Residential Electrical Inspector (CAE) Overcurrent Protection Lesson 18 Based on the 2014 OESC Updated: January 2017
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Residential Electrical Inspector (CAE)
Overcurrent Protection
Lesson 18
Based on the 2014 OESC
Updated: January 2017
Generally, Article 240 is used to determine how,
where and what size/type overcurrent protection is
required. Typically a circuit is protected at the source
with an overcurrent device. There are some
exceptions when Section 240.21 is applied.
Circuit breakers are the most common OC device but
often in remodels or with some disconnects, fuses are
used. Article 240 requires that conductors be
protected to their ampacity, in other words, the
overcurrent protection and conductor ampacity need
to match. There are exceptions found in Table 240.3
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S Type Fuse
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Edison Base
Cartridge
Blade
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Article 100 defines
overcurrent as: Any current in
excess of the rated current of
equipment or the ampacity of
a conductor. It may result
from overload, short circuit,
or ground fault.
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Article 100 Overcurrent Protective Device,
Branch-Circuit.
A device capable of providing protection for service,
feeder, and branch circuits and equipment over the
full range of over-currents between its rated current
and its interrupting rating. Branch-circuit
overcurrent protective devices are provided with
interrupting ratings appropriate for the intended use
but no less than 5000 amperes.
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Article 100 Overcurrent Protective Device,
Supplementary.
A device intended to provide limited overcurrent
protection for specific applications and utilization
equipment such as luminaires and appliances. This
limited protection is in addition to the protection
provided in the required branch circuit by the
branch-circuit overcurrent protective device.
Authors Note: An example is a furnace with circuit
breakers or an HVAC unit with fusing in the
disconnect
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Article 100 Overload.
Operation of equipment in excess of normal, full-
load rating, or of a conductor in excess of rated
ampacity that, when it persists for a sufficient length
of time, would cause damage or dangerous
overheating. A fault, such as a short circuit or ground
fault, is not an overload.
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0 20 0
An overload means loading a circuit beyond it’s overcurrent
rating resulting in the overcurrent device tripping.
As the load increases the conductors heat up. If the
overcurrent device was not installed or oversized and the
load kept increasing, eventually the conductor outer
insulation could melt and burn.
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Table 240.3 sends the
installer to other code
sections that may modify the
general requirements of
Article 240.
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210.20 Overcurrent Protection
Branch-circuit conductors and equipment shall be
protected by overcurrent protective devices that have
a rating or setting that complies with 210.20(A)
through (D).
(A) Continuous and Non-continuous Loads.
Where a branch circuit supplies continuous loads or
any combination of continuous and non-continuous
loads, the rating of the overcurrent device shall not
be less than the non- continuous load plus 125
percent of the continuous load.
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215.3 Overcurrent Protection
Feeders shall be protected against overcurrent in
accordance with the provisions of Part I of Article
240. Where a feeder supplies continuous loads or
any combination of continuous and non-continuous
loads, the rating of the overcurrent device shall not
be less than the non-continuous load plus 125
percent of the continuous load.
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240.4(G)
It is not uncommon to
see an HVAC unit with
an ampacity that does
not match a standard
fuse or breaker size or
sets a conductor size
smaller than typical for
an overcurrent device
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240.6 Standard Ampere Ratings
(A) Fuses and Fixed-Trip Circuit Breakers.
The standard ampere ratings for fuses and inverse
time circuit breakers shall be considered 15, 20, 25,
30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150,
175, 200, 225, 250, 300, 350, 400,
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240.10 Supplementary Overcurrent Protection
Where supplementary overcurrent protection is
used for luminaires, appliances, and other
equipment or for internal circuits and components
of equipment, it shall not be used as a substitute for
required branch-circuit overcurrent devices or in
place of the required branch-circuit protection.
Supplementary overcurrent devices shall not be
required to be readily accessible.
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An example of
supplementary
overcurrent
protection
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240.15 Ungrounded Conductors
(A) Overcurrent Device Required. A fuse or an
overcurrent trip unit of a circuit breaker shall be
connected in series with each ungrounded conductor.
A combination of a current transformer and
overcurrent relay shall be considered equivalent
to an overcurrent trip unit.
240.21 Location in Circuit
Overcurrent protection shall be provided in each
ungrounded circuit conductor and shall be located at
the point where the conductors receive their supply.
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240.24 Location in or on Premises
(A) Accessibility. Overcurrent devices shall be
readily accessible and shall be installed so that the
center of the grip of the operating handle of the
switch or circuit breaker, when in its highest position,
is not more than (6 ft. 7 in.) above
the floor or working platform.
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No higher than 6’ 7”
above the floor or
platform.
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Mounting Service Upside Down
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240.81 Indicating: Where circuit breaker handles
are operated vertically rather than rotationally or
horizontally, the “up” position of the handle shall
be in the “on” position.
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240.24(C) Not Exposed to Physical Damage.
Overcurrent devices shall be located where they will
not be exposed to physical damage.
Informational Note: See 110.11, Deteriorating
Agents.
240.24(D) Not in Vicinity of Easily Ignitable
Material. Overcurrent devices shall not be located in
the vicinity of easily ignitable material, such as in
clothes closets.
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240.24(E) Not Located in Bathrooms. In dwelling
units, dormitories, and guest rooms or guest suites,
overcurrent devices, other than supplementary
overcurrent protection, shall not be located in
bathrooms.
240.24(F) Not Located over Steps. Overcurrent
devices shall not be located over steps of a stairway.
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Homework Review Article 240
Review Section 310.104
Review Section 225.30
Review Section 225.39
Review Section 250.32
Lesson 18 Questions will be sent to you
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