Code Basics

Jan 1, 2000 12:00 PM, By James Stallcup, NEC and OSHA Consultant

Under certain conditions, you can ground all metal parts of enclosures used for wiring the elements of a separately derived system (SDS) by the grounded neutral conductor on the secondary side of the transformer. This includes the transformer's secondary conductors.

You use this neutral conductor in the same manner as we describe in Part 1 of this series; to ground the metal parts of noncurrent-carrying enclosures located on the supply side of the service equipment. You can use such a grounded conductor as a current-carrying neutral or as an equipment-grounding conductor, to serve as a low-impedance path for returning fault-currents caused by a ground-fault condition on the secondary side of the transformer of an SDS. To consider this conductor reliable , you must size it for both conditions and route it with the circuit conductors in a particular manner.

Using grounded neutral conductors on the secondary side of an SDS: Sec. 250-142(a)(3).In some situations, you can install low- and high-voltage feeder circuits from floor-to-floor in a high-rise building with transformers located on each floor. The transformers reduce the voltage to 120/240V or 120/208V for general use lighting and receptacle loads. You can install grounding either at the transformer or at the load served.

Let's look at the four parts of designing and installing the secondary bonding and grounding scheme of a transformer system:
1) Grounded conductor;
2) Bonding jumpers;
3) Grounding electrode conductor; and
4) Grounding electrode.

Let's discuss how to size, select, and use these elements to safely ground and bond an SDS and use the grounded neutral conductor as an equipment-grounding conductor during a ground-fault.

Grounded conductor brought to the SDS: Secs. 250-20(d), 250-24(b)(1), and (b)(2). You must install and run a grounded neutral conductor with the ungrounded phase conductors from the secondary side of the transformer to the panelboard. Grounded neutral conductors provide an effective path for fault currents if a phase-to-ground fault occurs in the electrical system. If a phase conductor faults to ground, fault current will flow from the point of fault through the grounded neutral conductor to the supply transformer. The fault current then returns through the phase conductor, and the overcurrent protection device for that faulted phase trips. The overcurrent protection device usually handles a fault current of 6 times to 10 times its rating. To satisfy this rule, you must size the grounded neutral conductor as large as the grounding electrode conductor, per Sec. 250-24(b)(1) and Table 250-66. For service entrance conductors larger than 1,100 kcmil copper or 1,750 kcmil aluminum, you must size the grounded conductor at least 121/2% of the area of the largest ungrounded phase conductor.

Bonding jumpers: Secs. 250-30(a)(1), 250-28(d), and 250-102(c). You must design and install the bonding jumpers based on the size and type of phase conductors supplying the panel, switch, or other electrical equipment connected to the secondary side of the transformer. You must size the jumpers as per Secs. 250-28(d) and 250-102(c).

If the ungrounded phase conductors are smaller than 1,100 kcmil for copper or 1,750 kcmil for aluminum, size the bonding jumper (either main or equipment) per Sec. 250-24(b)(1) and Table 250-66. The bonding jumpers must be at least 121/2% of the area of the largest conductor.

If the ungrounded phase conductors are larger than 1,100 kcmil for copper or 1,750 kcmil for aluminum, the bonding jumpers should normally be larger than the grounding-electrode conductor. Install and connect the bonding jumpers at any point on the SDS from the source to the first system disconnecting means or over-current protection device.

Grounding electrode conductor: Secs. 250-30(a)(2), 250-66(a) through (c), and Table 250-66. You must design and install the grounding electrode conductor based on the derived phase conductors supplying the panel, switch, or other electrical equipment connected from the secondary of the transformer. You must size the grounding electrode conductor per Secs. 250-66(a) through (c) or Table 250-66. Install and connect the grounding electrode conductor at any point on the SDS from the source to the first system disconnecting means or overcurrent protection device. If the kcmil rating is greater than 1,100 for copper or 1,750 for aluminum, the grounding electrode conductor will usually be smaller than the main and equipment-bonding jumper.

Grounding electrode: Secs. 250-30(a)(3), 250-50(a) through (d), and 250-52(c)(1) through (c)(3) and (d). Locate the grounding electrode conductor as near as possible, and preferably in the same area, to the grounding electrode conductor connection of the SDS. From the following three choices, select and install one of the following in the order in which they are listed (i.e. 1 through 3).
1) Nearest building steel
2) Nearest metal water pipe system
3) Other electrodes as specified in Secs. 250-50 and 250-52
4) Metal water pipe located in the area must be bonded to the grounded conductor per Sec. 250-104(a)(4).

In next month's issue, we'll discuss grounding separate buildings supplied from the service equipment of a main building.