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will become brittle and difficult to work with in cold temperatures.
This is of particular concern in northern climates where
maintaining systems that have PVC conduit installed in exposed
areas is a challenge.
At substation equipment, a transition from a non-metallic
conduit to a metallic conduit is usually made. Type RMC, or rigid
metal conduit (generally galvanized steel), is used for the above-
grade conduit connection to substation equipment in the yard.
NEC Article 344 applies to rigid metal conduit.
In northern climates, another consideration for conduit
installations to equipment in the yard is frost. Frost depths there
can reach up to four feet deep. Heaving related to the frost can
cause conduit to push or pull equipment cabinets. The NEC
provides articles on the use of these types of installations.
A few examples that may help minimize the upward movement
of conduit into enclosures include using flexible conduit or
expansion fittings, and site grading practices or local foundation
correction (i.e. deep sand layer).
The design engineer should consult with the requirements for
frost depth in each state, as it can also impact the burial depth of
the conduit.
While the substation design engineer will usually select the
conduit type, routing, and sizing of the conduit used for the
substation equipment in the yard, the conduit used within the
electrical equipment enclosure will be selected and installed by the
electrician or electrical equipment enclosure manufacturer.
The type of conduit generally used for this application is
electrical metallic tubing (EMT).
EMT will generally be routed on the walls inside the electrical
equipment enclosure. When installed with proper fittings, it can be
used as the equipment grounding conductor. However, running a
separate equipment grounding conductor with the phase
conductor(s) to lights or receptacles within the electrical
equipment enclosure is preferred.
The typical design of a conduit system will include conduit
burial depth, bending radius, calculations on conduit fill, and
determination of cable ampacity de-rating. Each of these criteria is
covered in the NEC. Along with these design considerations,
installation considerations are also addressed by the NEC, such as
how the conduit is to be supported and secured.

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