100 Amp Wire Size Calculator - NEC Ampacity and Voltage-Drop Sizing

A 100 amp wire size calculator tells you the smallest conductor that can safely carry a 100 amp feeder to a sub panel, workshop, or outbuilding. The answer is not a single number that you can memorize once: it depends on how far the wire runs, whether the supply is single-phase or three-phase, and whether you use copper or aluminum. Home centers sell a range of gauges, and picking one that is too small lets the conductor overheat at the 100 A ampacity limit, while picking one that is far too large wastes money on copper and forces a bigger conduit.

• • Main service entrance: Confirm the conductor size feeding a 100 A meter base or main breaker on a residential service before the inspector asks for the calculation.
• • Sub panel feed: Size the wire between a 200 A main panel and a 100 A sub panel in a garage, shop, or addition where the run can be long enough for drop to matter.
• • Outbuilding or shop: Pick the gauge for a long run to a detached building where voltage drop is the binding constraint rather than the ampacity floor.
• • Material trade-off: Compare copper versus aluminum when the conduit fill or budget changes the preferred choice, since aluminum needs a larger gauge for the same drop.

Two independent limits govern the size. The first is ampacity: NEC Table 310.16 requires at least 3 AWG copper or 1 AWG aluminum at 75 C terminations just to carry 100 A without overheating. The second is voltage drop, which becomes the controlling factor on longer runs and is the limit most people forget until their equipment misbehaves at the far end of the cable.

This calculator solves both at once. It computes the cross-section needed to keep the drop under your chosen percent of the supply voltage, applies the NEC ampacity floor so short runs never drop below the safe minimum, then converts the result to the next standard AWG gauge and reports the matching conduit trade size. When you know the 100 amp wire size you need for your exact run, you can buy the right spool the first time instead of guessing from a rule of thumb.

The 100 amp wire size question comes up for whole-service entrances, detached garages, and workshop feeds alike. Because the current is fixed at 100 A, the only variables left are distance, voltage, phase, and material, which keeps the calculation approachable while still respecting the electrical code.

If your feeder runs at a lower 220 volt single-phase load rather than a full 100 amp service, the 220V wire size calculator walks through the same voltage-drop method at a smaller current.

• Fixed current (100 A): The load current is held at 100 A, so sizing depends only on distance, voltage, phase, and material rather than a load you must enter.
• System voltage: The supply voltage sets the allowable drop budget; a higher voltage tolerates more absolute volts of drop for the same percent limit.
• Phase: Three-phase feeders use the square-root-of-three factor, so they drop less voltage for the same wire and can sometimes use a smaller gauge.
• One-way distance: Resistance accumulates over the full run length, so doubling the distance roughly doubles the voltage drop.
• Material resistivity: Copper (1.724e-8 ohm*m) conducts about 1.6 times better than aluminum (2.82e-8 ohm*m), which is why aluminum sizes run larger.

According to OSHA Electrical Standards, Occupational Safety and Health Administration electrical standards that back NEC-based conductor sizing.

According to ASTM B258 AWG standard nominal diameters and areas, Defines the AWG diameter-to-area relationship used to map the computed area to a standard gauge.

The voltage-drop term follows directly from resistance and current, so the Ohm's law calculator is the place to revisit how V = I x R produces the volts lost along the run.

The same area-and-drop logic applies at lower voltages where aluminum and copper differences are even more pronounced, as shown in the 24V wire size calculator.

1. 1 Set the system voltage: Pick your supply voltage (120, 208, 240, 277, or 480 V) from the dropdown; this sets the drop budget in volts.
2. 2 Choose single or three-phase: Select single-phase for residential services or three-phase for commercial feeders, since three-phase uses the square-root-of-three factor.
3. 3 Enter the one-way distance: Measure the run from the panel to the load in feet; this is the most common binding constraint on a 100 A feeder.
4. 4 Pick copper or aluminum: Choose the conductor material to compare sizes and conduit fill, remembering aluminum lands one to two gauges larger.
5. 5 Set the allowable drop: Leave at 3% for feeders, or raise it to 5% if local rules allow a looser budget on a marginal run.
6. 6 Read the gauge and conduit: The result shows the minimum AWG, cross-section, diameter, voltage drop, and conduit trade size so you can buy the right spool.

When the 100 A feeder serves a motor load, confirm the actual current draw with the three-phase motor amperage calculator before trusting the gauge.

• • Benefit: Avoids an undersized wire that overheats at the 100 A ampacity limit and trips or damages terminals over time.
• • Benefit: Prevents a long run from starving equipment through excessive voltage drop that shows up as dim lights or a sluggish motor.
• • Benefit: Compares copper and aluminum sizes side by side for cost and conduit fill, so you can weigh the savings against the larger gauge.
• • Benefit: Maps the result straight to a standard AWG gauge and conduit trade size, removing the guesswork between the math and the hardware aisle.
• • Benefit: Shows the actual resulting voltage drop in both volts and percent, so you can see how close you are to the 3% recommendation.

Knowing the real watt draw of the load helps validate that 100 A is the right current to size for, which the AC wattage calculator can confirm from voltage and power factor.

• • This calculator sizes for the conductor only; it does not account for breaker trip curves, ambient temperature derating, or conductor bundling that NEC 310.15 requires.
• • It assumes a copper or aluminum solid-round conductor at 20 C reference resistivity and does not model the temperature-dependent resistance rise that occurs under load.
• • Local amendments, conduit fill limits beyond single-conductor assumptions, and equipment terminal ratings may require a larger size than the one shown here, so confirm against the local inspection.

According to Voltage Drop Calculations (EC&M), Presents the single-phase and three-phase voltage-drop equations used here.