National Electrical Code Top Ten Tips:
Article 300 -- Wiring Methods
by Mark Lamendola
Please note, we do quote from copyrighted material. While the NFPA
does allow such quotes, it does so only for the purposes of education
regarding the National Electrical Code. This article is not a substitute
for the NEC.
These are the 10 NEC Article 300 items we deem most important, based
on the pervasiveness of confusion and the potential costs of same.
The language throughout NEC Article 300 misplaces the word
"only," but the meaning where this is misused is still clear
if you interpolate just a bit.
- NEC Article 300.3 addresses conductors and enclosures. (B)
requires all conductors of the same circuit to be in the same
wireway. One reason for this is the basic physics involved when the
electromagnetic fields of conductors interact. Many other reasons
make this requirement of great practical value.
An exception to it
does exist, and there are sometimes practical reasons for taking
advantage of that exception. (C) allows conductors at or below 600V
to be mixed in the same enclosure, cable, or raceway regardless of
their voltage. That mixing is safe from the standpoint of the NEC,
but the more of it you do the higher your risk of misoperation and
other problems. Good engineering practice demands separating wiring
systems as much as is practical. Thus, you would run 5V signal wires
in one wireway, 120V control wires in another, and 480V power in yet
another—even though you might terminate them all in one control
cabinet. Even inside the cabinet, you want to route and bundle the
wires so as to maintain the maximum separation that is reasonably
attainable. Motor drive power and output wiring deserves extra
attention in this regard.
- NEC Article 300.4 addresses protection against physical damage.
Many folks who run nonmetallic-sheathed cables (e.g., Romex) don’t
consider adding protection. In residential applications, this is
usually unnecessary, but a hole drilled off-center could easily
leave the wiring susceptible to puncture from a nail or screw driven
to support shelving, cabinets, or other wall-mounted objects.
- NEC Table 300.5 provides the minimum cover requirements for buried
cable of 0 to 600V.
- NEC Article 300.8. Raceways or cable trays containing electrical
conductors cannot contain elements of other systems—no water
pipes, gas pipes, or any other non-electrical system elements can
run in those electrical wireways. The intent of the NEC Article
300.8 also means, for example, running Romex through an A/C duct is
a Code violation.
- NEC Article 300.11 addresses the issues of securing and
supporting. Cables and raceways must have their own support—independent
of other systems. Their supporting structures cannot be piggybacked
onto other supports. For example, you can’t hang conduit from
ceiling grids, but you can clamp to the I-beams or rafters to hang
rod and strut specifically for the conduit.
(C) prohibits using wireways to support other wireways, cables, or non-electrical
equipment. Thus, using cable ties to secure the wiring for that new
PA system to conduit is a Code violation. A chief concern of NEC
Article 300.11 is that electrical wireways be independent. They may
share a support—for example, you can strap multiple conduits to a
strut suspended by two rods. But, you cannot then strap a strut to
those conduits and hang a secondary set of rods to support another
set of conduit or anything else.
- NEC Article 300.12 requires mechanical continuity of raceways and
cable sheaths.
- NEC Article 300.13 requires mechanical and electrical continuity
for conductors in raceways. In other words, you cannot have a splice
in a raceway (but you can have it in a box or conduit body that has
an accessible cover). NEC Article 300.5 does allow splices in
direct-buried conductors. That’s because you can use instruments
to locate the splices and you can excavate to get to them. However,
it’s much more difficult to do maintenance and inspection on
conductors that are in raceways.
- NEC Article 300.15 explains the exceptions noted in our comments
in the preceding item, and it addresses similar issues in 13
subheadings.
- NEC Article 300.19 and NEC Table 300.19(A) provide specifics on
conductor and raceway supports.
- NEC Article 300.20 requires conductors to be grouped together to
reduce heating (this takes advantage of magnetic field interaction
and cancellation). It contains two exceptions. (B) prescribes a
technique few people know about. In fact, when this appeared in
EC&M Magazine, many readers thought it was a hoax. It is not.
The technique involves cutting cooling slots in the holes through
which a single conductor passes. This item has an exception and an
FPN. It is worth becoming familiar with.
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