|NEC Article 240 - Overcurrent Protection
|By Mike Holt for EC&M Magazine
Extracted from Mike Holt's Book, "Understanding the 2002 NEC"
circuits and equipment from overcurrent involves more than simply selecting a
fuse or breaker from a table.
Article 240, divided into seven parts,
provides the requirements for selecting and installing Overcurrent Protection
Devices (OCPDs) (Figure 240-1). Before addressing those requirements, let's
review the basic concept. When current exceeds the rating of conductors or
equipment-due to overload, short circuit, or ground fault-you have overcurrent
[Article 100] (Figure 240-2). To protect conductors and equipment, you use
Note: Graphics are not contained in this newsletter.
OCPD protects equipment by opening when it detects an overload, short-circuit or
ground fault. Every piece of electrical equipment must have a short-circuit
current rating that permits the OCPD (for that equipment) to clear short
circuits or ground faults without extensive damage to the electrical components
of the circuit. [110.10]. You must also apply the other Articles referenced in
240.3, depending on the equipment you are protecting.
An OCPD protects a
circuit by opening when current reaches a value that will cause an excessive
temperature rise in conductors. The OCPD interrupting rating must be sufficient
for the maximum possible fault current available on the line-side terminals of
the equipment [110.9]. You'll find the standard ratings for fuses and fixed-trip
circuit breakers in 240.6 (Figure 240-10). The ampere rating of an adjustable
circuit breaker is the maximum possible long-time pickup current setting that is
permitted. A circuit breaker with restricted access to the adjusting means can
have an ampere rating equal to the long-time pickup current setting.
must protect conductors against overcurrent, based on the ampacities in 310.15.
As you might expect, this rule has exceptions. First, it does not pertain to
flexible cords, flexible cables, or fixture wires. Second, there's an extensive
list of "otherwise permitted" items in 240.4, Parts A through G-these include
fire pumps (A), tap conductors (F), and an extensive table of specific conductor
Don't confuse motor circuit protection
with motor overload protection. You'll find the requirements for both kinds of
motor-related protection in Article 430. You must protect motor circuit
conductors against short circuits and ground faults per 430.52 and 430.62
To size the branch-circuit conductor for a motor, refer to
Table 310.16, 430.22, and Table 430.150. To size the branch-circuit protection
for a motor, refer to 240.6(A), 430.52(C), and Table 430.150.
If you are
using motor controls, size and protect the motor control circuit conductors per
430.72. If you are using remote controls, protect your remote-control,
signaling, and power-limited circuit conductors against overcurrent per 725.23
You must install an OCPD in
series with each ungrounded conductor [240.20]. Circuit breakers must open all
ungrounded conductors of the circuit, unless the circuit meets one of the three
exception criteria detailed in 240.20(B):
Multiwire branch circuits
Single-phase line-to-line loads
Three-phase line-to-line loads.
Locate OCPDs so you prevent exposure to physical damage
[110.27(B)]. But also make the OCPDs readily accessible [240.24]. "Readily
accessible" means located so a person can reach the OCPD quickly without having
to climb over or remove obstacles (Figure 240-22). Supplementary OCPDs (often
used for luminaires, appliances, or for internal circuits and components of
equipment) do not need to be readily accessible [240.10] (Figure 240-23). OCPDs
located next to equipment they supply can be accessible by portable means (See
404.8(A) Ex. 2 and Figure 240-24).
Each occupant must have ready access
to all OCPDs protecting the conductors supplying that
Exception No. 1: Service and feeder OCPDs are not required to
be accessible to occupants of multiple-occupancy buildings r guest rooms of
hotels and motels, if electric maintenance is provided under continuous building
Exception No. 2: Branch-circuit OCPDs are not required to be
accessible to occupants of guest rooms of hotels and motels, if electric
maintenance is provided in a facility under continuous building
Be aware of two location restrictions:
OCPDs near easily ignitable material. For example, keep them out of clothes
closets (Figure 240-26).
Don't locate OCPDs in the bathrooms of dwelling
units or guest rooms of hotels or motels (Figure 240-27). Note that 230.70(A)(2)
prohibits locating the service disconnect in any bathroom.
Fuse requirements are in 240.50 through 240.61. Here,
we want to look at circuit breakers. All circuit breakers must meet the
Must be capable of being opened and closed by hand.
Nonmanual means of operating a circuit breaker-e.g., electrical (shunt rip) or
pneumatic operation-are permitted if the breaker can also be operated manually
Must clearly indicate whether they are in the open "off" or closed
"on" position. When the handle of a circuit breaker is operated vertically, the
"up" position of the handle is the "on" position. See 240.33, 404.6(C), and
Require an interrupting rating of 5,000A-unless marked
Must be marked with a voltage rating that corresponds
with their interrupting rating [240.85].
Ensure the breaker has sufficient
interrupting circuit rating for the available fault current. A breaker with
inadequate interrupting current rating could allow equipment destruction from a
line-to-line or line-to-ground fault-plus serious injury or even death. See
110.9 and Figure 240-33.
Circuit breakers used to switch 120V or 277V
fluorescent lighting circuits must be listed and marked "SWD" or "HID." Circuit
breakers used to switch high-intensity discharge lighting circuits must be
listed and marked "HID." UL 489 (Standard on Molded Case Circuit Breakers)
permits "HID" breakers to be rated up to 50A, whereas an "SWD" breaker is rated
up to 20A. The tests for "HID" breakers include an endurance test at 75 percent
power factor, whereas "SWD" breakers are endurance-tested at 100 percent power
factor. The contacts and the spring of an "HID" breaker are heavier duty, to
dissipate the increased heat from greater current flow during the HID ignition
You can use a breaker with a straight voltage rating (e.g., 240V)
on a circuit where the nominal voltage between any two conductors does not
exceed the voltage rating of the breaker. You can use a breaker with a slash
rating (e.g., 480Y/277V) on a solidly grounded circuit where the nominal voltage
of any one conductor to ground does not exceed the lower of the two values and
the nominal voltage between any two conductors does not exceed the higher value
[240.85]. You cannot use a 120/240V slash circuit breaker on the high-leg of a
solidly grounded 120/240V delta system, because the phase-to-ground voltage of
the high-leg is 208V-this exceeds the 120 line-to-ground voltage rating (Figure
Some key things to remember about OCPDs:
The purpose of
an OCPD is to protect conductors and equipment by opening the circuit.
OCPD protects against excess heat generated by overcurrent from such causes as
overload, short circuit, or ground fault.
Size OCPDs to keep conductors from
exceeding their ampacities.
Article 240 contains a listing of other Articles
you must apply, depending on the equipment you are protecting.
is full of exceptions and references to other Articles. So, be sure to start
with the basic concepts in mind and then read each of the seven sections as it
applies to your application.
Copyright © 2002 Mike Holt Enterprises,Inc.