How the wire size calculator decides
Picking a wire size is two separate checks, and the larger answer wins:
- Ampacity. The conductor must carry the load without overheating. The calculator multiplies continuous loads by 125%, then finds the smallest size in NEC Table 310.16 whose ampacity in your chosen temperature column meets that value. It also applies the small-conductor limits of 240.4(D): 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG copper (15 A and 25 A for 12 and 10 AWG aluminum).
- Voltage drop. The conductor must deliver enough voltage at the far end. Using the actual load current (not the 125% value) and NEC Chapter 9 Table 8 resistance, it finds the smallest size that keeps the drop at or under your limit, 3% by default.
On short runs ampacity controls; on long runs voltage drop usually controls. The result tells you which one did.
Formulas
Required ampacity = load × 1.25 (continuous) or load × 1.00
VD% = k × I × R × L ÷ 1000 ÷ V × 100, k = 2 (1-ph) or 1.732 (3-ph)
Worked examples
Example 1: 40 A load, 75 ft, 240 V
A 40 A non-continuous load on copper with 75 °C terminals. Table 310.16 shows 8 AWG copper at 50 A in the 75 °C column, so 8 AWG passes ampacity. Voltage drop on 8 AWG: 2 × 40 × 0.764 × 75 ÷ 1000 = 4.58 V, or 1.91% of 240 V. Both checks pass: 8 AWG copper.
Example 2: 48 A EV charger, 150 ft, 240 V
EV chargers are continuous loads (NEC 625.41), so the conductor needs 48 × 1.25 = 60 A. 6 AWG copper is 65 A at 75 °C. Voltage drop: 2 × 48 × 0.491 × 150 ÷ 1000 = 7.07 V, or 2.95%, just under 3%. Answer: 6 AWG copper on a 60 A breaker. A few more feet and voltage drop would push it to 4 AWG.
Example 3: 16 A at 120 V, 150 ft
Ampacity alone allows 12 AWG. But 12 AWG drops 2 × 16 × 1.93 × 150 ÷ 1000 = 9.26 V (7.7%). 8 AWG still drops 3.06%, so the calculator lands on 6 AWG for a 3% limit. This is a strong case for running the circuit at 240 V or adding a subpanel.
Choosing the temperature column
NEC 110.14(C) says the conductor ampacity you use cannot exceed the rating of the lowest-rated termination. For circuits of 100 A or less, or conductors 14 AWG through 1 AWG, use the 60 °C column unless the equipment is listed and marked for 75 °C, which most modern breakers and devices are. Above 100 A, use 75 °C. THHN's 90 °C rating is used as the starting point for derating, not as the final ampacity.
Quick reference: common circuit sizes (copper, 75 °C)
| Breaker | Minimum copper | Minimum aluminum |
|---|---|---|
| 15 A | 14 AWG | 12 AWG |
| 20 A | 12 AWG | 10 AWG |
| 30 A | 10 AWG | 8 AWG |
| 40 A | 8 AWG | 8 AWG* |
| 50 A | 8 AWG* / 6 AWG | 6 AWG* |
| 60 A | 6 AWG | 4 AWG |
| 100 A | 3 AWG | 1 AWG |
| 200 A | 3/0 | 250 kcmil |
*Only with all terminations rated 75 °C. Before voltage drop and derating. Dwelling services and feeders may use NEC 310.12 (83% rule) instead, which allows 2/0 copper or 4/0 aluminum for a 200 A service.
What this calculator does not cover
It does not apply ambient temperature correction, bundling adjustment, rooftop adders, motor rules (Article 430), or the 310.12 dwelling service table. It also does not size the equipment grounding conductor (Table 250.122). Use it to get the starting point, then apply the rules for your installation. For a conduit to carry the result, use the conduit fill calculator.