Electrical Systems Design-Grounding

What happens if there’s a high resistance or impedance in the ground-fault path?

The amount of current that flows is smaller and may not be large enough to trip the overcurrent protective device protecting the circuit.

According to 250.4(A)(5), what characteristics should ground paths have?

They shall be permanent and electrically continuous. They shall have the capacity to conduct any any fault currents that may be imposed. They shall provide a low-impedance path to ensure OCPD protection.

In an electrical context, what is ground?

Ground is the earth, represented as a conducting connection between electrical circuits or equipment and the earth. It can be intentional or accidental. Intentional connections are mae to ensure the benefits of grounding. 

What is a grounded conductor?

A grounded conductor is a conductor that has been intentionally connected or grounded. It can be a system conductor of a circuit conductor. It’s usually a neutral conductor, but not all electrical distribution systems use the grounded conductor as a neutral. 

What is an equipment grounding conductor?

An equipment grounding conductor (EGC) is an electrical conductor that provides a low-impedance path between electrical equipment and enclosures and the system grounded conductor and GEC. It provides a connection between the normally non-current-carrying metal parts of equipment to the system grounded conductor, the grounding-electrode conductor, or both. This connection usually occurs at the service equipment or at the source of power, such as a separately derived system, and helps to ensure the operation of the OCPD when fault conditions occur.

Why is the EGC connected to the system grounded conductor?

The EGC is connected to the system grounded conductor to facilitate the operation of the OCPD by providing a low-impedance ground-fault current path.

Can equipment function without an EGC?

Most equipment can work okay with a poor or no EGC, but they’re essential for life safety. So they really shouldn’t operate without an EGC.

What is a grounding electrode conductor, or GEC?

A grounding electrode conductor is the conductor that connects the grounding electrode(s) to the system grounded conductor and/or the equipment grounding conductor.

Where can the GEC be connected?

The GEC can be connected to the system grounded conductor and/or the EGC at the service equipment or at the source of a separately derived system. It’s only required to be made at one point on the grounding electrode system.

What can the GEC be made of?

The GEC may be constructed of aluminum, copper, or copper-clad aluminum; may be solid or stranded; and either insulated, covered, or bare per 250.62.

True or false: 3 AC systems of less than 50V must be grounded.

True

True/false: AC systems can be installed as either grounded or ungrounded systems.

True.

Under which conditions are AC systems operating at less than 50V required to be grounded?

The first condition, per 250.20(A)(1), involves systems that are supplied by transformers of circuits that operate at over 150 V to ground. These systems are required to be grounded to prevent the higher voltages of the primary from being imposed on the secondary side of the transformer in the case of a wiring fault.

The second condition under which an AC system of less than 50 V is required to be grounded occurs when the system is supplied from a transformer that is ungrounded. Per 250.20(A)(2), it is required that the low-voltage system be grounded because the supply system is ungrounded. Section 250.20(A)(3) requires that AC systems of less than 50 V be grounded when they are installed outside as overhead conductors.

When can the GEC be connected to the grounded conductor within the meter enclosure?

This can happen if the local utility permits it. The NEC lets us connect at the metering device or the service disconnecting means.

What does Section 250.24(A)(1) require?

Section 250.24(A)(1) requires that the system grounded conductor be connected to the GEC at any accessible point on the load end of the overhead service conductors, service drop, underground service conductors, or service lateral up to and including the grounded conductor terminal in the service disconnecting means.

What is the minimum size of the grounded conductor in electrical installations which use parallel service-entrance conductors?

the minimum size of the grounded conductor is based on the total circular-mil area of the conductors. It shall never be smaller than 12 1/2% of the largest service-entrance conductor.

What’s a separately derived system?

A separately derived system is an electrical source, other than a service, connected only to circuit conductors established by grounding and bonding connections. By definition, a separately derived system can have no direct electrical connection to the conductors of the supplying system. This also applies to the grounded conductor. If the grounded conductor is solidly connected, then there is not a separately derived system. Section 250.30 was revised in the 2020 NEC® to address installations in which multiple power sources of the same type are connected in parallel to form one system that supplies premises wiring. For the purpose of applying the requirements of 250.30, the multiple power sources shall be considered as a single derived system.

Where can the GEC and grounded conductor be connected?

The GEC and grounded conductor can be connected at the source of a separately derived system or at the first disconnecting means. A bonding jumper at both locations shall be permitted if doing so does not establish a parallel path for the grounded conductor per 250.30(A)(1), Exception 2.

Can ground references be transferred through windings of any kind?

No. The grounding connections that were in place for the service must be re-established.

What does 250.30(A)(1) say?

an unspliced system bonding jumper that complies with 250.28(A) to (D) must be installed between the EGC of the derived system and the grounded conductor. This connection shall be made at any point from the source of the separately derived system up to and including the first system disconnecting means or OCPD.

How do we size the bonding jumpers for separately derived systems?

We size them per 250.28(D). They can’t be smaller than the sizes shown in Table 250.102(C)(1). For conductors larger than 1100 kcmil Cu or 1750 kcmil Al, the bonding jumper must be no less than 12 1/2% of the area of the largest derived phase conductor.

How do we size the GECs for separately derived systems?

GECs for separately derived systems are sized based on the largest derived phase conductors per 250.30(A)(5). For single separately derived systems a GEC is required to connect the newly established grounded conductor to the grounding electrode. This connection must be made at the same point on the separately derived system where the system bonding jumper is connected. In some installations, such as a panelboard with main lugs only, the connection is made at the source of the separately derived system. The GEC is sized per 250.66. Instead of basing the calculation on the size of the largest service-entrance conductors, the calculation is based on the size of the largest derived phase conductors.

What does 250.30(A)(6) say?

Where multiple separately derived systems are installed on a floor or located on several floors of a multifloor building, it allows the use of a common GEC. It must be used to connect the separately derived systems to the grounding electrodes specified in 250.30(A)(4). The separately derived systems are then connected to the GEC by tap conductors.

Where are the requirements for grounding portable generators?

Section 250.34

What does 250.34(A)(2) say?

The frame of the generator shall be bonded to the equipment grounding terminals of the receptacles and to any non-current-carrying parts of equipment that is served from the generator. Bonding the receptacle terminals to the frame of the generator ensures that any equipment which is cord-and-plug connected to the generator is also connected to the generator frame which serves as the GEC.

When don’t we have to connect metallic elbows installed in nonmetallic raceways to the grounded system or electrode conductor?

When they’re at least 18” below grade.

True or false: we can install short sections of metal enclosures or raceways without connecting them to the EGC.

True. However, the NEC doesn’t define what constitutes a short section, so we should always consult the AHJ.

Do we have to connect permanently wired, fastened-in-place equipment with exposed non-current-carrying metal parts to the EGC?

Yes.

What does Section 250.112 do?

It lists specific equipment that shall be connected to the EGC if it contains exposed, noncurrent-carrying metal parts that are likely to become energized. There are no voltage limitations to these requirements.

What does Section 250.110 do?

Section 250.110 lists six general conditions where equipment which contains exposed non-current-carrying metal parts, and which is fastened in place or connected by a permanent wiring method shall be connected to the EGC. No specific equipment is mentioned. The requirements apply generally to all electrical installations.

Give one solution to the problem of grounding cord-and-plug-connected equipment. 

Double-insulated equipment. We can use it in some cases in lieu of a separate EGC. 

What does Subsection 250.114(1) to (4) contain?

Subsection 250.114(1) to (4) contains four conditions under which exposed, normally non-current-carrying metal parts of cord-and-plug-connected equipment shall be connected to the EGC. These are: the equipment is in a hazardous location, over 150V to ground applications, residential occupancies, and other-than-residential occupancies.

Where are the connections grounding electrical services usually made?

Electrical services are grounded by several connections that usually are made within or at the service equipment. The EGC shall be bonded to the grounded service conductor and the GEC per 250.130(A).

What does 250.118 do?

It provides for 14 types of EGCs.

How shall we ground all non-current-carrying metal parts of equipment, enclosures, etc?

They must be grounded. We can use a recognized EGC found in 250.118(2) to (14),or a method not specified in the powerpoint. 

Do new branch-circuit installations for ranges, clothes dryers, and ovens require an EGC?

Yes. New installations also require the frames to be grounded.

What methods can we use to bond service equipment?

There are 4 methods per 250.92(B). We can use a grounded service conductor. We can use threaded couplings or listed threaded hubs. We can use fittings for bonding purposes when they’re made wrenchtight. Threadless connections can be used where made up tight for metal raceways and metal-clad cables. We can use other listed devices. We can use bonding or grounding-type locknuts.

What does 250.94(A) say?

In 250.94(A), an intersystem bonding termination (IBT) must be supplied outside of enclosures to ensure an electrical connection from various communication systems to the service equipment. This section requires an external bonding termination for connecting intersystem bonding and grounding conductors. Section 250.94 requires that the IBT be accessible for inspection and termination and have the capacity to connect at a minimum of at least three intersystem bonding conductors.

What does Section 250.97 contain?

Bonding requirements for circuits over 250V. This section requires that electrical continuity be ensured by either threaded connections, threadless connections, bonding jumpers, or other devices. These are the same methods used for bonding the service equipment except for the grounded conductor.

What must be true of the main bonding jumper when it’s screw-only?

The screw shall be identified with a green finish and be visible when installed. It’s attached by a green screw or exothermic welds, listed pressure connectors, listed clams, or other listed means.

What’s an equipment bonding jumper?

An equipment bonding jumper (EBJ) is a conductor that connects two or more parts of the equipment grounding conductor.

What must equipment bonding jumpers and main bonding jumpers be made of?

. Like MBJs, EBJs must be constructed of copper, aluminum, copper-clad aluminum, or other corrosion-resistant materials.

What section contains provisions for bonding jumpers?

Section 250.102

How do we size line-side EBJs and MBJs?

Where the size of the service-entrance conductors exceeds 1100 kcmil Cu or 1750 kcmil Al, the size of the line-side EBJ or MBJ shall not be smaller than 12-1/2% of the phase conductors. If the service-entrance conductors are run in parallel, then the EBJs shall also be run in parallel. Unlike grounded conductors, EBJs, when run in parallel, shall be full-size conductors based on the largest conductor in each raceway or cable.

How do we size load-side EBJs?

Load-side EBJs are sized per 250.102(D) and selected based upon the values listed in Table 250.122. The bonding jumper size is determined by the size or rating of the OCPD ahead of the equipment. If the EBJ is to be used for more than one circuit in the raceway or cable, the size is selected from Table 250.122 based upon the largest OCPD protecting a circuit within the raceways or cables.

Do we have to bond single beams in dwelling units?

No.

What are the bonding requirements for exposed interior structural steel?

If this steel forms a metal frame of a building, it shall be bonded to the service equipment enclosure, the service grounded conductor, or the GEC. This section applies to structural steel that is interconnected to form a metal frame. The provisions of this section do not apply to a steel beam supported by concrete columns.

What does Section 250.24(A)(D) say?

Section 250.24(A)(D), which details the grounding connections required for an AC service that is to be grounded, requires that the GEC be connected to a grounding electrode that complies with Article 250. Part III contains the requirements for the grounding electrode system.

What do we do if none of the electrodes specified by 250.52(A)(1) to (3) are available or the only electrode is available is the metal underground water pipe?

In these cases, a supplemental electrode shall be installed. Section 250.52(A)(4) to (8) contains the requirements for made or other electrodes. An electrode is simply a means to permit current to enter the earth. Made electrodes can be designed to function much like the electrodes listed in 250.52(A)(1) to (3). As with these electrodes, made electrode connections shall be free of nonconductive coverings that may affect the impedance of the grounding path.

What does 250.64(E) say?

Per 250.64(E), the ferrous metal enclosures or raceways in which a GEC is installed shall be electrically continuous. This requires that both ends of the GEC enclosure be bonded to either the equipment or electrode or to the GEC itself to create a parallel path to ground.

How do we size the GEC?

The size of the GEC is selected from Table 250.66 and is based upon the largest service-entrance conductor for one phase. If the service-entrance conductors are paralleled, the equivalent area for a set of phase conductors shall be used as a basis for sizing the GEC.

How do we size the GEC to a ground rod or other made electrode?

The GEC to a ground rod or other made electrode need not be larger than 6 AWG Cu when it is the sole connection to the grounding electrode. The size of the GEC reflects, to some degree, the ability of the grounding electrode to dissipate current flow into the earth. For example, 250.66(A), (B), and (C) permit smaller-size GECs for specific types of grounding electrodes. For sole connections to a single or multiple made electrode, 250.66(A) permits the GEC to be 6 AWG Cu or 4 AWG Al. The GEC is not required to be larger than 4 AWG Cu per 250.66(B) for sole connections to a single or multiple concrete-encased electrode. For ground rings, 250.66(C) permits the GEC, which is a sole connection, to be the same size as the conductor used for the ground ring. In the 2017 NEC®, 250.66(A), (B), and (C) were revised to include bonding jumpers in the rules for maximum conductor sizes for GECs. The revision also added a sentence to each of the subparts that makes it clear that the maximum wires sizes included in the rules only apply where the GEC or bonding jumper does not extend to other types of electrodes that require a larger size of conductor.