100 Amp Wire Size Calculator

The short answer for 100 amps

For a general 100 amp circuit at the 75 C column of NEC Table 310.16, the wire is 3 AWG copper or 1 AWG aluminum. Those are the smallest sizes whose ampacity reaches 100 amps, and most breaker lugs and panel terminations carry a 75 C rating, so the 75 C column is the one that counts. Copper 4 AWG sits just below the mark at 85 amps and aluminum 2 AWG sits at 90 amps, so neither protects a plain 100 amp feeder without one of the exceptions below.

The rows interact. A garage subpanel fed on a 150 foot run takes at least 2 AWG copper or 1/0 aluminum, not the dwelling rule sizes, because a feeder that serves a garage does not serve the entire dwelling load. Start from the row that matches the job, then check voltage drop before buying wire.

Why 100 amps does not always mean a 100 amp wire

The NEC sizes conductors to the load, and the load rules cut in two directions. A feeder protected by a 100 amp breaker needs conductors rated for at least 100 amps, which is where 3 AWG copper comes from. But a load that runs for three hours or more counts as continuous, and 210.19(A)(1) and 215.2(A)(1) require the conductor to carry 125 percent of a continuous load: 100 x 1.25 = 125 amps, which pushes the copper up to 1 AWG (130 amps at 75 C) and the aluminum up to 2/0 (135 amps).

The other direction is the dwelling allowance. A single family service or a feeder that carries the whole dwelling load may be sized at 83 percent of the rating under NEC 310.12, so a 100 amp service needs conductors good for 100 x 0.83 = 83 amps. Copper 4 AWG at 85 amps and aluminum 2 AWG at 90 amps clear that bar, which is why 4 AWG copper shows up in so many 100 amp service installs.

What the wire never gets to do is sit below its breaker with no exception. Aluminum 2 AWG at 90 amps under a 100 amp breaker works only where the 83 percent rule applies or where the calculated load is at or below 90 amps and the local inspector accepts it. When in doubt, the next size up costs little and ends the argument.

NEC Table 310.16: the numbers behind the answer

Table 310.16 of the 2023 NEC lists allowable ampacities for insulated conductors in raceway or cable. The columns matter: the 60 C column applies to old style terminations, the 75 C column matches most modern breakers and lugs, and the 90 C column exists mainly as a starting point for derating, because 110.14(C) ties the usable rating to the coolest termination in the circuit.

Reading the table answers the common questions at a glance. Aluminum needs to sit about two sizes above copper for the same job, which the rows show: copper 3 AWG and aluminum 1 AWG both reach 100 amps. The 90 C numbers are higher, yet a wire picked from the 90 C column is still wrong if any lug on the path is rated 75 C. The full table lives in NFPA 70, the National Electrical Code (NFPA, free online access).

Voltage drop on a long 100 amp run

Ampacity is only half the sizing story; distance adds resistance loss. For a single phase run the drop is VD = 2 x K x I x L / CM, where K is 12.9 for copper and 21.2 for aluminum at 75 C, I is the current in amps, L is the one way length in feet and CM is the conductor area in circular mils (3 AWG copper is 52,620 circular mils). The informational notes behind 210.19 and 215.2 point to 3 percent for a single run as the working target.

Worked example: 100 amps over 100 feet of 3 AWG copper at 240 volts. The drop is 2 x 12.9 x 100 x 100 / 52620 = 4.9 volts, about 2.0 percent of 240. Stretch the same run to 150 feet and the drop grows to 7.4 volts, 3.1 percent, over the target. The fix is size, not heroics: each step up in gauge adds roughly 25 percent more area.

At 120 volts the same amps lose twice the percentage per foot, so the 3 AWG copper limit falls to about 73 feet. The calculator on this page runs these numbers for any length you enter and shows the size it picks when voltage drop, not ampacity, decides.

The 83 percent rule for a dwelling service

NEC 310.12 (310.15(B)(7) in the 2017 and 2020 editions) lets service conductors and whole house feeders of a single phase dwelling be sized at 83 percent of the service rating, on the logic that a residence never draws its full panel rating on every wire at once. For a 100 amp service the math is 100 x 0.83 = 83 amps, and two standard sizes clear it: copper 4 AWG at 85 amps and aluminum 2 AWG at 90 amps.

The limit matters in practice: a detached garage or an addition has its own load and its own panel, so its feeder is a plain 100 amp feeder and takes 3 AWG copper or 1 AWG aluminum. Inspectors apply the rule by edition year, and some jurisdictions amend it, so confirm the edition your permit is written against. Workplace wiring under OSHA 29 CFR 1910.304 follows the same conductor sizing logic for the sites it covers.

Grounds, neutrals and terminations

Three supporting conductors ride along with the hots. The equipment grounding conductor for a 100 amp overcurrent device is 8 AWG copper or 6 AWG aluminum under NEC 250.122, and its size follows the breaker rating, not the run length. The grounded conductor, the neutral, carries only the imbalance between the two hots of a 120/240 volt feeder, so 220.61 allows it to be smaller than the hots when the load calculation supports it, though services are commonly pulled at full size for margin.

Terminations decide the rest. Aluminum lugs need connectors listed AL/CU, torque to the marking on the equipment, and a coat of antioxidant joint compound on the exposed conductor. Cable assemblies make the shopping list short: copper SER in 3-3-3-5 or aluminum SER in 1-1-1-3 covers a plain 100 amp feeder, while the widely sold 2-2-2-4 aluminum SER carries 90 amps at 75 C and belongs only where the dwelling 83 percent rule applies or the load calculation comes in at or below 90 amps.

How to use the calculator, and what it leaves out

Enter the load in amps (100 is the default), the one way run length, the system voltage, the conductor material and the termination temperature, and check the continuous box if the load runs three hours or more. The results table shows the smallest wire that carries the ampacity, the voltage drop on that wire over your run, the smallest wire that holds 3 percent, the recommended pick between the two, the maximum length at 3 percent and the grounding conductor size. Change any input and the table updates.

The calculator leaves out the parts of Article 310 that need judgment: more than three current carrying conductors in a raceway trigger the 80 percent adjustment of 310.15(C)(1), ambient temperatures above 30 C (86 F) need the correction factors of Table 310.15(B)(1), and conduit fill beyond 40 percent follows Chapter 9. Local amendments, permit inspections and the edition your state adopts all sit on top of the tables shown here. For a service upgrade or anything inside the panel, the figures on this page are a planning aid; the installation itself belongs to a licensed electrician.