Transformer Sizes, Metal Clad Cables and More

Outside feeder grounding and bonding

Q: What grounding and bonding requirements apply to an outside feeder that supplies a woodworking hobby shop that is located in a detached building and is about 60 feet from, and supplied from, a single-family residence? The total calculated load is 38 amperes and the feeder is three-wire, 120 to 240 volts, single-phase underground in rigid metal conduit. Loads that are permanently connected are a wood lathe, belt sander, band saw, and lighting. Cord-and-plug connected tools include a screwdriver and drill. The feeder conductors will be three 8 AWG type THW insulated copper conductors.

Is the grounded-circuit conductor (neutral) required to be grounded at the feeder disconnect? What are the requirements for bonding the electrical system at the building served?

A: A grounding-electrode system must be used to connect the rigid metal conduit and building disconnecting means together and to the ground. The grounding-electrode conductor does not have to be larger than 8 AWG copper, but it must be installed in rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, electrical metallic tubing or cable armor. To avoid encasement of the grounding-electrode conductor, a 6 AWG copper or larger grounding-electrode conductor can be used. The requirement for protection of an 8 AWG copper grounding-electrode conductor is in 250.64(B).

The feeder neutral conductor cannot be grounded at the building served. This sentence appears in 250.32(B)(1): “Any installed grounded conductor shall not be connected to the equipment grounding conductor or to the grounding electrode(s).”

If the motor loads are not large enough to permit a 60-ampere overcurrent device to protect the 8 AWG Type THW copper conductors, 40-ampere overcurrent protection must be provided. However, the feeder disconnecting means cannot have a rating less than 60 amperes.

This minimum rating is required by 225.39(D). To comply with the overcurrent protection requirements of 240.4 for conductors and 225.39(D) for the minimum ampere rating of the disconnecting means, use a 60-ampere safety switch with 40-ampere fuses installed.

Other requirements for the feeder disconnecting means include marking, “Suitable for Use as Service Equipment,” and the disconnecting means must be installed at a readily accessible location nearest the point of entrance of the conductors.

Fire and jockey pump size

Q: What size transformer is required to supply a 208-volt, three-phase, 50-horsepower fire pump motor and a three-horsepower, 208-volt, three-phase pressure maintenance pump motor? The supply voltage is 480 volts, three phase. Nameplate full-load current for the fire pump motor is 140 amperes, and for the three-horsepower motor the full-load current is 10 amperes. What size overcurrent protection is required for the primary and secondary of the transformer?

A: Where the utility service voltage is different from the fire pump motor utilization voltage, a transformer that complies with 695.5 may be used. First of all, the transformer must be sized for at least 125 percent of the motor loads. Using the table values of motor full-load current from 430.250, 10.6 amperes is the figure for the three-horsepower motor. The table value of full-load current for a 50 horsepower motor is 143 amperes. Now, calculate the minimum kVA rating of the transformer: (143 +10.6) 1.25 x 208 x 1.73 divided by 1,000 = 69 kVA. A 75 kVA transformer satisfies the National Electrical Code (NEC) requirement.

Primary overcurrent protection must be no less than 600 percent of the sum of the full-load currents of the two motors. Therefore, the primary full-load current resulting from the motors is about 66 amperes (208 divided by 480 times 153.6). According to 695.5(B) primary overcurrent protection cannot be less than 600 percent of the full-load currents of the motors. Multiplying 66 by 600 results in a current of 396 amperes. This requires 400-ampere fuses or a 400-ampere circuit breaker.

Overcurrent protection on the secondary side of the transformer is prohibited by this sentence in 695.5(B): “Secondary overcurrent protection shall not be permitted.”

Feeder-conductor size is based on 125 percent of the full-load current of the fire pump motor and pressure maintenance pump motor, which is 143 + 10.6 x 1.25 = 192 amperes. Copper feeder conductors with 75ºC insulation and size 3/0 AWG are adequate for this installation. Branch circuit conductors for the fire pump motor must have an ampacity of 179 (143 x 1.25), and for the jockey pump, 13 (10.6 x 1.25).

Permission to size feeder and branch circuit conductors at 125 percent of full-load currents of the motors is granted by 695.6(C)(1), which reads as follows: “Fire Pump Motors and Other Equipment. Conductors supplying a fire pump motor(s) pressure maintenance pumps, and associated fire pump accessory equipment shall have a rating not less than 125 percent of the sum of the fire pump motor(s) and pressure maintenance pump motor(s) full-load current(s), and 100 percent of the associated fire pump accessory equipment.”

Although the overcurrent protection for the transformer does not comply with the requirements in 450.3, this paragraph appears in 450.1: “This article covers the installation of transformers dedicated to supplying power to a fire pump installation as modified by Article 695.”

Use of metal clad cable

Q: Is metal clad cable with interlocking metal tape and encapsulated in a nonmetallic material suitable for direct burial in the earth or concrete encasement?

A: Yes. If the cable is identified as suitable for direct burial, it may also be installed in concrete. This is what 330.12 says: “Uses Not Permitted. Type MC cable shall not be used where exposed to the following destructive corrosive conditions, unless the metallic sheath is suitable for the conditions:

(1) Where subject to physical damage;

(2) Direct burial in the earth;

(3) In concrete; FPN to item (3): MC cable that is identified for direct burial applications is suitable for installation in concrete.”

Also, 330.10 (A)(11) permits the use of Type MC cable in wet locations (burial in the earth) where any of the following conditions are met: the metallic covering is impervious to moisture; a lead sheath or moisture-impervious jacket is provided under the metal covering; the insulated conductors under the metallic covering are listed for use in wet locations.

Bottomless junction boxes

Q: Are listed junction boxes without bottoms installed in the earth recognized in the National Electrical Code? If the answer is yes, where are the rules for installation of these products?

A: These boxes are covered in 314.29 and 314.30. For underground installations, the contained wiring must be accessible without having to remove concrete, asphalt, earth, etc. An Exception permits gravel, light aggregate or noncohesive granulated soil above the cover if the location of the enclosure is properly marked.

The NEC uses the phrase “handhole enclosures” to identify these boxes. They must be designed and installed to withstand all loads likely to be imposed. For conductors of 600 volts or less, they must be sized to comply with 314.28(A)(2) Exception: Underground wiring entering or leaving the handhole does not have to be connected to the sides of the box. Handhole covers must be marked to identify their function, such as “electric.” Handhole covers must require tools to remove if they weigh 100 pounds or less, and exposed conductive surfaces must be bonded to comply with 250.96(A).

Number of service-conductor sets

Q: Does the National Electrical Code permit four sets of 2/0 AWG copper underground service conductors to supply 4- to 200-ampere service disconnects for four apartments? Do the service conductors have to be connected in parallel, or may each set of conductors be terminated in a separate 200-ampere switch?

A: This installation is permitted by the part of 230.2 that allows underground-service conductors 1/0 AWG and larger to be connected together at the supply end but not the load end. This is the sentence: “For the purpose of 230.40, Exception No. 2 only, underground sets of conductors, 1/0 AWG and larger, running to the same location and connected together at their supply end but not connected together at their load end shall be considered to be supplying one service.”

The supporting comment for this proposal indicated that the maximum available fault current would be reduced by acceptance of this proposal.

Exception No. 2 in 230.40 requires that the individual disconnecting means be grouped at one location. Various sections require identification of each switch to indicate the loads served.

Single-family neutral conductor

Q: A three-wire, single-phase, 120- to 240-volt service for a single-family residence has a calculated load of 198 amperes on the ungrounded conductors and 170 amperes maximum unbalanced current on the neutral. Is it permissible to use a 1/0 AWG Type THWN copper conductor for the neutral and 2/0 AWG Type THWN copper conductors for the ungrounded service-entrance conductors?

A: Yes, the elevated ampacities given in Table 310.15(B)(6) may be used for this installation. These conductor sizes are also permitted for the main power feeder between the service disconnect and the lighting and appliance branch circuit panelboard. Reduction in the size of the neutral conductor is permitted by 220.61(A) and (B). EC

FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at 504.734.1720 





About the Author

George W. Flach

Code Q&A Columnist
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 in...

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