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.

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.

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)

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).

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

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