Storage Battery Disconnects, Generators for Fire Pumps and More

By George W. Flach | Aug 15, 2009
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Article 230 Services; Article 240 Overcurrent Protection; Article 250 Grounding and Bonding; Article 330 Metal-Clad Cable: Type MC; Article 350 Liquidtight Flexible Metal Conduit: Type LFMC; Article 408 Switchboards and Panelboards; Article 409 Industrial Control Panels; Article 480 Storage Batteries; Article 695 Fire Pumps

Disconnect for storage batteries

The emergency power supply for a building consists of a bank of batteries operating at 120 volts. I installed a fused disconnect for the batteries in the adjacent room at the transfer switch. The inspector said I need a disconnect within sight of the bank of batteries. An in-sight disconnect is not required by Section 240.21(H). Is the inspector correct in his interpretation?

Yes, he is. Although, a disconnect within sight of the battery bank is not required by Section 240.21(H), which reads: “Battery Conductors. Overcurrent protection shall be permitted to be installed as close as practicable to the storage battery terminals in a non-hazardous location. Installation of the overcurrent protection within a hazardous location shall also be permitted.” However, a disconnect is required to be within sight of the batteries by Section 480.5. This section is new in the 2008 edition of the National Electrical Code (NEC) and reads: “Disconnecting Means. A disconnecting means shall be provided for all ungrounded conductors derived from a stationary battery system over 30 volts. A disconnecting means shall be readily accessible and located within sight of the battery system.”

Transformer grounding-electrode conductor size

What rules are used, and how is the grounding--electrode conductor sized from the secondary of a transformer to a buried metal water pipe that is more than 10 feet long? Please include a sample calculation.

Rules for grounding the secondary conductor (separately derived system) of a transformer are in Section 250.30, although other sections also may apply. Where the transformer supplies a grounded system, the grounded conductor should not be connected to normally noncurrent-carrying metal parts of the equipment, connected to equipment-grounding conductors or reconnected to ground on the load side of the point of grounding of a separately derived system.

An unspliced system-bonding jumper that is sized based on the derived phase conductors is used to connect the equipment-grounding conductors of the separately derived system to the grounded-circuit conductor. The main bonding jumper is sized to comply with Table 250.66.

Where an equipment-bonding jumper is run with the derived conductors of the transformer, it must be installed to comply with Section 250.102(C). This section also requires that the size of the equipment--grounding conductor be based on Table 250.66.

As an example, let’s take a 100-kVA, three-phase, four-wire, 480/277-volt transformer with rated secondary of 208Y/120-volt and secondary conductors of 250 kcmil copper with 75°C insulation. A buried metal water pipe is used for the grounding electrode along with the reinforcing steel in the building. According to Table 250.66, a 2 AWG copper grounding-electrode conductor and system-bonding jumper are required for this installation. Where an equipment-grounding conductor is run with the secondary conductors, it also must be at least a 2 AWG copper conductor.

Generator size for fire pump motor

What is the minimum size on-site generator required for backup protection for an electric motor drive fire pump? Does the generator have to provide locked-rotor current for the fire pump motor?

The on-site generator must be sized to start and run the fire pump motor. This means the generator must have adequate capacity to line-start the fire pump oven, though the fire pump controller provides reduced-voltage or part-winding starting of the fire pump motor. The generator must have sufficient capacity to start and accelerate the fire pump motor to operating speed when the controller is placed in emergency run mechanical starting. This position puts the fire pump motor across the line regardless of the normal starting method of the fire pump motor.

The sizes of the generator and overcurrent protective device do not have to be 600 percent of the fire pump motor full-load current. However, the generator and overcurrent protection must allow the fire pump motor to start and carry the load. Where the normal power supply to the fire pump is from the utility and it meets the reliability requirements of Section 695.3(A) and (1), an on-site generator is not required unless required by the authority having jurisdiction.

For larger generators that are capable of producing more than 225 percent of the fire pump motor full-load amperes, the circuit conductors in the building must be encased in a minimum of 2 inches of concrete or be protected by a fire-rated assembly listed to achieve a minimum fire rating of two hours, or it must be a listed electrical circuit protective system with a minimum a two-hour fire rating. This requirement for protection of fire pump supply conductors is in Section 695.6(B).

Supporting and securing Type MC cable between fluorescent luminaires

What is the maximum distance between supports of Type MC cable installed between ceiling-mounted fluorescent luminaires?

Generally, Type MC cable must be secured at intervals not exceeding 6 feet, but cables containing not more than four conductors and not larger than 10 AWG must be secured within 1 foot of every box, cabinet or other enclosure.

Type MC cable must be supported at intervals not exceeding 6 feet. However, this cable is permitted to be unsupported where the length is not more than 6 feet or 12 feet between support points, provided that the cable supplies luminaires or other electrical equipment in a suspended or accessible ceiling. Section 330.20(D) reads: “Type MC Cable shall be permitted to be supported where the cable is not more than 6 feet long from the last point of support to the connection point at the luminaires. Type MC cable fittings shall be permitted as a means of cable support to comply with this section.”

Therefore, 6 feet of unsupported cable from one luminaire and 6 feet of unsupported cable extending to the next fixture allow an unsupported length of 12 feet of cable between luminaires. So, the maximum unsupported length of type MC cable between luminaires is 12 feet.

Securing and supporting FMC

Does the NEC require liquidtight flexible metal conduit (LFMC) to be secured or supported where a 6-foot length is run between an outdoor air conditioning unit and a disconnecting means?

The answer is yes. LFMC must be secured within 1 foot of the air conditioner enclosure and within 1 foot of the disconnect switch enclosure. No additional support or securing is required in the 4-foot run of conduit between the points where the cable is secured. There are four exceptions to this requirement, but they do not apply to this installation. The rules for securing and supporting LFMC are in Section 350.30.

Identification of high leg in three-phase, four-wire delta system

When and how often is it necessary to identify the conductor with the higher voltage to ground on a three-phase, four-wire delta supply?

There is a requirement in Section 110.15 that requires special marking of the high leg in a four-wire delta system. It reads: “High-Leg Marking. On a four-wire, delta connected system where the midpoint of one phase winding is grounded, only the conductor or bus bar having the higher phase voltage to ground shall be durably and permanently marked by an outer finish that is orange in color or by other effective means. Such identification shall be placed at each point of the system where a connection is made if the grounded conductor is also present.”

For service conductors, the wire with the higher voltage to ground must be identified by an outer finish that is orange or other effective means at each termination or junction point. This requirement is in Section 230.56.

The phase arrangement on three-phase buses is required to be A, B, C from front to back, top to bottom or left to right as viewed from the front of the switchboard or panelboard. The conductor with the higher voltage to ground must be connected to the B phase. An exception to this requirement in Section 408.3(E) allows the high leg to have the same configuration as the meter where the meter is in the same section of a multi-section switchboard. In other words, the high leg does not have to be connected to the B phase under this condition.

A new part (F) of Section 408.3 requires field-marking of a switchboard or panelboard that contains a four-wire, delta system. This requirement reads: “High Leg Identification. A switchboard or panelboard containing a four-wire, delta-connected system where the midpoint of one phase winding is grounded shall be legibly and permanently field marked as follows: ‘Caution ____ Phase Has ____ Volts to Ground.’”

For industrial control panels, the phase arrangement for buses is the same as for switchboards and panelboards, and the conductor with the higher voltage to ground must be connected to the B phase (see Section 409.102(B)).

FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. Questions can be sent to [email protected].

About The Author

George W. Flach was a regular contributing Code editor for Electrical Contractor magazine, serving for more than 40 years. His long-running column, Code Q&A, is one of the most widely read in the magazine's history. He is a former chief electrical inspector for New Orleans and held many other prestigious positions in the electrical industry, including IAEI board of directors and executive committee. He passed away in August 2009.

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