6.3 Motors & Controllers (0503)

Motor Nameplate (NEC 430.7)

Every motor shall be marked with the data required by 430.7, including: manufacturer, voltage, full-load current (FLC) in amperes, horsepower (HP), phases, frequency, RPM, code letter (locked-rotor kVA/HP), design letter (B, C, D, or E), insulation class, ambient temperature rating, and NEMA frame size. The nameplate FLC is used for overload (running) protection; the Table 430.250/430.248 FLC (based on HP and voltage) is used for conductor sizing, short-circuit/ground-fault protection, and disconnect ratings. This distinction is the single most-tested motor rule.

Branch-Circuit Conductors (430.22)

Branch-circuit conductors supplying a single continuous-duty motor shall have an ampacity not less than 125% of the motor full-load current as given in Table 430.248 (single-phase) or Table 430.250 (3-phase).

Table 430.250 Excerpt — 3-Phase AC Induction Motor FLC (A)

Motor Branch-Circuit Short-Circuit and Ground-Fault Protection (430.52)

Table 430.52 sets the maximum percentage of motor FLC for the branch-circuit OCPD:

If the calculated value does not correspond to a standard size in 240.6(A), 430.52(C)(1) Exception No. 1 permits the next higher standard size. If the device still will not carry the starting current, Exception No. 2 permits an increase to at most 400% for an inverse-time breaker rated 100 A or less (300% above 100 A), 225% for a time-delay fuse, and 400% for a non-time-delay fuse rated 600 A or less. A further allowance lets an instantaneous-trip breaker go to 1100% for a NEMA Design B energy-efficient motor. These are maximums for short-circuit and ground-fault protection only — the running overload device is a different animal entirely.

Why the numbers look so large. A across-the-line motor start draws roughly six times full-load current for a few seconds. The branch-circuit device has to let that inrush pass without opening, so it is sized far above the running current; the overload relay, sized from the nameplate, is what protects the motor from a sustained overload.

Motor Overload (Running) Protection (430.31 - 430.44)

Overload protection responds to running overcurrent from mechanical overload or failure to start. It is sized from the nameplate FLC, not the table value.

• 430.32(A)/(C) — a separate overload device shall be rated at not more than:
  • 125% of nameplate FLC where the motor has a service factor (SF) ≥1.15 or a temperature rise of 40°C or less.
  • 115% of nameplate FLC for all other motors.
• 430.32(B) — a continuous-duty motor rated 1 hp or less that is automatically started must be protected against overload by one of the means in 430.32(B)(1) through (4): a separate overload device sized as in 430.32(A), a thermal protector integral to the motor, an impedance-protected motor, or the manufacturer's approved protection. A motor of 1 hp or less that is not automatically started and is within sight from the controller is permitted by 430.32(D) to be protected by the branch-circuit short-circuit and ground-fault device, provided that device does not exceed the values in Table 430.52.
• 430.37 — the number and location of overload units is set by Table 430.37: a three-phase AC motor requires an overload unit in each of the three phases.
• 430.40 — an overload relay used with a fuse or circuit breaker must be selected so its trip current is coordinated with the short-circuit device.

Motor Controllers (430.83 - 430.88)

• 430.83(A) — a motor controller must have a horsepower rating at the application voltage not lower than the horsepower rating of the motor; a branch-circuit inverse-time circuit breaker rated in amperes is also permitted as a controller, as is a molded-case switch.
• 430.83(C) — for stationary motors rated 2 hp or less and 300 V or less, the controller may be a general-use AC snap switch where the motor full-load current is not more than 80% of the ampere rating of the switch, or a general-use switch having an ampere rating not less than twice the motor full-load current.
• 430.87 — each motor must have its own individual controller, with an exception for a group of motors under 600 V driving several parts of a single machine, or protected by one overcurrent device, or in a single room within sight of the controller.
• 430.88 — a motor that is part of an approved assembly with no exposed live parts, and whose branch-circuit device is within sight, need not have a separate controller.

Disconnecting Means (430.102)

• 430.102(A) Controller disconnect — a disconnecting means must be located in sight from the controller location and must disconnect the controller. Several exceptions cover industrial process installations and cord-and-plug-connected motors.
• 430.102(B) Motor disconnect — a disconnecting means must be provided in sight from the motor location and the driven machinery location. The exception permits the disconnect to be out of sight only where it is capable of being individually locked in the open position per 110.25 and either the location is impracticable or introduces additional hazards, or the installation is under engineering supervision in an industrial setting.
• "In sight from" is an Article 100 definition: visible and not more than 15 m (50 ft) distant from the other equipment. Both halves of the test must be satisfied — a disconnect 60 ft away in plain view fails, and so does one 10 ft away behind a wall.
• 430.110(A) — the disconnecting means for a motor circuit rated 1000 V or less must have an ampere rating of at least 115% of the motor full-load current.

Control Circuits (430.71 - 430.74)

• 430.71 Scope — Article 430 Part VII covers motor control circuits.
• 430.72 Control-circuit OCPD — control-circuit conductors shall be protected by the branch-circuit OCPD or a separate OCPD sized per Table 430.72 where the conductor is sized per 430.72(B).
• 430.74 Control-circuit disconnect — a means shall be provided to disconnect the control circuit from its source.

Phase Converters (Article 455)

A phase converter produces 3-phase power from a single-phase supply. Article 455 governs phase-converter installations:

• 455.6 Conductors — the single-phase supply conductors to the phase converter, and the conductors from the converter to the load, are sized by the rules of 455.6(A) and (B); the manufactured (derived) phase conductor is not permitted to supply single-phase loads.
• 455.7 Overcurrent protection — the single-phase supply conductors and the phase converter must be protected against overcurrent.
• 455.8 Disconnecting means — a readily accessible disconnecting means must be provided ahead of the phase converter, with an ampere rating of at least 115% of the rated input current.
• 455.20 / 455.21 — a phase converter must have a nameplate giving the manufacturer, rated input and output voltages, frequency, and single-phase input full-load amperes; the terminal for the manufactured phase must be marked, and 455.21 requires that conductor to be identified at all termination, connection, and splice points.

Worked Example: Motor Branch-Circuit Sizing

A 10 HP, 460 V, 3-phase motor, NEMA design B, with nameplate FLC = 13.2 A and SF = 1.15.

1. Table 430.250 FLC = 14.0 A (use this for conductors and OCPD, not the 13.2 A nameplate).
2. Branch-circuit conductor (430.22): 125% × 14.0 A = 17.5 A → #12 AWG copper (Table 310.16, 75 °C column = 25 A) covers it. Note that the 35 A branch-circuit device computed in the next step exceeds the 20 A cap that 240.4(D) would normally impose on 12 AWG copper — and that is legal, because 240.4(G) sends motor-circuit conductors to Article 430 for their overcurrent protection instead of to 240.4(D). The overload relay, not the branch-circuit device, is what protects this conductor from overload.
3. Branch-circuit OCPD (inverse-time breaker, 430.52): 250% × 14.0 A = 35 A → standard 35 A breaker.
4. Overload (running) protection (430.32): nameplate FLC = 13.2 A, SF 1.15 → 125% × 13.2 = 16.5 A → use a 16 A (next lower) overload element.
5. Disconnecting means (430.102, 430.110): a disconnect in sight from both the motor and the driven machinery, rated not less than 115% of the table FLC (430.110(A)) — 1.15 × 14.0 = 16.1 A minimum, so a 30 A or 60 A disconnect switch with the required horsepower rating.

The same 14.0 A table value is used for the conductor (125%), OCPD (250%), and disconnect rating, while the 13.2 A nameplate is used only for the running overload (125%).