New York City Electrician Practice Test

Ampacity is the maximum current a conductor can carry safely based on the insulation’s temperature rating and the wiring table used by the code. For copper conductors, you read the ampacity from a table that lists values by insulation type and size. Type TW is an older thermoplastic insulation with a 60°C temperature rating, so you must use the 60°C column. For a 600 MCM copper conductor, the 60°C column shows an allowable ampacity of 350 A. That’s why 350 Amps is the correct value here. The other numbers would come from higher temperature-rated insulation (75°C or 90°C), which TW does not provide. In practice, you’d also consider de-rating for multiple conductors in a raceway or ambient temperature different from the assumed baseline, but the base allowable for this type and size is 350 A.