Transformer Accessibility, Derating Conductors and More

By George W. Flach | May 15, 2009
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Article 210 Branch Circuits; Article 240 Overcurrent Protection; Article 250 Grounding and Bonding; Article 310 Conductors for General Wiring; Article 450 Transformers and Transformer Vaults (Including Secondary Ties); Article 480 Storage Batteries

Aluminum conductors with AFCIs

Are any combination-type arc-fault circuit interrupters (AFCIs) available for connection to aluminum branch-circuit conductors?

I am not aware of any combination-type AFCIs that have terminals suitable for aluminum conductor terminations. Where there is adequate space in the enclosure, copper pigtails can be connected to the aluminum conductors and the copper wires can be connected to the AFCIs.

Wire connectors should be marked “AL-CU intermixed-dry locations” and be installed in accordance with the manufacturers’ instructions. It also may be necessary to provide oxide inhibitor and wire brush the wire before making the connection.

Accessibility of transformers

Is a 75 kVA dry-type transformer with -primary voltage of 480Y/277 volts and secondary of 120/208 volts considered to be readily accessible where the transformer is located on a platform 8 feet above the floor? The platform is large enough to provide working space around the transformer. My question concerns whether the transformer has to be readily accessible or is a permanent ladder to the platform required.

Transformers must be readily accessible to comply with Section 450.13 of the National Electrical Code (NEC), which states, “Accessibility: All transformers and transformer vaults shall be readily accessible to qualified personnel for inspection and maintenance or shall meet the requirements of 450.13(A) or 450.13(B).”

But Part (A) of 450.13 allows this installation without providing a ladder or permanent steps to the platform. It reads, “(A) Open Installations. Dry-type transformers 600 volts, nominal or less, located in the open on walls, columns, or structures, shall not be required to be readily accessible.” Therefore, a ladder or permanent staircase is not required.

Derating conductors on rooftops

What is the corrected ampacity of three 6 AWG copper conductors with Type THW insulation run exposed to the sun and within 4 inches of the rooftop of a manufacturing building? The wiring method is rigid metal conduit.

Because the person asking the question is from Miami, we will use the outdoor temperature of 91°F as shown in the Rooftop Ampacity Adjustment Tables provided by the Copper Development Association (CDA) with ambient temperatures for various cities taken from the ASHRAE Handbook. The Fine Print Note following Section 310.16(B)(2)(6) recognizes these ambient temperatures. For an ambient temperature of 91°F with the conduit located 4 inches above the roof, the temperature in the conduit is listed at 121°F.

With a CDA slide rule, the table should be set on the 114°F–122°F scale. Read down the 75°C column to 6 AWG copper for a corrected ampacity of 49 amperes. For increased ampacity without increasing conductor size, use Type THHW insulation. The 90°F insulation increases the ampacity of 6 AWG copper conductors to 62.

Grounding-electrode conductor size

What size grounding-electrode conductors are required for grounding a 200-ampere and three 100-ampere services? I would like to tap the grounding-electrode conductors from the three 100-ampere services to the grounding-electrode conductor from the 200-ampere switch. The service conductors are two sets of 250 kcmil copper conductors with Type THWN insulation.

I assume the service conductors will be 4/0 AWG copper for the 200--copper service and 2 AWG copper for the 100-ampere services. A 1/0 AWG copper grounding-electrode conductor may be run from the 200-ampere service to the metal water pipe grounding electrode. For the 100-ampere services, 8 AWG copper grounding-electrode conductors are adequate. Section 250.66 and Table 250.66 permit these sizes of grounding-electrode conductors.

A grounding-electrode conductor that is 8 AWG copper must be protected by installation in rigid metal conduit, intermediate metal conduit, rigid nonmetallic conduit, electrical metallic tubing or cable armor. This protection is required by 250.62(B).

Grounding-electrode conductor taps are permitted by 250.64(D). Part 1 of this section permits taps to the grounding-electrode conductor and states: “The common grounding electrode conductor shall be sized in accordance with 250.66 based on the sum of the circular mil areas of the longest ungrounded service-entrance conductor(s). Where the service-entrance conductors connect directly to a service drop or service lateral, the common grounding electrode conductor shall be sized in accordance with Table 250.66 Note 1. A tap conductor shall extend to the inside of each service disconnecting means enclosure. The tap conductors shall be connected to the common grounding electrode conductor by exothermic welding or with connectors listed as grounding and bonding equipment in such a manner that the grounding electrode conductor remains without a splice or joint.”

Notice that the connections from the 8 AWG grounding-electrode conductors must be made by exothermic welding or with connectors that are listed as grounding and bonding equipment.

Disconnecting means for batteries

In a small office building, batteries supply standby power. A fused disconnect switch was provided for the batteries at the manual transfer switch in an adjacent room. The electrical inspector has asked for a disconnect within sight of the batteries. Is this required by the NEC?

The inspector is correct. Section 480.5 is new in the 2008 edition of the NEC, and it states, “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.” For definitions of the terms “disconnecting means,” “readily accessible,” and “within sight” see Article 100 in the NEC.

This appears to be an optional standby system as covered by Article 702, but nothing in this article removes the requirement for a disconnecting means within sight of the batteries.

Grounding-electrode conductor size

What size grounding-electrode conductor is required for a vertical run of four floors in an office building to provide grounding of four 50-kVA, three-phase, 480 to 120/208-volt transformers (one on each floor)?

The rated secondary current of these transformers is about 140 amperes each (50,000 divided by 208 × 1.73). Assuming the secondary conductor sizes are all 1/0 AWG copper with Type THW insulation and all terminals are suitable for 75°C insulation, the total circular mil area of one phase conductor is given in Table 8 of Chapter 9 as 105,600 circular mils. Since there are four transformers, the total cross-sectional area is 422,400 circular mils (4 × 105,600). According to Table 250.66, a grounding-electrode conductor not smaller than 1/0 AWG copper would be required as the main grounding-electrode conductor. However, Section 250.30(4)(A) requires a minimum conductor size of 3/0 AWG copper for the common grounding-electrode conductor. This size must be run for the entire length because splices or joints are not allowed in the common grounding-electrode conductor.

One paragraph in Part (C) prohibits splices or joints in the common grounding--electrode conductor and reads, “Tap conductors shall be connected to the common grounding electrode in such a manner that the common grounding electrode conductor remains without a splice or joint.” Each tap-conductor size from each transformer is obtained from Table 250.66 based on the size of the secondary conductors. Therefore, a 6 AWG copper conductor must be provided from the neutral terminals of the transformers to the 3/0 AWG common grounding-electrode conductor. The rule for sizing these grounding-electrode conductors is in Section 250.30(4)(C).

Secondary overcurrent protection

Under what conditions is secondary overcurrent protection not required for transformers rated 600 volts or less?

According to Table 450.3(B), the following types of transformers do not require overcurrent protection of the secondary conductors where the primary current is less than 3 amperes and primary overcurrent protection does not exceed 300 percent of rated primary current, where the primary full load current is 9 amperes or more and the transformer has a two-wire primary and two-wire secondary, or a three-wire delta primary and three-wire delta secondary.

For the two-wire and delta-delta three-wire transformers, there are requirements in Article 240 that must be met so that secondary conductor overcurrent protection is not required.

Part of Sections 240.3(F) and 240.21(C)(1) that cover transformer secondary conductors permit elimination of secondary conductor overcurrent protection for the two types previously mentioned where the primary overcurrent protection complies with Section 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary voltage ratio.

ID conductors with different voltages

Are there any changes in the 2008 edition of the NEC for identification of conductors connected to different voltages?

Yes, there are requirements for identification of conductors connected to different voltage systems. Section 210.8(C) has this requirement: “Ungrounded Conductors. Where the premises wiring system has branch circuits supplied from more than one nominal voltage system, each ungrounded conductor of a branch circuit shall be identified by phase or line and system at all terminations, connections, and splice points. The means of identification shall be permitted to be by separate color coding, marking tape, tagging, or other approved means.” The method used to identify the branch-circuit conductors must be permanently posed on each branch circuit panelboard or documented in a manner that is readily available.

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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