What Size Wire for 100 Amps?

A 100-amp feeder or subpanel uses 1 AWG copper or 2/0 aluminum when sized for continuous load at the 75°C column. The math: 100 A × 1.25 = 125 A required, and 1 AWG copper (130 A) is the smallest size that clears it. Aluminum steps up to 2/0 (175 A) for the same duty because of its higher resistance and the common practice of leaving feeder margin.

There is a well-known nuance. NEC 310.12 allows a 83% "service conductor" rule for dwelling services and feeders that carry the entire load: a 100 A dwelling service can use 4 AWG copper or 2 AWG aluminum. For a general subpanel feeder not covered by that rule, 3 AWG copper (100 A) is the raw-ampacity match and 1 AWG the continuous-rated choice. The calculator above shows the continuous-rated answer; read the sections below to know which rule applies to your job.

For 100 amps you will see three different copper sizes quoted, and each is correct in its context. Understanding why prevents both over- and under-sizing.

• 3 AWG copper (100 A) — the raw 75°C ampacity match. Correct only if the load is non-continuous, which feeders rarely are.
• 1 AWG copper (130 A) — the continuous-load answer: 100 × 1.25 = 125 A, and 1 AWG is the smallest size above it. This is the safe default for a 100 A subpanel feeder.
• 4 AWG copper (85 A) — allowed specifically for a 100 A dwelling service or a feeder serving the whole dwelling under NEC 310.12, which permits conductors rated at 83% of the service rating (0.83 × 100 = 83 A ≤ 85 A). This is why utilities and inspectors accept 4 AWG on a 100 A house service.

Aluminum follows the same logic one to two sizes up: 2/0 for a continuous feeder, 2 AWG under the dwelling-service allowance. Always confirm which rule applies with your inspector, and verify the breaker with the breaker size calculator.

Example 1 — 100 A subpanel, 75 ft copper, continuous: required = 125 A → 1 AWG (130 A). Drop = 2 × 75 × 100 × 0.154 ÷ 1000 = 2.31 V = 0.96% of 240 V. Answer: 1 AWG copper, well within voltage-drop limits.

Example 2 — 100 A detached garage, 180 ft copper: 1 AWG meets ampacity, but drop = 2 × 180 × 100 × 0.154 ÷ 1000 = 5.54 V = 2.31%. Under 3%, so 1 AWG is fine; at 240 ft it would exceed 3% and want 2/0. Long feeders are where voltage drop, not ampacity, sets the size.

Example 3 — 100 A dwelling service, 40 ft copper: under NEC 310.12, 4 AWG copper (85 A) is permitted because 0.83 × 100 = 83 A. Drop at 100 A = 2 × 40 × 100 × 0.308 ÷ 1000 = 2.46 V = 1.03%. Answer: 4 AWG copper — but only under the dwelling-service allowance.

Example 4 — 100 A feeder, 120 ft aluminum: continuous → 2/0 aluminum (135 A ≥ 125 A). Drop = 2 × 120 × 100 × 0.0967 ÷ 1000 = 2.32 V = 0.97%. Answer: 2/0 aluminum. For a 200 A upgrade the same logic yields 4/0 copper or 250–350 kcmil aluminum.

Example 5 — 100 A shop feeder, 275 ft copper: ampacity says 1 AWG, but drop = 2 × 275 × 100 × 0.154 ÷ 1000 = 8.47 V = 3.53% — over the 3% branch target. Step up to 2/0 (0.0967 Ω): drop = 2 × 275 × 100 × 0.0967 ÷ 1000 = 5.32 V = 2.22%, which clears. This is the textbook long-feeder case: the conductor is two sizes larger than heat alone requires, purely to deliver usable voltage at a distant shop. If the shop later adds a 240 V compressor and welder, that 2/0 also leaves room to re-rate the feeder without a re-pull.

Service-entrance conductors have their own allowance (NEC 310.12) because they carry a diversified household load that rarely peaks at nameplate. The practical dwelling-service sizes:

• 100 A service — 4 AWG copper or 2 AWG aluminum (83% rule = 83 A).
• 125 A service — 2 AWG copper or 1/0 aluminum.
• 150 A service — 1 AWG copper or 2/0 aluminum.
• 200 A service — 2/0 copper or 4/0 aluminum (0.83 × 200 = 166 A).

Note that a 200 A service uses smaller conductors than a 200 A continuous feeder would, purely because of the dwelling allowance. When someone asks "what size wire for 200 amp service," the answer is 2/0 copper or 4/0 aluminum — a size that would be under-rated for a continuous 200 A industrial feeder. Always distinguish a service (utility-to-main) from a feeder (panel-to-subpanel), and from a branch circuit (breaker-to-load).

For the general sizing method behind all of these, use the wire size calculator, and check the neutral and ground with NEC 220.61 and 250.66/250.122.

On feeders — which are often long runs to detached garages, shops, or ADUs — voltage drop usually decides the size, not ampacity. The 3% branch and 5% total recommendations still apply:

For 1 AWG copper (0.154 Ω/1000 ft) at 100 A on 240 V, the 3% ceiling (7.2 V) is reached around 234 feet one-way. Step up to 2/0 (0.0967 Ω) and that extends past 370 feet. A detached shop 300 feet away therefore needs 2/0 copper even though 1 AWG is fine for heat. Aluminum, at ~1.6× resistance, upsizes sooner: 2/0 aluminum on a long run may need to become 4/0.

This is why experienced electricians frequently pull 2/0 copper or 4/0 aluminum for 100 A feeders by default — the conductor cost premium buys voltage headroom and room to upgrade to 125 A or 150 A later. Use the live drop readout above or the voltage drop calculator to find the crossover for your exact distance.

Feeders travel farther than branch circuits, so distance frequently drives the 100-amp conductor size past what heat alone requires. The reference below assumes a continuous 100-amp feeder in copper at 240 V against the 3% target, and complements the live calculation above. Read it alongside the service-versus-feeder distinction — dwelling services under NEC 310.12 start from the smaller 4 AWG rather than 1 AWG.

Notice how quickly the size climbs on long runs to a detached garage, shop, or accessory dwelling. A 250-foot feeder that would be perfectly safe on 1 AWG for ampacity must jump two sizes to 2/0 purely to keep the voltage usable at the far panel. This is why many electricians default to 2/0 copper or 4/0 aluminum for any 100-amp feeder leaving the main structure, buying both voltage headroom and room to re-rate to 125 or 150 amps later without pulling new conductors.

1. Enter feeder amps — 100 by default, or up to 200 to size a service upgrade.
2. Enter one-way run length. Feeders are often long; this drives the voltage-drop check that pushes 1 AWG to 2/0 on distant subpanels.
3. Choose copper or aluminum. Aluminum is extremely common for feeders and services because of cost and weight.

The tool returns the continuous-rated AWG (1 AWG copper for 100 A), required ampacity (125 A), and voltage drop. If your job qualifies for the NEC 310.12 dwelling-service allowance, you may use the smaller 4 AWG copper / 2 AWG aluminum instead — noted in the tables above. Always confirm with your local inspector, and match the overcurrent device with the breaker size calculator.

A 100-amp feeder is more than two hot conductors — the neutral and grounding conductors have their own sizing rules, and getting them wrong is a common inspection failure.

Neutral (grounded) conductor. Size the feeder neutral per NEC 220.61 based on the maximum unbalanced load. For a typical dwelling subpanel it is often the same size as the hots or one size smaller, but never smaller than the grounding-electrode requirements of 250.24(C). When in doubt, match the neutral to the ungrounded conductors.

Equipment grounding conductor. NEC Table 250.122 sets the equipment ground for a 100 A circuit at 8 AWG copper (or 6 AWG aluminum). This is a safety bond, not a current-carrying conductor under normal conditions, but it must be present in every feeder.

The 4-wire rule for separate structures. A subpanel in the same building as the main panel needs four conductors — two hots, a neutral, and an equipment ground — with the neutral isolated from ground at the subpanel and the neutral-to-ground bond made only at the service. Since the 2008 NEC, this four-wire rule also applies to subpanels in detached structures (garages, shops), which previously allowed a three-wire feeder with a local ground rod. Always run the separate ground and keep the neutral floating at the subpanel.

Grounding electrode. A detached structure fed by a 100 A feeder still needs its own grounding electrode system (ground rods or equivalent) bonded to the subpanel's ground bus. Confirm the breaker feeding the subpanel with the breaker size calculator and the conduit fit with the conduit fill calculator.

• Confusing service and feeder rules. 4 AWG copper is fine for a 100 A dwelling service but undersized for a general 100 A continuous feeder (use 1 AWG).
• Ignoring voltage drop on long runs. Past ~230 feet, 100 A on 1 AWG copper exceeds 3% — go to 2/0.
• Undersizing the neutral or ground. Size the grounded conductor per NEC 220.61 and the equipment ground per 250.122 (typically 8 AWG copper for a 100 A feeder).
• Forgetting the 4-wire feeder rule. Subpanels in separate structures need four conductors (two hots, neutral, ground) with the neutral isolated from ground at the subpanel.
• Using the 90°C column at the lugs. Terminate at 75°C; use 90°C only for derating.

For smaller circuits see wire size for 50 amps, and for the underlying method use the wire size calculator.