Top 50 NEC Rules

Sep 25, 2006 11:02 AM, By Mike Holt

250.24 Grounding and Bonding at Service Equipment

(A) Grounding. Services supplied from a utility transformer that is grounded to the earth must have the grounded neutral conductor grounded to a suitable grounding electrode [250.50] in accordance with the following:

(1) Accessible Location. A grounding electrode conductor must connect the grounded neutral conductor to the grounding electrode, and this connection can be made at any accessible location — from the load end of the service drop or service lateral up to and including the service disconnecting means. Some inspectors require the grounding electrode conductor to terminate at the meter enclosure, while others insist that the grounding electrode conductor terminate at the service disconnect. The Code allows this grounding (earthing) connection to be made at either of these locations.
(4) Main Bonding Jumper. When the grounded neutral conductor is bonded to the service disconnecting means [250.24(B)] by a bus bar [250.28], the grounding electrode conductor can terminate to either the grounded neutral terminal or the equipment grounding terminal within the service disconnect.
(5) Load-Side Neutral-to-Case Bonding. A neutral-to-case bond cannot be made on the load side of the service disconnect means, except as permitted for separately derived systems [250.30(A)(1)] or separate buildings [250.32(B)(2)] in accordance with 250.142. If an improper neutral-to-case bond is made on the load side of service equipment, dangerous objectionable current will flow on conductive metal parts of electrical equipment in violation of 250.6(A).

(B) Main Bonding Jumper. An unspliced main bonding jumper complying with 250.28 must be installed between the grounded neutral terminal and the metal parts of the service disconnecting means enclosure in accordance with 250.24(C).

(C) Grounded Neutral Conductor Required. Because electric utilities aren’t required to provide an equipment grounding (bonding) conductor to service equipment, a grounded neutral service must be run from the electric utility transformer to each service disconnecting means, and it must be bonded to each service disconnecting means as required by 250.24(B) [250.130(A)].

The grounded neutral service conductor provides the effective ground-fault current path to the power source to ensure that dangerous voltage from a ground fault will be quickly removed by opening the circuit protection device [250.4(A)(3) and 250.4(A)(5)].

A ground fault cannot be cleared to remove dangerous voltage on the metal parts, metal piping, and structural steel if the service disconnecting means enclosure is not bonded to the grounded neutral service conductor.

If the grounded neutral service conductor is open, a ground fault cannot be cleared, and metal parts will become and remain energized. In addition, if the grounded neutral conductor is opened, dangerous voltage will be present on metal parts under normal conditions, providing the potential for electric shock. For example: If the earth’s ground resistance is 25 ohms and the load’s resistance is 25 ohms, the voltage drop across each of these resistors would be ½ of the voltage source. Since the grounded neutral is bonded to the service disconnect, all metal parts will be elevated to 60V above the earth’s potential for a 120/240V system. This dangerous condition is of particular concern in buildings containing pools, spas, or hot tubs.

(1) Minimum Size Grounded Neutral Conductor. Because the grounded neutral service conductor is required to serve as the effective ground-fault current path, it must be sized so that it can safely carry the maximum fault current likely to be imposed on it [110.10 and 250.4(A)(5)]. This is accomplished by sizing the grounded neutral conductor in accordance with Table 250.66, based on the total area of the largest ungrounded conductor. In addition, the grounded neutral conductors must have the capacity to carry the maximum unbalanced neutral current in accordance with 220.61.
(2) Parallel Grounded Neutral Conductor. Where service conductors are paralleled, a grounded neutral conductor must be installed in each raceway, and it must be sized in accordance with Table 250.66, based on the total area of the largest ungrounded conductor in the raceway. In no case must the grounded neutral conductor in each parallel service raceway be less than 1/0 AWG [310.4]. In addition, the grounded neutral conductors must have the capacity to carry the maximum unbalanced neutral current in accordance with 220.61.

(D) Grounding Electrode Conductor. A grounding electrode conductor must connect the metal parts of service equipment enclosures to a suitable grounding electrode in accordance with Part III of Article 250.