jar546
CBO
I find #4 to be particularly interesting. Please read them all.
SUMMARY
1. Of the 17 different wiring systems tested, nine are currently addressed by the 2011 NEC Sec. 310.15(B)(3)(c) requirement for circular raceways exposed to sunlight on rooftops, e.g., EMT, RMC, and PVC, and eight are not, e.g., square raceway and cables. Using the maximum temperature rises from Table 5:
a. All of the wiring systems mounted 12 inches from the roof and 3-1/2 inches from the roof exceeded the NEC Table 310.(B)(3)(c) temperature adders of 25 oF and 30 oF respectively. For wiring systems mounted 12 inches from the roof, this ranged from 34.2 to 59.6 oF, and for wiring systems 3-1/2 inches from the roof this ranged from 35.1 to 65.0 oF.
b. For wiring systems mounted ½ inch off the roof, all of the cables, and the 4 inch square metal duct exceeded the 40 oF NEC temperature adder. Five of the nine circular raceways also exceeded this 40 oF adder. The range in temperature rises for wiring systems mounted ½ inch off the roof was 34.8 to 53.3 oF.
c. For wiring systems mounted on the roof, four of the cables exceeded the 60 oF temperature adder, and three of the circular raceways also exceeded this 60 oF adder. The range in temperature rises for wiring systems mounted on the roof was 43.8 to 71.4 oF.
2. In general, the smaller size wiring systems reached a hotter temperature than wiring systems of the same type but larger. This was especially true of the metallic wiring systems, and when the wiring systems were mounted directly on the roof. This was expected because the larger wiring systems had a larger ratio of surface area to mass, and thus took longer to heat up and cool down. This greater mass could also absorb the radiated heat from the sun (and the conducted heat from the roof) over a greater volume. See Tables 5 – 9.
3. The 1/0 SE cable operated much cooler than the similar sized 1/0 MC and 1/0 TC cables. This was expected due to the black outer jacked of the 1/0 MC and 1/0 TC cables having greater absorption properties, e.g., higher emissivity, than the lighter colored SE cable. See Boxplot No. 2. Ambient Temperature Adjustment for Raceway and Cable Systems Exposed to Sunlight on Rooftops page 26
4. Wiring systems mounted 3-1/2 inches off the white roof usually operated hotter than similar wiring systems mounted ½ inch off the black roof. This was likely because of the solar reflection off the white roof producing additional solar power that could be absorbed by the underside of the wiring system 3-1/3 inches from the roof. These results may have been different had the wiring systems mounted ½ inch off the roof been tested with a white roof. However, wiring systems mounted directly on the roof most always operated significantly hotter than wiring systems of the same type mounted off the roof. See Boxplot No. 1.
5. For all wiring systems mounted directly on the roof, the 90th percentile mean temperature rise above outdoor ambient for the period of time each day where the solar irradiance was greater than or equal to 1000 W/m2 was 48.8 oF with a standard deviation 6.8. For this case, maximum temperature rise above outdoor ambient for all wiring systems mounted directly on the roof with a 95% confidence interval (C.I.) would be 62.2 oF.
6. A statistical analysis of variance of the data for temperature rise above outdoor ambient showed that all test data from wiring systems mounted 0.5, 3.5, and 12.0 inches above the roof were not affected significantly by distance. Therefore, distances greater than 0.5 inches above the roof can be considered far from the roof and roof distance does not need to be a consideration in calculating temperature rise. For all wiring systems mounted 0.5, 3.5, and 12.0 inches above the roof, the 90th percentile mean temperature rise above outdoor ambient for the period of time each day where the solar irradiance was greater than or equal to 1000 W/m2 was 38.5 oF with a standard deviation 6.6. For this case, maximum temperature rise above outdoor ambient for all wiring systems mounted off on the roof with a 95% confidence interval (C.I.) would statistically be 51.4 oF.
Report by: Reviewed by: David A. Dini, P.E. Paul W. Brazis Jr., PhD Research Engineer
Underwriters Laboratories Inc.
SUMMARY
1. Of the 17 different wiring systems tested, nine are currently addressed by the 2011 NEC Sec. 310.15(B)(3)(c) requirement for circular raceways exposed to sunlight on rooftops, e.g., EMT, RMC, and PVC, and eight are not, e.g., square raceway and cables. Using the maximum temperature rises from Table 5:
a. All of the wiring systems mounted 12 inches from the roof and 3-1/2 inches from the roof exceeded the NEC Table 310.(B)(3)(c) temperature adders of 25 oF and 30 oF respectively. For wiring systems mounted 12 inches from the roof, this ranged from 34.2 to 59.6 oF, and for wiring systems 3-1/2 inches from the roof this ranged from 35.1 to 65.0 oF.
b. For wiring systems mounted ½ inch off the roof, all of the cables, and the 4 inch square metal duct exceeded the 40 oF NEC temperature adder. Five of the nine circular raceways also exceeded this 40 oF adder. The range in temperature rises for wiring systems mounted ½ inch off the roof was 34.8 to 53.3 oF.
c. For wiring systems mounted on the roof, four of the cables exceeded the 60 oF temperature adder, and three of the circular raceways also exceeded this 60 oF adder. The range in temperature rises for wiring systems mounted on the roof was 43.8 to 71.4 oF.
2. In general, the smaller size wiring systems reached a hotter temperature than wiring systems of the same type but larger. This was especially true of the metallic wiring systems, and when the wiring systems were mounted directly on the roof. This was expected because the larger wiring systems had a larger ratio of surface area to mass, and thus took longer to heat up and cool down. This greater mass could also absorb the radiated heat from the sun (and the conducted heat from the roof) over a greater volume. See Tables 5 – 9.
3. The 1/0 SE cable operated much cooler than the similar sized 1/0 MC and 1/0 TC cables. This was expected due to the black outer jacked of the 1/0 MC and 1/0 TC cables having greater absorption properties, e.g., higher emissivity, than the lighter colored SE cable. See Boxplot No. 2. Ambient Temperature Adjustment for Raceway and Cable Systems Exposed to Sunlight on Rooftops page 26
4. Wiring systems mounted 3-1/2 inches off the white roof usually operated hotter than similar wiring systems mounted ½ inch off the black roof. This was likely because of the solar reflection off the white roof producing additional solar power that could be absorbed by the underside of the wiring system 3-1/3 inches from the roof. These results may have been different had the wiring systems mounted ½ inch off the roof been tested with a white roof. However, wiring systems mounted directly on the roof most always operated significantly hotter than wiring systems of the same type mounted off the roof. See Boxplot No. 1.
5. For all wiring systems mounted directly on the roof, the 90th percentile mean temperature rise above outdoor ambient for the period of time each day where the solar irradiance was greater than or equal to 1000 W/m2 was 48.8 oF with a standard deviation 6.8. For this case, maximum temperature rise above outdoor ambient for all wiring systems mounted directly on the roof with a 95% confidence interval (C.I.) would be 62.2 oF.
6. A statistical analysis of variance of the data for temperature rise above outdoor ambient showed that all test data from wiring systems mounted 0.5, 3.5, and 12.0 inches above the roof were not affected significantly by distance. Therefore, distances greater than 0.5 inches above the roof can be considered far from the roof and roof distance does not need to be a consideration in calculating temperature rise. For all wiring systems mounted 0.5, 3.5, and 12.0 inches above the roof, the 90th percentile mean temperature rise above outdoor ambient for the period of time each day where the solar irradiance was greater than or equal to 1000 W/m2 was 38.5 oF with a standard deviation 6.6. For this case, maximum temperature rise above outdoor ambient for all wiring systems mounted off on the roof with a 95% confidence interval (C.I.) would statistically be 51.4 oF.
Report by: Reviewed by: David A. Dini, P.E. Paul W. Brazis Jr., PhD Research Engineer
Underwriters Laboratories Inc.