Wire Size Calculator
Size copper or aluminum conductors from known current, installation conditions, and an optional resistance-only voltage-drop screen.
Defaults: continuous portion 0 A · 86°F ambient · 3 CCC · rooftop off · voltage-drop screen off.
How to Use
- Enter actual operating current - Use the known load current before any continuous-load multiplier. Do not enter the breaker rating or a current already enlarged by 125%.
- Choose the supported wiring method and material - THHN/THWN-2 supports copper or aluminum in this raceway model. CalcShed supports copper NM-B only in this release.
- Confirm the termination basis - For THHN/THWN-2, use 60°C unless the lowest applicable connected equipment/termination basis is confirmed for 75°C. NM-B uses its own 60°C final ceiling.
- Set installation conditions when needed - Advanced settings accept the continuous portion, governing ambient, current-carrying-conductor count, and the exact supported direct-sun rooftop condition.
- Optionally include voltage drop - The resistance-only planning screen can force a larger conductor for the entered run, but it does not replace detailed AC impedance analysis.
Wire size is selected by separate checks, not one blanket multiplier. The calculator first checks the applicable 60°C/75°C final-basis ampacity against noncontinuous current plus 125% of the continuous portion. Separately, it applies ambient and current-carrying-conductor factors to the 90°C conductor ampacity and checks that adjusted result against the actual load. Ordinary small-conductor protection limits remain a separate screen, and optional resistance-only voltage drop can require a larger conductor.
How the Wire Size Is Chosen
Continuous and Noncontinuous Current Use Two Checks
Let I be the actual total current and C the continuous portion of that total. The ordinary sizing requirement is M = I + 0.25C. The applicable final-basis ampacity must be at least M.
The corrected/adjusted conductor ampacity is checked separately against the actual total current I. Applying the extra 25% again to this adjusted-ampacity comparison can unnecessarily force a larger conductor.
The calculator does not model the 100%-rated-assembly exception or the separately installed connector exception. Those are outside this ordinary workflow.
Ambient Temperature, Rooftops, and Final Temperature Limits
The THHN/THWN-2 workflow uses the 90°C Table 310.16 value as the permitted correction/adjustment basis, then limits the final result by the applicable 60°C or 75°C connected-equipment/termination basis.
For the exact supported direct-sun rooftop case, raceway or cable less than 3/4 inch above the roof receives a 60°F temperature adder before the 90°C correction factor is selected. The calculator does not generalize this into every rooftop or cable installation.
NM-B uses its 90°C conductor rating for permitted correction/adjustment but cannot exceed the 60°C final ampacity in this model.
90°C Ambient Correction Examples
| Ambient range | 90°C factor |
|---|---|
| 78-86°F | 1.00 |
| 87-95°F | 0.96 |
| 96-104°F | 0.91 |
| 105-113°F | 0.87 |
| 114-122°F | 0.82 |
| 123-131°F | 0.76 |
| 132-140°F | 0.71 |
The engine includes the full supported -40°F through 185°F mapping. Fractional values are mapped conservatively to the next warmer published band; they are not rounded down.
Current-Carrying-Conductor Adjustment
| Applicable CCC count | Adjustment factor |
|---|---|
| 1-3 | 100% |
| 4-6 | 80% |
| 7-9 | 70% |
| 10-20 | 50% |
| 21-30 | 45% |
| 31-40 | 40% |
| 41+ | 35% |
Neutral treatment depends on the actual circuit and load. Grounding and bonding conductors are not counted. The calculator accepts the applicable count you provide and does not infer code exceptions.
Equipment Termination Rating Is a Real Limit
The temperature basis is coordinated to the lowest applicable connected termination, conductor, or device. A higher insulation rating can be used for permitted correction/adjustment without turning a 60°C or 75°C equipment connection into a generic 90°C termination.
That is why the THHN/THWN-2 form asks for a confirmed 60°C or 75°C equipment termination basis rather than offering a generic 90°C option.
Worked Example A: Ordinary 20 A Copper THHN/THWN-2
| Check | Value | Result |
|---|---|---|
| Inputs | 20 A actual, 0 A continuous, 60°C basis, 86°F, 3 CCC | No correction or CCC reduction |
| 14 AWG | 60°C ampacity 15 A | Fails 20 A load-sizing requirement |
| 12 AWG | 60°C ampacity 20 A; 90°C basis 30 A | Passes ampacity and ordinary 20 A protection screen |
| Recommended size | 12 AWG copper | Smallest supported size passing every active check |
Worked Example B: 40 A Fully Continuous with 4 CCC
| Check | Value | Result |
|---|---|---|
| Load sizing requirement | I = 40 A, C = 40 A → M = 50 A | Final-basis ampacity must be at least 50 A |
| 8 AWG final basis | 75°C ampacity = 50 A | Passes the M check |
| 8 AWG adjusted basis | 55 A × 0.80 = 44 A | 44 A ≥ actual 40 A load |
| Recommended size | 8 AWG copper | Comparing 44 A against 50 A again would apply the continuous-load increase twice |
Worked Example C: Voltage Drop Governs
| Conductor | Table 8 resistance at 75°C | 120 V, 20 A, 100 ft one-way |
|---|---|---|
| 12 AWG Cu | 1.98 Ω/kft | 7.92 V · 6.60% |
| 10 AWG Cu | 1.24 Ω/kft | 4.96 V · 4.13% |
| 8 AWG Cu | 0.778 Ω/kft | 3.11 V · 2.59% |
Ampacity alone selects 12 AWG in this example. With the 3% resistance-only planning target enabled, 8 AWG becomes the first supported size that passes both ampacity and voltage drop.
Worked Example D: NM-B 60°C Ceiling
| Check | 10 AWG copper NM-B |
|---|---|
| Actual load | 30 A |
| 90°C correction/adjustment basis | 40 A |
| Final NM-B ceiling | 30 A at 60°C |
| Ordinary small-conductor protection screen | 30 A |
| Recommended size | 10 AWG Cu NM-B |
Selection Logic
For each supported conductor size, the calculator keeps thermal ampacity, protection, and optional voltage drop as separate checks:
Adjusted ampacity = A90 × 90°C ambient correction factor × CCC adjustment factor
Allowable ampacity = Minimum of adjusted ampacity and applicable final 60°C/75°C ceiling
Thermal pass = Final-basis ampacity ≥ M AND allowable adjusted ampacity ≥ I
Ordinary small-conductor protection = Separate 240.4(D) screen for supported 14/12/10 AWG cases; it does not replace thermal ampacity
DC / single-phase resistance-only VD = VD = 2 × I × R × one-way distance ÷ 1000
Balanced three-phase resistance-only VD = VD = √3 × I × R × one-way distance ÷ 1000
Pick rule = Return the first supported AWG/kcmil size that passes every active check
Internal comparisons use unrounded values. Display rounding happens only after the conductor has been selected.
2026 NEC and Physical-Data Sources
- Connected termination/device temperature limits and the use of higher conductor temperature ratings for permitted correction/adjustment are based on 110.14(C).NFPA 70 (2026) 110.14(C)
- The branch-circuit two-test continuous/noncontinuous conductor structure is based on 210.19(A); the feeder analogue used for this ordinary model is 215.4(A).NFPA 70 (2026) Articles 210 and 215
- Ordinary small-conductor protection limits are kept separate from Table 310.16 thermal ampacity values.NFPA 70 (2026) 240.4(D)
- Ampacity, ambient correction, current-carrying-conductor adjustment, neutral treatment, grounding/bonding count treatment, and the supported rooftop adder are based on Article 310.NFPA 70 (2026) 310.15 and Table 310.16
- Copper NM-B in this calculator uses the 334.80 90°C adjustment/correction basis with a 60°C final ceiling. Other NM conductor materials recognized by the NEC are deliberately outside CalcShed's current NM-B support.NFPA 70 (2026) 334.80 / 334.104
- The optional voltage-drop screen uses Chapter 9 Table 8 stranded-conductor DC resistance at 75°C. Table 9 demonstrates why AC resistance, reactance, arrangement, frequency, and power factor can require a more detailed model.NFPA 70 (2026) Chapter 9 Tables 8 and 9
Next Steps
Once the conductor passes this Wire Size planning screen, use the neighboring tools for the jobs they own:
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