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If heat is produced at a sufficient rate, the conductor temperature rises and the insulation can be damaged or ultimately the conductor itself can sag or melt. The ampacity rating for a conductor is based on the conductor diameter, material used (copper or aluminum), the rated maximum application temperature, and the installation conditions.
Wire that is approved to BS 6231 might not carry the UL and CSA ratings, if, for example, the wire is not suitable for use at the higher 105 °C temperature that is specified for those ratings. In that case, the wire is not tri-rated. According to UL 758, the maximum operating temperature of tri-rated cable is 105 °C. British Standard BS 6231 ...
A wire or cable has a voltage (to neutral) rating and a maximum conductor surface temperature rating. The amount of current a cable or wire can safely carry depends on the installation conditions. The international standard wire sizes are given in the IEC 60228 standard of the International Electrotechnical Commission.
A great deal of THWN wire is actually dual-rated, and meets THHN the specification as well, so may be used in wet environments up to 75°C or dry environments up to 90°C. An extended specification, THWN-2 permits use in wet locations and conductor temperatures up to 90°C simultaneously.
For wire sizes smaller than AWG No. 2 (33.6 mm 2, 0.0521 sq in), this term is also generally regarded as insignificant. R c , a {\textstyle R_{c,a}} is the effective thermal resistance between the conductor and the ambient conditions, which can require significant empirical or theoretical effort to estimate.
Insulation does not suddenly fail if the hot-spot temperature is reached, but useful operating life declines rapidly; a rule of thumb is a halving of life for every 10 °C temperature increase. Older editions of standards listed materials to be used for the various temperature classes.
Nichrome, a non-magnetic 80/20 alloy of nickel and chromium, is the most common resistance wire for heating purposes because it has a high resistivity and resistance to oxidation at high temperatures, up to 1,400 °C (2,550 °F). When used as a heating element, resistance wire is usually wound into coils.
These devices measure temperature, tension, sag and/or clearance from which the thermal rating is determined. Indirect measurement methods use weather stations and modeling. Direct methods give actual conductor state condition with respect to clearance rules and/or maximum conductor temperature, while indirect methods provide estimates based on ...
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