Final answer:
The ampacity of a No. 12 aluminum wire with RHW insulation installed in a raceway with 19 other wires is most accurately 7 A, as the NEC dictates derating to 50% for more than 9 conductors, which is 7.5 A. We typically select the nearest standard size of overcurrent protection device below this value, giving us 7 A in this case.
Step-by-step explanation:
The student is asking about the ampacity of a No. 12 aluminum wire with RHW insulation when installed in a raceway with 19 other wires. The term ampacity refers to the maximum current an electrical conductor can handle before deteriorating under the heat caused by electrical resistance, which is an important consideration in electrical wiring installations. According to the National Electrical Code (NEC), the base ampacity of No. 12 aluminum wire is generally 15 amps. However, when multiple wires are bundled together, as in a raceway, the ampacity must be derated due to the reduced ability to dissipate heat.
For 4-6 current-carrying conductors, the NEC suggests derating to 80%, 7-9 to 70%, and more than 9 to 50%. Therefore, with 20 conductors, the ampacity would be derated to 50% of 15 A, which is 7.5 A. But in practice, we use the nearest standard size of overcurrent protection device below the ampacity after derating, which usually brings us to either 7 or 5 amperes. Options 10 A and 15 A would be too high and disregard the need for derating. Thus, the most accurate answer, based on NEC guidelines, would likely be 7 A. Still, official guidelines and local regulations should be consulted to determine the exact ampacity as it may vary based on specific conditions.