This article was posted 05/22/2007 and is most likely outdated.

Article 240: Overcurrent Protection
 

 
Topic - NEC
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- Article 240: Overcurrent Protection

May 22, 2007
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Article 240: Overcurrent Protection

 

By Mike Holt for EC&M Magazine

 

Know what you’re protecting and how to protect it.

 

Article 240 provides the requirements for selecting and installing overcurrent protection devices (OCPDs). Depending on your application, other Articles may apply (see the Sidebar).

 

Overcurrent exists when current exceeds the rating of conductors or equipment. It can result from overload, short circuit, or ground fault.

 

  • An overload is a condition in which equipment or conductors carry current exceeding their rated ampacity. An example is plugging two 12.5A (1,500W) hair dryers into a 20A branch circuit.
  • A short circuit is the unintentional electrical connection between any two normally current-carrying conductors of a circuit, line-to-line or line-to-neutral.
  • A ground fault is an unintentional, electrically conducting connection between an ungrounded conductor of a circuit and the equipment grounding conductor, metallic enclosures, metallic raceways, metallic equipment, or earth. During a ground fault, dangerous voltages and abnormally large currents exist.

Circuits or equipment?

 

OCPDs protect circuits and equipment. But they protect circuits in one way, and equipment in another.

 

An OCPD protects a circuit by opening when current reaches a value that would cause an excessive temperature rise in the conductors. Using a water analogy, current rises like water in a tank—at a certain level, the OCPD shuts off the faucet. Think in terms of normal operating conditions that just get too far out of normal range. The interrupting rating must be sufficient for the maximum possible fault current available on the line-side terminals of the equipment [110.9]. You’ll find the standard ratings for fuses and fixed-trip circuit breakers in 240.6.

 

An OCPD protects equipment by opening when it detects a short circuit or ground fault. Every piece of electrical equipment must have a short-circuit current rating that permits the OCPDs (for that equipment) to clear short circuits or ground faults without extensive damage to the electrical components of the circuit [110.10]. Short circuits and faults aren’t normal operating conditions. Thus, the OCPDs for equipment have different characteristics than OCPDs for conductors.

 

Circuit protection

 

OCPDs come in standard sizes, which are listed in 240.6. When you size conductor OCPDs, you are trying to determine which of these standard sizes to use. Begin this sizing by determining the conductor ampacity and then making ampacity adjustments specified in 310.15 [240.4].

 

OCPD application will vary under the following circumstances:

 

Power Loss Hazard. Conductor overload protection is not required where circuit interruption would create a hazard (e.g., a fire pump). Short-circuit protection is still required.

 

Not Over 800A. You can use the next higher standard rating overcurrent device (above the ampacity of the ungrounded conductors being protected) if all of the following conditions are met:

  • The conductors do not supply multioutlet receptacle branch circuits.
  • The ampacity of a conductor, after ampacity adjustment and/or correction, doesn’t correspond to the standard rating of a fuse or circuit breaker in 240.6(A).
  • The protection device rating doesn’t exceed 800A.

For example, a 400A OCPD can protect 500 kcmil conductors, where each conductor has an ampacity of 380A at 75°C per Table 310.16. This “next size up” rule doesn’t apply to feeder tap conductors [240.21(B)] or secondary transformer conductors [240.21(C)].

 

Over 800A. If the OCPD exceeds 800A, the conductor ampacity (after ampacity adjustment and/or correction) must have a rating not less than the rating of the OCPD. For example, a 1,200A OCPD can protect three sets of 600 kcmil conductors per phase, where each conductor has an ampacity of 420A at 75°C per Table 310.16.

 

Small Conductors. Unless specifically permitted in 240.4(E) or (G), overcurrent protection must not exceed (after ampacity adjustment and/or correction):

  • 15A for 14 AWG copper.
  • 15A for 12 AWG aluminum.
  • 20A for 12 AWG copper.
  • 25A for 10 AWG aluminum.
  • 30A for 10 AWG copper.

Supplementary OCPDs

 

A “supplementary OCPD” provides limited overcurrent protection for specific applications and utilization equipment. It’s usually an internal fuse. Supplementary OCPDs are often used in luminaires, appliances, and equipment for internal circuits and components.

 

You cannot use a supplementary OCPD as the required branch-circuit OCPD [240.10]. A supplementary OCPD doesn’t have to be readily accessible [240.24(A)(2)].

 

Location in circuit

 

Install OCPDs at the point where the branch or feeder conductors receive their power. Exceptions exist in 240.21 (A) through (G). Here’s a summary of each of these, but be sure to read the details if the exception applies to your situation.

  • Branch circuits meeting 210.19 requirements are exempted from 240.21 location requirements. Examples include multiwire and range circuits.
  • You can’t make a tap from a tap.
  • The OCPDs for the primary side of a transformer provide protection for the secondary side, if certain conditions are met.
  • Service conductors are covered by 230.91.
  • Busway taps are covered by 368.17.
  • For motors, apply 430.28 and 430.53.
  • For generators, apply 445.12 and 445.13.

Location of OCPDs on premise.

 

Circuit breakers and fuses must be readily accessible [240.24]. “Readily accessible” means located so a person can reach it quickly without having to climb over (or remove) obstacles, or use a portable ladder. This rule does not prohibit the locking of panel doors or the placing of a padlock on a circuit breaker to restrict access [110.26].

 

Install OCPD enclosures such that 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. Four exceptions exist for this rule:

  • Busways as provided in 368.17(C).
  • Supplementary OCPDs [240.10].
  • OCPDs as described in 225.40 and 230.92.
  • OCPDs located next to equipment can be mounted above 6 ft 7 in., if accessible by portable means [404.8(A) Exception No. 2].

OCPDs must not be exposed to physical damage. Electrical equipment must be suitable for the environment. Give consideration to the presence of corrosives, which may deteriorate conductors or equipment [110.11].

 

Don’t locate OCPDs near easily ignitible material or in locations such as clothes closets. Don’t locate them in bathrooms of dwelling units or guest rooms (or guest suites) of hotels or motels. This rule also applies to the service disconnecting means, even in commercial or industrial facilities [230.70(A)(2)].

 

Enclosures

 

Enclosures containing OCPDs must be mounted in a vertical position unless this isn’t practical [240.33]. Circuit breaker enclosures can be horizontal if the circuit breaker is installed per 240.81.

 

240.81 specifies that where circuit breaker handles are operated vertically, the “up” position of the handle must be in the “on” position. So an enclosure that contains one circuit breaker can be mounted horizontally, but an enclosure that contains a panelboard or loadcenter with multiple circuit breakers on opposite sides of each other would have to be mounted vertically.

 

Also note that these enclosures are designed for lefthand operation, under the assumption that the operator is right-handed. The intended result is that the operator is standing to one side of the enclosure, rather than in front of it (and in the blast path) when operating it. Allow space for this when installing the enclosure.

 

Circuit Breakers

 

Circuit breakers must be capable of being opened and closed by hand [240.80]. Nonmanual means of operating a circuit breaker, such as electrical shunt trip or pneumatic operation, are permitted only if the circuit breaker can also be manually operated.

 

Circuit breakers used to switch 120V or 277V fluorescent lighting circuits must be listed and marked SWD or HID. Circuit breakers used to switch high-intensity discharge lighting circuits must be listed and marked HID.

 

UL 489, Standard for Molded Case Circuit Breakers, permits “HID” breakers to be rated up to 50A, but an “SWD” breaker may be rated only to 20A. The tests for “HID” breakers include an endurance test at 75 percent power factor, but “SWD” breakers are endurance-tested at 100 percent power factor. The contacts and the spring of an “HID” breaker are of a heavy duty material to dissipate the increased heat caused by the greater current flow in the circuit that occurs because the “HID” luminaire takes a minute or two to ignite the lamp.

 

Before you start any OCPD calculations, first determine if you’re trying to protect circuits or equipment. Next, determine if any other Articles apply for your application. Then you can plug in the numbers and select the correct OCPD.

 

Other Articles

Your application may fall under one of these other Articles:

  • Air-conditioning and refrigeration equipment, 440.22
  • Appliances, Article 422
  • Audio Circuits, 640.9
  • Branch Circuits, 210.20
  • Class 1, 2, and 3 Circuits, Article 725
  • Feeder Conductors, 215.3
  • Flexible Cords, 240.5(B)(1)
  • Fire Alarms, Article 760
  • Fire Pumps, Article 695
  • Fixed Electric Space-Heating Equipment, 424.3(B)
  • Fixture Wire, 240.5(B)(2)
  • Panelboards, 408.36(A)
  • Service Conductors, 230.90(A)
  • Transformers, 450.3
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Comments
  • Mike, we frequently see home owners install a new ceiling fan with out the use of a "Fan Box" or a wall switch. These fans are Hot Wired to the closest circuit available. This of course usually means that they will "Tap in to" the closest NM cable in the attic space with out the benifir of a j-box or connectors. Please educate all of us readers the code requirements for installing a ceiling fan with a light kit.

    John K. Erickson
    Reply to this comment

  • This is a good summary of the way things are now, but there's a change that's about to be made, in the 2008 NEC, to Articles 210.19 and 215.2 that allows the neutral conductor to be sized at only 100% of the load instead of 125% that's presently required.

    When you consider the " Not Over 800A" provision allowing the "..next higher standard rated..." OC device, in some cases the OC device rating will be 37% higher than the neutral conductor ampacity.

    That seems to be a step backward to me.

    James Wilson, PE
    Reply to this comment

  • I read your e-mail on overcurrent protection,ART 240: Piont of interest was art.240.4(E) or (G). Where mention 12 ga, or 10 ga, aluminum wire, I have seen this wire go out along time ago. As far as I know you can not buy it anymore so why do we still talk about it in this article. What's the name of the museum supply house?????????????

    dave bissell
    Reply to this comment

  • It's good to see the backstory on where and why these guidelines and rules came from.

    Matt
    Reply to this comment


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