Motor Full Load Amps (FLA) & NEC Sizing Calculator
Lookup exact NEC Table 430.248 (1-Phase) & 430.250 (3-Phase) motor Full Load Amps (FLA), size 125% branch circuit conductors, calculate maximum NEC 430.52 circuit breaker & fuse ratings, and estimate NEMA Code inrush starting current.
Electric Motor Specifications
Dictates motor starting inrush current
Thermal overload sizing threshold
Per NEC 430.6(A)(2), overload relays are sized from nameplate FLA!
NEC Table Ratings & Sizing Results
NEC Table 430 Full Load Current
TABLE 430.250
14.0 A
Mandatory for conductor & breaker sizing per NEC 430.6(A)(1)
Min Conductor (125% FLA)
#12 AWG Cu
Min 17.5 A (75°C THHN)
Max Circuit Breaker (NEC 430.52)
35 A
250% inverse time breaker
Max Thermal Overload (NEC 430.32)
16.5 A
125% of Nameplate (13.2 A)
Locked-Rotor Inrush (LRA)
74.1 A
5.3 × FLA starting spike
Recommended Disconnect Switch
15 HP Rated
NEC 430.110 Standard
≥ 115% FLA (16.1A)
Interactive NEC Article 430 Motor Branch Circuit Architecture
5 Fatal Traps & Electrician Motor Circuit Pitfalls
🚨 Trap 1: Sizing Wire from the Motor Nameplate Instead of NEC Tables (NEC 430.6(A)(1))
The number one reason electricians fail commercial inspections is reading the stamped FLA directly off the physical motor nameplate to size branch conductors and circuit breakers. NEC 430.6(A)(1) strictly forbids this! Conductors, switches, and overcurrent protection must be sized using the standardized full-load currents published in NEC Table 430.248 (Single-Phase) or Table 430.250 (Three-Phase). This guarantees that if the motor burns out in the future and is replaced by a less efficient brand with a higher nameplate current, the existing branch circuit wiring will never overheat or catch fire.
⚠️ Trap 2: Sizing Thermal Overload Heaters from NEC Tables Instead of Nameplate
The inverse of Trap 1: while wire and breakers must use the NEC table values, thermal overload protection (overload heaters and electronic relays per NEC 430.6(A)(2) and 430.32) MUST be sized using the ACTUAL physical motor nameplate rating! If a high-efficiency 10 HP motor has a nameplate of 12.0 A, but you set the overload relay to 125% of the NEC table value of 14.0 A (17.5 A), the motor can operate in a chronic 145% overload condition until its stator windings incinerate, and the overload will never trip!
⚡ Trap 3: Nuisance Tripping 100% Breakers on NEMA Code G Starting Inrush
Unlike resistive heaters, an AC induction motor is essentially a dead short circuit across the line at the instant of startup until rotor counter-electromotive force (back-EMF) develops. Standard NEMA Code G motors pull 6 times rated full-load current (600% inrush) for the first several seconds. Protecting a 14A motor with a standard 20A breaker causes instant thermal-magnetic tripping every time the contactor pulls in. This is why NEC Table 430.52 explicitly authorizes inverse time circuit breakers to be sized up to 250% of FLA (and up to 400% per 430.52(C)(1) Ex. 2 if required to start the load).
🔥 Trap 4: Single-Phasing Destruction on 3-Phase Motors
If one fuse blows or one phase conductor opens on a running three-phase induction motor, the rotating stator magnetic field collapses into an elliptical oscillating field. To maintain shaft horsepower, the current in the remaining two energized phase windings spikes to 173% of normal FLA. Even worse, negative-sequence currents induce massive rotor currents, heating the squirrel cage bars to over 600°F within two minutes. Always ensure the motor starter is equipped with three-phase electronic overload protection with built-in phase-loss detection.
📏 Trap 5: High Starting Voltage Drop Causing Inrush Contactor Chattering
On long feeder runs (e.g. 250+ feet to a wastewater lift station or quarry conveyor), sizing wire strictly for 125% running FLA can result in a 15% to 20% voltage drop during full-voltage motor starting. Because induction motor torque is proportional to the square of terminal voltage ($T \propto V^2$), a 20% voltage dip slashes starting torque by 36% ($0.80^2 = 0.64$). The motor stalls in locked-rotor mode, the magnetic starter coil drops out, picks up, and chatters violently, destroying contactor tips and burning out the stator.
First-Principles NEC Motor Circuit Derivations
1. Branch-Circuit Conductor Sizing (NEC 430.22)
Conductors supplying a single continuous-duty motor must have an ampacity rating not less than 125% of the motor full-load current rating from NEC Table 430.248 or 430.250:
Locked-Rotor Amperes ($I_{\text{LRA}}$) is derived from the NEMA kVA/HP code letter rating:
$I_{\text{LRA}} = \frac{\text{kVA/HP} \times \text{HP} \times 1000}{\sqrt{3} \times V_{LL}}$ (3-Phase) or $\frac{\text{kVA/HP} \times \text{HP} \times 1000}{V}$ (1-Phase).
Frequently Asked Questions
Why are circuit breakers allowed to be 250% of motor FLA?
Electric motors draw massive locked-rotor inrush currents (typically 600% of FLA) when starting across the line. A circuit breaker sized at 125% would trip magnetically on startup. In motor branch circuits, the breaker exists strictly to protect against ground faults and short circuits, while continuous running overload protection is provided by the thermal overload relays in the motor starter.
What is the difference between NEC Table 430.248 and Table 430.250?
NEC Table 430.248 covers single-phase alternating current motors (115V and 230V), whereas NEC Table 430.250 covers three-phase induction and synchronous motors across standard industrial voltages (208V, 230V, 460V, and 575V).
How do I size motor overload protection?
Under NEC 430.32, separate overload devices (such as overload relay heaters or electronic overload settings) must be sized based on the motor nameplate current rating, NOT the NEC tables: motors with a service factor (SF) of 1.15 or greater or a temperature rise not over 40°C are sized at a maximum of 125% of nameplate FLA; all other motors are capped at 115% of nameplate FLA.
What does a NEMA Code Letter mean on a motor nameplate?
NEMA Code Letters (ranging from A to V per NEMA MG-1 and NEC Table 430.7(B)) define the locked-rotor kVA per horsepower with the rotor locked at rated voltage and frequency. Most standard general-purpose industrial induction motors are Code G, corresponding to 5.6 to 6.29 kVA per horsepower.
Why does a 10 HP motor have different FLA at 230V vs 460V?
Because electrical power is proportional to voltage times current ($P = \sqrt{3} \times V \times I$), doubling the voltage from 230V to 460V cuts the required operating current exactly in half. For a 10 HP motor, Table 430.250 specifies 28.0 Amps at 230V, but only 14.0 Amps at 460V, allowing significantly smaller conductors and lower installation costs.
Frequently Asked Questions
Why are circuit breakers allowed to be 250% of motor FLA?+
Electric motors draw massive locked-rotor inrush currents (typically 600% of FLA) when starting across the line. A circuit breaker sized at 125% would trip magnetically on startup. In motor branch circuits, the breaker exists strictly to protect against ground faults and short circuits, while continuous running overload protection is provided by the thermal overload relays in the motor starter.
What is the difference between NEC Table 430.248 and Table 430.250?+
NEC Table 430.248 covers single-phase alternating current motors (115V and 230V), whereas NEC Table 430.250 covers three-phase induction and synchronous motors across standard industrial voltages (208V, 230V, 460V, and 575V).
How do I size motor overload protection?+
Under NEC 430.32, separate overload devices (such as overload relay heaters or electronic overload settings) must be sized based on the motor nameplate current rating, NOT the NEC tables: motors with a service factor (SF) of 1.15 or greater or a temperature rise not over 40°C are sized at a maximum of 125% of nameplate FLA; all other motors are capped at 115% of nameplate FLA.
What does a NEMA Code Letter mean on a motor nameplate?+
NEMA Code Letters (ranging from A to V per NEMA MG-1 and NEC Table 430.7(B)) define the locked-rotor kVA per horsepower with the rotor locked at rated voltage and frequency. Most standard general-purpose industrial induction motors are Code G, corresponding to 5.6 to 6.29 kVA per horsepower.
Why does a 10 HP motor have different FLA at 230V vs 460V?+
Because electrical power is proportional to voltage times current ($P = \sqrt{3} \times V \times I$), doubling the voltage from 230V to 460V cuts the required operating current exactly in half. For a 10 HP motor, Table 430.250 specifies 28.0 Amps at 230V, but only 14.0 Amps at 460V, allowing significantly smaller conductors and lower installation costs.