By Mike Holt for EC&M Magazine

 

Here's the follow up to yesterday's newsletter. This includes all of the answers to the questions sent, so you can see how you did.

Note: These questions are based on the 2014 NEC®. Any underlined text indicates a change to the Code rule for the 2014 NEC.

 

Q1. What are the grounding and bonding requirements for Class I locations?

A1. Because of the explosive conditions associated with electrical installations in hazardous (classified) locations [500.5], electrical continuity of metal parts of equipment and raceways must be ensured regardless of the voltage of the circuit.

(A) Bonding. Locknuts aren't suitable for bonding purposes in hazardous (classified) locations, therefore bonding jumpers or other approved means of bonding must be used. Such means of bonding apply to all intervening raceways, fittings, boxes, enclosures, and so forth between Class I locations and service equipment [501.30].

Author's Comment:

     Regardless of the circuit voltage, electrical continuity of metal parts of equipment and raceways in hazardous (classified) locations must be ensured by bonding-type locknuts, bushings, wedges, or bushings with bonding jumpers [250.92(B)(4)]; whether or not equipment grounding conductors of the wire type are installed in the raceway [250.100].

     A separate equipment grounding conductor isn’t required if a metal raceway is used for equipment grounding. Threaded couplings and hubs made up wrenchtight provide a suitable low impedance path [250.100].

(B) Bonding—Flexible Raceway. Flexible metal conduit and liquidtight flexible metal conduit must have an equipment bonding jumper of the wire type installed in accordance with 250.102.

Author's Comment:

     Bonding jumpers are sized in accordance with Table 250.122, based on the rating of the overcurrent device [250.102(D)], and where installed outside of a raceway, the length of bonding jumpers must not exceed 6 ft and they must be routed with the raceway [250.102(E)(2)].

 

Q2. What does the Code define as an Emergency System?

A2. Emergency Systems. Emergency power systems are those systems legally required and classed as emergency by a governmental agency having jurisdiction. These systems are intended to automatically supply illumination and/or power essential for safety to human life [700.2].

Note: Emergency power systems may also provide power to maintain life, fire detection and alarm systems, elevators, fire pumps, public safety communications systems, industrial processes where current interruption would produce serious life safety or health hazards, and similar functions.

 

Q3. What are the Code requirements for transfer equipment of Emergency Systems?

A3. Emergency System Capacity [700.4]

 (A) Capacity and Rating. An emergency power system must have adequate capacity to carry all emergency loads expected to operate simultaneously.

(B) Load Shedding. If an alternate power supply has adequate capacity, it’s permitted to supply emergency loads [Article 700], legally required standby loads [Article 701], and optional standby system loads [Article 702]. If the alternate power supply doesn’t have adequate capacity to carry the entire load, it must have automatic selective load pickup and load shedding to ensure adequate power in the following order of priority:

(1) The emergency circuits,

(2) The legally required standby circuits, and

(3) The optional standby circuits.

A temporary alternate source of power must be available whenever the emergency generator is out of service for more than a few hours for maintenance or repair.

Transfer Equipment [700.5]

 (A) General. Transfer equipment must be automatic, identified for emergency use, and approved by the authority having jurisdiction.

(C) Automatic Transfer Switches. Automatic transfer switches must be electrically operated, mechanically held, and listed for emergency power system use.

(D) Use. Transfer equipment must supply only emergency loads.

Author's Comment:

     Multiple transfer switches are required where a single generator is used to supply both emergency loads and other loads.

 

Q4. What locations are acceptable for the installation of overcurrent devices according to the NEC?

A4. Location of Overcurrent Devices [240.24]

 (A) Readily Accessible. Circuit breakers and fuses must be readily accessible, and they must be installed so the center of the grip of the operating handle of the fuse switch or circuit breaker, when in its highest position, isn’t more than 6 ft 7 in. above the floor or working platform, unless the installation is for:

(1) Busways, as provided in 368.17(C).

(2) Supplementary overcurrent devices aren’t required to be readily accessible [240.10].

(3) For overcurrent devices, as described in 225.40 and 230.92.

(4) Overcurrent devices located next to equipment can be mounted above 6 ft 7 in., if accessible by portable means [404.8(A) Ex 2].

(C) Not Exposed to Physical Damage. Overcurrent devices must not be exposed to physical damage.

Note: Electrical equipment must be suitable for the environment, and consideration must be given to the presence of corrosive gases, fumes, vapors, liquids, or chemicals that have a deteriorating effect on conductors or equipment [110.11].

(D) Not in Vicinity of Easily Ignitible Material. Overcurrent devices must not be located near easily ignitible material, such as in clothes closets.

(E) Not in Bathrooms. Overcurrent devices aren’t permitted to be located in the bathrooms of dwelling units, dormitories, or guest rooms or guest suites of hotels or motels.

Author's Comment:

     The service disconnecting means must not be located in a bathroom, even in commercial or industrial facilities [230.70(A)(2)].

(F) Over Steps. Overcurrent devices must not be located over the steps of a stairway.

Author’s Comment:

     Clearly, it’s difficult for electricians to safely work on electrical equipment that’s located on uneven surfaces such as over stairways.

 

Q5. What are the Code rules for sizing transformer secondary conductors?

A5.  (C) Transformer Secondary Conductors. A set of conductors supplying single or separate loads is permitted to be connected to a transformer secondary without overcurrent protection in accordance with 240.21(C)(1) through (6).

The permission of the “next size up” protection rule when the conductor ampacity doesn’t correspond with the standard size overcurrent protection device of 240.4(B) doesn’t apply to transformer secondary conductors.

(1) Protection by Primary Overcurrent Device. The primary overcurrent device sized in accordance with 450.3(B) is considered suitable to protect the secondary conductors of a 2-wire (single-voltage) system, provided the primary overcurrent device doesn’t exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.

Question: What’s the minimum size secondary conductor required for a 2-wire, 480V to 120V transformer rated 1.50 kVA with 60ºC terminals? 

(a) 16 AWG    (b) 14 AWG     (c) 12 AWG   (d) 10 AWG

Answer: (c) 12 AWG

Primary Current = VA/E

VA = 1,500 VA

E = 480V

Primary Current = 1,500 VA/480V

Primary Current = 3.13A

Primary Protection [450.3(B)] = 3.13A x 1.67

Primary Protection [450.3(B)] = 5.22A or 5A Fuse

Secondary Current = 1,500 VA/120V

Secondary Current = 12.50A

Secondary Conductor = 12 AWG, rated 20A at 60ºC, 
 [Table 310.15(B)(16)]

The 5A primary overcurrent device can be used to protect 12 AWG secondary conductors because it doesn’t exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.

20A secondary conductor ampacity x 120V/480V ratio = 5A which is equal to or less than the primary protection

 

(2) 10 Ft Secondary Conductors. Secondary conductors can be run up to 10 ft without overcurrent protection if installed as follows:

(1) The ampacity of the secondary conductor must not be less than:

a.  The calculated load in accordance with Article 220, and

b.  The rating of the overcurrent device at the termination of the secondary conductors

Ex: Listed equipment, such as a surge protection device, can have their conductors sized in accordance with the manufacturer’s instructions.

(2) The secondary conductors must not extend beyond the switchboard, switchgear, panelboard, disconnecting means, or control devices they supply.

(3) The secondary conductors are enclosed in a raceway.

(4) Not less than 10 percent of the rating of the overcurrent device protecting the primary of the transformer, multiplied by the primary-
to-secondary transformer voltage ratio.

(4) Outside Secondary Conductors of Unlimited Length. Outside secondary conductors can be of unlimited length, without overcurrent protection at the point they receive their supply, if they’re installed as follows:

(1) The conductors are suitably protected from physical damage in a raceway or manner approved by the authority having jurisdiction.

(2) The conductors must terminate at a single circuit breaker or a single set of fuses that limit the load to the ampacity of the conductors.

(3) The overcurrent device for the ungrounded conductors is an integral part of a disconnecting means or it’s located immediately adjacent thereto.

(4) The disconnecting means is located at a readily accessible location that complies with one of the following:

a.  Outside of a building.

b.  Inside, nearest the point of entrance of the conductors.

c.  If 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 must be installed in accordance with 240.21(B)(3).

(6) 25-Foot Secondary Conductor. Secondary conductors can be run up to 25 ft without overcurrent protection if they comply with all of the following:

(1) The secondary conductors have an ampacity not less than the value of the primary-to-secondary voltage ratio multiplied by one-third of the rating of the overcurrent device that protects the primary of the transformer.

(2) Secondary conductors terminate in a single circuit breaker or set of fuses rated no more than the tap conductor ampacity in accordance with 310.15 [Table 310.15(B)(16)].

(3) The secondary conductors are protected from physical damage by being enclosed in a manner approved by the authority having jurisdiction, such as within a raceway.

(D) Service Conductors. Service conductors must be protected against overload in accordance with 230.90 and 91.

(H) Battery Conductors. Overcurrent protection is installed as close as practicable to the storage battery terminals.