed in a commercial facility. Terminals are rated for 75°C. What is the minimum size THHN copper conductor required for the branch circuit?

• Step 1: Determine the Table FLA. Turn to NEC Table 430.248 (Single-Phase AC Motors). Locate 10 HP under the 230V column $\rightarrow$ 50 Amperes.

• Step 2: Calculate Minimum Required Ampacity (NEC 430.22). $\text{Required Ampacity} = 50\text{ A} \times 1.25 = 62.5\text{ Amperes}$

• Step 3: Select Conductor Size from Table 310.16. Look in the 75°C Copper column of Table 310.16:

  • #8 AWG Cu = 50 A (too small)
  • #6 AWG Cu = 65 A (meets or exceeds 62.5 A)

Answer: #6 AWG THHN Copper.

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Multi-Motor Feeder Conductor Sizing (NEC 430.24)

When a single feeder supplies a group of motors, NEC 430.24 dictates that the feeder conductors must have an ampacity sufficient to supply the largest motor load plus the sum of all other motor loads, plus any non-motor continuous and non-continuous loads.

$\text{Feeder Ampacity} = (1.25 \times \text{FLA}_{\text{largest motor}}) + sum \text{FLA}_{\text{other motors}} + (1.25 \times \text{Non-Motor Continuous}) + \text{Non-Motor Non-Continuous}$

+-------------------------------------------------------------------------+
|                   MULTI-MOTOR FEEDER CONDUCTOR CALCULATION              |
|                                                                         |
|   [Motor 1: 25 HP (34A)]  -->  34A x 1.25 = 42.5A (Largest Motor)     |
|   [Motor 2: 15 HP (21A)]  -->  21A x 1.00 = 21.0A                     |
|   [Motor 3: 10 HP (14A)]  -->  14A x 1.00 = 14.0A                     |
|   -------------------------------------------------------------------   |
|   Total Required Feeder Ampacity             = 77.5 Amperes             |
+-------------------------------------------------------------------------+

Identifying the 'Largest Motor'

The 'largest motor' is defined as the motor with the highest full-load current rating in Amperes from the NEC Tables—NOT necessarily the motor with the highest horsepower label.

[!NOTE] Multiple Motors of Identical Rating: If two or more motors on the feeder share the exact same highest FLA rating, choose only ONE of those motors to multiply by 125%, and add the remaining motors at 100% of their Table FLA.

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Worked Calculation Example 2: Multi-Motor Feeder Sizing

Question: An industrial distribution feeder supplies three 460-Volt, 3-phase motors: Motor A (25 HP), Motor B (15 HP), and Motor C (10 HP). All terminal connections are 75°C. What is the minimum ampacity and size of THWN copper feeder conductors required?

• Step 1: Look up NEC Table FLAs (Table 430.250 for 3-Phase AC).

  • Motor A (25 HP @ 460V) = 34 Amperes
  • Motor B (15 HP @ 460V) = 21 Amperes
  • Motor C (10 HP @ 460V) = 14 Amperes

• Step 2: Identify the largest motor and calculate feeder ampacity (NEC 430.24).

  • Largest motor = Motor A (34 A). $\text{Feeder Ampacity} = (34\text{ A} \times 1.25) + 21\text{ A} + 14\text{ A}$ $\text{Feeder Ampacity} = 42.5\text{ A} + 21\text{ A} + 14\text{ A} = 77.5\text{ Amperes}$

• Step 3: Select conductor from Table 310.16 (75°C Copper Column).

  • #6 AWG Cu = 65 A (too small)
  • #4 AWG Cu = 85 A (exceeds 77.5 A requirement)

Answer: Feeder Ampacity = 77.5 Amperes; Conductor Size = #4 AWG THWN Copper.

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Voltage Ratings & Special Duty Cycle Exceptions

Nominal Voltage Differences

If a motor is rated at 208V, use the 208V column in Tables 430.248 or 430.250. If a table only lists 230V ratings and the motor is operated on a 208V supply, NEC Table notes mandate increasing the 230V table FLA value by 10% for 208V calculations.

Non-Continuous Duty Cycle Motors (Table 430.22(E))

Motors used for short-time, intermittent, periodic, or varying duty (such as cranes, hoists, elevator motors, or valve operators) do not use the standard 125% multiplier. Instead, refer to Table 430.22(E) for duty cycle multipliers ranging from 85% to 200% depending on nameplate rating time classification.