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 ext indicates a change to the Code rule for the 2014 NEC.

Q1. What does the Code mean by “overcurrent”?
A1. Overcurrent exists when current exceeds the rating of equipment or the ampacity of a conductor, due to an overload, short circuit, or ground fault [Article 100].

Overload. An overload is a condition where equipment or conductors carry current exceeding their current rating [Article 100]. A fault, such as a short circuit or ground fault, isn’t an overload. An example of an overload is plugging two 12.50A (1,500W) hair dryers into a 20A branch circuit.

Short Circuit. A short circuit is the unintentional electrical connection between any two normally current-carrying conductors of an electrical circuit, either line-to-line or line-to-neutral.

Ground Fault. A ground fault is an unintentional, electrically conducting connection between an ungrounded conductor of an electrical circuit and the normally noncurrent-carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or the earth [Article 100]. During the period of a ground fault, dangerous voltages will be present on metal parts until the circuit overcurrent device opens.

Overcurrent devices protect conductors and equipment. Selecting the proper overcurrent protection for a specific circuit can become more complicated than it sounds. The general rule for overcurrent protection is that conductors must be protected in accordance with their ampacities at the point where they receive their supply [240.4 and 240.21]. There are many special cases that deviate from this basic rule, such as the overcurrent protection limitations for small conductors [240.4(D)] and the rules for specific conductor applications found in other articles, as listed in Table 240.4(G). There are also a number of rules allowing tap conductors in specific situations [240.21(B)]. Article 240 even has limits on where overcurrent devices are allowed to be located [240.24].

An overcurrent protection device must be capable of opening a circuit when an overcurrent situation occurs, and must also have an interrupting rating sufficient to avoid damage in fault conditions [110.9]. Carefully study the provisions of this article will help you provide sufficient overcurrent protection in the correct location.

Article 240 covers the general requirements for overcurrent protection and the installation requirements of overcurrent devices.

240.1 Informational Note: An overcurrent device protects the circuit by opening the device when the current reaches a value that’ll cause excessive or dangerous temperature rise (overheating) in conductors. Overcurrent devices must have an interrupting rating sufficient for the maximum possible fault current available on the line-side terminals of the equipment [110.9]. Electrical equipment must have a short-circuit current rating that permits the circuit’s overcurrent device to clear short circuits or ground faults without extensive damage to the circuit’s electrical components [110.10].

Q2. What is a current-limiting overcurrent device?
A2. An overcurrent device (typically a fast-acting fuse) that reduces the fault current to a magnitude substantially less than that obtainable in the same circuit if the current-limiting device wasn’t used. See 240.40 and 240.60(B) [240.2].

Author’s Comment:
A current-limiting fuse is a type of fuse designed for operations related to short circuits only. When a fuse operates in its current-limiting range, it’ll begin to melt in less than a quarter of a cycle, and it’ll open a bolted short circuit in less than half a cycle. This type of fuse limits the instantaneous peak let-through current to a value substantially less than what will occur in the same circuit if the fuse is replaced with a solid conductor of equal impedance. If the available short-circuit current exceeds the equipment/conductor short-circuit current rating, then the thermal and magnetic forces can cause the equipment circuit conductors, as well as the circuit equipment grounding conductors, to vaporize. The only solutions to the problem of excessive available fault current are to:

Install equipment with a higher short-circuit rating, or

Protect the components of the circuit by a current-limiting overcurrent device such as a fast-clearing fuse, which can reduce the let-through energy.

A breaker or a fuse does limit current, but it may not be listed as a current-limiting device. A thermal-magnetic circuit breaker typically clears fault current in less than three to five cycles when subjected to a short circuit or ground fault of 20 times its rating. A standard fuse will clear the same fault in less than one cycle and a current-limiting fuse in less than half of a cycle.

Q3. What are the Code rules for sizing overcurrent protection for conductors?
A3. Except as permitted by (A) through (G), conductors must be protected against overcurrent in accordance with their ampacity after ampacity correction and adjustment as specified in 310.15 [240.4].

(A) Power Loss Hazard. Conductor overload protection isn’t required, but short-circuit protection is required where the interruption of the circuit will create a hazard; such as in a material-handling electromagnet circuit or fire pump circuit.

(B) Overcurrent Devices Rated 800A or Less. The next higher standard rating of overcurrent device listed in 240.6 (above the ampacity of the ungrounded conductors being protected) is permitted, provided all of the following conditions are met:
(1) The conductors aren’t part of a branch circuit supplying more than one receptacle for cord-and-plug-connected loads.
(2) The ampacity of a conductor, after the application of ambient temperature correction [310.15(B)(2)(a)], conductor bundling adjustment [310.15(B)(3)(a)], or both, doesn’t correspond with the standard rating of a fuse or circuit breaker in 240.6(A).
(3) The overcurrent device rating doesn’t exceed 800A.

Author’s Comment:
This “next size up” rule doesn’t apply to feeder tap conductors [240.21(B)] or transformer secondary conductors [240.21(C)].

(C) Overcurrent Devices Rated Over 800A. If the circuit’s overcurrent device exceeds 800A, the conductor ampacity, after the application of ambient temperature correction [310.15(B)(2)(a)], conductor bundling adjustment [310.15(B)(3)(a)], or both, must have a rating of not less than the rating of the overcurrent device defined in 240.6.

(D) Small Conductors. Unless specifically permitted in 240.4(E) or (G), overcurrent protection must not exceed the following:
(1) 18 AWG Copper—7A
(2) 16 AWG Copper—10A
(3) 14 AWG Copper—15A
(4) 12 AWG Aluminum/Copper-Clad Aluminum—15A
(5) 12 AWG Copper—20A
(6) 10 AWG Aluminum/Copper-Clad Aluminum—25A
(7) 10 AWG Copper—30A

(E) Tap Conductors. Tap conductors must be protected against overcurrent as follows:
(1) Household Ranges and Cooking Appliances and Other Loads, 210.19(A)(3) and (4)
(2) Fixture Wire, 240.5(B)(2)
(3) Location in Circuit, 240.21
(4) Reduction in Ampacity Size of Busway, 368.17(B)
(5) Feeder or Branch Circuits (busway taps), 368.17(C)
(6) Single Motor Taps, 430.53(D)

(F) Transformer Secondary Conductors. 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 secondary conductor size 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 = 5.22A or 5A Fuse
Secondary Current = 1,500 VA/120V
Secondary Current = 12.50A
Secondary Conductor = 12 AWG, rated 20A at 60°C, [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.
5A< or = 20A x 120V/480V
5A< or = 20A x (0.25)
5A< or = 5A

(G) Overcurrent Protection for Specific Applications. Overcurrent protection for specific equipment and conductors must comply with the requirements referenced in Table 240.4(G).

Air-Conditioning and Refrigeration [Article 440]. Air-conditioning and refrigeration equipment, and their circuit conductors, must be protected against overcurrent in accordance with 440.22.

Author’s Comment:
Typically, the branch-circuit ampacity and protection size is marked on the equipment nameplate [440.4(A)].

Question: What size branch-circuit overcurrent device is required for an air conditioner (18A) when the nameplate indicates the minimum circuit ampacity is 23A, with maximum overcurrent protection of 40A? The nameplate specifies 60ºC terminals.
(a) 10 AWG, 40A protection (b) 10 AWG, 50A protection (c) 10 AWG, 60A protection (d) 10 AWG, 70A protection
Answer: (a) 12 AWG, 40A protection

Author’s Comment:
Air-conditioning and refrigeration nameplate values are calculated by the manufacturer according to the following:
♦ Branch-Circuit Conductor Size [440.32] 18A x 1.25 = 22.50A, 10 AWG rated 30A at 60ºC
♦ Branch-Circuit Protection Size [440.22(A)] 18A x 2.25 = 40.50A, 40A maximum overcurrent 
protection size [240.6(A)]
♦ Motors [Article 430]. Motor circuit conductors must be protected against short circuits and ground faults in accordance with 430.52 and 430.62 [430.51].

If the nameplate calls for fuses, fuses must be used to comply with the manufacturer’s instructions [110.3(B)].

Question: What size branch-circuit conductor and overcurrent device (circuit breaker) is required for a 7½ hp, 230V, three-phase motor with 75ºC terminals?
(a) 10 AWG, 50A breaker (b) 10 AWG, 60A breaker (c) a or b (d) none of these
Answer: (c) a or b
Step 1: Determine the branch-circuit conductor size [Table 310.15(B)(16), 430.22, and Table 430.250]:
FLC = 22A [Table 430.250]
22A x 1.25 = 28A, 10 AWG, rated 35A at 75°C
Step 2: Determine the branch-circuit protection size [240.6(A), 430.52(C)(1) Ex 1, and Table 430.250].
Inverse Time Breaker: 22A x 2.50 = 55A
Next size up = 60A

Motor Control [Article 430]. Motor control circuit conductors must be sized and protected in accordance with 430.72.

Q4. What is the Code rule for the overcurrent protection of flexible cords and fixture wires?
A4. Flexible cord must be protected by an overcurrent device in accordance with its ampacity as specified in Table 400.5(A)(1) or Table 400.5(A)(2). Fixture wires must be protected against overcurrent in accordance with their ampacity as specified in Table 402.5. Supplementary overcurrent protection, as discussed in 240.10, is permitted to provide this protection [240.5(A)].

240.5(B) Branch-Circuit Overcurrent Protection.
(1) Cords for Listed Appliances or Luminaires. If flexible cord is used with a specific listed appliance or luminaire, the conductors are considered protected against overcurrent when used within the appliance or luminaire listing requirements.

Author’s Comment:
The NEC only applies to premises wiring, not to the supply cords of listed appliances and luminaires.

(2) Fixture Wire. Fixture wires can be tapped to the following circuits:
(1) 20A–18 AWG, up to 50 ft of run length
(2) 20A–16 AWG, up to 100 ft of run length
(3) 20A–14 AWG and larger

(3) Extension Cord Sets. Flexible cord used in listed extension cord sets is considered protected against overcurrent when used within the extension cord’s listing requirements.

Q5. What does the Code recognize as standard ampere ratings for overcurrent protection devices?
A5. The standard ratings in amperes for fuses and inverse time breakers are: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1,000, 1,200, 1,600, 2,000, 2,500, 3,000, 4,000, 5,000 and 6,000 [240.6(A)] .

Additional standard ampere ratings for fuses include 1, 3, 6, 10, and 601.

Author’s Comment:
Fuses rated less than 15A are sometimes required for the protection of fractional horsepower motor circuits [430.52], motor control circuits [430.72], small transformers [450.3(B)], and remote-control circuit conductors [725.43].

(B) Adjustable Circuit Breakers. The ampere rating of an adjustable circuit breaker is equal to its maximum long-time pickup current setting.

(C) Restricted Access, Adjustable-Trip Circuit Breakers. The ampere rating of adjustable-trip circuit breakers that have restricted access to the adjusting means is equal to their adjusted long-time pickup current settings.

Q6. What is the Code requirement for supplementary overcurrent protection?
A6. Supplementary overcurrent devices must not be used as the required branch-circuit overcurrent device [240.10].

A supplementary overcurrent device isn’t required to be readily accessible [240.24(A)(2)].

Author’s Comment:
Article 100 defines a “Supplementary Overcurrent Device” as a device intended to provide limited overcurrent protection for specific applications and utilization equipment. This limited protection is in addition to the protection provided in the required branch circuit by the branch-circuit overcurrent device.

Q7. What is the Code rule for Ground-Fault protection of equipment?
A7. Service disconnects and feeder circuits rated 1,000A or more, supplied from a 4-wire, three-phase, 277/480V wye-connected system must be protected against ground faults in accordance with 230.95 [215.10 and 230.95].

The requirement for ground-fault protection of equipment doesn’t apply to [240.13]:
(1) Continuous industrial processes where a nonorderly shutdown will introduce additional or increased hazards.
(2) Installations where ground-fault protection of equipment is already provided.
(3) Fire pumps [695.6(H)].

Author’s Comment:
Article 100 defines “Ground-Fault Protection of Equipment” as a system intended to provide protection of equipment from ground faults by opening the overcurrent device at current levels less than those required to protect conductors from damage. This type of protective system isn’t intended to protect people, only connected equipment. See 215.10 and 230.95 for similar requirements for feeders and services.

Ground-fault protection of equipment isn’t required for emergency power systems [700.27] or legally required standby power systems [701.26].

 

Q8. What are the Code requirements for overcurrent protection for ungrounded conductors?
A8. A fuse or circuit breaker must be connected in series with each ungrounded conductor [240.15(A)].

Circuit breakers must automatically (and manually) open all ungrounded conductors of the circuit, except as follows [240.15(B)]:

(1) Multiwire Branch Circuits. Individual single-pole breakers with identified handle ties are permitted for a multiwire branch circuit that only supplies line-to-neutral loads.

Author’s Comment:
According to Article 100, “Identified” means recognized as suitable for a specific purpose, function, or environment by listing, labeling, or other means approved by the authority having jurisdiction. This means handle ties made from nails, screws, wires, or other nonconforming materials aren’t suitable.

Single-pole AFCI or GFCI circuit breakers aren’t suitable for protecting multiwire branch circuits. AFCI or GFCI circuit breakers for multiwire branch circuits must be of the 2-pole type.

(2) Single-Phase, Line-to-Line Loads. Individual single-pole circuit breakers rated 120/240V with handle ties identified for the purpose are permitted on each ungrounded conductor of a branch circuit that supplies single-phase, line-to-line loads.

(3) Three-Phase, Line-to-Line Loads. Individual single-pole breakers rated 120/240V with handle ties identified for the purpose are permitted on each ungrounded conductor of a branch circuit that serves three-phase, line-to-line loads on systems not exceeding 120V to ground.

Q9. Where does the Code require overcurrent protection to be located in a circuit?
A9. Except as permitted by 240.21(A) through (H), overcurrent devices must be placed at the point where the branch-circuit or feeder conductors receive their power. Taps and transformer secondary conductors aren’t permitted to supply another conductor (tapping a tap isn’t permitted).

(A) Branch-Circuit Taps. Branch-circuit taps are permitted in accordance with 210.19.

(B) Feeder Taps. Conductors can be tapped to a feeder as specified in 240.21(B)(1) through (B)(5). The “next size up protection rule” of 240.4(B) isn’t permitted for tap conductors.
(1) 10-Foot Feeder Tap. Feeder tap conductors up to 10 ft long are permitted without overcurrent protection at the tap location if the tap conductors comply with the following:
(1) The ampacity of the tap conductor must not be less than:
a. The calculated load in accordance with Article 220, and
b. The rating of the overcurrent device supplied by the tap conductors.

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

(2) The tap conductors must not extend beyond the equipment they supply.
(3) The tap conductors are installed in a raceway when they leave the enclosure.
(4) The tap conductors must have an ampacity not less than 10 percent of the rating of the overcurrent device that protects the feeder.

Note: See 408.36 for the overcurrent protection requirements for panelboards.

Example: A 400A breaker protects a set of 500 kcmil feeder conductors. There are three taps fed from the 500 kcmil feeder that supply disconnects with 200A, 150A, and 30A overcurrent devices. What are the minimum size conductors for these taps?

• 200A: 3/0 AWG is rated 200A at 75°C, and is greater than 10 percent of the rating of the overcurrent device (400A).

• 150A: 1/0 AWG is rated 150A at 75°C, and is greater than 10 percent of the rating of the overcurrent device (400A).

• 30A: 8 AWG rated 40A at 60°C. The tap conductors from the 400A feeder to the 30A overcurrent device can’t be less than 40A (10 percent of the rating of the 400A feeder overcurrent device.

240.21(B)(2) 25-Foot Feeder Tap. Feeder tap conductors up to 25 ft long are permitted without overcurrent protection at the tap location if the tap conductors comply with the following:

(1) The ampacity of the tap conductors must not be less than one-third the rating of the overcurrent device that protects the feeder.
(2) The tap conductors terminate in an overcurrent device rated no more than the tap conductor ampacity in accordance with 310.15.

(5) Outside Feeder Taps of Unlimited Length. Outside feeder tap conductors can be of unlimited length, without overcurrent protection at the point they receive their supply, if they comply with the following:

(1) The tap conductors are suitably protected from physical damage in a raceway or manner approved by the authority having jurisdiction.
(2) The tap conductors must terminate at a single circuit breaker or a single set of fuses that limits the load to the ampacity of the conductors.
(3) The overcurrent device for the tap conductors is an integral part of the disconnecting means, or it’s located immediately adjacent to it.

(4) The disconnecting means is located at a readily accessible location, either outside the building, or nearest the point of entry of the conductors.

240.21(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 (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.
5A< or = 20A x 120V/480V
5A< or = 20A x (0.25)
5A< or = 5A

(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.

Q10. What are the Code requirements for the location of overcurrent devices?
A10. 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 [240.24(A)]:

(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.
Informational 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.