Which of the following statements accurately describes the effect of insulation temperature rating on ampacity?

• A. False
• B. Not applicable
• C. True
• D. Only for service equipment

Answer: (C)

The amount of current a conductor can safely carry (ampacity) is set by how hot the insulation is allowed to get. The insulation temperature rating defines the maximum operating temperature the insulation can withstand. Since carrying current causes the conductor to heat up, the ampacity must stay within that insulation limit under the expected ambient conditions. Using insulation with a higher temperature rating generally allows a higher ampacity value from the NEC tables, provided terminations and equipment can handle that higher temperature. If you use a lower-temperature insulation, the usable ampacity must be reduced to keep the conductor within its insulation limit. This is why describing the effect as true is correct. It applies to conductors in general, not only to service equipment.