Wire Sizing for Circuit Breakers: NEC 310 Ampacity Tables and Derating Explained

Every circuit you install starts with the same question: what size wire do I need? Get it wrong, and you’re looking at a failed inspection, a callback, or worse — an overheated conductor behind a finished wall. Get it right, and the inspector signs off on the first visit.

Wire sizing for circuit breakers isn’t a single table lookup. It’s the convergence of ampacity ratings from NEC Table 310.16, temperature correction factors, conduit fill derating, terminal temperature limitations, and voltage drop — all filtered through the specific NEC edition your AHJ enforces. The correct wire size is the largest size required by any of these constraints.

This guide walks through the complete wire sizing process as the NEC intends it, with the table references and formulas you need to defend your work to any inspector.

Step 1: Determine Your Load Current

Before you touch Table 310.16, you need to know what the circuit is carrying. The starting point depends on the load type:

General loads — Calculate from the connected load in VA, then convert to amps: I = VA / V (single-phase) or I = VA / (V × 1.732) (three-phase)
Motor loads — Use the full-load current (FLC) from NEC Table 430.248 (single-phase) or 430.250 (three-phase), not the nameplate current. This is a common exam and inspection pitfall.
Continuous loads — Any load expected to run for 3 hours or more (NEC Article 100 definition). Conductors must be sized at 125% of the continuous load per NEC 210.19(A)(1). A 40A continuous load requires conductors rated for at least 50A.

For mixed loads, sum the continuous portion at 125% plus the non-continuous portion at 100%. This gives you the minimum required ampacity before any derating.

Step 2: Look Up Base Ampacity in NEC Table 310.16

NEC Table 310.16 is the primary ampacity reference for conductors rated 0–2000V in raceways, cables, or directly buried (based on an ambient temperature of 30°C / 86°F). The table has three temperature columns:

60°C column — Used for conductors with 60°C-rated insulation (TW, UF) and as the default for circuits 100A or less with standard equipment
75°C column — Used for conductors with 75°C-rated insulation (THWN, XHHW) and for equipment rated or marked for 75°C terminations
90°C column — Used for conductors with 90°C-rated insulation (THHN, THWN-2). This column is your starting point for derating calculations only

Here are the most commonly referenced ampacities for copper conductors with THHN/THWN-2 insulation:

AWG/kcmil	60°C (TW, UF)	75°C (THWN)	90°C (THHN)
14 AWG	15A	20A	25A
12 AWG	20A	25A	30A
10 AWG	30A	35A	40A
8 AWG	40A	50A	55A
6 AWG	55A	65A	75A
4 AWG	70A	85A	95A
3 AWG	85A	100A	115A
2 AWG	95A	115A	130A
1 AWG	110A	130A	145A
1/0 AWG	125A	150A	170A
2/0 AWG	145A	175A	195A
3/0 AWG	165A	200A	225A
4/0 AWG	195A	230A	260A
250 kcmil	215A	255A	290A
500 kcmil	320A	380A	430A

Terminal temperature limitation (NEC 110.14(C)): This is where many calculations go wrong. Even if you’re running THHN (90°C insulation), most equipment terminations are rated for 75°C. For circuits rated 100A or less, or conductors 14 AWG through 1 AWG, the NEC requires you to use the 60°C column unless the equipment is specifically listed and marked for 75°C terminations. Most modern breakers and panels are marked for 75°C, but always verify.

Step 3: Apply Temperature Correction Factors

Table 310.16 assumes a 30°C (86°F) ambient temperature. If your conductors run through an environment hotter than 30°C — attics in summer, rooftops, boiler rooms, or outdoor conduit in direct sun — you must derate using NEC Table 310.15(B)(1).

Key correction factors for 90°C-rated conductors (THHN/THWN-2):

Ambient Temp	90°C Correction Factor
31–35°C (87–95°F)	0.96
36–40°C (96–104°F)	0.91
41–45°C (105–113°F)	0.87
46–50°C (114–122°F)	0.82
51–55°C (123–131°F)	0.76
56–60°C (132–140°F)	0.71

Pro tip: Attic temperatures in southern states routinely hit 50–60°C (122–140°F) in summer. A 10 AWG THHN conductor rated at 40A in normal conditions drops to 32.8A at 50°C. If your circuit needs 30A, that 10 AWG still works — but barely. Always check the ambient conditions along the entire conductor run, not just at the panel.

Step 4: Apply Conduit Fill Adjustment Factors

When more than three current-carrying conductors share a raceway or cable for a continuous length exceeding 24 inches, heat buildup reduces each conductor’s capacity. NEC Table 310.15(C)(1) provides the adjustment factors:

Current-Carrying Conductors	Adjustment Factor
1–3	1.00 (no adjustment)
4–6	0.80
7–9	0.70
10–20	0.50
21–30	0.45
31–40	0.40
41+	0.35

Important: Neutral conductors that carry only unbalanced current from other conductors are not counted as current-carrying conductors. However, a shared neutral in a multiwire branch circuit with line-to-neutral loads on both phases does count as current-carrying. Equipment grounding conductors are never counted.

Step 5: The Derating Calculation Method

Here’s where the 90°C column earns its keep. When you need to apply derating factors, use this approach per NEC 310.15(B):

Start with the 90°C ampacity from Table 310.16
Multiply by the temperature correction factor from Table 310.15(B)(1)
Multiply by the conduit fill adjustment factor from Table 310.15(C)(1)
Compare the result to the terminal temperature column (60°C or 75°C) — use the lower of the two values

Worked Example: 40A Continuous Load in a Hot Attic

Suppose you need to run a 40A continuous load through an attic where ambient temperature reaches 45°C, with 6 current-carrying conductors in the conduit. Copper THHN conductors, equipment marked 75°C.

Minimum required ampacity: 40A × 1.25 = 50A (continuous load adjustment)

Try 6 AWG THHN (90°C base ampacity = 75A):

Temperature correction (45°C): 75A × 0.87 = 65.25A
Conduit fill adjustment (6 conductors): 65.25A × 0.80 = 52.2A
Terminal temp limit (75°C column for 6 AWG): 65A
Final ampacity: 52.2A (lower of 52.2A and 65A)
52.2A ≥ 50A required — 6 AWG passes

Without the 90°C starting point, you’d need to derate from the 75°C column: 65A × 0.87 × 0.80 = 45.2A — which fails the 50A requirement and forces you up to 4 AWG. The 90°C column saves you a wire size (and significant copper cost on long runs).

Step 6: Check Voltage Drop

After sizing for ampacity, calculate voltage drop to confirm compliance with the NEC recommendation of 3% maximum on branch circuits and 5% total (feeder + branch). While NEC 210.19(A) Informational Note frames this as a recommendation rather than a requirement, most AHJs enforce it.

The voltage drop formula for single-phase circuits:

VD = (2 × K × I × L) / CM

Where K = 12.9 for copper (21.2 for aluminum), I = load current in amps, L = one-way distance in feet, CM = circular mils of the conductor.

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

If voltage drop exceeds your target, increase the conductor size until it’s within limits. For long runs — warehouses, parking lot lighting, farm buildings — voltage drop frequently controls the wire size rather than ampacity. A 20A circuit on 12 AWG is fine for ampacity, but at 150 feet you’re already at 4.8% voltage drop on a 120V circuit. You’d need 10 AWG to stay under 3%.

Use our wire size calculator to check ampacity, derating, and voltage drop simultaneously — it shows which constraint controls the final size.

Step 7: Size the Circuit Breaker

The breaker protects the conductor, not the load. Per NEC 240.4, the standard overcurrent protection for conductors must not exceed the ampacity of the conductor after all adjustments.

The “next standard size up” rule (NEC 240.4(B)): Where the ampacity of the conductor does not correspond to a standard OCPD rating, the next higher standard rating is permitted — but only up to 800A. Standard breaker sizes per NEC 240.6(A): 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800A.

Small conductor protection (NEC 240.4(D)): Regardless of any calculation, 14 AWG is limited to 15A OCPD, 12 AWG to 20A, and 10 AWG to 30A. No exceptions for standard branch circuits.

Example: Your 6 AWG conductor from the attic example has a final derated ampacity of 52.2A. The next standard breaker size down is 50A. You’d use a 50A breaker. You could apply 240.4(B) and use a 60A breaker only if the unmodified ampacity (before derating) permits it — but since your derated ampacity is 52.2A, a 50A breaker is the correct choice to protect the conductor at its actual capacity.

Step 8: Size the Equipment Grounding Conductor

Don’t forget the EGC. NEC Table 250.122 sizes the equipment grounding conductor based on the rating of the overcurrent protective device, not the wire size:

OCPD Rating	Copper EGC Size
15A	14 AWG
20A	12 AWG
30A	10 AWG
40A	10 AWG
60A	10 AWG
100A	8 AWG
200A	6 AWG

For the 50A breaker in our example, the EGC would be 10 AWG copper per Table 250.122.

Common Wire Sizing Mistakes

After years of inspections and exam grading, these are the errors that come up repeatedly:

Using the 90°C column directly for breaker sizing — The 90°C value is for derating math only. Your final ampacity is capped by the terminal temperature rating. A 12 AWG THHN does not get a 30A breaker.
Forgetting the 125% continuous load rule — A 40A continuous load needs 50A of conductor capacity, not 40A.
Using nameplate amps for motors — Motor circuits use the NEC FLC tables (430.248/430.250), not nameplate current. Nameplate current can differ significantly.
Ignoring conduit fill derating — That home run with 8 circuits in one conduit? Each conductor loses 30% of its ampacity with 7–9 current-carrying conductors.
Calculating voltage drop at 100% load but sizing conductor at 125% — Voltage drop is calculated at the actual load current, not the 125% sizing current.
Wrong NEC edition — Always confirm which edition your AHJ has adopted. Table numbers and some ampacity values change between editions.

Quick Reference: Wire-to-Breaker Sizing Table

For standard installations (30°C ambient, 3 or fewer conductors in raceway, 75°C-rated equipment, copper THHN/THWN-2 conductors):

Wire Size	75°C Ampacity	Typical Max Breaker	Common Applications
14 AWG	20A	15A*	Lighting, general receptacles
12 AWG	25A	20A*	Kitchen, bathroom, garage receptacles
10 AWG	35A	30A*	Dryers, water heaters, A/C units
8 AWG	50A	50A	Ranges, large A/C, sub-panels
6 AWG	65A	60A	Sub-panels, large equipment
4 AWG	85A	80A	Sub-panels, commercial feeders
3 AWG	100A	100A	100A sub-panels
2 AWG	115A	110A	Large sub-panels
1/0 AWG	150A	150A	150A services, large feeders
2/0 AWG	175A	175A	175A services
4/0 AWG	230A	200A	200A residential services

*NEC 240.4(D) limits: 14 AWG to 15A, 12 AWG to 20A, 10 AWG to 30A regardless of ampacity.

Important: This table applies only under standard conditions. Any temperature correction, conduit fill derating, or long-run voltage drop may require upsizing the conductor beyond these values.

The Bottom Line

Wire sizing is the fundamental NEC calculation, and getting it right means checking every constraint: ampacity from Table 310.16, terminal temperature limits from 110.14(C), temperature correction from Table 310.15(B)(1), conduit fill adjustment from Table 310.15(C)(1), voltage drop per 210.19(A) Informational Note, and OCPD sizing per Article 240. The controlling factor varies by installation — a short run in a cool basement is ampacity-controlled, while a long run through a hot attic is likely voltage-drop-controlled. Check them all, document which one controls, and the inspector will have nothing to question.