NEC Wire Size & Ampacity Calculator (310.16)
Is romex inside conduit derated?
NM cable used inside conduit is uncommon and most inspectors will push back, but yes — wherever conductors share a raceway, NEC 310.15(C)(1) ambient + adjustment factors apply, and NM is still capped at the 60°C column per 334.80. The combined effect is usually worse than just pulling THHN/THWN in conduit and sizing from the 75°C column. If you actually need NM-through-conduit (a short sleeve through a wall is the common case), the short raceway exception in 310.15(C)(1) waives the bundling derate.
Run the numbers for your project
Use the NEC Wire Size & Ampacity Calculator (310.16) to get an exact answer for your inputs.
Open the calculatorRelated questions
- What wire size do I need for a 50-amp circuit?
- Does NEC require 75°C or 90°C ampacity?
- How do I derate wire for high ambient temperature?
- What size wire do I need for a 50 amp hot tub?
- Do I have to upsize wire for a long run?
- Does a VFD or EV charger count as a continuous load?
- How many wires can I put in conduit before I have to derate?
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