12.1 Motor Nameplate vs. NEC Tables & Motor Full-Load Current
Key Takeaways
- Under NEC 430.6(A)(1), motor branch-circuit conductors, disconnect switches, and short-circuit/ground-fault protective devices MUST be sized using the Full-Load Current (FLC) values published in NEC Tables 430.247 through 430.250, never the motor nameplate FLA.
- Motor nameplate Full-Load Amperes (FLA) is used exclusively for sizing motor overload protection devices under NEC Article 430, Part III (NEC 430.32) and selecting thermal protector ratings.
- The NEC mandates Table FLC values for circuit design to ensure standardized safety margins that accommodate worst-case motor replacement across different manufacturers, varying efficiencies, and power factors.
- System operating voltages correspond to specific table columns: 120V circuits use the 115V column, 240V circuits use the 230V column, and 480V circuits use the 460V column in Tables 430.248 and 430.250.
- When motors operate at nominal voltages not listed in the tables (such as 200V or 208V when not explicitly listed), the current must be mathematically converted using the inverse voltage relationship: I_actual = I_table * (V_table / V_actual).
12.1 Motor Nameplate vs. NEC Tables & Motor Full-Load Current
Exam Fast Fact: The single most common motor calculation trap on the Colorado Journeyman Electrician examination is using the motor nameplate current to size branch-circuit conductors, disconnect switches, or circuit breakers. Under NEC 430.6(A)(1), you are legally required to discard the nameplate current and use the values from NEC Tables 430.247, 430.248, 430.249, or 430.250 for all conductor and short-circuit/ground-fault protection calculations. Nameplate Full-Load Amperes (FLA) is used only when sizing motor overload protection (Part III).
National Electrical Code (NEC) Article 430 is the longest and most heavily tested specialized equipment article on the Colorado Journeyman Electrician exam. Unlike general branch circuits where loads are straightforward resistances or known continuous volt-amperes, electric motors present severe inductive characteristics, extreme starting inrush currents (locked-rotor current running 600% or more of running current), and mechanical heat accumulation. To safely govern these dynamics, Article 430 establishes a strict two-tier current standard that every journeyman must master.
The Core Motor Rule: NEC 430.6(A)(1)
NEC 430.6(A)(1) establishes the foundational division of current ratings. When engineering, inspecting, or wiring any standard motor circuit, two distinct current values exist for the exact same physical machine:
- Table Full-Load Current (FLC): The standardized current values published at the end of Article 430 in Tables 430.247 (DC), 430.248 (Single-Phase AC), 430.249 (Two-Phase AC), and 430.250 (Three-Phase AC).
- Nameplate Full-Load Amperes (FLA): The actual, measured running current stamped into the metal data plate riveted to the motor housing by the manufacturer.
Application Breakdown: Table FLC vs. Nameplate FLA
| Motor Circuit Component | NEC Citation | Current Source to Use | Reason for Requirement |
|---|---|---|---|
| Branch-Circuit Conductors | NEC 430.22 | NEC Tables (FLC) | Standardized conductor capacity ensures safe replacement if motor is swapped. |
| Feeder Conductors | NEC 430.24 | NEC Tables (FLC) | Sized for worst-case motor group interchangeability. |
| Branch Short-Circuit & Ground-Fault OCPD | NEC 430.52 | NEC Tables (FLC) | Prevents nuisance tripping on standard locked-rotor inrush. |
| Feeder Short-Circuit & Ground-Fault OCPD | NEC 430.62 | NEC Tables (FLC) | Coordinates distribution protection across multiple motors. |
| Disconnecting Means Rating (Amperes & HP) | NEC 430.110 | NEC Tables (FLC) | Ensures switch blades and contacts interrupt worst-case locked rotor. |
| Motor Controller Rating (Horsepower) | NEC 430.83 | NEC Tables (FLC) | Contactors sized to withstand standard operational switching arcs. |
| Motor Overload Protection (Heaters / Relays) | NEC 430.32 | Motor Nameplate (FLA) | Protects the specific internal insulation and windings of that exact machine. |
| Thermal Protector Selection | NEC 430.32(A)(2) | Motor Nameplate (FLA) | Directly monitors physical temperature profile of the specific motor frame. |
Why Does the NEC Mandate Table FLC Over Nameplates?
Apprentices and journeymen frequently ask: Why would the Code force an electrician to ignore the real, measured nameplate current on the actual machine bolted to the concrete pad and instead look up a theoretical value in a code table?
The answer is based on interoperability, safety margins, and motor replacement life cycles:
- Worst-Case Manufacturer Variance: Electric motors of identical horsepower, voltage, and speed vary widely in efficiency and power factor depending on manufacturer design, bearing tolerances, and core steel quality. A high-efficiency premium motor may have a nameplate FLA of 24 amperes, whereas a standard replacement motor from another brand might draw 27 amperes. If the original installation's conductors, conduit, and circuit breakers were sized strictly for 24 amperes, replacing the motor after a burnout would dangerously overload the conductors or cause nuisance breaker trips.
- Standardized Infrastructure: By mandating Table FLC, the NEC guarantees that all upstream raceways, conductors, switches, and circuit breakers are sized conservatively to accommodate any off-the-shelf standard motor of that horsepower rating.
- Why Overloads Use Nameplate Current: Conversely, the overload relay (such as thermal heater elements or electronic solid-state overloads inside the starter) exists exclusively to protect the winding insulation of that specific physical motor. If you sized the overload relay using the higher Table FLC value rather than the nameplate FLA, the motor could run continuously in an overloaded, overheating condition, breaking down its insulation varnish and burning out without the overload ever tripping.
Navigating the NEC Motor Tables
Article 430 contains four dedicated Full-Load Current tables. You must know where each is located and how to cross-reference them instantly during the PSI examination:
- Table 430.247: Direct-Current (DC) Motors (1/4 HP to 200 HP; 90V, 120V, 180V, 240V, 500V, 550V).
- Table 430.248: Single-Phase Alternating-Current Motors (1/6 HP to 10 HP; 115V, 200V, 208V, 230V).
- Table 430.249: Two-Phase Alternating-Current Motors (4-Wire) (1/2 HP to 150 HP; 115V, 230V, 460V, 575V, 2300V).
- Table 430.250: Three-Phase Alternating-Current Motors (Induction, Squirrel-Cage, Wound-Rotor) (1/2 HP to 500 HP; 115V, 200V, 208V, 230V, 460V, 575V, 2300V).
System Voltage vs. Table Voltage Harmonization
In the field and on exam questions, system supply voltages are typically stated as nominal supply voltages (e.g., 120V, 208V, 240V, 480V, 600V). However, motor utilization equipment is designed to operate at utilization voltage levels that account for internal distribution voltage drops. The NEC tables reflect these rated utilization voltages. You must correlate system voltages to table headings as follows:
| Nominal System Distribution Voltage | NEC Table Rated Voltage Heading | Notes & Exceptions |
|---|---|---|
| 120 Volts, 1-Phase | 115 Volts (Table 430.248) | Standard 120V circuits use the 115V column. |
| 208 Volts, 1-Phase | 208 Volts (Table 430.248) | Listed directly in Table 430.248. |
| 240 Volts, 1-Phase | 230 Volts (Table 430.248) | Standard 240V residential/commercial circuits use the 230V column. |
| 208 Volts, 3-Phase | 208 Volts (Table 430.250) | Read directly from the 208V column. |
| 240 Volts, 3-Phase | 230 Volts (Table 430.250) | 240V delta or wye systems use the 230V column. |
| 480 Volts, 3-Phase | 460 Volts (Table 430.250) | 480Y/277V systems use the 460V column. |
| 600 Volts, 3-Phase | 575 Volts (Table 430.250) | 600V industrial systems use the 575V column. |
Key FLC Values from Table 430.248 (Single-Phase AC Motors)
| Horsepower | 115 Volts (FLC) | 200 Volts (FLC) | 208 Volts (FLC) | 230 Volts (FLC) |
|---|---|---|---|---|
| 1/2 HP | 9.8 A | 5.6 A | 5.4 A | 4.9 A |
| 3/4 HP | 13.8 A | 7.9 A | 7.6 A | 6.9 A |
| 1 HP | 16.0 A | 9.2 A | 8.8 A | 8.0 A |
| 1.5 HP | 20.0 A | 11.5 A | 11.0 A | 10.0 A |
| 2 HP | 24.0 A | 13.8 A | 13.2 A | 12.0 A |
| 3 HP | 34.0 A | 19.6 A | 18.7 A | 17.0 A |
| 5 HP | 56.0 A | 32.2 A | 30.8 A | 28.0 A |
| 7.5 HP | 80.0 A | 46.0 A | 44.0 A | 40.0 A |
| 10 HP | 100.0 A | 57.5 A | 55.0 A | 50.0 A |
Key FLC Values from Table 430.250 (Three-Phase AC Motors)
| Horsepower | 208 Volts (FLC) | 230 Volts (FLC) | 460 Volts (FLC) | 575 Volts (FLC) |
|---|---|---|---|---|
| 3 HP | 10.6 A | 9.6 A | 4.8 A | 3.9 A |
| 5 HP | 16.7 A | 15.2 A | 7.6 A | 6.1 A |
| 7.5 HP | 24.2 A | 22.0 A | 11.0 A | 9.0 A |
| 10 HP | 30.8 A | 28.0 A | 14.0 A | 11.0 A |
| 15 HP | 46.2 A | 42.0 A | 21.0 A | 17.0 A |
| 20 HP | 59.4 A | 54.0 A | 27.0 A | 22.0 A |
| 25 HP | 74.8 A | 68.0 A | 34.0 A | 27.0 A |
| 30 HP | 88.0 A | 80.0 A | 40.0 A | 32.0 A |
| 40 HP | 114.0 A | 104.0 A | 52.0 A | 41.0 A |
| 50 HP | 143.0 A | 130.0 A | 65.0 A | 52.0 A |
| 75 HP | 211.0 A | 192.0 A | 96.0 A | 77.0 A |
| 100 HP | 273.0 A | 248.0 A | 124.0 A | 99.0 A |
Voltage Conversion Formulas for Unlisted Voltages
Occasionally, a motor operates on an atypical distribution voltage or an unlisted nominal voltage (for instance, a 200V motor when only 230V is tabulated, or an older 220V system). Because electric motors deliver a constant mechanical shaft horsepower, current is inversely proportional to voltage.
The Inverse Voltage Formula
When calculating FLC for an operating voltage not directly listed in Table 430.248 or 430.250:
FLC_actual = FLC_table * (V_table / V_actual)
Worked Example: Voltage Conversion
Scenario: A 10 HP, 3-phase induction motor is connected to an older industrial system operating at 220 volts. Table 430.250 lists currents for 208V (30.8 A) and 230V (28.0 A), but not 220V.
- Step 1: Select the nearest standard table voltage: 230 V, where FLC_230V = 28.0 A.
- Step 2: Apply the inverse voltage formula:
FLC_220V = 28.0 A * (230 V / 220 V) = 28.0 * 1.0455 = 29.27 A - Result: The calculated table FLC to use for conductor and breaker sizing is 29.3 amperes.
Common Exam Traps & Practical Pitfalls
| Practical Scenario | Correct NEC Application | Common PSI Examination Trap |
|---|---|---|
| Conductor Sizing Question: The question provides a 15 HP, 460V motor with a nameplate FLA of 18 A and Table 430.250 FLC of 21 A. | Size conductors using 21 A (Table FLC). Minimum ampacity = 21 A * 1.25 = 26.25 A. | Electricians reflexively grab the nameplate 18 A, calculate 18 * 1.25 = 22.5 A, and choose the wrong wire size. |
| Overload Sizing Question: The same 15 HP motor has 18 A nameplate FLA and 21 A Table FLC, with Service Factor 1.15. | Size overloads using 18 A (Nameplate). Maximum overload = 18 A * 1.25 = 22.5 A. | Electricians use Table FLC (21 A) and calculate 21 * 1.25 = 26.25 A, which would permit the motor to overheat and destroy itself. |
| 208V vs. 230V Table Selection: Sizing a branch circuit for a motor connected to a 208Y/120V commercial network. | Use the dedicated 208V column in Table 430.248 or Table 430.250. | Accidentally reading the 230V column because 200-class motors are roughly 240V, underestimating current by 10% to 15%. |
According to NEC 430.6(A)(1), which motor current value must be used when sizing the branch-circuit conductors and short-circuit protective devices for a continuous-duty 3-phase squirrel-cage induction motor?
For which of the following motor circuit calculations does the National Electrical Code explicitly require using the motor nameplate full-load current (FLA) instead of the NEC table values?
An electrician is inspecting a single-phase AC induction motor connected to a nominal 240-volt branch circuit. The nameplate indicates 5 HP, 230 V, and 24 A. What current value must be used to size the branch-circuit conductors in accordance with NEC Table 430.248?