De-rating: Outdoor Temperatures for Various U.S. Cities
The National Electrical Code, and good design practice, requires derating of wires and cables installed in ambient temperature conditions that are higher than those upon which the listed ampacities of Table 310.15 are based. Ambient temperature is understood to mean the temperature of the air surrounding the conductor. For conductors in conduit, the ambient to which the insulated conductor is subjected is the temperature of the air inside the conduit.
To help determine the temperature insulation will be subjected to—under unloaded conditions—CDA has compiled a table of Outdoor Temperatures for Various U.S. Cities, as well as temperatures inside conduit on and at various heights above rooftops.
For each city, the table contains:
- 2% ASHRAE design temperature for each location, averaged for June through August*
- Likely temperatures inside roof top conduit (unloaded) exposed to direct sunlight:
- On the roof, and up to ½ inch above roof
- Greater than ½ inch and up to 3½ inches above roof
- Greater than 3½ inches and up to 12 inches above roof
- Greater than 12 inches and up to 36 inches above roof
- On the roof, and up to ½ inch above roof
- ASHRAE's reported 'extreme annual design condition'—i.e. the maximum dry-bulb temperature reading for the location over the last 30 years.
- Go to the Table or Download PDF Version of the Table [45 Kb]
or
- This table is also available in booklet form. Please order Publication A6116 from the Publications List.
Derating Booklet - Also available is a free pocket-sized slide tool helps to easily determine adjusted ampacities for 60°C, 75°C and 90° C wire. Has an abbreviated list of cities with their 2% design temperatures; makes a perfect addition to the booklet. Please order Publication A6115 from the Publications List.
Pocket-sized Slide - View the entire Electrical Publications list online.
Copper in Building Wire
ATTENTION!
Home Owners, Co-op Owners, Condominium Owners Apartment Renters.
Do You Need An Electrician?
There are 10 Questions to ask yourself about your Home Electrical System.
Order Building Wire educational and technical publications & video.
Have questions regarding Structured Wiring, a component being incorporated into more and more homes? For an overview, order a FREE copy of Structured Wiring For Today's Homes CD-ROM - Homeowner Edition online or call 800-CDA-DATA .
