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Code Quiz

Jan 8, 2009 4:50 PM, By Steven Owen

What are the minimum size current-carrying copper conductors required for the outlet that serves a professional xenon projector? The projector current rating is 55A. The projector is not considered to be a continuous load. The terminals are marked as 60°C.

1. No. 8 THHN
2. No. 6 THHN
3. No. 4 THHN
4. No. 3 THHN

Answer: B

As per 540.13, "Conductors supplying outlets for arc and xenon projectors of the professional type shall not be smaller than 8 AWG and shall have an ampacity not less than the projector current rating. Conductors for incandescent projectors shall conform to normal wiring standards, as noted in 210.24."

As noted in this example, the nameplate ampere rating of the projector is 55A and the terminals are marked with a temperature rating of 60°C. Section 110.14(C) requires the rating of the lowest rated component of a circuit to be used. In this case, the 60°C rating of the terminals is lower than the 90°C rating of the THHN insulation. From the 90°C column of Table 310.16, a No. 8 THHN is listed as 55A, however, we have no adjustments to make, and the terminals are not marked as 90°C. Therefore, we cannot use the No. 8 THHN rated at 55A from the 90°C column, rather, we must use the 60°C column because of the terminal rating of 60°C.

From the 60°C column, a 6 AWG conductor is rated 55A, which is the minimum size conductor allowed for this example.

Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at necexpert@aol.com.

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