Change of Plans

By Mark C. Ode | Dec 15, 2006




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The price of copper has been increasing at an incredible rate over the past few years; the reasons for the increase is not an issue that will be discussed here, but the resulting price escalation affects the electrical industry since many of the electrical conductors used in construction are copper. Many contractors have inserted price-escalation clauses in their contracts to help defray the differences in the cost of copper between the time the contractor bids the job, the bid is accepted and the purchase of the copper conductors. In some cases, aluminum conductors are being substituted for copper conductors resulting in redesign of the service and feeder conductors after the bid is accepted. In other cases, the contractor or engineer is using different ampacity tables permitting higher ampacity of the same size conductor.

To analyze the conversion from copper to aluminum or to use a conductor at a greater than normal ampacity, various sections of the National Electrical Code (NEC) must be studied to ensure proper compliance. Article 310 is an extremely important article for conductor ampacity and contains the building blocks necessary to determine the ampacity of conductors to be used for general construction wiring. Article 310 covers general requirements for conductors, their type designations, insulations, markings, mechanical strengths, ampacity ratings and the uses of conductors. It does not apply to conductors that form an integral part of equipment, such as motors, motor controllers and similar equipment. For example, the internal wiring of a dishwasher would not be covered, but the branch circuit wiring would be.

Generally, Section 310.15(A)(1) in Article 310 permits conductor ampacity to be determined by ampacity tables provided in Section 310.15(B) or under engineering supervision, as provided in Section 310.15(C) using the Neher/McGrath formula. The formula uses a series of calculations, taking into account all heat sources and any thermal resistances between the heat sources and free air to calculate heat transfer. Section 310.15(B) covers ampacities for conductors rated 0 to 2,000 volts, as specified in Table 310.16 through Table 310.19, Allowable Ampacity, and Tables 310.20 and 310.21, Ampacity, as modified by (B)(1) through (B)(6) for various correction factors.

Moreover, Tables 310.16 and 310.17 seem to be causing some confusion based on their usage within the industry. Table 310.16 covers allowable ampacities of insulated conductors of not more than three current-carrying conductors in a raceway, cable or directly buried in earth based on an ambient temperature of 30°C (86°F). Table 310.17 covers allowable ampacities of single-insulated conductors in free air based on an ambient air temperature of 30°C (86°F).

The decision to use Table 310.16 or Table 310.17 is usually based on the method of installation. If you install the conductors in a raceway, base the ampacity of the conductors on Table 310.16. If you install single conductor cables in a single layer with one cable space between cables in an uncovered cable tray, use Table 310.17. Between these tables, the difference in permitted ampacity for the same size conductors is very apparent. For example, using the 75°C column in Table 310.16 for 500 Kcmil XHHW copper, the allowable ampacity would be 380 amperes in a raceway. When using the same cable in a cable tray, the allowable ampacity would be 620 amperes, or 240 amperes higher than the conductor installed in a raceway.

The problem occurs where the higher ampacity for a cable is used based on Table 310.17, and the conductor is then terminated to electrical equipment. Equipment terminations are usually based on the ampacity of Table 310.16, as 110.14(C)(1) states, and unless the equipment is listed and marked otherwise, conductor ampacities for equipment termination provisions should be based on Table 310.16. This creates a dilemma since the 500 Kcmil termination lug within the equipment is based on a Table 310.16 ampacity of 380 amps, but the cable is being used at an ampacity of 620 amps as permitted by 392.11(B)(3).

However, Section 310.15(A)(2) exception states “where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 10 feet or 10 percent of the circuit length figured at the higher ampacity, whichever is less.” However, this exception only applies to conductors and not to the termination. The answer to the problem this raises seems to be the connection of a splicing device, in our example where one 500-Kcmil conductor at 620 amps could be converted to two 350 Kcmil conductors at 310 amps each for a total of 620 amps.

A thorough understanding is necessary since misuse of any aspects of the NEC can be extremely costly and potentially dangerous. It is essential to understand the concepts whenever contemplating a change.

About The Author

ODE is a retired lead engineering instructor at Underwriters Laboratories and is owner of Southwest Electrical Training and Consulting. Contact him at 919.949.2576 and [email protected]

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