In Article " Overcurrent Protection – Part One " which was an Introduction to Overcurrent Protection, I provide the basic information needed for best understanding of the Overcurrent protection.
Also, In Article " Overcurrent Protection – Part Two ", I answered the following questions:
- How to size the overcurrent protection devices (OCPDs)?
- How to select the proper overcurrent protection for certain applications?
Also, In Article " Overcurrent Protection – Part Three ", I answered the following questions:
- How to select the proper overcurrent protection devices (OCPDs) for certain application?
In Article " Overcurrent Protection – Part Four ", I answered the following questions:
- Where to locate the overcurrent protection devices (OCPDs)?
And I answer this question for the first two cases from the following cases:
- Branch-Circuit Conductors.
- Feeder Taps.
- Transformer Secondary Conductors.
- Service Conductors.
- Busway Taps.
- Motor Circuit Taps.
- Conductors from Generator Terminals.
- Battery Conductors.
Today, I will explain the proper locations for the overcurrent protection devices (OCPDs) for other cases in above.
3- Transformer Secondary Conductors
A set of conductors feeding a single load, or each set
of conductors feeding separate loads, shall be permitted to be connected to a
transformer secondary, without overcurrent protection at the secondary, as
specified in the following conditions:
Conditions for application of each case are summarized in the below table.
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Cases for Conductors permitted to be connected to
Transformer Secondary without overcurrent protection at the secondary
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Conditions of case application
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1- Protection by Primary Overcurrent Device.
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The conductors supplied by transformer secondary
shall be permitted to be
protected by overcurrent protection provided on the
primary (supply) side of the transformer with the following conditions:
Conductors supplied by the secondary side of a
single-phase transformer
having a 2-wire (single-voltage) secondary, or
A
three-phase, delta-delta connected transformer having a
3-wire (single-voltage) secondary,
Primary protection is in accordance with 450.3 and
does not exceed the value determined by multiplying the secondary conductor
ampacity by the secondary-to-primary transformer voltage ratio.
Single-phase (other than 2-wire) and multiphase
(other than delta-delta, 3-wire) transformer secondary conductors are not
considered to be protected by the primary overcurrent protective device.
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2- Transformer Secondary Conductors Not over 3 m (10
ft) Long.
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If the
length of secondary conductor does not exceed 3 m (10 ft) and complies with
all of the following:
(1) The
ampacity of the secondary conductors is :
a. Not
less than the combined calculated loads on the circuits supplied by the
secondary conductors, and
b. Not
less than the rating of the device supplied by the secondary conductors or
not less than the rating of the overcurrent-protective device at the
termination of the secondary conductors
(2) The
secondary conductors do not extend beyond the switchboard, panelboard,
disconnecting means, or control devices they supply.
(3) The
secondary conductors are enclosed in a raceway, which shall extend from the
transformer to the enclosure of an enclosed switchboard, panelboard, or
control devices or to the back of an open switchboard.
(4) For
field installations where the secondary conductors leave the enclosure or
vault in which the supply connection is made, the rating of the overcurrent
device protecting the primary of the transformer, multiplied by the primary
to secondary transformer voltage ratio, shall not exceed 10 times the
ampacity of the secondary conductor.
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3- Industrial Installation Secondary Conductors Not
over 7.5 m (25 ft) Long.
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For industrial installations only, where the length
of the secondary conductors does not exceed 7.5 m (25 ft) and complies with
all of the following:
(1) Conditions of maintenance and supervision ensure
that only qualified persons service the systems.
(2) The ampacity of the secondary conductors is not
less than the secondary current rating of the transformer, and the sum of the
ratings of the overcurrent devices does not exceed the ampacity of the
secondary conductors.
(3) All overcurrent devices are grouped.
(4) The secondary conductors are protected from
physical damage by being enclosed in an approved raceway or by other approved
means.
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4- Outside Secondary Conductors.
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Where
the conductors are located outdoors of a building or structure, except at the
point of load termination, and comply with all of the following
conditions:
(1) The
conductors are protected from physical damage in an approved manner.
(2) The
conductors terminate at a single circuit breaker or a single set of fuses
that limit the load to the ampacity of the conductors. This single
overcurrent device shall be permitted to supply any number of additional
overcurrent devices on its load side.
(3) The
overcurrent device for the conductors is an integral part of a disconnecting
means or shall be located immediately adjacent thereto.
(4) The
disconnecting means for the conductors is installed at a readily accessible
location complying with one of the following:
a.
Outside of a building or structure
b.
Inside, nearest the point of entrance of the conductors
c.
Where installed in accordance with 230.6, nearest the point of entrance of
the conductors.
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5- Secondary Conductors from a Feeder Tapped
Transformer.
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Transformer secondary conductors installed in
accordance with 240.21(B)(3) shall be permitted to have overcurrent
protection as specified in that section.
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6- Secondary Conductors Not over 7.5 m (25 ft) Long.
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Where
the length of secondary conductor does not exceed 7.5 m (25 ft) and complies
with all of the following:
(1) The
secondary conductors shall have an ampacity that is not less than the value
of the primary-to-secondary voltage ratio multiplied by one-third of the
rating of the overcurrent device protecting the primary of the transformer.
(2) The
secondary conductors terminate in a single circuit breaker or set of fuses
that limit the load current to not more than the conductor ampacity that is
permitted by 310.15.
(3) The
secondary conductors are protected from physical damage by being enclosed in
an approved raceway or by other approved means.
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Example#1:(240.21(C)(6):Secondary Conductors Not over 7.5 m (25 ft) Long)
Apply the 25 ft secondary conductor protection provisions of 240.21(C)(6) to a transformer rated 75 kVA, 3-phase, 480-volt primary to 208Y/120-volt secondary. The transformer primary overcurrent protective device is rated 125 amperes. Determine the minimum secondary conductor size for this installation.
Solution:
Step #1: Determine 1/10 of the primary OCPD rating using the following calculation:
125 A / 10 = 12.5 A
Step #2: Determine the line-to-line primary to secondary
Voltage ratio = 480 V/ 208 V = 2.31
Step #3: Minimum ampacity for ungrounded transformer secondary conductor:
12.5 A X 2.31 = 29 A
From Table 310.15(B)(16): 10 AWG copper THWN has 30 A from 60°C column A 10 AWG copper conductor is permitted to be tapped from the secondary of this transformer with primary overcurrent protection rated 125 amperes. The load supplied by this secondary conductor cannot exceed the conductor’s allowable ampacity from Table 310.15(B)(16) coordinated with the temperature rating of the conductor terminations in accordance with 110.14(C)(1)(a).
4- Service
Conductors
Service conductors shall be permitted to be protected by
overcurrent devices in accordance with 230.91.
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5- Busway
Taps
Busways and busway taps shall be permitted to be protected
against overcurrent in accordance with 368.17.
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6- Motor
Circuit Taps
Motor-feeder and branch-circuit conductors shall be permitted
to be protected against overcurrent in accordance with 430.28 and 430.53,
respectively.
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7- Conductors
from Generator Terminals
Conductors from generator terminals that meet the size
requirement in 445.13 shall be permitted to be protected against overload by
the generator overload protective device(s) required by 445.12.
Part of Article 445.13 states that the ampacity of the
conductors from the generator terminals to the first distribution device
containing overcurrent protection shall not be less than 115% of the
nameplate current rating of the generator. If the generator is rated 800
amps, then the conductors should be sized not less than 920 amps.
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8- Battery
Conductors
Overcurrent protection shall be permitted to be installed as
close as practicable to the storage battery terminals in an unclassified location.
Installation of the overcurrent protection within a hazardous (classified)
location shall also be permitted.
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Location of Overcurrent protection Devices (OCPDs) in or on Premises:
1- Accessibility
As per NEC Section 240.24(A), OCPDs must be readily accessible
and must be installed so the center of the grip of the operating handle, when
in its highest position, isn’t more than 6 ft, 7 in. above the floor or
working platform. The four exceptions to this height requirement are:
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| Fig (1) |
2- Occupancy
As per NEC Section 240.24(B), each occupant shall have ready
access to all overcurrent devices protecting the conductors supplying that
occupancy, unless otherwise permitted in the following cases:
(1) Service and Feeder Overcurrent Devices.
Where electric service and electrical maintenance are provided
by the building management and where these are under continuous building
management supervision, the service overcurrent devices and feeder
overcurrent devices supplying more than one occupancy shall be permitted to
be accessible only to authorized management personnel in the following:
(2) Branch-Circuit Overcurrent Devices.
Where electric service and electrical maintenance are provided
by the building management and where these are under continuous building
management supervision, the branch-circuit overcurrent devices supplying any
guest rooms or guest suites without permanent provisions for cooking shall be
permitted to be accessible only to authorized management personnel.
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3- Additional
Notes
OCPDs must not be:
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| Fig (2): providing physical protection to electrical equipment by the use of bollard posts. |
4- General
Notes
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In the next Article, I will explain Conductors Ampacity Calculations. Please, keep following.
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