Does NEC require 75°C or 90°C ampacity?

Conductors can be rated 90°C, but terminations on most devices are listed for 75°C — so you size from the 75°C column unless the equipment is specifically listed for 90°C terminations. Use 90°C ampacity only as the starting point for derating calculations.

How do I derate wire for high ambient temperature?

NEC Table 310.15(B)(1) gives an ambient temperature correction factor — multiply the 90°C ampacity by the factor for your installation temperature, then verify the corrected value still exceeds 125% of the continuous load.

What size wire do I need for a 50 amp hot tub?

For a hot tub fed by NM-B (Romex), NEC 334.80 forces sizing from the 60°C column, so you need #6 AWG copper for a continuous 50 A load. If you run THHN/THWN-2 in conduit you can drop to #8 AWG copper because terminations are listed at 75°C and the 75°C column allows 50 A on #8. Always confirm the spa-pack manual — many require GFCI protection and a specific minimum wire size that supersedes NEC.

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.

Do I have to upsize wire for a long run?

Ampacity tables do not care about distance, so you size conductors for ampacity first. But voltage drop is a separate check (NEC informational notes recommend 3% on branch circuits and 5% total), and on runs longer than about 75–100 ft you often need to bump up one wire size to keep voltage drop in range. Use the voltage-drop calculator alongside ampacity sizing to confirm.

Does a VFD or EV charger count as a continuous load?

Yes for EV charging: NEC 625.41 treats EVSE as a continuous load, so overcurrent protection and conductors are sized at 125% of the charger rating. A 40 A Level 2 charger therefore lands on a 50 A circuit, typically with #6 copper conductors. VFD loads are not called out the same way, but any load expected to run at full output for three hours or more is continuous under NEC 210.19(A)(1).

How many wires can I put in conduit before I have to derate?

Three current-carrying conductors share a raceway with no adjustment. Four to six get 80%, seven to nine get 70%, and ten to twenty get 50% per NEC Table 310.15(C)(1). Neutral counting depends on system type and harmonics. Conduit fill limits in Chapter 9 are separate from ampacity derating, so you need to check both.

NEC Wire Size & Ampacity Calculator (310.16)

What wire size do I need for a 50-amp circuit?

For a continuous 50-amp load on copper THHN at 75°C terminations, NEC Table 310.16 calls for #6 AWG (65A allowable ampacity after 125% sizing). Use #4 AWG aluminum for the same load. Always derate for ambient temperature above 30°C and for more than three current-carrying conductors in a raceway.

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Use the NEC Wire Size & Ampacity Calculator (310.16) to get an exact answer for your inputs.

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