Wire Size for a 200-Foot, 30-Amp Circuit (Voltage Drop)

Ampacity is not the constraint here

A 30-amp circuit only needs #10 copper on ampacity. NEC 240.4(D) caps #10 copper overcurrent protection at 30 amps, and the 60°C column of Table 310.16 lists #10 copper at exactly 30 amps. On a short run that is the whole story. Push the same conductor out to 200 feet and voltage drop, not heat, sets the wire size. These are two independent checks and you size for the worse of the two.

Voltage drop scales with length and current and falls as the conductor cross-section grows. Doubling the run doubles the drop; going up one full AWG size (which roughly increases circular-mil area by 26%) cuts it by about that fraction. Over 200 feet the minimum-ampacity conductor is nowhere near the 3% target.

The formula and constants

For conductors below #2 AWG the NEC K-factor form is the standard method and the one this site's calculator uses:

K is the copper constant 12.9 (aluminum 21.2) at 75°C, I is 30 amps, L is the 200-foot one-way length, and CM is the conductor circular-mil area from NEC Chapter 9 Table 8. The 2× factor is the round trip: current leaves on the ungrounded conductor and returns on the grounded one. Circular mils used below, straight from Table 8: #10 = 10,380, #8 = 16,510, #6 = 26,240, #4 = 41,740.

Copper on a 240V circuit

Every copper example shares the same numerator: 2 × 12.9 × 30 × 200 = 154,800. Only the circular-mil denominator changes.

#8 copper carries 30 amps with room to spare on ampacity (50 amps in the 75°C column) yet still misses the 3% branch recommendation at this length. #6 copper is the first size that clears it. That gap between the ampacity-legal size and the voltage-drop-practical size is the entire reason long circuits get upsized.

The same run on 120V

Halving the voltage doubles the percentage for an identical drop in volts, because VD% is measured against the source. #6 copper that read 2.46% at 240V reads 5.90 / 120 = 4.92% at 120V. That clears the 5% combined ceiling but blows past the 3% branch target. A 120V 30-amp load at 200 feet wants #4 copper (154,800 / 41,740 = 3.71 V, or 3.09%), and in practice most 30-amp loads at this distance are wired at 240V specifically to halve the drop.

If you are pulling aluminum

Aluminum swaps K = 21.2 for 12.9, so the numerator becomes 2 × 21.2 × 30 × 200 = 254,400. #6 aluminum gives 254,400 / 26,240 = 9.70 V, or 4.04% at 240V, which fails the 3% target. Step to #4 aluminum: 254,400 / 41,740 = 6.10 V, or 2.54% at 240V. So the 240V answer is #6 copper or #4 aluminum for a 3% result.

Reference table (240V, 30 A, 200 ft one way, unity PF)

These figures use unity power factor, the conservative choice for a resistive-worst-case branch. A calculator that assumes 0.9 power factor (common for residential air conditioning) reports slightly lower drops. For the reasoning behind the 3% and 5% figures and how to compute the percentage for any circuit, see how to calculate voltage drop percentage.

NEC reference

NEC 2020. Circular-mil areas from Chapter 9 Table 8. The 3% branch and 5% combined figures are the recommendations in the Informational Notes to 210.19(A)(1) and 215.2(A)(1); they are advisory in the NEC itself though some jurisdictions adopt them as amendments. Ampacity minimum from Table 310.16 and the small-conductor rule in 240.4(D).

Related

• Voltage Drop Calculator: test any conductor, current, and length against the 3% and 5% targets.
• How to Calculate Voltage Drop Percentage: the 3% rule and the method behind these numbers.
• Wire Size Calculator: size a conductor for ampacity and voltage drop together.