What Size Wire for a 100 Amp Subpanel?

The short answer

With terminations rated 75°C — most modern panelboards and breakers — a 100 A subpanel feeder needs 3 AWG copper or 1 AWG aluminium as an absolute minimum. That comes straight from NEC Table 310.16: 3 AWG copper and 1 AWG aluminium are both rated exactly 100 A at 75°C, which is what a 100 A overcurrent device requires.

If your equipment is rated 60°C only, the minimum jumps to 1 AWG copper (110 A at 60°C) or 1/0 AWG aluminium (100 A at 60°C). That is NEC 110.14(C)(1), which limits conductors for circuits of 100 A or less to the 60°C column unless the equipment is listed and identified for higher-ampacity conductors. It is also why two electricians can disagree about this answer while both being right about the table.

That table answers the ampacity question. It does not answer the distance question, and on a run out to a detached shop or garage, the distance is usually what you are really asking about.

Why ampacity comes first

Ampacity is a safety limit, not a performance target. It answers "can this conductor carry this current without its insulation failing?" Voltage drop answers a different question — "will the load at the far end get enough volts?" A conductor can be perfectly fine for voltage drop and still be illegal to put on a 100 A breaker, because nothing about voltage drop says the wire can carry 100 A.

So the order is always: pick the size the ampacity table allows, then check voltage drop, then go bigger if the drop fails. Never smaller. Table 310.16 assumes no more than three current-carrying conductors in a raceway or cable, at 30°C ambient; if you bundle more or run through a hot attic, apply the derating of 310.15 before you start. The whole table — every size, the 90°C column, and the ambient and bundle correction factors — is reproduced on the wire ampacity chart if you want to read the numbers yourself.

240.4(B) is the rule that trips people up. It permits the next higher standard overcurrent device where a conductor's ampacity does not match a standard rating. 4 AWG copper is 85 A at 75°C, and the next standard device above 85 A is 90 A — so 4 AWG copper can sit on a 90 A breaker, but not on a 100 A one. "85 A is basically 100 A" is not how the section works.

The 83% rule does not apply to your subpanel

This is the single most common piece of bad advice on the subject. The 83% allowance lives in NEC 310.12 — the section that was 310.15(B)(7) before the 2023 code renumbered it. It applies to service conductors and to feeder conductors supplying the entire load of a one-family dwelling, or the entire load of an individual dwelling unit in a two-family or multifamily building. 83% of 100 A is 83 A, which 4 AWG copper (85 A) satisfies — and that is exactly where the "4 AWG for 100 A" claim comes from.

A subpanel feeder in a garage, a workshop, or a second floor does not supply the entire load of the dwelling. It supplies part of it, downstream of the main. 310.12 does not reach it, so the full 100 A ampacity applies and you are back to 3 AWG copper or 1 AWG aluminium. If the feeder is effectively an extension of the service supplying everything, that is a different code conversation and one for an electrician to sign off on.

Distance: holding a 3% drop

The formula this site's voltage drop calculator uses is Vd = 2 × L × I × R ÷ 1,000 for single-phase, where L is the one-way run in feet, I is the load in amps, and R is the conductor resistance in ohms per 1,000 ft from NEC Chapter 9 Table 8 (75°C, uncoated). The 2 accounts for current going out and coming back, so enter the one-way distance.

The 3% target is 7.2 V on a 240 V feeder. Strictly, the 3% branch and 5% total figures are informational notes to 210.19(A)(1) and 215.2(A)(1), not mandatory rules — 90.5(C) says notes are not enforceable. In practice, though, a feeder to a detached building is exactly the case where the note's advice is worth following: a shop with a table saw, a welder, or a well pump will tell you loudly when it has been starved of voltage.

Here is what that works out to for a feeder carrying the full 100 A, using Table 8 resistances (3 AWG Cu 0.245, 2 AWG Cu 0.194, 1 AWG Cu 0.154, 1 AWG Al 0.253, 1/0 Al 0.201, 2/0 Al 0.159 Ω per 1,000 ft):

Note how little the answer moves with distance compared to how much it moves with material. Copper only climbs two sizes across 200 ft, because the feeder starts at 3 AWG for ampacity anyway and every step up is a big jump in cross-sectional area.

Worked example — 150 ft of copper to a shop

A 100 A subpanel in a detached shop, 150 ft from the house panel, 240 V single-phase, and you intend to use the full 100 A eventually.

Step 1 — ampacity. A 100 A breaker needs a conductor rated at least 100 A. 3 AWG copper is 100 A at 75°C. That is the floor.

Step 2 — voltage drop on 3 AWG. Vd = 2 × 150 × 100 × 0.245 ÷ 1,000. Do it in pieces if you want to check by hand: 2 × 150 = 300 ft of conductor; 300 ÷ 1,000 = 0.3; 0.3 × 0.245 = 0.0735 Ω of loop resistance; 0.0735 × 100 A = 7.35 V. As a percentage, 7.35 ÷ 240 × 100 = 3.06% — just over the line.

Step 3 — step up. 2 AWG copper has R = 0.194: 0.3 × 0.194 = 0.0582 Ω, × 100 A = 5.82 V = 2.43%, and 2 AWG copper carries 115 A at 75°C. That works on both checks, so 2 AWG copper is the answer at 150 ft.

The cost of getting this wrong in the other direction is real: on 3 AWG at a genuine 100 A, the run burns 100 A × 7.35 V = 735 W as heat inside the wall, and the tools see 232.6 V instead of 240 V. One size up turns that into 582 W and 234.2 V. But note the assumption — 100 A continuous is worst case. If your Article 220 load calculation says the real calculated load is 60 A, then 3 AWG at 150 ft drops 2 × 150 × 60 × 0.245 ÷ 1,000 = 4.41 V, or 1.84%, and 3 AWG is perfectly adequate. Size the voltage drop for the calculated load, not for the breaker handle. That is where most of the money is saved on long runs, and it is the default assumption the calculator lets you change.

Copper or aluminium?

Aluminium is about 1.6× the resistance of copper at the same size, so it needs to be one to two sizes larger for the same job — the table above shows it plainly, where 1 AWG aluminium does the work of 3 AWG copper. It is also cheaper per amp, which is why long service and feeder runs often use it. The old reputation for loose connections is a termination problem, not a wire problem: lugs must be listed for aluminium (AL/CU), the conductor must be stripped and torqued to spec, and the anti-oxidant compound that used to be mandatory is now typically built into the connector. Get the terminations right and aluminium is a legitimate choice.

Don't forget the ground wire

The equipment grounding conductor is sized from the rating of the overcurrent device, not from the phase conductors, per NEC Table 250.122. For a 100 A breaker that is a minimum 8 AWG copper or 6 AWG aluminium. Sizing it to match the hots is a common and expensive oversize.

If the subpanel is in a separate building, two more rules apply. NEC 250.32(A) requires the separate structure to have its own grounding electrode system — ground rods — connected to the feeder's equipment ground. And the subpanel keeps its neutral and equipment ground on separate bars: the main bonding jumper or bonding screw stays out, so the neutral is grounded at one point only, back at the service. The feeder is four wires — two hots, a neutral, and a separate equipment ground. The old three-wire arrangement, where the neutral doubles as the ground, survives only as a narrow exception for existing installations under 250.32(B); it is not something to design a new run around, and it is the mistake inspectors find most often on garage subpanels.

Common mistakes

• Applying the 83% rule to a subpanel feeder. It belongs to service conductors and whole-dwelling feeders only (NEC 310.12, formerly 310.15(B)(7)). Using it here is how people end up quoting 4 AWG copper for a 100 A breaker that cannot legally carry 4 AWG copper.
• Sizing on ampacity alone. 3 AWG copper is legal at 200 ft and still drops 2 × 200 × 100 × 0.245 ÷ 1,000 = 9.8 V, or 4.1%. Legal wire, unhappy welder.
• Sizing on voltage drop alone. A 2/0 conductor makes the drop beautiful and does nothing about the 100 A breaker feeding a circuit whose lugs are the real limit. Both checks, then the larger conductor.
• Assuming 6 AWG is a 100 A wire. It is 55 A at 60°C and 65 A at 75°C. This myth survives because of the old service table and because 6 AWG is genuinely common on 60 A feeders.
• Forgetting derating. Four or more current-carrying conductors in a conduit, or a hot attic run, cut the allowed ampacity under 310.15. A feeder that was fine on paper can fail inspection in a crowded conduit.
• Bonding the neutral in a detached-building subpanel. Two ground paths and a shared neutral means neutral current can travel on the grounding system. Separate the bars, install the ground rods, run the fourth wire.
• Guessing the load. Every number here starts with the calculated load from Article 220. "I'll just feed it with 100 A" is a design decision, and a 100 A feeder running a 40 A load is money sitting in the wall.

Run your own numbers — the voltage drop and wire size calculator walks Table 8 for your distance, load, voltage and material, and applies the Table 310.16 ampacity floor and the 240.4(D) small-conductor caps on top, so it will not hand you a size that the breaker cannot legally protect. It also has the 60/75°C terminations selector that decides whether your answer is 3 AWG or 1 AWG.

Subpanel wire size FAQ

Related pages

• What size wire for a 200 amp service or subpanel? — the same two checks at 200 A, where the 83% rule does apply to a dwelling service but not to a subpanel feeder
• Voltage drop & wire size calculator — NEC Chapter 9 Table 8
• Breaker size chart — NEC 240.6(A), 240.4(D) and 210.20 — the device rating side of the same question, plus the per-appliance circuits
• How to calculate concrete for a slab — for the pad under the shop or garage
• Rebar & footing calculator — reinforcement for the slab and footings
• Drain pipe slope calculator — for the trench alongside the conduit

Code references are to the 2023 NEC (NFPA 70) unless noted; Table 8 resistance values and Table 310.16 ampacities were re-checked 2026-09-16. Adopted editions and local amendments vary — this page is a working reference, not a substitute for a licensed electrician or your AHJ. No load calculation is performed for you.