At first, it seems odd to have a provision for CTs in Article 110—especially
when there is a specific article covering
transformers. But as stated in Exception
No. 1 under 450.1, installations of CTs
are not covered in Article 450. Since
Article 450 does not cover CTs, the best
article for this provision is Article 110.
As stated in 110.23, unused CTs associated with potentially energized circuits
shall be short-circuited. Figure 1 shows
a CT with an ammeter in the secondary
circuit. This is a permitted installation
because the ammeter is the load and
the secondary circuit is not open (see
In the electrical world, the result of
having a short circuit is usually bad. But
when it comes to CTs, having an open
secondary circuit while the transformer
is energized can be dangerous. It is pos-
sible to have high voltage produced on the
secondary side of a CT if the secondary
terminals or conductors are not short-cir-
cuited. This potential can be dangerous to
personnel and can also damage the CT. In
Figure 2, the ammeter was removed with-
out secondary-circuit conductors being
short-circuited. As long as there is current
flowing through the conductor (primary)
passing through the CT, there will be a
voltage potential on the secondary-circuit
conductors. Because the ammeter was
removed and the secondary-circuit con-
ductors have not been short-circuited,
this is a Code violation (see Figure 2).
When an ammeter needs to be disconnected and removed from a CT, make
sure to short-circuit the CT’s secondary-circuit conductors first. For example, an
ammeter supplied by secondary-circuit
conductors of a CT needs to be replaced.
A jumper is installed to short-circuit the
secondary-circuit conductors of the CT
before the ammeter is removed from
the circuit. After the secondary-circuit
conductors have been short-circuited,
it is safe to disconnect and remove the
ammeter (see Figure 3).
In installations where CTs are used,
shorting blocks are often installed.
Shorting blocks help make the task of
short-circuiting the secondary conductors of a CT easy and safe.
The next section in Article 110 pertains to available fault current at the
service. As stated in 110.24(A), service
equipment in other than dwelling units
shall be legibly marked in the field with
the maximum available fault current.
Except for dwellings (single-, two- and
multifamily), service equipment shall be
marked to show the maximum available
fault current at that point in the electrical system. Knowing the maximum
available fault current is important for
those responsible for designing, installing, inspecting and maintaining that
particular electrical system.
To determine the maximum available
fault current at the service equipment, a
calculation must be performed. This section says the field markings shall include
the date the fault-current calculation was
performed and be of sufficient durability
to withstand the environment involved.
While this section says the sign or label
must be able to endure the environment, ILL
COD E IN F O C U S BY CHARLES R. MILLER
THE SECOND ARTICLE in the National Electrical Code (NEC), Article 100, contains
definitions that are essential to the proper Code application. Article 110 contains
many requirements that are essential to understanding and applying requirements
located throughout the Code. Some of the subjects that Article 110 covers are mentioned numerous times throughout the NEC. The NEC does not mention other
subjects as often, such as current transformers (CTs).
Requirements, Part XV
FIGURE 1 CURRENT TRANSFORMER WITH AN AMMETER
This is a permitted installation because the ammeter is the load; therefore, the
secondary circuit is not open.
Unused C Ts associated with potentially energized circuits shall be short-circuited (110.23).
C T with an ammeter in the secondary circuit