What size equipment grounding conductor is required in each raceway for an 800A 500 Kcmil feeder paralleled in two raceways?

From 250.24(c) (2) Parallel Conductors. Where the service-entrance phase conductors are installed in parallel, the size of the grounded conductor shall be based on the total circular mil area of the parallel conductors as indicated in this section. Where installed in two or more raceways, the size of the grounded conductor in each raceway shall be based on the size of the ungrounded service-entrance conductor in the raceway but not smaller than 1/0 AWG.
FPN: See 310.4 for grounded conductors connected in parallel.(C)

There are no exceptions to this rule? So if you are running two 4/0 calbes in parallel and there ground wires were #6 but with the run had to change to #3 because the 4/0 cables are used to keep VD under 3%, you still have to make the ground wires 1/0?

I believe in the case you are presenting, you are speaking of equipment grounding conductors which would be handled differently from the groundED conductor being installed with the service.

By: Dennis Bordeaux | Apr 20, 2016

Supply-Side grounded and bonding conductors are sized from Table 250.102(C)(1) in the 2014 NEC. In earlier Codes, all these conductors, as well as the Grounding Electrode Conductor, were selected from Table 250.66.

Table 250.102(C)(1) from the 2014 NEC.

It is very helpful to have a separate Table for selecting the size of the Grounded Conductor, Main Bonding Jumper, System Bonding Jumper, and Supply-Side Bonding Jumper. These conductors are installed to carry fault current and return it to the electrical source. The Grounding Electrode Conductor, on the other hand, is not intended to carry fault current. Its job is to connect the grounded conductor and equipment to the grounding electrode.

According to an Informational Note in 250.102: The term supply conductors includes ungrounded conductors that do not have overcurrent protection on their supply side and terminate at service equipment or the first disconnecting means of a separately derived system.

The Grounded Conductor, Main Bonding Jumper, System Bonding Jumper, and Supply-Side Bonding Jumper are critical to the safe operation of the electrical system. They are the vital link back to the electrical supply that will cause the overcurrent device to trip and clear a fault. The bigger the service, or the larger the transformer on a separately derived system, the bigger a fault can be, and the bonding jumpers must be large enough to handle any fault that is possible on the system.

The Grounded Conductor, Main Bonding Jumper, System Bonding Jumper, and Supply-Side Bonding Jumper are selected based on the size of the ungrounded conductors at the service or the source of a separately derived system. As the size of the ungrounded conductors increases, so does the size of the bonding jumpers.

Conductors: grounded conductor, system bonding jumper, and grounding electrode conductor.

An important difference between using Table 250.102(C)(1) for selecting the right size Bonding Jumpers, and Table 250.66 for selecting the right size for the Grounding Electrode Conductor, is that a Grounding Electrode Conductor never has to be larger than 3/0 AWG. No matter how big the service or separately derived system gets, the Grounding Electrode Conductor never has to be larger than 3/0 AWG. You would think that for services where the ungrounded conductors were larger than 1100 kcmil the corresponding size of the Grounding Electrode Conductor would have to increase. But it doesn’t. Because the Grounding Electrode Conductor is not expected to carry fault current, 3/0 AWG is the largest it ever has to be.

Not so with the conductors that are sized from Table 250.102(C)(1). For services or separately derived systems where the ungrounded conductors are larger than 1100 kcmil, the Grounded Conductor, Main Bonding Jumper, System Bonding Jumper, and Supply-Side Bonding Jumper have to increase in size in proportion to the size of the ungrounded conductor.

Note 1 to Table 250.120(C)(1) says that for ungrounded phase conductors larger than 1100 kcmil copper or 1750 aluminum the size of conductors selected from the Table must have an area not less than 12 ½ percent of the area of the largest ungrounded supply conductor or the equivalent area for paralleled supply conductors.

For example, the equivalent area of four ungrounded, paralleled 500 kcmil conductors is 2000 kcmil. 12 ½ percent of 2000 kcmil is 250 kcmil. For a service that uses four paralleled 500 kcmil conductors per phase, the minimum size main bonding jumper is 250 kcmil.

Summary: Table 250.102(C)(1) is used to select the Grounded Conductor, Main Bonding Jumper, System Bonding Jumper and Supply-Side Bonding Jumper, based on the size of the largest ungrounded phase conductor. These bonding jumpers are intended to carry fault current. The size of the selected bonding conductors increase as the size of the ungrounded phase conductors increases, no matter how large the ungrounded conductors are.   Table 250.66 is used to select the size of the Grounding Electrode Conductor. It is not intended to carry fault current and never has to be larger than 3/0 AWG.

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