National Electrical Code Top Ten Tips: Article 665 -- Induction and Dielectric Equipment
When people from outlying suburbs or rural areas first visit big cities in
the American north during the winter, they are often surprised to see many
sidewalks have no ice. In Chicago, where ice is a serious winter problem, there
are plenty of electrically-heated sidewalks. That type of electrical heating is
covered by Article 426, not Article 665.
- Article 665 pertains to industrial and scientific applications. It
excludes medical, dental, and other applications [665.1].
- You can't install this equipment in a hazardous location, unless it and
the associated wiring are approved for that particular classification of
hazardous location [665.4].
- The output circuit can be isolated from ground [665.5]. This does not
mean ungrounded. Nor does it mean driving a separate ground rod. Consult
IEEE-142 (the Green Book) for more information on isolated grounding.
- If your system has multiple control points, these must be interlocked
such that the system can be operated from only one of these at a time
- If you provide a foot switch, it must have a cover to prevent accidental
- When determining conductor ampacity for multiple pieces of equipment,
add up the nameplate ratings of the largest group of equipment capable of
simultaneous operation. Then add to that the standby currents of the
remaining machines (use the full nameplate rating, if the standby rating
isn't on the nameplate. This is your minimum ampacity [665.10(A)].
- The rules for at disconnect for this kind of equipment are similar to those for motor
disconnects. For example, you must install a disconnect within sight of the equipment and it
must be capable of being locked in the open position
- Ensure the interconnection components are guarded [665.19]. Generally,
the suitable guards for this purpose are factory-provided along with the
equipment, but sometimes removed or not installed. Ensure they are installed
per factory requirements.
- All control panels shall be of deadfront construction [665.21].
- The "grounding and bonding" requirements are in 665.26, and they are
brief. As this subsection says, conform to Article 250, Parts II and V. But
also, see the Article 100 definitions of grounding and bonding. Note that
Part V of Article 250 requires that you create a metallic path, which is
If you ground where you should bond, you will leave
hazardous differences of potential that will probably get somebody killed.
The NEC still has not fixed this persistent language deficiency, but now
that you know what it really means you can provide a safe installation.