RES Technical Corner by Brett Eliasz, RES Director

Technical Corner

This month’s article focuses on ambient temperature adjustments for raceways and cables exposed to sunlight on or above a rooftop. It will also speak to some of the changes over the past few NEC Code cycles, which are shown below.

You will see that these tables are very close and did not appear to change between these cycles. However, the NEC 2017 has removed these tables and offers a 60 degree F adder to ambient temp only when the bottom of the raceway or cable is located less than 7/8” above the roof.

The 2017 code also adds the exception that using type XHHW-2 cable eliminates the need to comply with any ambient temperature adjustments.

Example

A circuit for a 100A calculated load will need to be routed directly to the roof without any spacing from the roof surface. Loads are not continuous and the neutral should not be considered a current carrying conductor. Location is Rochester NY. Determine minimum size conductor using 90 degree C copper THWN-2 conductors. Use NEC 2017.

Per ASHRAE standards the ambient temperature for Rochester NY is 86 deg F.

The temp adder is 60 degrees F per NEC 310.15(B)(3)(c). So, the design temp is 86 + 60 = 146 deg F

Look in table 310.15(B)(2)(a) and the 90 deg C column to find a .65 adjustment factor.

Take 100A and divide by .65 to get 153.8 Amps

Select a conductor not less than 153.8 from table 310.15(B)(16) using the 90 degree C copper columns to get: #1/0. Check the corresponding 75 deg C column to ensure this value of 150A is larger than the anticipated load of 100A, which it is.

Hopefully this article finds you well and can be used as a reference for your project needs.

If anyone would like to contribute to the RES magazine and add an article or would like to request information on a specific topic (not limited to Electrical) just email me at beliasz@bergmannpc.com. As always, any comments are appreciated…! Thank you for reading.

Brett Eliasz, P.E., LEED AP BD+C , RES Director

res - technical corner FEBRUARY 2020 The ROCHESTER ENGINEER | 7