Use Your Metal

By Mark C. Ode | Dec 15, 2005
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The failure of a service-entrance bus duct supplying power to a major grocery store data center and corporate facility created a dilemma for store management. Since this facility is the 24-hour support system for all store transactions, maintaining power to the data center and the corporate complex is critical for continued business operation. The long delivery time for a 4,000-ampere replacement bus duct prompted immediate repair by the installation of parallel conductors in a metal wireway, not in a bus duct.

The distance from the utility company service point at the tap box to the service equipment in the building was approximately 50 feet. Ten 600-Kcmil THHN/THWN copper conductors were installed in parallel, for each phase and grounded conductor of the 277/480-volt, three-phase, 4-wire system, in a metal wireway in accordance with Article 376 in the 2005 National Electrical Code (NEC).

Due to access and connection configurations within the service switchboard, all of the grounded conductors were installed within the wireway first. Then, one conductor for each phase was connected from the tap box to the service switchboard and measured to establish the necessary length for the other parallel conductors.

After taking the measurement of each phase conductor length, the remaining conductors were installed in the wireway by phase but without maintaining any specific arrangement of conductor groups.

Remember, Section 310.4 requires each paralleled phase, polarity (for DC circuits), neutral or grounded conductor be the same length, conductor material, size in circular mil area, insulation type, termination, and, where installed in separate raceways or cables, the raceways or cables must have the same physical characteristics.

Conductors of one phase, polarity, neutral or grounded conductor, however, are not required to have the same physical characteristics of those of another phase, polarity, neutral or grounded circuit conductor. The purpose of maintaining the same physical characteristics for these phases, neutrals or grounded conductors is to ensure the same basic current will flow in each paralleled conductor of the set.

Upon installation completion, the service conductors were re-energized. The entire load to the service switchboard was also re-energized through the paralleled conductors and the wireway. Conductor current readings were then measured on all conductors connected to the system with measurements taken individually on each conductor.

At this point, high current readings were noted on some conductors within each phase group and low current on other conductors within the same phase group. This difference of current in individual conductors within a phase group meant some conductors were operated in excess of their permitted ampacity, thus operating hot, while other conductors had low current and were operating cold. Minor adjustments of the conductors within the wireway did not solve the problem of the imbalance of the current in the various conductors.

If these service-entrance conductors had been installed in a cable tray, Section 392.8(D) would have required the single conductor cables comprising each phase, neutral or grounded conductor in a paralleled configuration to be installed in groups consisting of not more than one conductor per phase, neutral or grounded conductor.

Therefore, each group would have a single conductor from Phase A, B, C and one from neutral or ground. This type of installation prevents current imbalances in the paralleled conductors due to inductive reactance.

To solve the problem for the grocery store data center and corporate complex installation, each cable in the wireway was disconnected and the DC resistance of the cable was measured to ensure there was minimal difference in the resistances of the cables in each phase.

The cables were then reinserted into the wireway by bundling the cables with Phase A, B, C and grounded conductor into groups with sufficient air gaps between each bundled group to ensure proper air circulation and subsequent cooling.

The system was re-energized and the individual cables were again monitored for ampacity for two days with an infrared scan performed on the cables under load conditions. Not only were the current readings on the conductors relatively the same, the system operated much cooler than previously noted. Problem solved.

Unlike cable trays, installing paralleled conductors in groups of Phase A, B, C and a grounded conductor within a wireway is not a requirement.

However, since the same phenomena of current imbalances from the inductive reactance noted within a cable tray would also seem to apply in a metal wireway, providing this grouping within a wireway, auxiliary gutter or similar ferrous metal enclosures should be done. In fact, adding this requirement for a metal wireway to the 2008 NEC might be a good idea. EC

ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or at [email protected]. 



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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