What size copper wire for a 200 amp breaker 300 feet away?
| Wire size | 4/0 AWG |
|---|---|
| Smallest by ampacity aloneNEC 310.16, 75°C copper | 3/0 AWG |
| Voltage drop at this size7.3 V of 240 V over 300 ft | 3.05% |
| GuidelineNEC 210.19(A) informational note — a recommendation, not a rule | 3% |
| Ampacity of 4/0 AWGcarrying 200 A | 230A |
| Circular milsthe number the voltage-drop formula actually uses | 211,600 |
Notes
- Two separate questions, and only one of them is code. Ampacity asks whether the wire survives the current — that is NEC Table 310.16 and it is mandatory. Voltage drop asks whether useful voltage reaches the far end — that is a 3% recommendation, not a requirement. At 300 ft the second question is the one that bites: 3/0 AWG is legal but arrives 3.8% low.
- NEC 240.4(D) overrides the ampacity table for small wire. 14 AWG is capped at a 15 A breaker, 12 AWG at 20 A and 10 AWG at 30 A, no matter what 310.16 shows. Reading the ampacity column alone and stopping there is the single most common mistake in this whole subject.
- Aluminum needs roughly two sizes up on the same load. It carries less current per gauge and has about 1.6 times the resistance, so both constraints move together. It is cheaper per foot and it is what utilities use for service entrances — but every termination has to be listed for aluminum, and antioxidant compound is not optional.
- Distance is the variable people leave out. The same 200 A circuit takes 3/0 AWG in a panel-room subpanel and 4/0 AWG out to a detached garage. Measure the actual conductor path — around the joists and up the wall — not the straight line across the yard.
- Temperature column matters too. These figures use the 75°C column, which is what the terminations on ordinary breakers and lugs are rated for. A 90°C cable does not let you use the 90°C column unless everything it lands on is also rated 90°C, and on residential gear it almost never is.
200 A over 300 feet in copper
4/0 AWG. Voltage drop lands at 3.05, against a 3% guideline.
Two questions, and only one of them is code
Ampacity asks whether the wire survives the current. That is NEC Table 310.16 and it is mandatory.
Voltage drop asks whether useful voltage reaches the far end. That is a 3% recommendation in an informational note — not a requirement.
Short runs are decided by the first. Long runs are decided by the second, and that is why an ampacity chart on its own gives the wrong answer past about 100 feet.
The rule that overrides the table
NEC 240.4(D) caps the breaker on small wire no matter what the ampacity column shows:
| Wire | Maximum breaker |
|---|---|
| 14 AWG | 15 A |
| 12 AWG | 20 A |
| 10 AWG | 30 A |
Reading 310.16 alone and stopping there is the most common mistake in this subject.
Aluminum runs about two sizes larger
It carries less current per gauge and has roughly 1.6 times the resistance, so both constraints move the same way.
It is cheaper per foot and it is what utilities use for service entrances. But every termination has to be listed for aluminum, and antioxidant compound is not optional.
Measure the wire, not the yard
The same 200 A circuit takes one size in a panel-room subpanel and another out to a detached garage.
Measure the actual conductor path — around the joists, up the wall, along the trench — not the straight line across the lawn.
The temperature column
These figures use the 75°C column, which is what the terminations on ordinary breakers and lugs are rated for.
A 90°C cable does not let you use the 90°C column unless everything it lands on is rated 90°C too — and on residential gear it almost never is.
Nearby sizes
Read more
- What Size Wire for 150 Amp Service, Subpanel or BreakerThe answer depends on what the wire feeds. Wire that carries the whole house can be smaller than wire that feeds a garage subpanel on the same 150 amp breaker.
- Voltage Drop — Why a Long Circuit Needs Bigger Wire Than the Amps SuggestThe wire is big enough to be safe and too small to be useful, and those are two different calculations.
- Which Appliances Need a Dedicated Circuit, and What SizeSome dedicated circuits come from the NEC, some from the appliance's installation instructions, which the NEC tells installers to follow. Either way the answer is on the nameplate.
- Kitchen Circuit Requirements: How Many Circuits a Kitchen NeedsMost tripping in older kitchens comes from one fact: the toaster, kettle and microwave share a single 15 or 20 amp circuit that was never meant for them.