Picking a wire size is two questions: can it carry the load without overheating (ampacity) and can it deliver useful voltage at the far end of the run (voltage drop). Get either one wrong and you've either created a fire risk or a piece of equipment that can't start.

Step 1: ampacity (NEC 310.16)

Table 310.16 gives the allowable ampacity for insulated conductors in raceway, cable, or earth, at 86°F (30°C) ambient, with 3 or fewer current-carrying conductors. The most common copper sizes at 75°C terminal rating:

• 14 AWG Cu — 20 A (15 A breaker max per 240.4(D))
• 12 AWG Cu — 25 A (20 A breaker max per 240.4(D))
• 10 AWG Cu — 35 A (30 A breaker max per 240.4(D))
• 8 AWG Cu — 50 A
• 6 AWG Cu — 65 A
• 4 AWG Cu — 85 A
• 3 AWG Cu — 100 A
• 2 AWG Cu — 115 A
• 1 AWG Cu — 130 A
• 1/0 AWG Cu — 150 A
• 2/0 AWG Cu — 175 A
• 3/0 AWG Cu — 200 A
• 4/0 AWG Cu — 230 A

These are 75°C column values — the column virtually all modern terminations are rated to. If a piece of equipment specifically calls for 60°C terminations (small breakers under 100 A occasionally do), drop to that column.

Step 2: derate for ambient and conductor count

310.16 ampacity is at 30°C ambient with ≤3 current-carrying conductors. Real installs almost never hit that. Apply the correction factors from 310.15(B)(1) (ambient) and 310.15(C)(1) (conduit fill) and multiply.

Adjusted ampacity = base × ambient × fill

Ambient correction (75°C, common values):
  21–25°C → 1.05
  26–30°C → 1.00     ← table baseline
  31–35°C → 0.94
  36–40°C → 0.88
  41–45°C → 0.82
  46–50°C → 0.75
  51–55°C → 0.67

Adjustment for >3 CCC's in same raceway:
  4–6   conductors → 0.80
  7–9   conductors → 0.70
  10–20 conductors → 0.50
  21–30 conductors → 0.45

Step 3: voltage drop (NEC 210.19 informational + Chapter 9 Table 9)

NEC recommends keeping branch-circuit voltage drop under 3% and total feeder + branch under 5%. The fast-and-clean formula uses circular mil area (CMA):

Single-phase:        Vd = (2 × K × I × L) / CMA
Three-phase:         Vd = (1.732 × K × I × L) / CMA

K   = 12.9 for copper (Ω·CMA / ft, 75°C)
      21.2 for aluminum
I   = load current (A)
L   = one-way length of run (ft)
CMA = circular mils of conductor

12 AWG  =  6,530 CMA
10 AWG  = 10,380 CMA
8  AWG  = 16,510 CMA
6  AWG  = 26,240 CMA
4  AWG  = 41,740 CMA
3  AWG  = 52,620 CMA
2  AWG  = 66,360 CMA
1  AWG  = 83,690 CMA
1/0     = 105,600 CMA
2/0     = 133,100 CMA
3/0     = 167,800 CMA
4/0     = 211,600 CMA

Table 9 in Chapter 9 gives more accurate AC impedance numbers (it accounts for inductive reactance, which the CMA formula doesn't). For sub-150 ft runs at unity power factor the difference is small — for long runs or motor loads, use Table 9.

Worked example: 50 A subpanel, 175 ft run, 240 V single-phase

Load:           50 A continuous (kiln, garage subpanel, etc.)
Continuous:     50 × 1.25 = 62.5 A breaker / wire (NEC 215.2)
Run length:     175 ft
Material:       copper, 75°C, in conduit, ≤3 CCC

Ampacity check:
  62.5 A → 6 AWG Cu (65 A) clears 310.16

Voltage drop check at 6 AWG:
  Vd = (2 × 12.9 × 50 × 175) / 26,240
     = 225,750 / 26,240
     = 8.6 V
  % drop = 8.6 / 240 = 3.6%   ← over the 3% target

Bump to 4 AWG:
  Vd = (2 × 12.9 × 50 × 175) / 41,740
     = 225,750 / 41,740
     = 5.4 V
  % drop = 5.4 / 240 = 2.25%  ← clean

Final: 4 AWG Cu THHN/THWN-2, in 1" EMT (or 3/4" tight), with #10 EGC.

This is the pattern: ampacity says 6 AWG, voltage drop forces 4 AWG. On a 50 A run shorter than ~100 ft you'd usually still land at 6 AWG.

The continuous-load 125% rule (NEC 210.19, 215.2, 230.42)

If a load runs continuously for 3+ hours (think EV charger, kiln, big lighting circuit), the wire and breaker have to be sized at 125% of the load. A 40 A continuous EV charger needs a 50 A breaker and wire rated for at least 50 A.

Common mistakes

• Sizing on the 90°C column when the breaker is rated 75°C. Always size to the lowest-rated termination in the circuit.
• Ignoring voltage drop on 100+ ft runs. Code recommends, doesn't require it — but motors will tell you.
• Forgetting to derate when running multiple circuits in the same conduit.
• Using copper formulas on aluminum. K is almost double for aluminum (21.2 vs 12.9).
• Treating the 125% continuous rule as optional. The breaker manufacturer's listing usually requires it.

Run it on your phone

The ElectricianCalc app handles all of this — ampacity with full 310.15 derating, voltage drop using Chapter 9 Table 9, conduit fill with mixed conductors, motor FLA + OCPD, box fill, service load. 100% offline, no ads, no accounts. Free on the App Store and Google Play.

Related

• ElectricianCalc — NEC calculator on iOS + Android
• How to calculate speeds and feeds (Manufacturing)

Note: Practical reference for licensed electricians and qualified persons. Always verify against the current adopted edition of the NEC and any local amendments. Not a substitute for plan review or AHJ inspection.