fwiw, there are twist on wire nuts rated for underground use. I watched in
amazement when an electrician used them in an outdoor lighting application,
where the ground was always wet.
Then there's this: with pumps that are submerged in a well, I had seen
underwater electrical connections protected with multiple layers of electrical
The NEC requires that wire connectors or splicing means installed on
conductors for direct burial shall be listed for such use.
Here are some of the rules:
110.14 (B) Splices. Conductors shall be spliced or joined with splicing
devices identified for the use or by brazing, welding, or soldering with a
fusible metal or alloy. Soldered splices shall first be spliced or joined so
as to be mechanically and electrically secure without solder and then be
soldered. All splices and joints and the free ends of conductors shall be
covered with an insulation equivalent to that of the conductors or with an
insulating device identified for the purpose.
Wire connectors or splicing means installed on conductors for direct burial
shall be listed for such use.
300.5 (C) Underground Cables Under Buildings. Underground cable installed
under a building shall be in a raceway that is extended beyond the outside
walls of the building.
(D) Protection from Damage. Direct-buried conductors and cables shall be
protected from damage in accordance with (1) through (5).
(1) Emerging from Grade. Direct-buried conductors and enclosures emerging
from grade shall be protected by enclosures or raceways extending from the
minimum cover distance required by 300.5(A) below grade to a point at least
2.5 m (8 ft) above finished grade. In no case shall the protection be
required to exceed 450 mm (18 in.) below finished grade.
(2) Conductors Entering Buildings. Conductors entering a building shall be
protected to the point of entrance.
(3) Service Conductors. Underground service conductors that are not encased
in concrete and that are buried 450 mm (18 in.) or more below grade shall
have their location identified by a warning ribbon that is placed in the
trench at least 300 mm (12 in.) above the underground installation.
(4) Enclosure or Raceway Damage. Where the enclosure or raceway is subject
to physical damage, the conductors shall be installed in rigid metal
conduit, intermediate metal conduit, Schedule 80 rigid nonmetallic conduit,
(5) Listing. Cables and insulated conductors installed in enclosures or
raceways in underground installations shall be listed for use in wet
(E) Splices and Taps. Direct-buried conductors or cables shall be permitted
to be spliced or tapped without the use of splice boxes. The splices or taps
shall be made in accordance with 110.14(B).
(F) Backfill. Backfill that contains large rocks, paving materials, cinders,
large or sharply angular substances, or corrosive material shall not be
placed in an excavation where materials may damage raceways, cables, or
other substructures or prevent adequate compaction of fill or contribute to
corrosion of raceways, cables, or other substructures.
Where necessary to prevent physical damage to the raceway or cable,
protection shall be provided in the form of granular or selected material,
suitable running boards, suitable sleeves, or other approved means.
(G) Raceway Seals. Conduits or raceways through which moisture may contact
energized live parts shall be sealed or plugged at either or both ends.
FPN: Presence of hazardous gases or vapors may also necessitate sealing of
underground conduits or raceways entering buildings.
(H) Bushing. A bushing, or terminal fitting, with an integral bushed opening
shall be used at the end of a conduit or other raceway that terminates
underground where the conductors or cables emerge as a direct burial wiring
method. A seal incorporating the physical protection characteristics of a
bushing shall be permitted to be used in lieu of a bushing.
(I) Conductors of the Same Circuit. All conductors of the same circuit and,
where used, the grounded conductor and all equipment grounding conductors
shall be installed in the same raceway or cable or shall be installed in
close proximity in the same trench.
Exception No. 1: Conductors in parallel in raceways or cables shall be
permitted, but each raceway or cable shall contain all conductors of the
same circuit including grounding conductors.
Exception No. 2: Isolated phase, polarity, grounded conductor, and
equipment grounding and bonding conductor installations shall be permitted
in nonmetallic raceways or cables with a nonmetallic covering or nonmagnetic
sheath in close proximity where conductors are paralleled as permitted in
310.4, and where the conditions of 300.20(B) are met.
(J) Ground Movement. Where direct-buried conductors, raceways, or cables are
subject to movement by settlement or frost, direct-buried conductors,
raceways, or cables shall be arranged to prevent damage to the enclosed
conductors or to equipment connected to the raceways.
FPN: This section recognizes "S" loops in underground direct burial to
raceway transitions, expansion fittings in raceway risers to fixed
equipment, and, generally, the provision of flexible connections to
equipment subject to settlement or frost heaves.
(K) Directional Boring. Cables or raceways installed using directional
boring equipment shall be approved for the purpose.
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