NEC Fundamentals - Wire Sizing & OCPD

Sizing wire comes down to one question: will this conductor carry its load all day without overheating, and will the breaker or fuse protect it? The National Electrical Code (NEC) answers that with a few tables and rules. These rules appear on every job and on every licensing exam. This guide shows the order to use them in. OCPD stands for overcurrent protective device, which means a breaker or a fuse.

What You Will Be Able to Do

• Read Table 310.16 and pick the right temperature column.
• Apply the termination rule and the small-conductor rule.
• Correct ampacity for hot locations and for several conductors in a raceway.
• Size a circuit for a continuous load.
• Calculate voltage drop and decide whether to upsize.

Table and section numbers here match recent editions. Your state or city may enforce an older edition, and local changes apply. Confirm yours before you rely on a number.

Ampacity and Table 310.16

Ampacity is the current a conductor can carry continuously without going over its temperature rating. Table 310.16 lists ampacities for up to three current-carrying conductors at an ambient temperature of 30 degrees C (86 degrees F).

Aluminum has its own columns with lower values. For example, 2 AWG aluminum is 90 amps at 75 degrees C, and 4/0 aluminum is 180 amps.

The columns match insulation ratings. THHN is a 90 degree C wire. THWN is a 75 degree C wire, and THWN-2 is 90 degrees C. NM cable has 90 degree C conductors, but it must be used at the 60 degree C ampacity.

Three Limits on Every Conductor

Three rules work together. The lowest result wins.

Limit 1: The Termination

A conductor is only as cool as its hottest connection. NEC 110.14(C) limits ampacity to the temperature rating of the terminal at either end.

• Circuits of 100 amps or less, or 14 through 1 AWG: use the 60 degree C column, unless the equipment is listed and marked for 75 degrees C.
• Circuits over 100 amps, or conductors larger than 1 AWG: use the 75 degree C column.

Most modern breakers and lugs are marked 75 degrees C. So THHN does not get you 90 degree ampacity at the breaker. For a 10 AWG THHN conductor on 75 degree terminals, the usable value is the 35 amps in the 75 degree column.

Limit 2: The Small-Conductor Rule

NEC 240.4(D) sets a cap on common small sizes, whatever the table says:

So 12 AWG shows 25 amps at 75 degrees C and 30 amps at 90 degrees C, but it still goes on a 20-amp breaker. Motors, air conditioning and a few other loads have their own rules that override this one.

Limit 3: Conditions of Use

The table assumes 30 degrees C and no more than three current-carrying conductors together. Conditions outside that require you to reduce the ampacity.

Ambient temperature correction. Hot attics, rooftops and boiler rooms reduce what a conductor can carry.

Conductor count adjustment. Conductors in a raceway heat each other. Table 310.15(C)(1) applies when more than three current-carrying conductors are together.

Equipment grounding conductors do not count. A neutral that carries only unbalanced current does not count, but a neutral does count when most of the load is electronic, or when the circuit uses two phases and the neutral of a wye system.

The order of operations. Start with the column of the wire's insulation, usually 90 degrees C for THHN. Apply both factors to that value. Then compare the result against the termination limit and the OCPD rule. The lowest number is your usable ampacity.

Worked Example: Hot Attic, Crowded Conduit

Seven current-carrying 10 AWG THHN conductors run in a conduit through an attic at 46 degrees C. The terminals are rated 75 degrees C.

1. Start with the 90 degree column: 40 amps.
2. Apply the temperature factor: 40 x 0.82 = 32.8 amps.
3. Apply the count factor: 32.8 x 0.70 = 22.96 amps.
4. Compare with the termination limit of 35 amps. The adjusted value is lower, so 22.96 amps is the ampacity.
5. Check the OCPD: 240.4(D) would allow up to 30 amps, but the wire is only good for about 23 amps. Under 240.4(B), you may go up to the next standard size, which is 25 amps, as long as the load does not exceed the adjusted ampacity.

Continuous Loads

A continuous load runs at its maximum current for three hours or more. For these loads, both the conductor and the breaker must be sized at 125 percent of the continuous load, plus 100 percent of any noncontinuous load. Sections 210.19, 210.20 and 215 set this rule.

Another way to say it: a standard breaker can carry only 80 percent of its rating continuously. A 20-amp breaker can carry 16 amps continuously.

Worked Example: A Continuous Load

A set of commercial lights draws 13 amps for 10 hours a day.

1. Multiply: 13 x 1.25 = 16.25 amps.
2. The next standard breaker is 20 amps.
3. Use 12 AWG copper, which gives 25 amps at 75 degrees C, and 20 amps maximum under 240.4(D).

Common continuous loads include commercial lighting, EV chargers, storage water heaters of 120 gallons or less, and electric heat. Most general receptacle circuits in homes are not treated as continuous.

Picking the OCPD

Breakers and fuses come in standard sizes, listed in NEC 240.6: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200 and up. If the conductor's ampacity falls between sizes, and the circuit is 800 amps or less, 240.4(B) allows the next size up, provided the circuit does not feed multi-outlet receptacle branch circuits for cord-and-plug loads. The small-conductor rule still applies.

Voltage Drop

Current through a long conductor loses voltage. The NEC does not require a specific limit in most cases, but informational notes suggest 3 percent on a branch circuit and 5 percent total for the feeder and branch circuit together. Low voltage makes motors run hot and lights dim.

For single-phase circuits:

VD = (2 x K x L x I) / CM

• K is about 12.9 for copper and 21.2 for aluminum.
• L is the one-way length in feet.
• I is the current in amps.
• CM is the conductor's area in circular mils, from Chapter 9, Table 8. 12 AWG is 6,530, 10 AWG is 10,380 and 8 AWG is 16,510.

For three-phase circuits, replace the 2 with 1.732.

Worked Example: A Long Run

A 120-volt load draws 16 amps at the end of a 120-foot run.

1. Try 12 AWG: (2 x 12.9 x 120 x 16) / 6,530 = 7.6 volts. That is 6.3 percent, so it is too high.
2. Try 10 AWG: (2 x 12.9 x 120 x 16) / 10,380 = 4.8 volts. That is 4.0 percent, still above 3 percent.
3. Try 8 AWG: (2 x 12.9 x 120 x 16) / 16,510 = 3.0 volts. That is 2.5 percent, so it passes.

If you upsize hot conductors for voltage drop, the equipment grounding conductor must grow in proportion, under 250.122(B).

Equipment Grounding Conductors

The EGC is sized from the OCPD rating, not the load, using Table 250.122. 15 amps needs 14 AWG copper, 20 amps needs 12 AWG, 30 through 60 amps need 10 AWG, 100 amps needs 8 AWG and 200 amps needs 6 AWG.

Sizing Common Circuits

The method stays the same. Find the load, apply 125 percent if continuous, choose the conductor, then the OCPD.

For air conditioners and heat pumps, the nameplate gives the minimum circuit ampacity (MCA) and maximum overcurrent protection (MOCP). Use those numbers. Motors follow Article 430, where the rules are different and the breaker is often much larger than the wire would otherwise allow.

Common Mistakes

Check Your Understanding

1. A 12 AWG conductor runs through a very hot space, and its adjusted ampacity works out to 17 amps. May you use a 20-amp breaker?

Under 240.4(B), an adjusted ampacity of 17 amps may round up to 20 amps if the load is no more than 17 amps and the circuit is not a multi-outlet general branch circuit. Check with your supervisor for how the rule applies.

2. A continuous load draws 24 amps. What is the minimum OCPD?

24 x 1.25 = 30 amps, so a 30-amp device.

3. Why can you not use the 40-amp value for 10 AWG THHN on a typical breaker?

The breaker's terminal limits you to the 75 degree column, and 240.4(D) limits 10 AWG to 30 amps in most cases. The 90 degree value is used only as the starting point for derating.

4. A 240-volt, 30-amp load is 200 feet from the panel. Would 10 AWG copper stay under 3 percent drop?

(2 x 12.9 x 200 x 30) / 10,380 = 14.9 volts, which is 6.2 percent. No. Upsize, and increase the EGC in proportion.

Key Takeaways

• Start with the load, then the conductor, then the OCPD.
• The termination rating usually limits you to the 75 degree column, or the 60 degree column for NM.
• 14, 12 and 10 AWG are limited to 15, 20 and 30 amps.
• Derate from the insulation's column. Then compare with the termination limit.
• Continuous loads need 125 percent.
• Keep voltage drop near 3 percent on long runs, and upsize the EGC along with the hots.