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Mike Holt for EC&M Magazine
Don't use the Equipment Grounding Conductor for grounding.
The Equipment Grounding Conductor (EGC) does not connect anything directly to the earth (ground). Does that surprise you? From the definition of EGC in Article 100, we see that the EGC connects the non-current carrying metal parts of electrical systems to the system grounded conductor or the grounding electrode conductor (or both). Thus, the EGC is really not a grounding conductor at all, but a bonding conductor.
From Article 100, we see that bonding is the "permanent joining of metallic parts" and grounding is a connection "to earth." So we have a distinction, here. If you are joining metallic parts, you are bonding. If you are connecting to earth, you are grounding.
This isn't just semantics. Understanding
this difference will help you avoid such mistakes as driving an "isolated ground"
rod rather than running a bonding jumper to the system grounded conductor or the grounding
electrode conductor (or both).
Article 250 provides a list of fourteen types of components you can use (singly or in combination) as an EGC [250.118]. As you might expect, item #1 on the list is a conductor. This can be wire or busbar made of copper, aluminum, or copper-clad aluminum. It can be bare or insulated.
The next three items (2, 3, and 4 on the list) are types of metallic nonflexible raceway: Rigid Metal Conduit, Intermediate Metal Conduit, and Electrical Metallic Tubing.
Chapter 3 in the NEC devotes an Article to each of these: 342, 344, and 358, respectively. These raceways may outperform the conductors within them as bonding paths, especially at high frequencies.
Four varieties of metallic flexible raceway occupy spots 5, 6, 7, and 8, respectively. They are flexible metal conduit, liquidtight flexible metal conduit, flexible metallic tubing, and Type AC armored cable.
Listed flexible metal conduit. FMC (Article 348) installations must meet four requirements (Figure 250-s118(05) 01 CC250-29.cdr):
Listed liquidtight flexible metal conduit. LFMC (Article 350) installations must meet five requirements (Figure 250-s118(06) CC250-31.cdr):
To qualify as an EGC, Flexible metallic tubing (Article 360) must terminate in fittings listed for grounding. The circuit conductors in the tubing must be rated at 20A or less, and the FMC must be no more than 6 ft long.
Type AC Cable (Article 320) is manufactured with an internal bonding strip that is in direct contact with the interlocked metal armor. The combination of the bonding strip and the interlocked metal armor makes the cable suitable as an effective ground-fault current path (Figure 250-172).
The ninth item on the list is the copper metal sheath of Mineral Insulated cable (Article 332). Take care with MI that you don't expose the sheath to corrosion.
Type MC Cable, where listed and identified for grounding, is a tenth option. But, pay attention to whether it is interlocked, smooth, or corrugated tube type:
You can use metallic cable tray as an EGC, if continuous maintenance and supervision ensure that qualified persons service it [392.3(C)], and it meets two other requirements [392.7]:
You can use cablebus framework, if it meets the requirements of 370.3.
You can use other electrically continuous metal raceways listed for bonding, such as metal wireways.
Your fourteenth option is a surface metal raceway, if it's listed for grounding.
Some ID, please
Individually covered or insulated EGCs must have a continuous outer finish that is green (or green with one or more yellow stripes). Conductors with insulation that is green (or green with one or more yellow stripes) cannot be used for an ungrounded conductor or grounded neutral conductor (Figure 250-s119 02 CC250-32.cdr).
An insulated or covered EGC larger than 6 AWG may be permanently reidentified at the time of installation at every point where the EGC is accessible [250.119]. But, you don't have to identify EGCs larger than 6 AWG in conduit bodies.
EGC identification must encircle the EGC. You can choose from three ways to accomplish the identification:
Sizing the EGC
Size wire EGCs per Table 250.122 based on the ampere rating of the circuit-protection device. In no case does the EGC have to be larger than the circuit conductors (Figure 250-176).
The EGC must be capable of safely conducting any ground-fault current likely to be imposed on it [110.10 and Table 250.122]. Otherwise, the EGC may burn clear before the protective device responds.
Suppose you increase ungrounded conductor size to accommodate voltage drop, excessive heating from harmonic currents, fault-current studies, or future capacity. What about the EGC size? Any time you increase the size of ungrounded circuit conductors-regardless of the reasons-you must proportionately increase the size of the EGC. The following problem illustrates this requirement.
If you increase the size the ungrounded conductors for a 40A circuit from 8 AWG to 6 AWG, you would also need to increase the size of your EGC. If your EGC was 10 AWG, what size would it need to be now (Figure 250-178)?
The circular mil area of 6 AWG is 59 percent greater than that of 8 AWG (26,240 cmil/16,510 cmil) [Chapter 9, Table 8].
The EGC for a 40A protection device can be 10 AWG (10,380 cmil) [Table 250.122], but you must increase it in size by a multiplier of 1.59.
Multiple Circuits. When multiple circuits are installed in the same raceway or cable, you need only one EGC in the raceway. Size it per Table 250.122 based on the largest overcurrent device protecting the circuit conductors in the raceway (Figure 250-179).
Parallel Runs. When circuit conductors are run in parallel [310.4], you must install an EGC with each parallel conductor set. Size it based on the ampere rating of the circuit-protection device per Table 250.122 (Figure 250-181)-unless ground-fault protection is installed for cables systems.
If ground-fault protection is installed, size the EGC based on the ampere rating of the ground-fault protection per Table 250.122 where ground-fault protection of equipment is installed (Figure 250-s122F2 NEW.cdr). The installation must meet the three following conditions, or you must size the EGC per the first method:
Feeder Tap Conductors. Size EGCs for feeders based on the ampere rating of the circuit-protection device ahead of the feeder, per Table 250.122. In no case does the EGC need to be larger than the circuit conductors (Figure 250-s122G CC250-33.cdr).
Don't let the "grounding" part of the EGC confuse you. It's really the equipment bonding conductor. Don't think in terms of "grounding" equipment-that is, connecting it to the earth . Think in terms of connecting it to the system grounded conductor or the grounding electrode conductor (or both). Remember, the low-impedance fault-current path you create is necessary for overcurrent protection devices to operate. Earth is not a low-impedance fault-current path.
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