Service Equipment- Overcurrent Protection, NEC 230-90
Where Required.
 | Each ungrounded service conductor shall have overload protection. (a) Ungrounded Conductor. Such protection shall be provided by an overcurrent device in series with each ungrounded service conductor having a rating or setting not higher than the allowable ampacity of the conductor. Exception No. 1: For motor-starting currents, ratings in conformity with Sections 430-52, 430-62, and 430-63 shall be permitted. Exception No. 2: Fuses and circuit breakers with a rating or setting in conformity with Section 240-3(b) or (c) and Section 240-6. Exception No. 3: Not more than six circuit breakers or six sets of fuses shall be permitted as the overcurrent device to provide the overload protection. The sum of the ratings of the circuit breakers or fuses shall be permitted to exceed the ampacity of the service conductors, provided the calculated load in accordance with Article 220 does not exceed the ampacity of the service conductors. Exception No. 4: Fire Pumps. Where the service to the fire pump room is judged to be outside of buildings, these provisions shall not apply. Overcurrent protection for fire pump services shall be selected or set to carry locked-rotor current of the motor(s) indefinitely. Exception No. 5: As permitted by Note 3 to Ampacity Tables of 0 to 2000 volts for 120/240-volt, 3-wire, single-phase dwelling services. A set of fuses shall be considered all the fuses required to protect all the ungrounded conductors of a circuit. Single-pole circuit breakers, grouped in accordance with Section 230-71(b), shall be considered as one protective device. |
(b) Not in Grounded Conductor. No overcurrent device shall be inserted in a grounded service conductor except a circuit breaker that simultaneously opens all conductors of the circuit.