What Size Wire for a 200 Amp Service?

The short answer

It depends on what the 200 A breaker is protecting, and the code treats the two cases very differently:

If that looks like two sizes too many for one question, that is the point. The lower row is the conservative answer that is always legal. The upper row is a genuine code allowance that applies to exactly one case — a service (or feeder) that supplies the entire load of a one-family dwelling — and gets misapplied constantly when someone reads it off a chart for a garage subpanel.

Why there are two answers: the 83% rule

NEC 310.12 (it was 310.15(B)(7) before the 2023 code renumbered it) says that for a service or feeder rated 100 through 400 A, the conductors supplying the entire load of a dwelling shall be permitted to have an ampacity not less than 83 percent of the service rating. The reasoning is diversification: a dwelling's connected load is a set of appliances that do not all run at once, so the service conductors do not need to carry the full rating continuously.

For a 200 A service, the arithmetic is 200 A × 0.83 = 166 A. So the conductor only needs an ampacity of 166 A, not 200 A. Reading Table 310.16 at 75°C, 2/0 copper is 175 A and 4/0 aluminium is 180 A — the two smallest sizes that clear 166 A. That is where the widely repeated "2/0 copper or 4/0 aluminium for 200 A" answer comes from, and it is correct for a service.

The code also publishes this as a lookup table, Table 310.12, so you do not have to run the 83% multiplication yourself. Its 200 A row reads 2/0 copper and 4/0 aluminium. Two conditions matter: the table is for a 120/240 V, single-phase system, and it may only be used where no ampacity adjustment or correction factors are required (310.12(A) and (B)). If the conductors are bundled with others or run through a hot space, do the derating first and the 83% shortcut is off the table.

Why a subpanel gets the bigger wire

310.12 reaches a conductor only when it supplies the entire load of the dwelling. A subpanel in a garage, a shop, or a second floor supplies part of the load, downstream of the main. The whole-load condition is not met, so the allowance is not available and full ampacity applies: a 200 A breaker needs a conductor rated at least 200 A. That means 3/0 AWG copper (200 A at 75°C) or 250 kcmil aluminium (205 A at 75°C).

240.4(B) does not rescue the smaller size here, and the reason is subtle. That rule lets a conductor sit on the next higher standard device where its ampacity does not match a standard rating. 2/0 copper is 175 A — and 175 A is a standard rating in 240.6(A). Because the ampacity already corresponds to a standard device, there is nothing to round up to, so 2/0 copper stays on a 175 A breaker. (Compare 4 AWG copper at 85 A, where 85 A is not a standard rating and 240.4(B) does let it reach a 90 A device.) The full standard-rating list is on the breaker size chart.

Which temperature column?

For a 200 A circuit the answer is the 75°C column. NEC 110.14(C)(1)(b) directs conductors for circuits rated over 100 A to the 75°C ampacity unless the equipment is listed and identified for a higher rating. Almost all modern panelboards and breakers are rated 75°C, so 2/0 copper (175 A), 3/0 copper (200 A) and 250 kcmil aluminium (205 A) are read from that column. The 60°C column that decides the small-conductor feeder questions never comes into play at 200 A, which is one fewer trap than the sub-100 A case.

Distance: holding a 3% drop

Ampacity sets the floor; distance sets the reality. The formula this site's voltage drop calculator uses is Vd = 2 × L × I × R ÷ 1,000 for single-phase, one-way length in feet, R from NEC Chapter 9 Table 8 (75°C, uncoated, ohms per 1,000 ft). On a 240 V system, 3% is 7.2 V. Strictly, the 3% and 5% figures are informational notes to 210.19(A)(1) and 215.2(A)(1), not enforceable rules — 90.5(C) says notes are not binding. In practice a long feeder to a shop is exactly where the note earns its keep.

Here is what the arithmetic gives for a feeder carrying the full 200 A, starting from the ampacity minimum in each material (Table 8 resistances: 3/0 Cu 0.0766, 4/0 Cu 0.0608, 250 kcmil Al 0.0847, 300 kcmil Al 0.0707 Ω per 1,000 ft):

Both materials hold the ampacity minimum through 200 ft and step up once at 250 ft. Copper climbs from 3/0 to 4/0; aluminium from 250 kcmil to 300 kcmil. The failing steps that force the bump are 3/0 Cu at 250 ft = 2 × 250 × 200 × 0.0766 ÷ 1,000 = 7.66 V (3.19%, over the line) and 250 kcmil Al at 250 ft = 2 × 250 × 200 × 0.0847 ÷ 1,000 = 8.47 V (3.53%).

Note that the service case starts from a smaller ampacity minimum — 2/0 copper or 4/0 aluminium — but voltage drop reimposes a floor once the run is long. On 2/0 copper at 200 A, 150 ft drops 5.80 V (2.42%) and 200 ft drops 7.74 V (3.22%) — so a 200 ft service run on the 83% conductor has to come back up to 3/0 copper anyway. The two answers converge as distance grows.

Worked example — 200 ft to a detached shop

A 200 A subpanel in a detached shop, 200 ft from the house panel, 240 V single-phase, and you intend to use the full 200 A eventually. This is a subpanel, not a whole-house service, so the 83% rule is off the table.

Step 1 — ampacity. A 200 A breaker needs a conductor rated at least 200 A. At 75°C that is 3/0 AWG copper (200 A) or 250 kcmil aluminium (205 A). That is the floor.

Step 2 — voltage drop on 3/0 copper. Vd = 2 × 200 × 200 × 0.0766 ÷ 1,000. In pieces: 2 × 200 = 400 ft of conductor; 400 ÷ 1,000 = 0.4; 0.4 × 0.0766 = 0.03064 Ω of loop resistance; 0.03064 × 200 A = 6.13 V. As a percentage, 6.13 ÷ 240 × 100 = 2.55% — inside the 3% target. 3/0 copper works at 200 ft. In aluminium, 250 kcmil gives 0.4 × 0.0847 = 0.0339 Ω, × 200 A = 6.78 V = 2.82%, also inside.

Now push the same load to 250 ft and the picture changes. On 3/0 copper: loop resistance 2 × 250 × 0.0766 ÷ 1,000 = 0.0383 Ω, × 200 A = 7.66 V = 3.19% — just over. One size up, 4/0 copper is 0.0304 Ω × 200 A = 6.08 V = 2.53%, and it carries 230 A at 75°C. In aluminium, 250 kcmil at 250 ft = 8.47 V (3.53%), stepping to 300 kcmil at 7.07 V (2.95%). At 250 ft the honest answer is 4/0 copper or 300 kcmil aluminium.

The cost of getting this wrong is real but easy to overstate. At a genuine 200 A, 3/0 copper over 250 ft burns 200² × 0.0383 = 1,532 W as heat inside the run; 4/0 cuts that to 1,216 W. But note the assumption — 200 A continuous is worst case. If your Article 220 calculation says the real load is 120 A, then 3/0 copper at 250 ft drops 2 × 250 × 120 × 0.0766 ÷ 1,000 = 4.60 V, or 1.92%, and 3/0 is perfectly adequate. Size the voltage drop to the calculated load, not to the breaker handle.

Copper or aluminium?

Aluminium is about 1.6× the resistance of copper at the same size, so it needs to be two sizes larger for the same job — the table shows 250 kcmil aluminium doing the work of 3/0 copper. It is also markedly cheaper, which is why most 200 A service entrance conductors and long feeder runs are aluminium. The old reputation for loose connections is a termination problem, not a wire problem: the lugs must be listed for aluminium (AL/CU), the conductor stripped clean and torqued to the manufacturer's spec, and anti-oxidant compound applied where the connector requires it. Done correctly, aluminium is a legitimate, code-compliant choice. Copper is smaller and stiffer and easier to terminate in tight gutters, which is why some electricians still prefer it, particularly inside a crowded panel.

Don't forget the ground wire

The equipment grounding conductor for a 200 A feeder is sized from the rating of the overcurrent device, not from the phase conductors, per NEC Table 250.122: for a 200 A breaker that is a minimum 6 AWG copper or 4 AWG aluminium. Sizing it to match the hots is a common and pointless oversize.

A service is a different conductor again. The wire from the service disconnect to the ground rods is the grounding electrode conductor, sized from the service conductors per NEC Table 250.66, with a separate small-conductor exception for a rod-only connection. Mixing up Table 250.122 and Table 250.66 is one of the most common errors on service work.

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 survives only as a narrow exception for existing installations under 250.32(B); it is not something to design a new 200 A run around.

Common mistakes

• Using the 83% rule on a subpanel. It belongs to a service or feeder supplying the entire load of a dwelling (NEC 310.12). A garage, shop, or outbuilding subpanel does not qualify, so the full 200 A applies.
• Relying on 240.4(B) to put 2/0 copper on a 200 A breaker. 2/0 copper is 175 A, and 175 A is already a standard rating, so there is no next-standard-size round-up. It stays on a 175 A device.
• Sizing on ampacity alone. 3/0 copper is legal at 300 ft and still drops 2 × 300 × 200 × 0.0766 ÷ 1,000 = 9.19 V, or 3.83%. Legal wire, unhappy welder.
• Sizing on voltage drop alone. A conductor chosen purely to make the drop pretty can still be too small for the 200 A breaker — ampacity is the floor you may never go below.
• Quoting a single national "200 amp wire size". The right answer is a pair — service versus subpanel — and a local amendment or a hot, bundled run can move it again. State the assumptions.
• Confusing the equipment ground with the grounding electrode conductor. A feeder's EGC is Table 250.122; a service's GEC is Table 250.66. They are not the same wire.
• Bonding the neutral in a detached-building subpanel. Two ground paths and a shared neutral let neutral current travel on the grounding system. Separate the bars, install the ground rods, run the fourth wire.

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 the breaker cannot legally protect. Pair it with the NEC Table 310.16 ampacity chart to see every size and column at once.

200 amp service wire size FAQ

Related pages

• What size wire for a 100 amp subpanel? — the same two checks at half the load, where the 83% rule trap is even easier to fall into
• Voltage drop & wire size calculator — NEC Chapter 9 Table 8 arithmetic for your own run length
• Wire ampacity chart — NEC Table 310.16 — every size from 14 AWG to 2000 kcmil, all three temperature columns
• Breaker size chart — NEC 240.6(A) and 240.4(B) — the standard ratings the round-up rule works from
• Conduit fill chart — NEC Chapter 9 — sizing the raceway that a 2/0 or 3/0 service pull has to pass through

Code references are to the 2023 NEC (NFPA 70) unless noted. Table 310.12 and Table 250.122 were re-checked 2026-10-05 against two independent reproductions; Table 8 resistances and Table 310.16 ampacities are the set verified 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.