What Size Wire Do I Need? The 3 Rules Wire Charts Skip

2026-10-24

What size wire do I need? Work out the current the circuit will carry, then take the smallest wire in NEC Table 310.16 whose ampacity in the right temperature column is at least that current, and apply the small-conductor caps in NEC 240.4(D). For an ordinary copper branch circuit, that gives 14 AWG for 15 A, 12 AWG for 20 A and 10 AWG for 30 A.

Most wire gauge charts in search results stop at those three pairs. They rarely say which column of the table the numbers came from, why 14 AWG is listed at 20 A and still can't go on a 20 A breaker, or that a load running for hours needs extra headroom. Those details change the answer more often than the chart does, so this guide goes through them with numbers you can check.

What size wire do I need: start with the current

If you already know the amps, from the breaker you plan to use or the equipment nameplate, start there. If you only know watts, convert them first. For a single-phase load with a power factor of 1, amps = watts ÷ volts. For three-phase, divide the watts by √3 × line voltage × power factor. That's the standard load-current formula from electrical design handbooks such as ABB's installation handbook.

Take a 5,000 W heater on a 230 V supply. It draws 21.74 A. That's just over 20 A, so 12 AWG copper, which is good for 20 A, is already out. The answer in the 60°C column is 10 AWG, rated 30 A.

Heaters and other resistive loads have a power factor of 1. Motors and compressors don't, so take the power factor from the nameplate or, better, use the rated amps directly.

Then ask how long the load runs. The NEC calls a load continuous if its maximum current is expected to last three hours or more. Water heaters and EV chargers are typical examples. Under 210.19(A)(1), the conductor must carry the noncontinuous load plus 125% of the continuous load, and 210.20(A) sets the same floor for the breaker. An example from IAEI Magazine: 8 A of continuous load plus 10 A of noncontinuous load calls for a 20 A breaker, because the 8 A counts as 10 A.

You'll often see this described as the 80% rule: a breaker shouldn't carry more than 80% of its rating continuously. It's the same requirement seen from the breaker's side, and it applies to continuous loads, not to every circuit.

60°C or 75°C: which column of Table 310.16

Table 310.16 lists three ampacities for every size, for conductors rated 60°C, 75°C and 90°C. Which column you may use depends on the weakest part of the circuit, and that's usually the terminals on the breaker and the device. Under 110.14(C), circuits rated 100 A or less, or wired with 14 through 1 AWG, use the 60°C column unless the equipment is marked for 75°C. NM-B cable, the usual Romex in houses, is held to the 60°C column too.

The column matters more as the wire gets bigger. For 100 A of copper, the 60°C column gives 1 AWG, rated 110 A. Some charts list 3 AWG for 100 A, which is the 75°C value. That's two sizes apart for the same current, and both charts present their number as the answer.

The 75°C column is legitimate when both the conductor and every termination are rated for it, which is common on larger circuits and feeders. An IAEI example sizes a 150 A circuit with 75°C terminations at 1/0 copper, and the table gives the same result. The 90°C column doesn't let you drop to a smaller wire on ordinary equipment, because the termination limits in 110.14(C) still apply.

Why 14 AWG at 20 A still means 12 AWG

Look at the 75°C copper column and you'll see 14 AWG at 20 A, 12 AWG at 25 A and 10 AWG at 35 A. Charts copied from that column make 14 AWG look fine for a 20 A circuit. It isn't. Section 240.4(D) caps the overcurrent protection for small conductors regardless of what the table says:

• copper: 15 A for 14 AWG, 20 A for 12 AWG and 30 A for 10 AWG;
• aluminum: 15 A for 12 AWG and 25 A for 10 AWG.

So a 20 A copper load still lands on 12 AWG even in the 75°C column, and the table's 25 A for that wire counts as 20 A. For 30 A, 10 AWG copper is the answer in both columns, and its 35 A in the 75°C column counts as 30 A.

Aluminum trips people up the same way. For 25 A in the 75°C column, 10 AWG aluminum works: the table lists 30 A, and the cap brings it down to 25 A, which is just enough. For 30 A of aluminum in the 60°C column, you need 8 AWG. A chart that gives one aluminum size for 30 A without naming the column is guessing for you.

What the ampacity table assumes

Every number above comes with conditions written into the table's heading: an ambient temperature of 30°C (86°F) and no more than three current-carrying conductors in the raceway, cable or earth. Real installations break those assumptions all the time.

• A cable through a hot attic sits above 30°C for much of the year, and the correction factors in 310.15(B) reduce its ampacity.
• A conduit shared by several circuits holds more than three current-carrying conductors, and the adjustment factors in 310.15(C)(1) reduce ampacity again.
• A long run loses voltage along the way. The table doesn't look at length at all, so a run to a detached garage or a well pump needs a separate voltage drop check, and often a larger wire.

Each of these can only push the wire size up, never down. That's why a chart answer is a minimum, not a final choice.

There's also the far end of the table. In the version our calculator uses, the largest copper size is 500 kcmil, rated 380 A in the 75°C column. Ask it for 400 A and it says the load is above the table maximum instead of guessing. Loads that size usually mean parallel conductors and a proper design.

A quick check before you buy wire

Run through the steps in this order:

• Find the current: nameplate amps, or watts converted with the right voltage and power factor.
• Multiply the continuous part of the load by 1.25.
• Pick the column your terminations allow, which is usually 60°C for household circuits up to 100 A.
• Read the smallest size whose ampacity covers the current, and apply the 240.4(D) caps to 14, 12 and 10 AWG.
• Ask whether heat, bundled circuits or distance push you up a size.

This is a reference check for one circuit, not a wiring design. Areas adopt different NEC editions, and some amend them. Anything you actually install should be checked by a licensed electrician or your local inspector.

What to calculate next

The wire gauge calculator below follows the same steps. Enter amps, or watts with voltage and power factor, choose copper or aluminum and the 60°C or 75°C column, and it returns the smallest AWG size from NEC Table 310.16, 2023 edition. When the 240.4(D) cap applies, it shows both the table ampacity and the capped value, so you can see why the size went up. It doesn't apply the 125% factor for you, so enter continuous loads already multiplied by 1.25. A metric mode gives sizes in mm² for readers who work outside the US.

If you're still working out the wattage, the appliance electricity cost calculator takes the same watts and shows what the load costs to run per hour, per day and per year.