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EGCs for Energy-Efficient Motors: The basic rules for sizing this equipment type

By Michael Johnston | Apr 15, 2024
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A few circumstances can cause an increase in the minimum size of wire-type equipment grounding conductors (EGCs), including voltage drop issues as addressed in 250.122(B) and conductor withstand ratings to meet the requirements in 110.10. 

A few circumstances can cause an increase in the minimum size of wire-type equipment grounding conductors (EGCs), including voltage drop issues as addressed in 250.122(B) and conductor withstand ratings to meet the requirements in 110.10. The rating of the motor short-circuit and ground-fault protective device also affects minimum motor circuit sizes. Minimum size requirements for EGCs in motor circuits are in 250.122(D)(1) and (D)(2).

Sizing a wire-type EGC

The basic rule is that the EGC (wire-type) be sized no smaller than determined by 250.122(A), based on the rating of the branch circuit short circuit and ground-fault protective device of the motor circuit, based on Section 430.52. Remember that the branch circuit short circuit and ground-fault protective device is usually sized larger to carry the motor’s starting current (locked rotor), which affects the size of a wire-type EGC.

When an instantaneous-trip circuit breaker or motor short-circuit protector is selected as the overcurrent protective device for a motor circuit, a wire-type EGC must be sized no smaller than provided in 250.122(A), using the maximum rating of a dual-element time-delay fuse selected for branch circuit short circuit and ground-fault protection in accordance with 430.52(C)(1) Exception No. 1.

Section 430.52(C) has been revised in the 2023 National Electrical Code to include requirements for Design B premium efficiency motors. According to NEMA, these have largely replaced energy-efficient motors because they have been mandated for many motor horsepower applications for 1 through 500 since December 2016.

Federal regulations now require a higher efficiency than previously available from Design B motors. Design B and Design B premium efficiency motors are energy-­efficient designs with low-impedance and a high inrush current. Where the setting specified in Table 430.52(C)(1) is not sufficient for the motor’s starting current, the setting of an instantaneous-trip circuit breaker is permitted to be increased up to 1,700% of the motor full-load current for Design B energy­-efficient and Design B premium.

Trip settings above 1,100% for these motors are permitted where an engineering evaluation determines a need. In such cases, it is not necessary to first apply an instantaneous­-trip circuit breaker at 1,100%. A listed self-protected combination motor controller is permitted in lieu of the devices specified in 430.52(C)(1). The adjustable instantaneous-trip setting is not permitted to exceed 1,700% of the full-load current rating for Design B energy-efficient and Design B premium efficiency motors. The higher trip levels should reduce nuisance trips. To improve usability, the exceptions were rewritten in positive language. This revision has two key concerns that can affect installations of motor branch circuit EGCs.

Grounding, bonding, current path

EGCs perform three important functions in the electrical safety system. The first is grounding. 

EGCs are intended to act as a conductive path that connects equipment to ground (the earth). The EGC extends the ground connection to various points in the electrical system because it is generally installed with feeders or branch circuits. Grounding’s role is to place a conductive object (equipment) at or as close to earth (ground) potential as possible. The EGC limits aboveground potential (voltages) on conductors and equipment enclosures during normal operation and abnormal conditions, such as a ground-fault or short-circuit event.

Another important EGC function is bonding, as indicated in the informational note following its definition.

The EGC’s third role is to serve as part of an effective ground-fault current path during abnormal events such as ground faults. The conductor must be capable of carrying the fault current back to the source for the time it takes the overcurrent protective device to open and clear the event from the circuit. Sections 250.4(A)(5) and 250.4(B)(4) provide the performance requirements and criteria for an effective ground-fault current path.

Ideally, there will never be a ground fault on the circuit, but insulation breakdown or failure can occur and result in a ground fault. Insulation can be in the form of a dielectric material or air space, such as the space between busbars in a switchboard or panelboard. A ground fault can also result from human error or accidents, such as dropping a conductive tool in an electrical enclosure during work on energized equipment. The EGC must be capable of withstanding higher levels of ground-fault current to perform its all-important safety function.

The NEC provides the minimum sizing requirements for wire-type EGCs, meaning one must do at least that much. Some conditions also warrant the EGC to be larger.

shutterstock / Technicsorn Stocker

About The Author

A man, Mike Johnston, in front of a gray background.

Michael Johnston

NECA Executive Director of Codes and Standards

JOHNSTON is NECA’s executive director of codes and standards. He is a member of the NEC Correlating Committee, NFPA Standards Council, IBEW, UL Electrical Council and NFPA’s Electrical Section. Reach him at [email protected].

 

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