All questions and answers are based on the 2011 NEC.

Q. What are the grounding electrode requirements for a separately derived system, such as a transformer? [250.30(A)(4)]

A. First of all, let’s define a separately derived system. According to Art. 100, a separately derived system is a wiring system whose power is derived from a source where there’s no direct electrical connection to the supply conductors of another system. Transformers are considered separately derived when the primary conductors have no direct electrical connection from circuit conductors of one system to circuit conductors of another system, other than connections through the earth, metal enclosures, metallic raceways, or equipment grounding conductors, as shown in Fig. 1.

A generator having transfer equipment that switches the neutral conductor — or one that has no neutral conductor at all — is a separately derived system and must be grounded and bonded in accordance with 250.30(A). Note 2 in this section states: For non-separately derived systems, see 445.13 for the minimum size neutral conductors necessary to carry fault current.

Separately derived systems must be grounded and bonded in accordance with (A)(1) through (A)(8) [250.30(A)]. A neutral-to-case connection must not be made on the load side of the system bonding jumper, except as permitted by 250.142(B). Caution: Dangerous objectionable neutral current will flow on conductive metal parts of electrical equipment as well as metal piping and structural steel, in violation of 250.6(A), if more than one system bonding jumper is installed — or if it’s not located where the grounding electrode conductor terminates to the neutral conductor.

(1) An unspliced system bonding jumper must be installed at the same location where the grounding electrode conductor terminates to the neutral terminal of the separately derived system — either at the separately derived system or the system disconnecting means, but not at both locations [250.30(A)(5)]. A system bonding jumper is the connection between the neutral conductor and supply side bonding jumper or equipment grounding conductor or both at a separately derived system [Art. 100].

Where the system bonding jumper is installed at the source of the separately derived system, the jumper must connect the neutral conductor of the derived system to the supply-side bonding jumper and the metal enclosure of the source (transformer case), as shown in Fig. 2. Where the system bonding jumper is installed at the first disconnecting means of a separately derived system, the jumper must connect the neutral conductor of the derived system to the supply-side bonding jumper and the metal disconnecting means enclosure, as shown in Fig. 3.

A system bonding jumper is a conductor, screw, or strap that bonds the metal parts of a separately derived system to the system neutral point [Art. 100 Bonding Jumper, System], and it’s sized to Table 250.66 in accordance with 250.28(D). Danger: During a ground fault, metal parts of electrical equipment, as well as metal piping and structural steel, will become and remain energized, providing the potential for electric shock and fire if the system bonding jumper isn’t installed.

(2) Supply-side bonding jumper. If the separately derived system and the first disconnecting means are located in separate enclosures, a supply-side bonding jumper must be run to the derived system disconnecting means. The supply-side bonding jumper can be a nonflexible metal raceway, a wire, or a bus.

If the supply-side bonding jumper is of the wire type, it must be sized in accordance with Table 250.66, based on the area of the largest ungrounded derived system conductor in the raceway or cable. If the supply-side bonding jumper is a bus, it must have a cross-sectional area no smaller than required by Table 250.66.

(3) If the system bonding jumper is installed at the disconnecting means instead of at the source, the following requirements apply when sizing the system neutral conductor:

• Sizing for single raceway. Because the neutral conductor of a derived system serves as the effective ground fault current path for ground fault current, it must be routed with the ungrounded conductors of the derived system and be sized not smaller than specified in Table 250.66, based on the area of the ungrounded conductor of the derived system.

• Parallel conductors in two or more raceways. If the conductors from the derived system are installed in parallel in two or more raceways, the neutral conductor of the derived system in each raceway or cable must be sized not smaller than specified in Table 250.66, based on the area of the largest ungrounded conductor of the derived system in the raceway or cable. In no case is the neutral conductor of the derived system permitted to be smaller than 1/0 AWG [310.10(H)].

If the system bonding jumper is installed at the disconnecting means instead of at the source, an equipment bonding conductor must connect the metal parts of the separately derived system to the neutral conductor at the disconnecting means in accordance with 250.30(A)(2).

(4) Grounding electrode. The grounding electrode must be as near as practicable, and preferably in the same area where the system bonding jumper is installed and be one of the following:

1) Metal water pipe electrode, within 5 ft of the entry to the building [250.52(A)(1)].

2) Metal building frame electrode [250.52(A)(2)].

Exception No. 1: If the electrodes specified in 250.30(A)(4) aren’t available, one of the following electrodes can be used:

• A concrete-encased electrode encased by not less than 2 in. of concrete, located horizontally near the bottom or vertically, and within that portion of concrete foundation or footing that’s in direct contact with the earth [250.52(A)(3)].

• A ground ring electrode encircling the building/structure, buried not less than 30 in. below grade, consisting of at least 20 ft of bare copper conductor not smaller than 2 AWG [250.52(A)(4) and 250.53(F)].

• A ground rod electrode having not less than 8 ft of contact with the soil meeting the requirements of 250.52(A)(5) and 250.53(G)].

• Other metal underground systems, piping systems, or underground tanks [250.52(A)(8)].

Note 1: Interior metal water piping in the area served by separately derived systems must be bonded to the separately derived system in accordance with 250.104(D).

(5) Grounding electrode conductor, single separately derived system. The grounding electrode conductor must be sized in accordance with 250.66, based on the area of the largest ungrounded conductor of the derived system. A grounding electrode conductor must connect the neutral terminal of a separately derived system to a grounding electrode of a type identified in 250.30(A)(4) at the same point on the separately derived system where the system bonding jumper is connected.

System grounding also helps reduce fires in buildings as well as voltage stress on electrical insulation, thereby ensuring longer insulation life for motors, transformers, and other system components. To prevent objectionable neutral current from flowing [250.6] onto metal parts, the grounding electrode conductor must originate at the same point on the separately derived system where the system bonding jumper is connected [250.30(A)(1)].

Exception No. 1: If the system bonding jumper is a wire or bus bar, then the grounding electrode conductor is permitted to terminate to either the neutral terminal or the equipment grounding terminal, bar, or bus in accordance with 250.30(A)(1).

Exception No. 2: Separately derived systems rated 1kVA or less are not required to be grounded (connected to the earth).

(6) Grounding electrode conductor, multiple separately derived systems. Where there are multiple separately derived systems, a grounding electrode conductor tap from each separately derived system to a common grounding electrode conductor is permitted. This connection is to be made at the same point on the separately derived system where the system bonding jumper is connected [250.30(A)(1)].

Exception No. 1: If the system bonding jumper is a wire or bus bar, then the grounding electrode conductor tap can terminate to either the neutral terminal or the equipment grounding terminal, bar, or bus in accordance with 250.30(A)(1).

Exception No. 2: Separately derived systems rated 1kVA or less are not required to be grounded (connected to the earth).

 

Q. What are the GFCI protection requirements of space-heating cables?

A. GFCI protection is required for electric space-heating cables that are embedded in concrete or poured masonry floors of bathrooms, kitchens, and hydromassage bathtub locations [424.44(G)]. See 680.28(C)(3) for restrictions on the installation of radiant-heating cables for spas and hot tubs installed outdoors.