National Electrical Code ® (NEC ® ) (ANSI/NFPA 70-1999) & OSHA

The National Electrical Code® (NEC®) (ANSI/NFPA 70-1999) has as its purpose the practical safeguarding of persons, buildings, and their contents from hazards arising from the use of electricity. It contains provisions considered necessary for safety.

Its scope includes the electric conductors and equipment installed, for example, within or on public or private buildings, industrial substations and mobile homes.

It does not cover installations under the exclusive use of electric utilities, mines, and certain other exceptions. The NEC as a recommendation for safe practice is adopted by most of the states, cities, and towns in the United States as the governing electrical code, and is enforced by the local or state-approved authority.

OSHA, 29 CFR, Chapter XVII, Part 1910 is concerned with all establishments engaged in the manufacture of products for interstate commerce. Part 1910 (Subpart S—Electrical) of the regulation has essentially adopted the NEC and incorporates its requirements for electrical installations.

The edition of the NEC adopted by OSHA is not generally the same as that enforced by local authority. NFPA Std 70E is generally the standard that influences the OSHA changes.

The adoption of the NEC in the public and private sector, even though excluding installations under the exclusive control of electric utilities when used in connection with the generation, transmission, and distribution of electric energy, essentially represents requirements for practically all residential, commercial, and industrial installations. For this reason, NEC requirements, as they apply to motors and motor circuits, are included.

The NEC specifies overload devices used to protect motors, motor-control apparatus and motor branch-circuit conductors against excessive heating due to overload and failure to start (see ANSI/NFPA 70-1996, Article 430, Part C).

The NEC further specifies devices intended to protect the motor, motor-control
apparatus, and branch-circuit conductors against overcurrents due to short circuits and grounds (see ANSI/NFPA 70-1996, Article 430, Part D).

In motor branch circuits, it is customary to provide for these functions separately, where the running overload protective device protects against motor overloads and locked rotor, and the overcurrent protective device (as a separate device) provides protection against short circuits and ground faults.

The NEC specifies maximum current rating or setting in relation to motor nameplate full-load current for the motor running overload protective device, since the requirement includes the need to monitor the maximum continuous motor branch-circuit current.

Of the several means recognized for providing motor running overload protection, the two basic approaches in common use are a separate overcurrent device that is responsive to motor current, and a thermal protector integral with the motor (see ANSI/NFPA 70-1996, Articles 430–432).

For continuous duty motors, a separate overload device responsive to motor current is specified in ANSI/ NFPA 70-1996, Articles 430–432, to be rated or selected to trip at no more than 125% of rated full-load current for motors with a marked temperature rise not over 40 °C, or with a service factor not less than 1.15 and not higher than 115% of full-load current rating for all other motors.

For continuous duty motors having a thermal protector integral with the motor, ANSI/NFPA 70-1996, Articles 430–432 specify that the thermal protector shall be approved for use with the motor that it protects on the basis that it will prevent dangerous overheating of the motor due to overload and failure to start.

For motors rated more than 1 hp, in addition to protecting against excessive temperature, the thermal protector is specified to limit the combination of motor and protector to an ultimate trip current in the following manner.

Where full-load current does not exceed 9 A, the trip current should not exceed 170% of motor full-load current. For motors with full-load current values between 9.1 A and 20 A, the trip current should not exceed 156% of motor full-load current. For motors with a full-load current value of 20 A, the trip current should
not exceed 140% of motor full-load current.

Other overload protection recognized by ANSI/NFPA 70-1996, Articles 430–432(a) (4), involves the use of embedded temperature detectors used in conjunction with intermediate devices that cause motor current to be interrupted.