Overcurrent Protection – Part Five


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:

  1. Branch-Circuit Conductors. 
  2. Feeder Taps. 
  3. Transformer Secondary Conductors. 
  4. Service Conductors. 
  5. Busway Taps. 
  6. Motor Circuit Taps. 
  7. Conductors from Generator Terminals. 
  8. 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:

  1. Protection by Primary Overcurrent Device.
  2. Transformer Secondary Conductors Not over 3 m (10 ft) Long.
  3. Industrial Installation Secondary Conductors Not over 7.5 m (25 ft) Long.
  4. Outside Secondary Conductors.
  5. Secondary Conductors from a Feeder Tapped Transformer.
  6. Secondary Conductors Not over 7.5 m (25 ft) Long.

 Conditions for application of each case are summarized in the below table.





Cases for Conductors permitted to be connected to Transformer Secondary without overcurrent protection at the secondary
Conditions of case application
1- Protection by Primary Overcurrent Device.
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.

2- Transformer Secondary Conductors Not over 3 m (10 ft) Long.
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.

3- Industrial Installation Secondary Conductors Not over 7.5 m (25 ft) Long.
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.

4- Outside Secondary Conductors.
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.

5- Secondary Conductors from a Feeder Tapped Transformer.

Transformer secondary conductors installed in accordance with 240.21(B)(3) shall be permitted to have overcurrent protection as specified in that section.

6- Secondary Conductors Not over 7.5 m (25 ft) Long.
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.







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.






5- Busway Taps

Busways and busway taps shall be permitted to be protected against overcurrent in accordance with 368.17.






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.






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.







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.






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:

  1. Busways, as provided in 368.17(C),
  2. Supplementary OCPDs as described in 240.10 (see fig.1),
  3. OCPDs described in 225.40 and 230.92,
  4. OCPDs located next to equipment, if accessible by portable means.






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:

  • Multiple-occupancy buildings
  • Guest rooms or guest suites


(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.






3- Additional Notes

OCPDs must not be:
  1. Exposed to physical damage (see fig.2),                                                 
  2. Located near easily ignitable material,
  3. Located in the bathrooms of dwelling units, dormitories, or guest rooms/guest suites of hotels or motels,
  4. Located over stairway steps.







Fig (2): providing physical protection to electrical equipment by the use of bollard posts. 



4- General Notes

  • To protect Overcurrent protection Devices (OCPDs) from physical damage, overcurrent devices can be installed in enclosures, cabinets, cutout boxes or equipment assemblies. They can also be installed in panelboards or control boards that are in rooms or enclosures free from dampness, easily ignitable material and accessible only to qualified personnel. Also, the operating handle shall be accessible without opening the door or cover.

  • Fuses and circuit breakers should be located or shielded so that people will not be burned or otherwise injured by their operation. Handles or levers of circuit breakers and similar parts that may move suddenly in such a way that persons in the vicinity are likely to be injured by being struck by them shall be guarded or isolated.







In the next Article, I will explain Conductors Ampacity Calculations. Please, keep following.






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