Industrial machinery

The second CNC mill arrived on a flatbed in a crate taller than Tyler. Unlike the compressor or the saws, it came as a complete machine: spindle motor, axis drives, coolant pump, chip conveyor, way-lube pump, control computer and lighting, all wired inside its own cabinet by the builder. Dana's job was not to size each motor; the machine's builder had done that. Her job was to read the machine's nameplate, bring it a properly sized supply circuit with a disconnect and short-circuit protection, and confirm the machine could survive the fault current available at its location. That is the subject of Article 670, one of the shortest articles in the code and one of the most practical.

This lesson covers the scope of Article 670 and how it relates to NFPA 79, the nameplate information a machine must carry, sizing the supply conductors, overcurrent protection, the disconnecting means, short-circuit current ratings and the available fault current, and where Article 430 still applies on a machine job.

Scope: where the NEC stops and NFPA 79 begins

Article 670 covers the definition of industrial machinery, the nameplate data required, and the size and overcurrent protection of supply conductors to the machine. Article 100 defines industrial machinery as a power-driven machine, not portable by hand while working, used to process material by cutting, forming, pressure, electrical, thermal or optical techniques, lamination, or a combination of these, including associated equipment used to transfer material or tooling and to inspect, test or package. The wiring inside the machine, from the supply terminals onward, is governed by NFPA 79, which the machine builder follows. The electrician installing the machine follows the NEC up to the machine's supply terminals.

The machine nameplate

Section 670.3(A) requires a permanent nameplate on each machine with the information needed to supply it safely. In general terms the nameplate lists the supply voltage, number of phases, frequency and full-load current; the maximum ampere rating of the short-circuit and ground-fault protective device; the ampere rating of the largest motor, from the motor nameplate, or the largest load; the short-circuit current rating of the machine's industrial control panel; and the electrical diagram number or the number of the index to the diagrams. The full-load current on the nameplate is not less than the sum of the full-load currents of all motors and other equipment that may be in operation at the same time under normal conditions of use.

The CNC mill's nameplate, as invented for this course, reads: 480 V, 3 phase, 60 Hz; full-load current 52 A; largest motor 22 A; maximum short-circuit protective device 80 A; short-circuit current rating 18 kA; diagram number EL-4471.

Supply conductors

Section 670.4(A) sets the minimum ampacity of the supply conductors. In general terms, the conductors must have an ampacity of not less than 125% of the full-load current of all resistance heating loads, plus 125% of the full-load current of the highest-rated motor, plus the sum of the full-load currents of all other connected motors and apparatus that may be in operation at the same time, based on their duty cycle. In practice, the nameplate full-load current already sums the loads, so the calculation adds an extra 25% of the largest motor to the nameplate total.

Where a machine includes resistance heating, such as a plastic molding machine with barrel heaters, the heating load is also taken at 125%. Exam questions on Article 670 often give the nameplate full-load current and the largest motor, and expect exactly this calculation.

Overcurrent protection

Section 670.4(C) requires the supply conductors to be protected by an overcurrent device, and the machine's nameplate states the maximum rating of the short-circuit and ground-fault protective device. That marked value is a limit: the supply breaker or fuses may not exceed it. Where the machine is supplied by a branch circuit, the protection must also comply with the branch-circuit rules, and where the machine has a single motor, the motor rules of Article 430 can apply to the supply. When the machine includes its own overcurrent device for the supply conductors at the machine, 670.4(C) treats the supply conductors as a feeder protected by that device under the stated conditions. On exams, the most common question asks which value limits the supply overcurrent device, and the answer is the nameplate maximum.

The disconnecting means

Section 670.4(B) requires a disconnecting means for the machine, and it may be the machine's own supply circuit disconnect, mounted on the machine, as NFPA 79 normally requires. A machine supplied by more than one source needs a disconnect for each, with markings. Many machines arrive with an integral through-door disconnect handle, lockable in the off position. If the machine has one, the installer usually adds a separate disconnect only when local rules or the facility's lockout program require it, or when the supply circuit length and route make a disconnect in sight useful. Article 430's in-sight rules still apply to any separately installed motor outside the machine.

Short-circuit current ratings

Section 670.5 addresses the machine's short-circuit current rating. Industrial machinery must not be installed where the available fault current exceeds its short-circuit current rating, and the available fault current at the machine's supply terminals must be documented and available to the AHJ. This is one of the most practical rules in the article: a machine's control panel may be rated only 5 kA or 10 kA, while the fault current at a machine near a large transformer can be 30 kA or more. A machine installed beyond its rating can explode its control components during a fault.

If the available fault current had exceeded the rating, the options would be a current-limiting device ahead of the machine selected to bring the let-through current within the rating under the manufacturer's guidance, more conductor length or a transformer to add impedance, or a machine with a higher rating. Similar rules apply to industrial control panels under Article 409, air-conditioning equipment, and other equipment marked with a short-circuit current rating.

Where Article 430 still applies

Article 670 does not replace Article 430 for every motor on a machine job. Motors installed separately from the machine, such as a remote coolant chiller pump or a central dust collector, follow Article 430 in full. A feeder serving several machines follows the feeder rules, combining each machine's nameplate data as the motor feeder rules of 430.24 and 430.62 suggest. Machines that are really a single motor with a simple starter may be treated under Article 430 directly. On exams, read whether the question calls the equipment industrial machinery with a nameplate or a separately installed motor.

def machine_supply(fla_total, largest_motor, heating=0.0):
    return fla_total + 0.25 * largest_motor + 0.25 * heating

print("CNC mill supply conductors:", machine_supply(52, 22), "A")
print("Molding machine:", machine_supply(70, 30, heating=24), "A")
print("Feeder to both mills:", machine_supply(52 + 38, max(22, 18)), "A")

available_ka, sccr_ka = 14.2, 18
print("Available", available_ka, "kA vs SCCR", sccr_ka, "kA:", "OK" if available_ka <= sccr_ka else "exceeds rating")

CNC mill supply conductors: 57.5 A
Molding machine: 83.5 A
Feeder to both mills: 95.5 A
Available 14.2 kA vs SCCR 18 kA: OK

Commissioning a machine

Connecting the supply is not the end of the job. Before the machine builder's technician starts the mill, the electrician confirms a short list of items, and exam questions sometimes ask about them because they are where field mistakes happen.

• Voltage at the supply terminals is within the machine's rated tolerance, with the machine's transformer taps set correctly if it has one.
• Phase rotation matches the machine's requirement, checked with a rotation meter, so pumps and spindles turn the right way.
• The equipment grounding conductor is connected to the machine's protective bonding terminal, often marked PE, and its size suits the supply overcurrent device under Table 250.122.
• The supply disconnect locks in the off position and is labeled.
• The available fault current label or documentation is in place for the inspector.

On the Delgado mill, the rotation check caught a problem: the new feeder's phases were landed in the order C, B, A at the machine, so the coolant pump ran backward during the builder's first test. Swapping two supply conductors at the disconnect fixed it. Dana added a line to her standard checklist: confirm phase rotation before the machine's own startup test, not after.