Electrician Prep

Wire Size Calculator

NEC 310.16

Find the right wire size for your circuit. Accounts for ampacity, temperature derating, bundling, and voltage drop.

About this calculator

Wire sizing requires balancing four factors: ampacity for the load, voltage drop on the run length, temperature rating of the terminations, and any derating for ambient temperature or conductor bundling. NEC 310.16 is the starting table; 110.14(C) limits you to the 60°C or 75°C column based on terminal ratings; 310.15(B) and (C) handle derating. This calculator runs all four checks and reports the smallest conductor that satisfies them.

Formula

The required ampacity is the load current divided by the product of all derating factors. The wire chosen must have a base ampacity (from NEC 310.16) at or above that required value at the temperature column matching your terminations.

Required ampacity = load amps ÷ (temp correction factor × adjustment factor)

Reference: NEC 310.16, 310.15, 110.14(C), 240.4

How to use

  1. Enter the load current in amps and choose copper or aluminum.
  2. Pick the conductor insulation type (THHN, THWN-2, XHHW, etc.).
  3. Set the termination temperature rating - 60°C for most residential, 75°C for most commercial.
  4. Enter ambient temperature and the number of current-carrying conductors in the raceway.
  5. Optionally enter circuit length to also check voltage drop.
  6. The calculator reports the smallest wire size that passes ampacity, derating, and (if entered) voltage drop.

Worked example

Setup

A 50 A continuous load on a 75°C terminal in a raceway with 6 current-carrying conductors at 30°C ambient, copper THWN-2.

Calculation

Continuous load adjustment: 50 × 1.25 = 62.5 A required. Adjustment factor for 4-6 conductors is 0.80 (Table 310.15(C)(1)). Temperature correction at 30°C is 1.00. Required ampacity = 62.5 ÷ 0.80 = 78.1 A. From NEC 310.16, 75°C copper, #4 AWG is rated 85 A.

Answer

#4 copper THWN-2 satisfies the load. Use a 70 A breaker (next standard up from 62.5 A continuous calculation) or stay at 60 A if the load actually pulls less than 50 A.

Frequently asked questions

Which temperature column do I use - 60, 75, or 90°C?
Use the lower of the conductor insulation rating and the termination rating. NEC 110.14(C) restricts most equipment to the 60°C column for circuits 100 A or less (or smaller than #1 AWG) and the 75°C column for larger circuits, unless the equipment is specifically marked for 90°C terminations.
Can I use the 90°C column at all?
You can use the 90°C ampacity as the starting point for derating calculations, but the final ampacity (after derating) cannot exceed what the 60°C or 75°C terminations allow. So 90°C wire helps with bundled or hot-environment runs, but does not let you push more current through cooler terminals.
Does this calculator handle parallel conductors?
No. NEC 310.10(G) lets you parallel conductors 1/0 AWG and larger under specific conditions; this calculator sizes a single conductor per circuit. For parallel runs, divide the load current by the number of parallel sets and size each conductor for that share.