Wire Gauge & Conduit Fill Calculator
A free wire gauge calculator and conduit fill calculator built on the NEC wire ampacity chart, NEC 310 derating tables, and NEC Chapter 9 fill rules. Use it to size copper or aluminum conductors, check voltage drop, and verify EMT, PVC, RMC, or IMC conduit fill — all in one place.
Cable Sizing (Ampacity + Voltage Drop)
Determine appropriate cable size based on both current capacity and voltage drop requirements.
NEC Requirements
- NEC 210.19: Voltage drop should not exceed 3% for branch circuits (5% total)
- NEC 310.15: Ampacity must be derated for temperature and bundling
- Continuous loads: Conductors must be sized at 125% of load current
- Always select larger size if two criteria require different sizes
Need this validated by a licensed PE?
A free calculator gets you in the ballpark. For permit-stamped, defensible work, ClarkTE delivers a PE-stamped wire-sizing report from a working engineer — typically within 48 hours of receiving your one-line and load data.
NEC wire ampacity chart (Table 310.16, copper, 75°C)
The wire sizing chart below shows ampacity for copper conductors with 75°C insulation (THW, THWN) before any temperature or bundling derate. Run the calculator above for the derated values your install actually requires.
| Wire size (AWG / kcmil) | Copper 75°C ampacity | Typical use |
|---|---|---|
| 14 AWG | 20 A | 15 A lighting branch circuits |
| 12 AWG | 25 A | 20 A receptacle circuits |
| 10 AWG | 35 A | 30 A appliance circuits |
| 8 AWG | 50 A | 40 A range circuits |
| 6 AWG | 65 A | 60 A subpanels, large appliances |
| 4 AWG | 85 A | 80 A feeders |
| 3 AWG | 100 A | 100 amp wire size for short feeders |
| 2 AWG | 115 A | 100 A service with margin |
| 1/0 AWG | 150 A | 150 A residential service |
| 2/0 AWG | 175 A | Light commercial feeders |
| 3/0 AWG | 200 A | 200 A residential service |
| 4/0 AWG | 230 A | Service entrance, commercial |
| 350 kcmil | 310 A | 300 A commercial feeders |
Values shown are NEC Table 310.16 base ampacity at 30°C ambient. Real-world conductor selection requires temperature correction (Table 310.15(B)(2)(a)) and bundling adjustment (Table 310.15(B)(3)(a)). The calculator above applies both.
PVC conduit fill chart and EMT conduit fill quick reference
NEC Chapter 9 caps conduit fill at 53% for one conductor, 31% for two, and 40% for three or more. The conduit wire fill chart below lists how many THHN conductors fit in common trade sizes at the 40% three-or-more rule. Use the conduit fill calculator above for mixed-size pulls or non-THHN insulation.
| Conduit trade size | 12 AWG THHN (PVC) | 10 AWG THHN (PVC) | 12 AWG THHN (EMT) | 10 AWG THHN (EMT) |
|---|---|---|---|---|
| 1/2" | 8 | 5 | 9 | 5 |
| 3/4" | 15 | 9 | 16 | 10 |
| 1" | 25 | 15 | 26 | 16 |
| 1-1/4" | 43 | 27 | 45 | 28 |
| 1-1/2" | 59 | 37 | 61 | 38 |
| 2" | 98 | 62 | 100 | 63 |
Source: NEC Chapter 9 Table 1 (max fill) and Table 5 (THHN cross-sectional area). Numbers are conductor counts, not wire pairs or circuits.
Common wire sizing questions
What size wire do I need for 100 amps?
For a 100 amp feeder at 75°C, NEC Table 310.16 lists 3 AWG copper or 1 AWG aluminum as the smallest size that meets ampacity. Most installers default to 2 AWG copper (115 A) or 1/0 AWG aluminum to leave headroom for derating, voltage drop, and future load growth. Always run the calculator with your actual ambient temperature and conductor count — it will catch cases where 3 AWG fails the derate.
Why does the wire amp chart change with temperature?
Conductor insulation has a maximum operating temperature (60°C, 75°C, or 90°C). When ambient temperature rises, less of the insulation's thermal headroom is available to dissipate heat from the conductor itself, so ampacity drops. NEC 310.15(B)(2)(a) publishes the correction factors. Hot attics and rooftop conduit runs routinely cut 75°C ampacity by 20–30%.
Is there a separate DC wire size calculator?
The same calculator works for DC sizing. Set system type to single phase, set voltage to your DC bus voltage (12, 24, 48, 380, etc.), and use a power factor of 1.0. The voltage-drop math collapses to the standard DC formula (2 × I × R × L / 1000) automatically. For solar and battery-bank applications, most installers target 2% voltage drop, not 3%.
How is this wire length calculator different from a simple voltage-drop tool?
A wire length calculator typically only checks voltage drop. Code-compliant cable sizing has to satisfy ampacity (with derating) AND voltage drop, then pick the larger of the two answers. The "Cable Sizing" tab above does both passes and reports each minimum so you can see which constraint is driving the size.
Sizing medium- or high-voltage cable for a real install?
This calculator covers branch and feeder sizing for low-voltage circuits. For 5 kV through 230 kV cable — power factor testing, partial discharge diagnostics, VLF testing, and PE-stamped reports — ClarkTE's specialty testing and substation services teams handle the hard cases.
Wire and conduit FAQ
What size wire do I need for 40 amps?
For a 40 A continuous load at 75°C copper, NEC Table 310.16 requires 8 AWG (50 A ampacity, derates with margin to handle the 125% continuous-load multiplier). For aluminum, use 6 AWG. The calculator above will confirm based on your actual ambient temperature and conductor count.
What size wire for 200 amps?
A 200 A residential service typically uses 3/0 AWG copper or 4/0 AWG aluminum at 75°C, sized to the panel's lug temperature rating. For commercial feeders or longer runs, you may need to upsize for voltage drop. Run the calculator with your distance and voltage to confirm.
What is conduit fill, and where does the 40% rule come from?
NEC Chapter 9 Table 1 caps conduit fill at 53% for one conductor, 31% for two, and 40% for three or more. The limits ensure heat dissipation, allow for jam-resistant pulls, and prevent insulation damage. The conduit fill calculator applies these limits across EMT, PVC, RMC, and IMC.
Does this work as a DC wire size calculator?
Yes. Set system type to single-phase, voltage to your DC bus voltage (12, 24, 48, 380 V), and power factor to 1.0. The voltage-drop math collapses to the standard DC formula (2 × I × R × L / 1000) automatically. Most DC installs target 2% voltage drop instead of 3%.
Why does this calculator differ from a simple wire length calculator?
A wire length calculator usually only checks voltage drop. Code-compliant cable sizing has to satisfy ampacity (with derating) AND voltage drop, then pick the larger of the two. The Cable Sizing tab does both passes and reports each minimum so you can see which constraint drives the size.