Detecting failure in electrical systems

Circuit protection, as defined by NFPA 70: National Electrical Code, can be interpreted in many ways, depending on building load and use. Without an effective circuit-protection scheme, a building’s electrical system is prone to myriad failures.


This article has been peer-reviewed.Learning objectives

  • Understand the codes and standards and knowingly apply them with the same intent with which they were written.
  • Examine how to safeguard human lives and infrastructure from the hazards of electricity, specifically in the form of circuit protection.

One of the largest concerns when designing a building’s electrical distribution system is the application of a code compliant and safe circuit protection scheme. Circuit protection in its most basic form is quite simple: The designer will introduce an element which is meant to fail in order to protect the users and the greater system. This element is typically in a piece of equipment where it can readily be replaced or serviced in the event that it is needed. This seemingly simplistic definition becomes increasingly convoluted as the designer begins to examine the overall circuit-protection scheme on their new project and how it relates to the NFPA 70: National Electrical Code (NEC).

Without an effective circuit-protection scheme, a building’s electrical system is prone to myriad failures. Since its inception in 1897, the NEC has analyzed electrical hazards and provided an outline for safeguarding against them. Circuit protection is described in the NEC is the use of an overcurrent protection device (OCPD), typically a circuit breaker or fuse, to protect downstream conductors and their loads. In short, if there is an event downstream, such as an overload or fault, the breaker or fuse will open the circuit. These devices’ capabilities range from basic features to advanced functionality with reporting, alarm modules, relaying schemes, or electronically tunable trip settings.

Regardless of building or occupancy type, the first step in any standardized circuit-protection scheme is identifying the hazards of the specific project and designing an approach for these hazards.

Overcurrent rated 800 amperes or less

NEC 240.4(B) outlines the use of current-limiting OCPDs under 800 amps. Assuming there are no cord and plug receptacles on the circuit being protected, the OCPD is selected by using a corresponding standard breaker size compared against the conductor derated ampacity. If this value falls between standard OCPD sizes (standard sizes located in Table 240.6(A)), the NEC allows you to size to the next OCPD standard size. For branch circuits with cord and plug loads, refer to NEC 210 and specifically NEC 210.20 for OCPD selection, as it outlines continuous versus non-continuous loads, receptacle circuits, etc. Of course, the application of NEC 240 is also dependent on the load being served, i.e., motors (NEC 430), transformers (NEC 450), etc.

Figure 1: This diagram shows the examples for overcurrent devices rated 800 amps or less and details some additional examples. Each feeder was calculated using the same code references as the previous two sections. The ground and conduit size has been incl

For example, an electrical engineer has a 120/208 V, 3-phase, 4-wire panelboard with 20 kVA of continuous load and 20 kVA of non-continuous. This represents an ampacity of roughly 120 amps. The engineer also has determined the standard wire sizes and ampacities for this new project and has determined that no derating need occur inside the building when using 75°C copper thermoplastic heat and water-resistant nylon coated wire (THWN) with four conductors inside each electrical metallic tubing (EMT) conduit in free air (three current-carrying wires, NEC 310.15(B)(5)(a)) and no concerns about voltage drop. Armed with this knowledge, the designer begins to calculate the feeder size and OCPD. Below are a few examples that also include wire ampacities per NEC 310, conduit sizes per NEC Table 9, and ground sizing per NEC Table 250.122.


  • 20 kVA L Code
  • 20 kVA N Code
  • Total = 20 kVA x 1.25 + 20 kVA = 45 kVA
  • Current = 45 kVA at 208 V/3 phase = 125 amps
  • Wire ampacity: #1 is rated for 130 amps
  • Next OCPD: 150 amps
  • Feeder: 1½ in.—Four #1 and one #6 Ground (refer to Figure 1 for more examples).

Overcurrent more than 800 amps

NEC 240.4(C) outlines the use of current-limiting OCPDs over 800 amps and is more prescriptive than NEC 240.4(B). This code section states that the OCPD must be of equal or greater ampacity to the conductors being protected. With an OCPD larger than 800 amps, the designer is typically sizing a distribution board feeder or large transformer.

Example (assuming the same conditions as the previous example and that no derating need occur for wiring inside the building):


  • (Two) panelboards
  • Panelboard 1: 250 kVA N Code and 20 kVA of L Code
  • Panelboard 2: 400 kVA N Code and 100 kVA of L Code
  • Total = 120 kVA x 1.25 + 650 kVA = 800 kVA
  • Current = 800 kVA at 480 V/3-phase = 963 amps
  • Wire ampacity: #400 KCM is rated for 335 amps; running (three) parallel sets allows a wire ampacity of 1,005 amps
  • OCPD that is equal or less than ampacity: 1,000 amps
  • 1,000 amps: (three) 4 in—each with four #400 KCM and one #1/0 Ground (refer to Figure 1 for more examples).


Figure 2: This schedule walks through the 75 kVA transformer OCPD example as well as several others. Per the previous feeder schedule, conduit, equipment-ground, and transformer-ground sizes are provided as a courtesy. Wire ampacity was selected based on NTransformer circuit protection is outlined within NEC 450, specifically 450.3. A typical commercial facility will have utilization voltages of 120/208 V and 277/480 V with an indoor transformer; these voltages will use NEC Table 450.3(B) for transformer OCPDs of 1,000 V or less.

There are two distinct configurations of circuit protection for transformers: primary-only protection or primary and secondary protection. These requirements are outlined in NEC 240.21. Primary-only protection is outlined within NEC 240.21(C)(1) and is allowed for some single-phase or multiphase delta-delta connected transformers. More commonly, primary and secondary protection is required for the transformer. For these requirements, NEC 240.21(C) shall apply for different scenarios depending on the length of the secondary conductors.

For example, a facility is equipped with a 75-kVA transformer with a 480 V, 3-phase, 3-wire primary and 120/208V, 3-phase, 4-wire secondary. The transformer is indoors with a secondary feeder that is 15 ft long, thus it is required to have primary and secondary protection (NEC 240.4(F), 240.21(C)(6)).

Primary full load amps (FLA): 75,000 VA/480 V x 1.73 = 90.3 amps

  • FLA x 2.5 = 225.8
  • Primary OCPD: up to 250% of FLA, up to 225 amps per NEC Table 450.3(B), recommended 125 amps

Secondary FLA: 75,000 VA/208 V x 1.73 = 208.4 amps

  • FLA x 1.25 = 260.5
  • Secondary OCPD: up to 125% of FLA, up to 275 amps per NEC Table 450.3(B), recommended 225 amps

A few notes: 125 amps is recommended as the primary OCPD; however, this OCPD should be plotted against the transformer inrush current because a lower size could result in nuisance tripping of the primary OCPD. In addition, the secondary OCPD should be coordinated against any high inrush loads to ensure nuisance tripping from the load does not occur.

Outdoor installations and installations of more than 1,000 V were not covered but are outlined within NEC 450.

See Figure 2.

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