All Code references are based on the 2005 National Electrical Code.
Because utilities provide grounded AC services and most facilities have at least one utility service connection, a grounded AC service most likely provides power to your premises wiring system. When you have one, your premises wiring system must have a grounding electrode conductor connected to the grounded service conductor [250.24(A)].
This brings up the question of how to comply with grounding electrode conductor requirements. Because a grounding electrode conductor must connect the grounded conductor to the grounding (earthing) electrode, the question of how expands to include where. Can you make this connection anywhere?
Location, location, location. Some inspectors require the grounding electrode conductor to terminate to the grounded conductor terminal at the meter enclosure. Other inspectors require the grounding electrode conductor to terminate to the grounded terminal at the service disconnect.
The Code says you can make this connection at any accessible location, from the load end of the service drop or service lateral up to and including the service disconnecting means [250.24(A)(1)] (Fig. 1). The choice then becomes an engineering decision that balances such factors as installation costs, available space, and maintenance issues.
Connections. In this day of increased demand for uninterrupted power, many facilities are dual-fed. This means separate lines run to the same services; such services are referred to as “double-ended.” If the dual feeds are in a common enclosure (or grouped together in separate enclosures) and they employ a secondary tie, you can use a single grounding electrode connection to the tie point of the grounded conductors from each power source [250.24(A)(3)].
Whether your service is double-ended or not, you must install an unspliced main bonding jumper between the grounded terminal and any metal on the service disconnecting means enclosure. Ensure the bonding jumper and installation comply with 250.28 and 250.24(C), respectively.
Your main bonding jumper is probably a wire or busbar. If you've installed this jumper from the grounded conductor terminal (or bus) to the equipment-grounding terminal (or bus) in the service equipment, the NEC allows you to connect the grounding electrode to the same equipment-grounding terminal (or bus or bar) to which you connected the main bonding jumper [250.24(A)(4)].
Bonding of service equipment. A neutral-ground bond anywhere other than at service equipment is a common cause of power quality problems. Such a bond creates ground loops, which allow undesired current to circulate in the system. Power quality problems often lead to the discovery and removal of such a bond. But don't wait for power quality problems to reveal the bond.
Another concern makes corrective action imperative. Load side neutral-ground bonds allow objectionable current to flow on conductive metal parts of electrical equipment — thereby violating 250.6(A). This objectionable current can cause lethal electric shock. And it sets the stage for inadvertent flashovers, overheating of equipment, and other problems stemming from the presence of electricity in the wrong place.
So don't make (or allow) a neutral ground connection on the load side of the service disconnect [250.24(A)(5)]. However, exceptions to this rule (250.142) allow you to make such a connection for:
Separately derived systems if you follow the requirements of 250.30(A)(1).
Separate buildings if you follow the requirements of 250.32(B)(2).
Grounded conductor. Electric utilities don't typically provide an equipment-grounding (bonding) conductor to service equipment, and they aren't required to do so. Thus, you must run a grounded conductor from the electric utility transformer to each service disconnecting means [250.24(B) and 250.130(A)] (Fig. 2).
The grounded service conductor provides the effective ground-fault current path to the power source winding. This path ensures that opening the circuit protection device will quickly remove dangerous ground-fault voltage from the circuit [250.4(A)(3) and 250.4(A)(5)].
The earth's resistance is too great for it to be an effective bonding jumper. Very little fault current returns to the power source winding if the earth is the only fault-current return path. But let's suppose the earth is your only fault-current return path option. What would the consequences be? For one thing, the circuit overcurrent protection device wouldn't open and clear the ground fault. Consequently, metal parts like metal piping and structural building steel would become — and remain — energized to circuit voltage (Fig. 3). The system then poses a high risk of shock, arc flash, and fire.
You can calculate, for example, the voltage on a metal enclosure due to an open service grounded conductor. Forensic engineers often crank out these kinds of numbers when investigating a fatality case or damage to a facility. It's easier just to comply with NEC requirements and eliminate such a voltage in the first place.
So it's obvious you need a grounded conductor, but how big should it be? Remember, this grounded service conductor serves as the effective ground-fault current path. Thus, you must size it so it can safely carry the maximum fault current likely to be imposed on it [110.10 and 250.4(A)(5)]. Size the grounded conductor per Table 250.66 — based on the total area of the largest ungrounded conductor. The grounded conductor must also have the capacity to carry the maximum unbalanced current, per 220.61.
To test your understanding of the concept, consider the following scenario:
What's the minimum size grounded service conductor required for a 480V, 3-phase service, where the ungrounded service conductors are 500 kcmil and the maximum unbalanced load is 100A?
The unbalanced load requires a 3 AWG grounded service conductor — rated for 100A at 75°C per Table 310.16 [220.61]. However, the grounded service conductor can't be smaller than 1/0 AWG (Table 250.66). This minimum size requirement ensures the conductor will accommodate the maximum fault current likely to be imposed on it. Thus, the real answer is 1/0 AWG.
If you parallel your service conductors, do you use just the one conductor or do you parallel your grounded conductor the way you parallel the current-carrying conductors? The answer is neither.
First, you must install a grounded conductor in each raceway whenever you parallel your service conductors.
Second, you can't simply divide your grounded conductor into two smaller equal conductors. You would satisfy the requirement that the grounded conductor must have the capacity to carry the maximum unbalanced current per 220.61, but it could also result in a grounded conductor that's too small for a given raceway.
To eliminate such a problem, size each grounded conductor per Table 250.66 — based on the total area of the largest ungrounded conductor in the raceway. Note that regardless of the number you come up with, the grounded conductor in each parallel service raceway can never be less than 1/0 AWG (310.4).
Let's review with another quick quiz:
What's the minimum size grounded service conductor required for a 480V, 3-phase service installed in two raceways, where the ungrounded service conductors in each raceway are 350 kcmil and the maximum unbalanced load is 100A?
The unbalanced load requires only a 3 AWG grounded service conductor, per Table 310.16 (220.61). However, the grounded service conductor in each raceway can't be smaller than 2 AWG (Table 250.66) [250.24(C)(2)]. This ensures it will accommodate the maximum fault current likely to be imposed on it. But ungrounded service conductors run in parallel can't be smaller than 1/0 AWG (310.4), so the answer is 1/0 AWG per raceway.
Properly grounding and bonding service equipment improves safety while eliminating a common cause of power quality problems. You just have to make the right connections in the right places. If you think in terms of providing a low-impedance ground-fault path back to the source, you'll have no problem.