Temperature Derating Factors for Wire Ampacity

Why Derating Matters

Electrical conductors generate heat proportional to the square of the current flowing through them (I²R losses). When ambient temperatures are elevated or when multiple conductors share a conduit, the heat cannot dissipate as effectively. Without derating, the conductor temperature can exceed the insulation rating, leading to accelerated degradation, insulation failure, and potential fire.

The NEC requires derating to ensure conductors operate within their rated temperature limits under actual installation conditions. Failing to apply derating factors is one of the most common code violations found in electrical inspections—and one of the most dangerous.

Ambient Temperature Correction Factors (NEC 310.15(B)(2))

NEC Table 310.15(B)(2) provides correction factors for ambient temperatures above 30°C (86°F). The ampacity values in Table 310.16 are based on a 30°C ambient. When the ambient temperature where the wire is installed exceeds this baseline, the ampacity must be multiplied by the corresponding correction factor.

The correction factors differ depending on which temperature column of Table 310.16 you are using as the base. This is why the 90°C column is valuable for derating—it provides the most headroom before the corrected ampacity falls below the 75°C or 60°C values.

Correction Factors for 60°C Insulation

Ambient Temperature	Correction Factor
21–25°C (70–77°F)	1.08
26–30°C (79–86°F)	1.00
31–35°C (88–95°F)	0.91
36–40°C (97–104°F)	0.82
41–45°C (106–113°F)	0.71
46–50°C (115–122°F)	0.58
51–55°C (124–131°F)	0.41
56–60°C (133–140°F)	0.00 (not usable)

Correction Factors for 75°C Insulation

Ambient Temperature	Correction Factor
21–25°C (70–77°F)	1.05
26–30°C (79–86°F)	1.00
31–35°C (88–95°F)	0.94
36–40°C (97–104°F)	0.88
41–45°C (106–113°F)	0.82
46–50°C (115–122°F)	0.75
51–55°C (124–131°F)	0.67
56–60°C (133–140°F)	0.58
61–65°C (145–149°F)	0.47
66–70°C (151–158°F)	0.33
71–75°C (160–167°F)	0.00 (not usable)

Correction Factors for 90°C Insulation

Ambient Temperature	Correction Factor
21–25°C (70–77°F)	1.04
26–30°C (79–86°F)	1.00
31–35°C (88–95°F)	0.96
36–40°C (97–104°F)	0.91
41–45°C (106–113°F)	0.87
46–50°C (115–122°F)	0.82
51–55°C (124–131°F)	0.76
56–60°C (133–140°F)	0.71
61–65°C (145–149°F)	0.65
66–70°C (151–158°F)	0.58
71–75°C (160–167°F)	0.50
76–80°C (169–176°F)	0.41
81–85°C (178–185°F)	0.29
86–90°C (187–194°F)	0.00 (not usable)

Conductor Bundling Derating (NEC 310.15(B)(3)(a))

When more than three current-carrying conductors are installed in a raceway or cable, they must be derated to account for the increased heat buildup. The more conductors packed together, the less heat can escape, and the lower the allowable current. NEC Table 310.15(B)(3)(a) provides the following adjustment factors:

Number of Current-Carrying Conductors	Adjustment Factor	Percent of Table Ampacity
1–3	1.00	100% (no adjustment)
4–6	0.80	80%
7–9	0.70	70%
10–20	0.50	50%
21–30	0.45	45%
31–40	0.40	40%
41–50	0.35	35%
51–60	0.30	30%
61–70	0.275	27.5%
71–80	0.25	25%
81–90	0.225	22.5%
91–100	0.20	20%

Which Conductors Count as Current-Carrying?

Not every conductor in a conduit counts toward the bundling derating. The following rules apply:

• Count as current-carrying: All ungrounded (hot) conductors, all grounded (neutral) conductors that carry current (such as in single-phase multiwire branch circuits with unbalanced loads).
• Do NOT count as current-carrying: Equipment grounding conductors (green or bare), isolated neutrals in balanced three-phase circuits (4-wire wye), and spare or inactive conductors.
• Multiwire branch circuits: In a single-phase multiwire branch circuit (shared neutral), the neutral does carry current and counts as a current-carrying conductor.
• Three-phase circuits: In a balanced 3-phase, 4-wire wye circuit, the neutral carries only the third harmonic current. If the third harmonic exceeds 50% of the fundamental, the neutral counts as a current-carrying conductor per NEC 310.15(B)(5)(c).

How to Apply Derating: Step-by-Step Example

Let's walk through a practical example to see how derating works in the real world.

The 90°C Column Advantage

When applying derating factors, always start with the 90°C column of NEC Table 310.16, even if your terminations are rated for only 75°C. This is because NEC 110.14(C)(1) limits the final ampacity at the termination to the 75°C or 60°C value, but the derating is applied to the 90°C base value first. Here's why this matters:

• If you start with the 90°C column and the derated value is still above the 75°C column value, you can use the wire at the 75°C ampacity.
• If the derated value falls between the 60°C and 75°C columns, you can use the wire at the 60°C ampacity.
• Only if the derated value falls below the 60°C column value do you need to upsize the wire.

This "headroom" approach often allows you to stay with a smaller wire size than if you had started derating from the 75°C or 60°C column directly.

Common High-Temperature Installation Locations

Certain locations routinely exceed the 30°C ambient baseline and require temperature derating:

Location	Typical Summer Temperature	Derating Impact
Attic space (directly under roof)	50–65°C (122–149°F)	Severe — often requires 1–2 wire size increase
Roof-mounted conduit (black pipe)	60–75°C (140–167°F)	Extreme — may need 2–3 size increase
South-facing exterior wall conduit	40–50°C (104–122°F)	Moderate — 1 size increase typical
Boiler room / mechanical room	40–55°C (104–131°F)	Moderate to severe
Commercial kitchen ceiling plenum	40–50°C (104–122°F)	Moderate
Underground (direct burial)	20–25°C (68–77°F)	None (below baseline)
Conditioned interior space	20–30°C (68–86°F)	None

Common Bundling Scenarios

Here are typical residential and commercial scenarios where bundling derating applies:

Scenario	Current-Carrying Conductors	Adjustment Factor
3 circuits (6 wires + ground) in 3/4" EMT	6	80%
4 circuits (8 wires + grounds) in 1" EMT	8	70%
6 circuits (12 wires + grounds) in 1-1/4" EMT	12	50%
Large commercial conduit with 15 circuits	30	45%
Single circuit (2 wires + ground) in conduit	2	100% (no adjustment)

Exceptions to Derating Rules

There are a few notable exceptions where bundling derating does not apply or is modified:

• Type NM (Romex) cable: NEC 334.80 provides specific ampacity ratings for NM cable that account for typical bundling in residential construction. NM cable is rated at the 60°C column regardless of insulation type.
• Type SE cable: NEC 338.10 provides specific rules for service entrance cable ampacity.
• Adjustment factor tables with specific conductor counts: Some applications have adjusted tables that account for typical bundling (e.g., Type AC cable, Type MC cable).
• Luminaires (lighting): Conductors in luminaires follow different rules per NEC 410.

Combining Both Derating Factors

When both ambient temperature and bundling conditions apply, the correction factors are multiplied together. The combined derating factor is:

Combined Factor = Ambient Temperature Correction × Bundling Adjustment

For example, in an attic at 50°C (factor 0.82 with 90°C insulation) with 7 conductors in a conduit (factor 0.70):

Combined Factor = 0.82 × 0.70 = 0.574

This means the wire's base ampacity (from the 90°C column) must be divided by 0.574 to find the minimum required ampacity, which is equivalent to multiplying the required load by 1.74 (1 ÷ 0.574). In practical terms, you may need to upsize the wire by 1–2 gauge sizes to compensate.

Quick Reference: Combined Derating Factors (90°C Base)

This table shows common combined derating factors for quick reference:

Ambient Temp	4–6 Conductors (0.80)	7–9 Conductors (0.70)	10–20 Conductors (0.50)	21–30 Conductors (0.45)
30°C (baseline)	0.800	0.700	0.500	0.450
35°C	0.768	0.672	0.480	0.432
40°C	0.728	0.637	0.455	0.410
45°C	0.696	0.609	0.435	0.392
50°C	0.656	0.574	0.410	0.369
55°C	0.608	0.532	0.380	0.342
60°C	0.568	0.497	0.355	0.320
65°C	0.520	0.455	0.325	0.293
70°C	0.464	0.406	0.290	0.261

Multiply the base ampacity (90°C column) by the factor to get the derated ampacity. The result must equal or exceed your circuit load current.

When to Use a Calculator

Derating calculations involve multiple variables—insulation type, ambient temperature, conductor count, and load type—that interact in complex ways. Our Wire Size Calculator applies all NEC 310.15(B) correction factors automatically. Enter your ambient temperature and conductor count, and the calculator determines the correct wire gauge with all derating applied.

Frequently Asked Questions

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