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SAWHORSE
:window.close()'>REDUCED AMPACITY
The ampacity of wire or cable is the current that it is allowed to safely carry. Calculating ampacity requires a detailed engineering effort, because it is affected not only by the construction details of the wire or cable but also by the environment. The ampacity ratings published by NEC are reliable, but only if the usage conditions correspond to the calculation premises. Often, this may not be true.
The temperature attained by a conductor is governed by the interplay between the heat produced (due to flow of current) and the heat lost (due to convective cooling). The latter, however, depends not only on details of the cable but also on anything that affects convective cooling, e.g., presence of other adjacent conductors and presence of thermal insulation.
Most are aware that it is unsafe to stick electric cords under carpets, mattresses or similar environments since they may overheat. Electricians who install NM-type house-wiring cables generally have no basis to know that they may be creating an overt fire hazard. This hazard normally arises because thermal insulation is used in a wall or ceiling cavity where the cabling is located (Table 3).
Goodson et al23 became concerned upon finding houses under construction, where NM cables suffered charring damage. They then conducted a study where they measured the short-term temperatures obtained on cables in wall cavity spaces. They noted that the standards for circuit breaker performance do not require tripping if the sustained load is less than 120% of the rating. Thus, they considered that the appropriate current flow to consider for establishing the thermal hazard is 120% of the breaker rating.
Using 90°C-rated NM cables, a single run of cable in an uninsulated cavity space registered 114°C, which is well above the 90°C limit (even setting aside Stricker's finding that a 90°-rated cable may fail at 71°C). As insulation was added to the cavity spaces and as additional runs of NM cable were placed adjacent to each other - as fully permissible in the NEC - the situation got progressively worse. With three parallel runs of cable and fiberglass thermal insulation, for example, a temperature of 211°C was reached.
Goodson et al were not the first researchers to be alerted to this problem. Already in 1980, Evans24 at NBS studied the temperatures developed on cables covered by fiberglass thermal insulation. His study was less extensive and involved only single runs of cable, but he too reported that actual temperatures exceeded the rated 90°C value.
http://www.fpemag.com/archives/article.asp?issue_id=54&i=451
Just read this in an FPE article and thought what problems will new buildings have with green and energy codes and possible fires.
The ampacity of wire or cable is the current that it is allowed to safely carry. Calculating ampacity requires a detailed engineering effort, because it is affected not only by the construction details of the wire or cable but also by the environment. The ampacity ratings published by NEC are reliable, but only if the usage conditions correspond to the calculation premises. Often, this may not be true.
The temperature attained by a conductor is governed by the interplay between the heat produced (due to flow of current) and the heat lost (due to convective cooling). The latter, however, depends not only on details of the cable but also on anything that affects convective cooling, e.g., presence of other adjacent conductors and presence of thermal insulation.
Most are aware that it is unsafe to stick electric cords under carpets, mattresses or similar environments since they may overheat. Electricians who install NM-type house-wiring cables generally have no basis to know that they may be creating an overt fire hazard. This hazard normally arises because thermal insulation is used in a wall or ceiling cavity where the cabling is located (Table 3).
Goodson et al23 became concerned upon finding houses under construction, where NM cables suffered charring damage. They then conducted a study where they measured the short-term temperatures obtained on cables in wall cavity spaces. They noted that the standards for circuit breaker performance do not require tripping if the sustained load is less than 120% of the rating. Thus, they considered that the appropriate current flow to consider for establishing the thermal hazard is 120% of the breaker rating.
Using 90°C-rated NM cables, a single run of cable in an uninsulated cavity space registered 114°C, which is well above the 90°C limit (even setting aside Stricker's finding that a 90°-rated cable may fail at 71°C). As insulation was added to the cavity spaces and as additional runs of NM cable were placed adjacent to each other - as fully permissible in the NEC - the situation got progressively worse. With three parallel runs of cable and fiberglass thermal insulation, for example, a temperature of 211°C was reached.
Goodson et al were not the first researchers to be alerted to this problem. Already in 1980, Evans24 at NBS studied the temperatures developed on cables covered by fiberglass thermal insulation. His study was less extensive and involved only single runs of cable, but he too reported that actual temperatures exceeded the rated 90°C value.
http://www.fpemag.com/archives/article.asp?issue_id=54&i=451
Just read this in an FPE article and thought what problems will new buildings have with green and energy codes and possible fires.
