A control circuit that is part of industrial machinery, even machinery listed as a whole, must meet any applicable grounding requirements.

We have received questions from three states regarding the applicability of the grounding rules in Art. 250 to control circuitry that is factory-supplied by manufacturers of very large equipment. The equipment in question comes with preassembled control panels with comparatively large control transformers, running in the 2 to 5kVA range.

One question specifically dealt with large plastic extrusion machinery that takes pelletized plastic material, melts, and then extrudes a huge bubble of plastic; the process controls the thickness of the resulting film that is made into trash bags and similar products. The bubble might be three stories tall, and the machinery includes many motors, heaters, blowers, etc. The supply to the control panel consisted of a 400A 480V 3-wire feed with a No. 3 equipment grounding conductor.

The control panel, as supplied, had a 2kVA 480V to 120V control transformer. The secondary was grounded, but to the equipment grounding terminal of the supply system. We have heard of other similar setups, but with the secondary ungrounded. In each case, the wiring arrangements have been defended on the basis that the wiring on the equipment is factory-supplied and beyond the scope of the Code.

The EC&M panel's response

It is untrue to suggest that the Code doesn't apply to construction features of manufactured equipment. If that were true, much of Art. 422 wouldn't exist.

The Code exists to fulfill its mandate in Sec. 90-1(a), the "practical safeguarding of persons and property from hazards arising from the use of electricity." To do so, it must place some limits on the construction features of equipment connected to electric systems. Within those policy limits, product standards provide a level playing field for manufacturers. The realm of the product standard is the set of criteria under which "examinations for safety made under standard conditions" are performed, as described in the first paragraph of Sec. 90-7.

Now we agree that testing laboratories need not enforce all of the field-wiring rules in the Code, where its reexamination process can accomplish the same objectives. A product standard requirement for maintained spacing from sharp edges might very well substitute for a field requirement for an insulating bushing. If the Code calls for a listed product, that is a product tested to such "appropriate designated standards" as defined in Art. 100.

In general, these standards are carefully drawn to follow the policy set by the Code. Code rules that govern the relationship between a product and the wiring system, as are at issue here, must be observed. This equipment uses Class 1 control circuits and power circuits that extend out from the control panel for great distances. The hazards of improperly grounded control circuits on such equipment cannot be addressed in its construction features.

A close look at the second paragraph of Sec. 90-7 shows that the scope of internal field inspections is limited, but not actually excluded or prevented. Nothing in any part of Sec. 90-7 says that internal wiring isn't subject to the NEC. To say otherwise would be to allow the writer of a product standard to effectively rewrite portions of the NEC to suit. In fact, qualified testing laboratories, such as UL that routinely write product standards, take pride in maintaining representation on Code-making panels so they can stay close to the reasoning behind Code requirements. Nevertheless, no system is perfect, and every now and then something shows up on the market that shouldn't. If there is clear and convincing evidence of NEC noncompliance, then the Authority Having Jurisdiction should reject the product as submitted and so advise the listing authority, but with an open mind. The listing authority may have goofed, but it may also have compelling reasons for its listing.

Years ago, the data processing industry tried many different approaches in misguided attempts to escape from the safety requirements in Art. 250. One of those attempts was to say that since their circuits were part of listed systems, when those circuits were run out from a power distribution unit they weren't actually premises wiring, and the Code didn't apply. They were completely wrong then, and we think that saying the wiring on this plastic extrusion equipment isn't premises wiring would be making the same mistake.

Industrial machinery has specific rules

Furthermore, the machinery in question is clearly industrial machinery as defined in Sec. 670-2. Although per Sec. 670-4(b) this equipment will "be considered as an individual unit and therefore shall be provided with a disconnecting means" that section also provides for such equipment to be fed with a feeder in such cases, and therefore its control panel is not the outlet.

Since the control panel isn't the outlet, the supply conductors to the various motors, heaters, etc. are branch-circuit conductors. Branch circuit conductors are premises wiring, and premises wiring is subject to any system grounding rules in Art. 250.


As covered in Sec. 250-5(d), a separately derived premises wiring system, "if required to be grounded, ... shall be grounded in accordance with Section 250-26." The 120V control circuit(s) fed by the two-winding control transformer are separately derived and require, per Sec. 250-5(b)(1), a system grounding connection in this case. Note, however, that if the supply is a transformer with a primary not over 1000V, there is an exception allowing ungrounded control circuits. The exception requires qualified maintenance and supervision and a showing that continuity of control power is required. In addition, ground detectors must be installed on the control system.

Otherwise, the control panel will need a means to attach a main bonding connection and a grounding electrode conductor to one side of the control circuit. This will be true any time the control circuit transformer exceeds 1kVA or supplies other than control or signaling circuits as provided in the Exceptions Nos. 1 to Sec. 250-26(a) and-26(b). Those exceptions still require system grounding, but allow for control circuit grounding to the supply equipment grounding conductor. If, as in this case, that threshold is exceeded, then the grounding electrode conductor must run to a fully qualified grounding electrode per Sec. 250-26(c). In this type of industrial location, that would probably be effectively grounded building steel or the nearest metal water piping.

Remember, a system grounding connection provides a local grounding reference for a system. In this case, the reference has nothing to do with the ability to return fault current, and its size (No. 8 minimum) has to do with mechanical permanence.

Be careful about assuming a listing. For example, if the control panel were put together in a listed shop covered by UL's reexamination service, it might well carry a listing without the overall equipment having been listed. However, even if the entire equipment were listed as described, it would still need to be wired and grounded in accordance with the NEC.