How to Size a Wire to NEC Requirements

Step 1: Establish the design current

Size to the load, not to the breaker that happens to be in the panel. Take continuous load at 125 percent and non-continuous load at its actual current, then add them. A load is continuous when it is expected to operate at its maximum current for three hours or more: commercial lighting, most electric heat, a compressor on a long duty cycle. A 44 A continuous load is a 55 A sizing problem.

Working backwards from an existing breaker is the most common shortcut and the one that produces the most undersized conductors, because the breaker rating says nothing about the conditions the conductor is installed in.

Step 2: Fix the material and the insulation

Copper or aluminum, and the insulation type. The insulation temperature rating decides which column of the ampacity table is read: 90 degrees C for THHN and XHHW-2, 75 degrees C for THWN and for XHHW in a wet location, 60 degrees C for TW and for conductors inside a nonmetallic-sheathed cable.

Two points that get reversed in the field. The column is set by the conductor, not by the equipment it lands on. And a 90 degree C insulation does not deliver 90 degree C ampacity to the load; it delivers a higher starting number to derate from, which is worth one size on a bad pull and nothing at all on an easy one.

Step 3: Read the tabulated ampacity

The table value assumes two things: 30 degrees C ambient, and no more than three current-carrying conductors in the raceway or cable. Both are stated as conditions of use, not as typical cases. An 8 AWG copper THHN conductor is 55 A in the 90 degree C column, and that figure is valid for a single circuit in a conditioned space and nowhere else.

Take the tabulated value as the starting point for the arithmetic, never as the answer.

Step 4: Apply the ambient correction

Multiply the tabulated value by the correction factor for the actual ambient temperature in the column being used. Ambient means the air surrounding the raceway, which is not the thermostat setting. An attic under a dark roof runs well above 100 degrees F on a summer afternoon; a boiler room, a commercial kitchen ceiling and a south-facing exterior wall all sit above the 86 degrees F the table assumes.

At 104 degrees F the 90 degree C column loses 9 percent, the 75 degree C column 12 percent, and the 60 degree C column 18 percent. Higher-rated insulation is penalised least, which is the practical reason THHN dominates. Past the top of the tabulated range there is no factor at all: the conductor is not permitted, and no extrapolation is defensible.

Step 5: Apply the conductor count adjustment

Count the current-carrying conductors sharing the raceway and multiply by the adjustment factor: 80 percent for four to six, 70 percent for seven to nine, 50 percent for ten to twenty. Three two-wire circuits in one conduit is six current-carrying conductors and takes 0.8, not three conductors taking 1.0.

What counts is narrower than the conductor count. Ungrounded conductors always count. A grounded neutral counts only where it carries current beyond the unbalanced current of the other conductors, which excludes an ordinary multiwire branch circuit on a single-phase system. The equipment grounding conductor never counts. That last point matters twice over, because the conduit fill calculation uses a different count that does include it.

Ambient correction and the count adjustment compound. Six conductors at 104 degrees F in the 90 degree C column leaves 0.91 multiplied by 0.8, or 72.8 percent of the tabulated value.

Step 6: Cap at the termination rating

The corrected ampacity may not exceed the tabulated value in the column matching the lowest-rated termination at either end of the circuit. This is a ceiling applied after the multipliers, not another factor in the chain.

A 6 AWG THHN copper conductor derates from 75 A in the 90 degree C column and is then capped at 65 A, the 75 degree C value, when it lands on 75 degree C lugs. Under benign conditions the cap governs and the 90 degree C column bought nothing; under a hard derate the multipliers land below the cap and the cap never binds.

Unless equipment is marked otherwise, terminations on equipment rated 100 A or less are treated as 60 degree C, and larger equipment as 75 degree C. Very little equipment is listed for 90 degree C terminations. Read the label rather than assuming, because on small conductors this ceiling decides the size more often than derating does.

Applying the cap first and then derating from the capped figure is the single most common arithmetic error in conductor sizing. It yields a smaller conductor than the code requires and will not survive a plan review that checks the work.

Step 7: Check overcurrent protection and voltage drop

The overcurrent device protects the conductor, so its rating is bounded by the conductor's usable ampacity, and devices only exist in standard ratings. Where the ampacity falls between two of them, the next size up is permitted on a circuit supplying no receptacle outlets at ratings of 800 A or less.

Two limits override everything above. On 14, 12 and 10 AWG the small conductor rule caps protection at 15, 20 and 30 A for copper and 15 and 25 A for aluminum, regardless of what the ampacity table shows: 12 AWG copper reads 30 A in the 90 degree C column and is still a 20 A conductor. And a conductor that satisfies ampacity can still be unusable on a long run, because voltage drop scales with length while ampacity does not. Beyond roughly 100 feet at branch-circuit voltages, drop governs the size more often than heat does. Run the wire size calculator for the drop check; it reports which of the two constraints decided the result.

Where the procedure goes wrong

• Deriving from the breaker. The breaker is an output of this procedure, not an input to it.
• Reading the termination column from the start. Costs a size on long feeders, every time.
• Counting conductors instead of current-carrying conductors. Overstates the derate on multiwire branch circuits and understates it when a neutral does carry harmonic current.
• Using the room temperature as the ambient. The raceway is in the attic, not in the conditioned space below it.
• Stopping at ampacity on a long run. Voltage drop is not a code violation, but the equipment at the far end does not care about that distinction.
• Ignoring the grounding conductor after an upsize. Where the ungrounded conductors are increased for voltage drop, the equipment grounding conductor is increased proportionally.

What this procedure does not cover

Motor conductors are sized from the full-load current tables in Article 430 at 125 percent, and motor branch-circuit protection is deliberately set well above the conductor ampacity. Air-conditioning and refrigeration equipment follows the minimum circuit ampacity and maximum device rating on its own nameplate. Photovoltaic circuits, conductors in free air, parallel sets and taps each carry additional rules. None of them are covered here, and applying this seven-step procedure to them will produce a wrong answer with confidence.