Wire is sized to the load's amps, using NEC Table 310.16 ampacities — but the number you can actually use is capped by the termination temperature (usually the 75°C column for breakers and lugs, and the 60°C column for NM-B/Romex). The everyday results: 14 AWG for 15 A, 12 AWG for 20 A, 10 AWG for 30 A, 8 AWG for 40 A, 6 AWG for 50–55 A copper. Continuous loads add 25%, hot spaces and bundled wires derate the ampacity down, and long runs may need an upsize for voltage drop. Aluminum runs about two sizes larger than copper for the same load.
Picking a gauge? The wire size calculator selects the minimum conductor for your load with the full Table 310.16 chain — termination limits, ambient and bundling derates, the continuous-load uplift, and an optional voltage-drop check — free, no signup. This guide explains why the “ampacity” on a chart isn't always the number you can use.
📋 The base ampacities (Table 310.16)
NEC Table 310.16 lists how many amps each conductor can carry, in three temperature columns (60°C, 75°C, 90°C). For copper at 75°C, the common household sizes are:
- 14 AWG → 20 A (but used at 15 A, see below)
- 12 AWG → 25 A (used at 20 A)
- 10 AWG → 35 A (used at 30 A)
- 8 AWG → 50 A · 6 AWG → 65 A · 4 AWG → 85 A
Aluminum carries less for the same gauge, so it runs about two sizes larger — a load that takes 6 AWG copper needs 4 AWG aluminum.
🌡️ The termination-temperature cap
This is the rule that trips people up. Even though a conductor's insulation may be rated 90°C, you can only use the ampacity of the lowest-rated termination in the circuit (NEC 110.14(C)). Most breakers and lugs are rated 75°C, so you use the 75°C column. And NM-B cable (Romex) is limited to the 60°C column regardless of its insulation (334.80) — which is why 6 AWG copper NM-B is only good for 55 A, not 65.
Small-conductor rule. NEC 240.4(D) caps the overcurrent protection regardless of ampacity: 14 AWG copper maxes at a 15 A breaker, 12 AWG at 20 A, 10 AWG at 30 A. That's why 14 AWG is a 15 A wire even though the table shows 20 A.
📉 Derating: heat and bundling
The table ampacities assume 30°C (86°F) ambient and no more than three current-carrying conductors together. Two things reduce them:
- Ambient temperature — a hot attic (say 50°C) applies a correction factor well below 1.0, so the usable ampacity drops.
- Bundling — four or more current-carrying conductors in the same conduit or cable get an adjustment factor (80% for 4–6, 70% for 7–9, and so on).
A wire that's fine in open air can be undersized in a packed conduit in a hot attic. The calculator applies both factors so the gauge is right for where the wire actually runs.
➕ Continuous loads & voltage drop
A continuous load (three hours or more — an EV charger, some heating) requires the conductor and breaker at 125% of the load (210.19). And on a long run, the wire that's big enough for ampacity may still drop too much voltage.
The NEC's 3% branch-circuit voltage-drop figure is a recommendation (an Informational Note, not enforceable), but it's good practice on long runs — a motor or heater starved by low voltage runs hot and inefficient. Check it with the voltage drop calculator; the wire-size tool can factor it in too.
✅ The bottom line
Size wire to the load's amps from Table 310.16, then apply the caps: the termination column (75°C for most gear, 60°C for NM-B), the 240.4(D) small-conductor breaker limits, ambient and bundling derates, and the 125% continuous-load uplift. Go up a size or two for aluminum, and check voltage drop on long runs.
Ampacity values are NEC 2023 (unchanged in 2020/2026), but local amendments and install conditions vary — confirm with a licensed electrician and your AHJ. Get the gauge from the wire size calculator, and pair it with the breaker size calculator for the matching protection.
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