Overcurrent Protection, Tamper-Resistant Receptacles and More

Article 210 Branch Circuits; Article 220 Branch Circuit, Feeder, and Service Calculations; Article 240 Overcurrent Protection; Article 250 Grounding and Bonding; Article 330 Metal-Clad Cable: Type MC; Article 422 Appliances; Article 450 Transformer and Transformer Vaults (Including Secondary Ties)

Grounding-electrode conductor size and location

I installed a service that consists of one 200-ampere disconnect and three 100-ampere disconnects in separate enclosures. A wireway, which contains service-entrance conductors, supplies these disconnects. I used a grounding electrode with taps from each 100-ampere disconnect. The service consists of three 3 AWG Type THWN copper conductors for the 100 ampere disconnects and one set of 3/0 AWG Type THWN copper conductors for the 200-ampere disconnect. What size grounding--electrode conductor is required?

There are numerous ways to ground this service. A single grounding---electrode conductor may be connected to the grounded service conductor in the wireway. The size of this grounding--electrode conductor is obtained by taking the sum of the service--entrance conductors and using this total circular mil area to size the grounding--electrode conductor to be connected to the buried metal water pipe. The size of the service--entrance conductors is obtained by adding the areas of the service-entrance conductors together to get the actual size of the service-entrance conductors entering the wireway: 3 AWG copper has a circular mil area of 52,620 and 3/0 AWG has a circular mil area of 167,800. Adding these together provides an equivalent size of the -service-entrance conductors of 325,660 circular mils (3 × 52,620 + 167,800). According to Table 250.66, the grounding-electrode conductor cannot be smaller than 2 AWG copper. This method of connecting the grounding-electrode conductor to the service-entrance conductors is in compliance with 250.64(D)(4).

The 2 AWG copper conductor also can be run from the 200-ampere disconnect under the three 100-ampere disconnects. Taps are made from each 100-ampere switch to the 2 AWG copper grounding--electrode conductor. Table 250.66 requires an 8 AWG copper conductor for the 3 AWG copper service-entrance conductors. Taps must be made by exothermic welding or with connectors that are listed as grounding and bonding equipment. This method of grounding is recognized by 250.64(D)(1).

An individual grounding-electrode conductor may be connected to each disconnect, one 4 AWG copper conductor from the 200-ampere disconnect to the water pipe and three 8 AWG copper conductors for the three 100-ampere disconnects. If an 8 AWG copper grounding--electrode conductor is used for the 100-ampere disconnects, it must be protected from physical damage by installing it in rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, electrical metallic tubing or cable armor. This requirement appears in 250.63(B).

The metal water pipe used as an electrode must be supplemented with a concrete-encased electrode where present or other electrodes mentioned in 250.52.

Transformer secondary protection

Under what conditions is secondary overcurrent protection for transformers not required?

There are many rules that allow omission of overcurrent protection for the secondary windings of a transformer, but secondary overcurrent protection for secondary conductors usually is required.

Table 450.3 lists overcurrent protection rules for transformers with primary currents of less than 9 amperes and 9 amperes or more. Where the primary current is 9 amperes or more and primary overcurrent protection does not exceed 125 percent of primary rated current, secondary overcurrent protection is not required. A transformer with a full load current of less than 9 amperes and protected by an overcurrent device not exceeding 167 percent of primary rated current and secondary current above or below 9 amperes does not need overcurrent protection. Protection is required unless the rated secondary current is 2 amperes or less; then overcurrent protection is required that cannot exceed 300 percent of secondary rated current.

Overcurrent protection is not required for Class 2 and Class 3 transformers where the power source does not exceed the voltages listed in Table 11(A) of Chapter 9.

Control transformers used with motor controllers may not need secondary overcurrent protection. Section 430.72 and Table 430.72 provide information on control-circuit conductor size and overcurrent protection.

There is no requirement for secondary winding protection where the transformer primary and secondary windings have two wires or where three-wire, three-phase delta primary to three-phase, three-wire delta secondary transformers are used. Permission to eliminate this secondary winding overcurrent protection is allowed by part (F) of 240.4. A 25-kVA transformer with a 240-volt primary and 120-volt secondary is an example. Single-phase primary current equals kilovolt-amperes divided by volts (25,000 divided by 240 = 105 amperes). Primary overcurrent protection installed is 125 amperes. Secondary conductor size cannot be less than 250 amperes (125 × 240 divided by 120). According to Table 310.16, a 250 kcmil copper conductor with Type THWN insulation is the minimum size conductor permitted to comply with 240.4.

Although overcurrent protection is not required to protect the secondary conductors of the transformer, the conductors connected to the secondary must be protected to comply with the various rules in Article 240.

Separate branch circuit in bedrooms

Is there anything in the 2008 edition of the National Electrical Code (NEC) that requires a separate branch circuit in each bedroom in a single-family residence? The plan examiner has requested a separate branch circuit for each bedroom. One bedroom measures 9-by-12 feet, two bedrooms measure 12-by-2 feet and the master bedroom measures 12-by-14 feet.

No, there is not. The minimum load required by 220.12 and Table 220.12 is 3 volt-amperes per square foot. Therefore, the minimum load for the small bedroom is 2.7 amperes (9 × 12 × 3 divided by 120). For the two 12-by-12 bedrooms, the current is 7.2 amperes (12 × 12 × 2 × 3 divided by 120). For the 12-by-14 bedroom, the load is 4.2 amperes (12 × 13 × 3 divided by 120) . The total calculated minimum load for four bedrooms is 14.1 amperes (2.7 + 7.2 + 4.2). These 4 bedrooms may be connected to a single 15- or 20-ampere branch circuit to satisfy the Code.

Remember the NEC is a minimum standard. It provides installations that are essentially free from hazard but may not be efficient, convenient or adequate for good service.

The plan reviewer may be enforcing a local amendment to the 2008 edition of the NEC, or he/she may be asking for one branch circuit for each bedroom to reduce the troubleshooting time should an arcing fault occur in a bedroom. Otherwise, requiring one branch circuit for each bedroom is a design consideration and is not required by the NEC.

Tamper-resistant receptacles

Did Code-Making Panel 18 consider the difficulty elderly or impaired people might have when inserting an attachment cap (plug) in a 15- or 20-ampere, 125-volt tamper-resistant receptacle? These receptacles are now required by 406.11 in all dwelling units.

Yes, it did. The panel accepted this proposal, stating, “The panel is concerned about the possible increased insertion force required for our aging population. The panel requests data concerning the amount of force necessary to insert a plug into the shutter and the amount of force necessary to fully insert a plug into a tamper-resistant receptacle.” During the public comment period, NEMA submitted a public comment, which states, “The typical insertion forces observed could be characterized as follows: when the plug blades are initially inserted into a tamper--resistant receptacle, a small force of approximately 1–1.5 pounds is required to overcome the initial resistance of the -tamper-resistant mechanism. This followed by a drop in force as the plug blades have opened the tamper-resistant mechanism and are passing through. As insertion continues, at the point where the blades reach and become engaged with the receptacle contacts, the force increases. This is where the maximum force is observed.

“The typical insertion force varied from 10–20 pounds, depending on the design of the receptacle. There was an appreciable difference in insertion force between -tamper-resistant receptacles and receptacles without the tamper-resistant mechanism. The overall force required to open the receptacle contacts are far greater than the force exerted by the tamper-resistant mechanism.”

These tests indicate that insertion of a plug into a tamper-resistant receptacle should not be a problem for physically impaired and elderly people.

Securing Type MC cable

Does the NEC permit 12 feet of MC cable between fluorescent luminaires in a suspended ceiling with a strap to secure the -cable 6 feet from each luminaire?

Yes, this support meets the requirement of Section 330.30(D), which states, “Unsupported Cables. Type MC Cable shall be permitted to be unsupported where the cable (2) is not more than 1.8 m (6 ft) in length from the last point of support to the point of connection to the luminaires or other electrical equipment and the cable and point of connection are within an accessible ceiling. For the purpose of this section, Type MC Cable fittings shall be permitted as a means of cable support.”

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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