What wire size is needed for a 200 amp subpanel 300 feet away?

250 kcmil copper keeps voltage drop at 2.6 percent at the full 200 amperes. 3/0 copper meets the Table 310.16 ampacity requirement but drops 3.8 percent over that distance.

Is voltage drop a code requirement?

No. The 3 percent feeder guidance is an informational note to NEC 215.2(A)(2), which is not enforceable text, though many designers and AHJs apply it as a standard.

Do I have to upsize the ground wire when I upsize for voltage drop?

Yes. NEC 250.122(B) requires the equipment grounding conductor to be increased proportionally to the increase in circular mil area of the ungrounded conductors.

Is 350 kcmil copper right for a 200 amp subpanel 300 feet away?

It works, and it is larger than it needs to be. For a 200 ampere, 240 volt single phase feeder at 300 feet, ampacity is satisfied by 3/0 copper and voltage drop is satisfied by 250 kcmil copper. Choosing 350 kcmil buys margin at real cost, since the conductors get heavier, the conduit gets bigger, and the terminations get harder.

Step one: ampacity

Table 310.16 at the 75 degrees C column gives 3/0 copper an ampacity of 200 amperes. Section 110.14(C)(1)(b) limits you to the 75 degrees C column for circuits over 100 amperes or conductors larger than 1 AWG unless the terminations are listed for 90 degrees C. Section 215.2(A)(1) requires feeder conductors to have an ampacity not less than the larger of the calculated load or the sum of the noncontinuous load plus 125 percent of the continuous load. So 3/0 copper THWN-2 is the ampacity minimum.

Step two: voltage drop

Voltage drop is not a mandatory NEC requirement. The informational note to 215.2(A)(2) recommends that feeder conductors be sized to limit voltage drop to 3 percent, with a combined feeder and branch circuit total not exceeding 5 percent. Most engineers and many AHJs treat that as a design standard.

Use VD equals 2 times K times I times D divided by CM, where K is about 12.9 for copper, I is the current in amperes, D is the one way distance in feet, and CM is the circular mil area of the conductor. The numerator here is 2 times 12.9 times 200 times 300, which equals 1,548,000.

3/0 copper is 167,800 circular mils, giving 9.2 volts, which is 3.8 percent of 240. 4/0 copper is 211,600 circular mils, giving 7.3 volts, which is 3.0 percent and sits right on the line. 250 kcmil is 250,000 circular mils, giving 6.2 volts, which is 2.6 percent. 350 kcmil is 350,000 circular mils, giving 4.4 volts, which is 1.8 percent.

So 250 kcmil copper is the smallest conductor that clears 3 percent at the full 200 amperes. If your calculated load under Article 220 is really 140 amperes rather than 200, then 3/0 copper drops to 2.7 percent and everything above it is unnecessary.

The rest of the package

The neutral is sized from the maximum unbalanced load per 220.61, with the 250.102(C)(1) floor. The equipment grounding conductor comes from Table 250.122, which gives 6 AWG copper for a 200 ampere overcurrent device. Section 250.122(B) then requires the EGC to be increased in size proportionally to the increase in circular mil area of the ungrounded conductors when they are upsized for voltage drop. Going from 3/0 to 250 kcmil is a factor of 1.49, so 6 AWG at 26,240 circular mils becomes 39,098 circular mils, which lands on 4 AWG copper.

Underground means a wet location, so use THWN-2 or XHHW-2 per 300.5 and 310.10(C). For three 250 kcmil THWN-2 at 0.3970 square inches each plus a 4 AWG EGC at 0.0824 square inches, the total is 1.2734 square inches. Chapter 9 Table 4 gives 2 inch Schedule 40 PVC a 40 percent fill of 1.316 square inches, so 2 inch PVC fits. Three 350 kcmil at 0.5242 each is 1.5726 square inches and needs 3 inch. That is another reason not to oversize without a reason.

Field takeaway

Run 250 kcmil copper THWN-2 with a 4 AWG copper EGC in 2 inch Schedule 40 PVC, burial depth per Table 300.5. Aluminum is worth pricing too, since 500 kcmil aluminum gives similar performance at a fraction of the material cost.