temperature rating for Romex
For derating purposes you can use the 90C column for NM-B, but you cannot exceed 15A, 20A, or 30A overcurrent protection for 14, 12, 10 gauge wire , respectively (with some exceptions). For example, if you have four 12/2 cables bundled together (8 conductors) you derate the wire by 70% using the 90C column. Since .7x30=21, you are still above 20 and may use a 20A breaker. However, if you bundle five 12/2, you now must derate 50%, reducing the ampacity to 15A and requiring you to use a 15A breaker to protect the circuits.