Conductor Sizing with Ampacity Correction and Adjustment

Reading a conductor size from a table is a beginner's task. The hard part starts when a raceway holds nine conductors in a hot room, or a feeder passes both load tests and then fails against the breaker. Conductors have several limits at once, and the smallest one wins. A senior electrician knows the order to check them in and can defend the answer to an inspector. This walks through that order with real numbers, including a case where the usual shortcut gives the wrong size. It assumes you know what ampacity is and how to read Table 310.16.

What You Will Be Able to Do

• Apply ambient correction and bundling adjustment together.
• Count current-carrying conductors correctly.
• Hold the result to the terminal temperature limit.
• Check the breaker against the adjusted ampacity.
• Handle rooftops, short raceways, mixed conditions and parallel sets.
• Size a raceway and check voltage drop.

Section numbers follow the 2023 National Electrical Code (NEC). Older editions put these tables under different numbers, so check the adopted edition and any local changes. The final size on a real job is approved by the licensed person responsible for it.

The Limits on Every Conductor

A size has to pass all of these.

The words are easy to mix up. Correction deals with hot surroundings. Adjustment deals with conductors heating each other in a raceway or cable.

Start in the Right Column

Heat-resistant conductors such as THHN are rated 90 degrees C. You may start correction and adjustment from the 90-degree column, but the final answer can never be higher than the terminal limit allows. Most equipment terminals are rated 75 degrees C. So the 90-degree column gives you more room for derating, and the 75-degree column still caps the result.

Factors Used Here

Counting Current-Carrying Conductors

A wrong count gives the wrong factor, so check it first.

• Every hot conductor counts.
• Equipment grounding conductors never count.
• A neutral that carries only unbalanced current usually does not count, such as on a 120/240-volt circuit or a balanced four-wire wye.
• A neutral on a three-wire circuit taken from a four-wire wye counts, because it carries about the same current as the hots.
• A neutral on a four-wire wye where most of the load is nonlinear counts, because harmonic current flows in it. Nonlinear loads include electronic drivers and power supplies.

Worked Example: Counting a Raceway

An EMT raceway holds three hots on different phases of a 208Y/120 system, one shared neutral, and one equipment grounding conductor. The load is mostly LED drivers.

1. Three hots: 3.
2. Neutral, because the load is nonlinear: 1.
3. Equipment grounding conductor: 0.

That is four, so the 80 percent factor applies. If the load were plain heaters, the count would be three and no adjustment would apply.

The Check Order for a Feeder

For a feeder, 215.2(A)(1) gives two separate tests, and they are not stacked together.

1. Without derating: the conductor must carry 125 percent of the continuous load plus 100 percent of the rest. Use the terminal column.
2. With derating: after correction and adjustment, the conductor must still carry the actual load.
3. Overcurrent device: check it against the adjusted ampacity under 240.4.

Worked Example: The Size That Passes and Fails

A feeder in a 40 degree C room shares a raceway with one other three-phase circuit, for six conductors carrying current. Its load is 120 amps continuous and 20 amps noncontinuous, on 75-degree terminals. Conductors are copper THHN.

Test 1. (120 x 1.25) + 20 = 170 amps. In the 75-degree column, 2/0 is good for 175 amps. Pass.

Test 2. The combined factor is 0.91 x 0.80 = 0.728. In the 90-degree column, 2/0 is 195 amps, and 195 x 0.728 = 142 amps. The actual load is 140 amps. Pass.

Overcurrent device. The device must also handle the load, so it has to be at least 170 amps, and the next standard size is 175 amps. Under 240.4(B), you may go up to the next standard size only from the conductor's adjusted ampacity. The next size above 142 is 150 amps. A 175-amp breaker is too big for this conductor. Fail.

Try 3/0. 225 x 0.728 = 163.8 amps. The next standard size is 175 amps, so the 175-amp breaker is allowed.

The answer is 3/0. Someone who stopped at the two load tests would have installed 2/0 and failed inspection, or left a conductor unprotected.

Worked Example: A Warm Branch Raceway

Nine 12 AWG THHN conductors carry current in a 38 degree C space, and each circuit carries 16 amps.

1. 90-degree base: 30 amps.
2. Combined factor: 0.91 x 0.70 = 0.637.
3. Adjusted ampacity: 30 x 0.637 = 19.1 amps.
4. The 16-amp load fits.
5. NEC 240.4(D) limits 12 AWG to 20 amps, and the next-size-up rule lets a 20-amp breaker sit on 19.1 amps.

So 12 AWG works with a 20-amp breaker. If the load were 20 amps, it would not, because the conductor must carry the load after derating.

Terminal Limits in Practice

A 6 AWG THHN conductor shows 75 amps in the 90-degree column. With no derating at all, you still cannot use 75 amps on 75-degree terminals. The limit is the 75-degree figure, which is 65 amps. That is the most common misuse of the 90-degree column.

Terminals on equipment rated 100 amps or less are often assumed to be 60-degree unless marked otherwise, so read the equipment. Many modern devices are marked 75 degrees, and the marking controls.

Rooftops

Sunlight heats a raceway on a roof. Under 310.15(B)(2), where a raceway or cable sits less than 7/8 inch above the roof in direct sun, add 33 degrees C to the outdoor temperature. Some conductors, such as XHHW-2, are exempt.

On a 38 degree C day, a raceway laid on the roof is treated as 71 degrees C. That cuts a 90-degree conductor to about half its table value. Support the raceway at least 7/8 inch above the roof and the adder goes away. A few inches of hardware can save several wire sizes.

Short Runs and Mixed Conditions

Short Raceways

Adjustment factors do not apply to conductors in a raceway no longer than 24 inches, such as a short nipple between two enclosures. Ambient correction still applies. Find this in 310.15(C)(1).

One Section Is Worse Than the Rest

Sometimes a small part of a run passes through a hot room. The rule in 310.14(A)(2) says the lower ampacity governs, with an exception. The higher ampacity may be used where the worse section is no longer than 10 feet or 10 percent of the circuit length, whichever is less.

Worked Example: A Boiler Room Crossing

An 80-foot feeder crosses 6 feet of a hot boiler room.

1. 10 percent of 80 feet is 8 feet.
2. The lower limit is 8 feet, compared with 10 feet.
3. 6 feet is less than 8 feet, so the exception can apply.

If the crossing were 9 feet, it would fail, even though it is under 10 feet. Read the exception in your edition before you rely on it, and check the terminals.

Parallel Sets

Conductors 1/0 and larger may be run in parallel under 310.10(G). Each set must match in length, material, size and insulation, and they must be terminated the same way.

Worked Example: A 400-Amp Feeder

Two sets of 3/0 copper THHN feed a 400-amp device, on 75-degree terminals.

• Separate raceways: three conductors carry current in each. 200 x 2 = 400 amps. That passes.
• One raceway: six conductors carry current, so 80 percent applies. 225 x 0.80 = 180 amps each, or 360 amps total. The next standard size above 360 is 400 amps, so the device passes only if the load is 360 amps or less.

One raceway saves pipe and costs capacity. A lead decides this before the job is priced.

Raceway Fill

Chapter 9 limits fill to 40 percent of the raceway area for three or more conductors. Add the conductor areas from Table 5, and compare to Table 4.

For nine 12 AWG conductors and one 12 AWG ground: 10 x 0.0133 = 0.133 square inches. That is over the 1/2-inch limit, so use 3/4 inch. A raceway can meet fill and still need adjustment factors, because they answer different questions.

Voltage Drop

The NEC's 3 percent and 5 percent figures are in informational notes, so they are recommendations in general. Some equipment rules, such as fire pumps, set real limits. Drop still matters, because low voltage heats motors and fails electronics.

For a single-phase circuit, you can estimate it with this formula, where K is about 12.9 for copper and CM is the conductor's area in circular mils:

Voltage drop = (2 x K x amps x length in feet) / CM

Worked Example: A Long Garage Feed

A 40-amp, 240-volt load sits 150 feet away.

1. On 8 AWG (16,510 CM): (2 x 12.9 x 40 x 150) / 16,510 = 9.4 volts, or 3.9 percent.
2. On 6 AWG (26,240 CM): (2 x 12.9 x 40 x 150) / 26,240 = 5.9 volts, or 2.5 percent.

The 8 AWG size passes on ampacity and misses the recommended limit. The customer decides, with your advice, unless a local rule or the equipment instructions set the number.

Common Mistakes

Check Your Understanding

1. A designer puts all the circuits for a floor into one large pipe to save labor, because it meets fill. What do you warn them about?

Fill and adjustment are separate checks. More conductors in one raceway lowers the ampacity of each, so a raceway that meets fill can still need larger wire.

2. A 1/0 THHN conductor has an adjusted ampacity of 118 amps. What is the largest breaker allowed by the next-size-up rule?

125 amps, since 125 is the next standard size above 118, and the device is under 800 amps. The load must still be 118 amps or less.

3. A circuit runs in 45 degree C air with three conductors. What is the factor for a 90-degree conductor?

0.87. No adjustment applies with three conductors.

4. An old disconnect rated 60 amps has no temperature marking on its lugs. What do you check before relying on a higher column?

The equipment itself. Terminals on equipment rated 100 amps or less are often assumed to be 60-degree unless marked, so read the marking and size from the column it supports.

5. A 30-amp, 240-volt load is 200 feet away on 8 AWG copper. What is the voltage drop?

(2 x 12.9 x 30 x 200) / 16,510 = 9.4 volts, or 3.9 percent.

Key Takeaways

• A conductor has several limits, and the smallest one wins.
• Correction is for heat, and adjustment is for conductors in a group.
• You may derate from the 90-degree column, but terminals cap the result.
• Check the breaker against the adjusted ampacity, not just the load.
• Count the neutral when it carries harmonic current or serves a three-wire circuit from a wye.
• Rooftops, short raceways and mixed runs each have their own rules.