Motor Calculations for the NEC Exam: FLC, Conductor, and OCP Sizing

Motor calculations account for about 15% of the Calculations portion of the Texas journeyman electrician exam, which translates to roughly 4 questions. These questions tend to be multi-step problems that require you to follow a specific process using several NEC tables and sections.

The good news is that the process is the same every time. Once you learn the steps and know which tables to use, motor problems become one of the more predictable calculation types on the exam. The key rule that trips up most candidates: always use the NEC table values for full-load current, never the motor nameplate.

The Motor Calculation Roadmap

Every motor circuit calculation follows this sequence:

Find the full-load current (FLC) from NEC tables
Size the branch-circuit conductors
Size the branch-circuit overcurrent protection
Size the overload protection
(For multiple motors) Size the feeder conductors and feeder overcurrent protection

Let us walk through each step.

Step 1: Find the Full-Load Current (FLC)

This is the most critical step, and it is where the biggest exam trap lies.

Rule: Use the NEC tables, NOT the motor nameplate.

The NEC provides full-load current tables based on motor horsepower and voltage:

Table 430.247: DC motors
Table 430.248: Single-phase AC motors
Table 430.250: Three-phase AC motors
Table 430.249: Two-phase AC motors (rare)

The motor nameplate shows the actual operating current of that specific motor, which may be higher or lower than the table values. The NEC uses standardized table values for sizing conductors and overcurrent protection to ensure consistency and adequate protection regardless of the specific motor installed.

The one exception: Nameplate current IS used for sizing overload protection (Step 4). This is the only place where the nameplate value applies.

Example: Finding FLC

A 10 HP, 3-phase, 208V motor. What is the FLC?

Look up Table 430.250: For 10 HP at 208V, FLC = 30.8A

Note that 208V is not listed directly in some tables. If your table lists 200V instead, use that column. Some editions list 208V separately. Always check the specific table in the NEC edition you are using.

Step 2: Size the Branch-Circuit Conductors

NEC 430.22 requires that branch-circuit conductors for a single motor have an ampacity of not less than 125% of the motor's full-load current (from the NEC table, not the nameplate).

Formula: Minimum conductor ampacity = FLC x 1.25

Then select the conductor from Table 310.16 based on the appropriate temperature column (60C, 75C, or 90C depending on the terminal temperature rating and any derating conditions).

Example

Using our 10 HP, 3-phase, 208V motor with FLC = 30.8A:

Minimum conductor ampacity = 30.8 x 1.25 = 38.5A

From Table 310.16 (75C column, copper): #8 AWG = 50A. This exceeds 38.5A.

Minimum conductor size: #8 AWG copper

Step 3: Size the Branch-Circuit Overcurrent Protection

Motor branch-circuit overcurrent protection is sized per Table 430.52, which provides maximum percentages based on the type of motor and the type of protective device.

Table 430.52 Key Values

| Motor Type | Dual-Element Fuse | Non-Time Delay Fuse | Inverse Time Breaker | Instantaneous Breaker | |---|---|---|---|---| | Single-phase | 175% | 300% | 250% | 800% | | AC polyphase (squirrel cage) | 175% | 300% | 250% | 800% | | AC polyphase (wound rotor) | 150% | 150% | 150% | 800% | | DC | 150% | 150% | 150% | 250% |

The calculation: Multiply the FLC by the percentage from Table 430.52, then select the next standard overcurrent device size from 240.6(A).

Important rules:

If the calculated value does not correspond to a standard OCPD size, you may round up to the next standard size (430.52(C)(1), Exception No. 1)
If the next standard size is not sufficient for the motor to start (nuisance tripping), you may increase the size per the maximum values in Table 430.52(C)(1), Exception No. 2
The maximum OCPD size from Table 430.52 is a ceiling, not a target. You should use the smallest device that allows the motor to start without nuisance tripping

Example

Our 10 HP, 3-phase motor with FLC = 30.8A, using an inverse time circuit breaker:

OCPD = 30.8 x 2.50 = 77A

Next standard size from 240.6(A) = 80A

If we were using a dual-element (time-delay) fuse instead: OCPD = 30.8 x 1.75 = 53.9A Next standard size = 60A

Step 4: Size the Overload Protection

Overload protection prevents the motor from overheating during extended operation at currents above its rated capacity. Unlike branch-circuit overcurrent protection, overload protection is sized from the motor nameplate current, not the NEC table FLC.

NEC 430.32 provides the sizing rules:

| Motor Type | Overload Size | |---|---| | Motors with service factor 1.15 or greater | 125% of nameplate FLA | | Motors with temperature rise not exceeding 40C | 125% of nameplate FLA | | All other motors | 115% of nameplate FLA |

If the overload size calculated does not allow the motor to start, it may be increased to a maximum of 140% (for motors with 1.15 SF or 40C rise) or 130% (for all other motors).

Example

Our 10 HP, 3-phase motor has a nameplate FLA of 28A and a service factor of 1.15.

Overload = 28 x 1.25 = 35A

If the service factor were 1.0: Overload = 28 x 1.15 = 32.2A

Step 5: Multiple Motor Calculations

When a feeder supplies multiple motors, the calculation changes.

Feeder Conductor Sizing (430.24)

The feeder conductor ampacity must be at least: 125% of the largest motor FLC + 100% of all other motor FLCs

Example

A feeder supplies three motors:

Motor A: 10 HP, 3-phase, 208V, FLC = 30.8A
Motor B: 5 HP, 3-phase, 208V, FLC = 16.7A
Motor C: 3 HP, 3-phase, 208V, FLC = 10.6A

Largest motor = Motor A at 30.8A

Feeder ampacity = (30.8 x 1.25) + 16.7 + 10.6 = 38.5 + 16.7 + 10.6 = 65.8A

From Table 310.16 (75C column): #4 AWG copper = 85A. This works.

Or #6 AWG copper = 65A. This does NOT work (65 < 65.8).

Minimum feeder conductor: #4 AWG copper

Feeder Overcurrent Protection (430.62)

The maximum feeder OCPD rating is: Largest motor branch-circuit OCPD + sum of FLCs of all other motors

Using our example with inverse time breakers:

Largest motor OCPD = 80A (calculated in Step 3) Other motors: 16.7 + 10.6 = 27.3A

Feeder OCPD = 80 + 27.3 = 107.3A

Next standard size = 110A

Common Exam Question Types

Type 1: Single Motor Branch Circuit

"Size the conductors, overcurrent protection, and overload for a [HP], [phase], [voltage] motor."

Follow Steps 1-4 in order. This is the most straightforward motor problem.

Type 2: Multiple Motor Feeder

"Three motors are supplied by a single feeder. Size the feeder conductors and overcurrent protection."

Follow Step 5. Remember: 125% of the LARGEST motor, plus 100% of all others.

Type 3: Combined Motor and Non-Motor Loads

"A panel serves two motors and a 30A lighting load. Size the feeder."

Calculate the motor feeder load per 430.24, then add the non-motor loads at 100% (or with appropriate demand factors if applicable).

Type 4: Motor with Derating Conditions

"A motor circuit runs through a conduit with 6 current-carrying conductors in an ambient temperature of 40C."

Calculate the conductor size per Step 2, then apply temperature correction and conduit fill adjustment factors from Article 310. The required ampacity before derating = (FLC x 1.25) / (temperature factor x adjustment factor).

Quick Reference: The Four NEC Tables for Motor Calculations

| Table | What It Tells You | When to Use It | |---|---|---| | 430.248 or 430.250 | Full-load current (FLC) | Always the first step. Use for conductor and OCPD sizing. | | 310.16 | Conductor ampacity | After calculating 125% of FLC | | 430.52 | Maximum OCPD percentage | For branch-circuit protection sizing | | 240.6(A) | Standard OCPD sizes | To select the actual device size |

The Five Motor Calculation Rules to Memorize

FLC from NEC tables, NOT nameplate (for conductors and OCPD)
Conductors at 125% of FLC (430.22)
OCPD from Table 430.52, round up to next standard size (430.52)
Overload from nameplate FLA, at 115% or 125% depending on service factor (430.32)
Multiple motors: 125% of largest FLC + 100% of all others (430.24)

If you memorize these five rules and know where to find the tables, you can solve any motor calculation the exam throws at you.

Practice Strategy

Motor calculations are best learned through repetition. Work through these practice scenarios:

Size a single-phase motor branch circuit (5 HP, 230V, single-phase)
Size a three-phase motor branch circuit (25 HP, 460V, three-phase) with both inverse time breaker and dual-element fuse options
Size a feeder for three motors of different sizes
Size a motor circuit where the conductors pass through conduit with more than 3 current-carrying conductors

For each problem, write out all four values: conductor size, OCPD size, overload size, and (where applicable) feeder sizing. Check your answers against the NEC tables.

Practice motor calculations with our exam simulator

Sources:

National Fire Protection Association - NFPA 70: National Electrical Code, 2023 Edition
Texas Department of Licensing and Regulation (TDLR) - Electrician Exam Information
TDLR - Electrician Exam Statistics, Fiscal Year 2025
ExpertCE - Journeyman Electrician Exam Prep: Calculations You Must Know
PSI Exams - TDLR Electrician Exam Candidate Information