What size wire do I need? The whole run, in order

Take the load, add 25 percent of whatever portion is continuous, and find a conductor whose ampacity in your termination's temperature column meets that number. Then check the small-conductor cap, size the overcurrent device, size the equipment grounding conductor from that device, and finally check voltage drop over your actual run length. Voltage drop is what usually forces the size up on long runs.

There is no single table that answers this. Six separate rules each set a floor, and the conductor you pull is whichever one lands highest. Doing them out of order means redoing them, because sizing up for one reason can change the input to another.

1. Continuous load — the 125 percent step

A load running three hours or more is continuous. The conductor and the overcurrent device both get sized to the non-continuous load plus 125 percent of the continuous portion. A 16 A load that is entirely continuous is a 20 A sizing problem, not a 16 A one.

2. Ampacity — and which column you are allowed to read

This is where most people go straight to the 90°C column because THHN is a 90°C insulation. You generally cannot terminate there. Under 110.14(C), equipment rated 100 A or less, and conductors 1 AWG and smaller, are typically limited to the 60°C or 75°C column depending on what the equipment is listed for. The 90°C column is for derating math, not for picking the final size.

For 12 AWG copper that means 25 A, not the 30 A printed in the 90°C column — and a 20 A breaker after the next rule. The gap between the column you read and the column your insulation is rated for is where most oversizing mistakes start.

3. The small-conductor cap

Regardless of what the ampacity table says, 240.4(D) caps the overcurrent device on the smallest copper conductors — 15 A on 14 AWG, 20 A on 12 AWG, 30 A on 10 AWG. This is why 12 AWG copper reading 25 A in the 75°C column still lands on a 20 A breaker. There are listed exceptions for specific loads such as motors, but the general branch circuit does not get to ignore it.

4. The overcurrent device

Standard device sizes step 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 and up. When the calculated ampacity falls between two standard sizes, 240.4(B) generally lets you round up to the next standard size for circuits rated 800 A or less — but not past the small-conductor cap, and not for every load type. Round up only when the rule actually applies to what you are feeding.

5. The equipment grounding conductor

The EGC is sized from the overcurrent device, not from the phase conductor. The table jumps in a way that catches people out.

6. Voltage drop — the one that actually decides long runs

Voltage drop is not a code requirement in the way the five checks above are; it appears in informational notes recommending roughly 3 percent on a branch circuit and 5 percent total. It is still the check that most often forces the conductor up a size or two, and it is the only one that depends on how far you are running.

Because it scales with length, it changes the answer late. Size the conductor by ampacity first, then check drop at your real one-way distance — if it fails, step up and recheck. Stepping up for voltage drop can also affect the equipment ground under the proportional-increase rule, which lives at 250.122(D) in the 2026 and 250.122(B) in the 2020 and 2023.

The order matters

1. 1

   Load

   Non-continuous plus 125 percent of continuous.

2. 2

   Ampacity

   In the termination column, after any derating.

3. 3

   Cap

   Apply 240.4(D) if the conductor is 14, 12 or 10 AWG copper.

4. 4

   Device

   Next standard size, where 240.4(B) permits it.

5. 5

   Ground

   From the device rating, using 250.122.

6. 6

   Distance

   Check drop at the real length; step up and repeat if it fails.

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