100-Amp Wire Size Interactive Calculator

Running a 100-amp circuit—whether it’s for a subpanel, a machine, or a commercial kitchen—comes down to wire selection. The wire size isn’t just about safety codes; it’s about real-world reliability. Undersize it, and you’re asking for tripped breakers, hot wires, or voltage sag that can throw off your equipment. This tool uses your choices (material, insulation, length, ambient temp, and setup) to handle minimum gauge, voltage drop, resistance, power loss, and conduit fill. You’ll find the math, an industry case, details on temperature derating, and an FAQ all below.

What is 100-amp wire sizing?

100-amp wire sizing is simply picking a wire that can handle 100 amps without overheating or too much voltage loss. Actual size depends on wire material, insulation type, the way you install it, and especially how far that current has to go.

Simple Explanation

Think of wire as a pipe—make it too small, you choke the current, get voltage drop, and heat builds up. For a 100-amp circuit, the wire needs enough cross-section to do the job without dropping voltage or running hot. If you’re going long distance, you need a bigger wire, no way around it.

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Visual Diagram

100-Amp Wire Size Interactive Calculator

How to Use This Calculator

1. Pick what you want to solve—wire size, voltage drop, max length, max current, or conduit fill.
2. Fill in your actual numbers for amps, system voltage, wire material, insulation temp, length, ambient temperature, configuration, and load type.
3. If you're checking conduit fill, enter how many wires are in the conduit.
4. Hit Calculate for your answer.

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Wire Sizing Equations

Here’s how to calculate voltage drop, losses, and ampacity for a 100-amp circuit:

Simple Example

Inputs: 100A continuous load, 240V single-phase, copper, 75°C insulation (THWN), 100 ft one-way, 30°C ambient.

Minimum: 2 AWG copper (ampacity 115A × 1.0 = 115A, clears the required 125A for continuous load).
Voltage drop: 2 × 100A × 0.000194 Ω/ft × 100 ft = 3.88V (1.62%)—below 3% code guideline.
Power loss: (100A)² × (2 × 0.000194 × 100) = 388W.

Theory & Practical Applications

Sizing wire for 100 amps is a nuts-and-bolts calculation in any serious electrical install. Undersize it, and you risk both safety and nuisance failures—bad voltage drop, breakers tripping, or actual fire risk. NEC tables work if you’re in textbook conditions: right ambient, short runs, three wires in a pipe. Out in the field, you often get higher temps, longer runs, tightly packed conduits, and load profiles that push right up to the breaker limit. A 100-amp 240V circuit runs 24kW; that’s a full house, a decent commercial kitchen, or a busy shop’s main tool bus. So, you don’t cut corners picking wire sizes with this much current.

Ampacity and Temperature Derating

Wire ampacity is the steady current it can carry without the insulation getting too hot. NEC Table 310.15(B)(16) lists 3 AWG copper with 75°C insulation or 1 AWG aluminum as your minimum for 100 amps—but that’s at 30°C ambient and three or fewer wires in the conduit. Real jobs don’t always give you that.

With higher ambient temperature, you lose cooling and have to discount table ampacity. For example, 40°C ambient drops derating to 0.88, so 3 AWG copper (100A) now really only does 88A—too small for 100A. That means you need to upsize to 2 AWG copper, base ampacity 115A, which stays above 100A even after derating. Warehouses, rooftops, hot attics, anywhere with direct sun on conduit—figure on standard wire size not working without recalculating for temperature.

Voltage Drop Economics and Equipment Performance

Code says try to keep voltage drop below 3% for each circuit. 5% total from the panel to the equipment is the upper limit. That isn’t a hard legal number but push above it and you get wasted energy and more stress on motors and electronics. For a 100A motor feeder, a 4% drop burns 960W continuously—that adds to your electric bill quickly, especially 24/7. Motors especially hate low voltage: less torque, more current, and a spiral of heat-rise. Drives, LEDs, controllers? Even more finicky. If you want to avoid callbacks and keep things running smooth, shoot for well below 3% voltage drop, sometimes 2% or even 1.5% for critical electronics. For a 100A, 240V run of 100ft, you’re looking at 1 AWG copper to hit that low drop, not 3 AWG.

Aluminum Versus Copper Conductor Selection

Aluminum’s cheaper than copper (sometimes half), but you’ll always have to go up at least two sizes. For 100A, 1/0 AWG aluminum is the rough equal of 3 AWG copper on ampacity, but it’ll have more voltage drop since resistance is higher (about 1.6×). The bigger wire means you need larger conduit and special connectors rated for AL, plus anti-oxidant paste on the lugs. For long feeders (100+ ft), aluminum often closes the cost gap—lighter, so labor’s easier, and price per foot is much less. But the connections take more attention: aluminum moves under load, so plan to check torque on those lugs over time, and never try to mix aluminum wires direct into old copper-only terminals. Also, hot environments boost resistance in aluminum more than copper, so you lose even more margin if ambient is high.

Conduit Fill and Derating for Multiple Conductors

Pack more than three current-carrying wires into a pipe and they heat each other up. NEC says derate: four to six—ampacity drops to 80%, up to nine—drops to 70%. Neutral counts as current-carrying if it handles unbalanced current (like in three-phase with neutral). Forgetting the derating is common, especially in branch circuits. For example, for a three-phase 100A feeder with a neutral (four wires), 3 AWG copper now only gives you 80A when derated—so you must bump to 2 AWG or larger.

Physically, you can’t fill the pipe up, either: NEC allows up to 40% of conduit’s area once you have three or more wires. The insulation matters—THHN is much thinner than THW, so more fits. Pay attention—try to squeeze too many #3 wires in a 3/4” pipe with the wrong insulation, and you’ll run out of space. Always confirm both ampacity and physical fill before pulling wire.

Continuous Load Multiplier and Safety Margins

Any circuit running over three hours needs wires rated for at least 125% of the load. So, your “100A continuous” load requires a wire good for 125A—before derating for ambient or conduit fill. For example, 3 AWG copper's base rating is 100A; not enough. 2 AWG (115A) often isn’t either if you have to derate for heat or fill, so 1 AWG (130A) or 1/0 (150A) is common for tough conditions. Don't cheap out here—the difference in wire costs is small compared to the risks you take with overheated conductors or nuisance breaker trips. Worst case, undersizing might mean a fire.

Three-Phase Systems and Voltage Drop Calculations

Three-phase voltage drop formulas use the √3 factor because the currents are out of phase and the load spreads across all three wires. For the same current and wire, you get slightly less voltage drop for three-phase than single-phase. For a 100A, 480V, 180ft run on 3 AWG copper, your calculated drop is only about 1.33%—within most job specs. If you switch that to single-phase, the drop gets worse and may blow past the 3% guide, pushing you to upsize wire. Also, keep in mind, your real delivered power at the equipment depends on power factor—so for loads like big motors with poor power factor, you’re still sizing for “worst-case” amps even if the real power is lower.

Worked Example: Industrial Machine Feeder Design

Say you’ve got to feed a 100A continuous-rated industrial machine, 180ft from a 480V, three-phase panel, with 38°C ambient. Load: a 75 HP motor (94% efficient, 0.88 power factor) plus 12 kW of extras.
First, calculate the real current. Motor: 75 × 0.746 / 0.94 ≈ 59.53 kW. Motor amps: 59,530 / (√3 × 480 × 0.88) ≈ 81.3A. Plus 12,000W of auxiliaries: 12,000 / (√3 × 480 × 1.0) = 14.4A. So, total ≈ 96A.
Apply continuous load factor: 96 × 1.25 = 120A. At 38°C ambient, factor drops to 0.91. So, you need a base rating of at least 120 / 0.91 ≈ 132A.
From tables: 1 AWG copper at 75°C is 130A – not enough; 1/0 AWG copper is next up at 150A. Derated, that’s 136.5A. For 1/0, resistance is 0.122mΩ/ft.
Voltage drop: √3 × 96 × 0.000122 × 180 ≈ 3.67V (0.76%). That’s fine.
For aluminum: 3/0 AWG is the next match. Same process, slightly higher resistance, calculated drop is 0.79%.
Power loss: Copper—(96A)² × (2 × 0.000122 × 180) = 407W. Aluminum—420W.
Annual energy cost difference over 8760 hours is just $13.66 (at $0.12/kWh).
Conduit fill for four 1/0 copper wires is 0.742 in² total. You need 1½" EMT minimum. For 3/0 aluminum, you’ll need 2" EMT.
Bottom line: for this job, copper is simpler for connections, but aluminum saves about $450 in materials. Over 20 years, copper nearly catches up in cost via energy use, and avoids the hassle of aluminum lugs and torque checks. Both work; the right answer depends on your install preferences.

Frequently Asked Questions

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