Voltage drop at recommended size
10.83 V · 4.51%
√3·K·I·L ÷ CM = 10.83 V on a 240 V circuit — 4.51% drop.
Formula: VD = 2·K·I·L/CM (1φ) · √3·K·I·L/CM (3φ) — NEC 210.19(A)(1) IN 4
Calculator 10 / NEC 210.19(A)(1) · 215.2(A)(1)
Voltage drop
Size conductors so branch-circuit and feeder voltage drop stay within the 3% / 5% guidance, while never going below the NEC Table 310.16 ampacity floor.
Circuit and conductor inputs
System voltage
Phase
Conductor material
Target max drop
Terminal temperature rating (ampacity floor)
The Table 310.16 column used for the ampacity floor. Per NEC 110.14(C), terminations rated ≤ 100 A are typically 60°C unless marked 75°C; larger equipment is typically 75°C. The 90°C column is not used for termination ratings.
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VoltForgeVoltage Drop Report
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Inputs
| System voltage | 240 V |
| Phase | Three-phase |
| Load current | 40 A |
| One-way length | 200 ft |
| Material | Copper (K = 12.9) |
| Target max drop | 5% |
| Terminal rating | 75°C |
Results
Recommended minimum size
8 AWG
Governed by: ampacity (the ampacity size is the larger one). 8 AWG is also the first size with ampacity ≥ 40 A in the 75°C column (50 A).
NEC Table 310.16 (ampacity) · NEC 210.19(A)(1) IN 4 (voltage drop guidance)
Engineering disclaimer: This design aid is not a substitute for engineering judgment. Confirm final design with a licensed professional engineer and the authority having jurisdiction.
Voltage drop at recommended size
10.83 V · 4.51%
√3 × K (12.9) × I (40 A) × L (200 ft) ÷ CM (16510) = 10.83 V, which is 4.51% of 240 V.
Formula: VD = 2·K·I·L/CM (1φ) · √3·K·I·L/CM (3φ) · NEC 210.19(A)(1) IN 4
Recommended minimum size
8 AWG
VD target 5% needs 8 AWG (4.51%), while the 75°C ampacity floor for 40 A needs 8 AWG (50 A). ampacity (the ampacity size is the larger one) governs → 8 AWG.
NEC Table 310.16 · NEC 210.19(A)(1) IN 4 · 215.2(A)(1)
Ampacity floor: the recommendation is never below NEC Table 310.16 — the 75°C copper column requires 8 AWG (50 A ≥ 40 A). Voltage drop sets the minimum; ampacity is satisfied.
Voltage drop by conductor size
| Size | Circular mils | VD (V) | VD (%) | Status |
|---|---|---|---|---|
| 14 AWG | 4110 | 43.49 | 18.12% | — exceeds target |
| 12 AWG | 6530 | 27.37 | 11.41% | — exceeds target |
| 10 AWG | 10380 | 17.22 | 7.18% | — exceeds target |
| 8 AWG | 16510 | 10.83 | 4.51% | ✓ recommended |
| 6 AWG | 26240 | 6.81 | 2.84% | ✓ VD OK |
| 4 AWG | 41740 | 4.28 | 1.78% | ✓ VD OK |
| 3 AWG | 52620 | 3.40 | 1.42% | ✓ VD OK |
| 2 AWG | 66360 | 2.69 | 1.12% | ✓ VD OK |
| 1 AWG | 83690 | 2.14 | 0.89% | ✓ VD OK |
| 1/0 AWG | 105600 | 1.69 | 0.71% | ✓ VD OK |
| 2/0 AWG | 133100 | 1.34 | 0.56% | ✓ VD OK |
| 3/0 AWG | 167800 | 1.07 | 0.44% | ✓ VD OK |
| 4/0 AWG | 211600 | 0.84 | 0.35% | ✓ VD OK |
| 250 kcmil | 250000 | 0.71 | 0.30% | ✓ VD OK |
| 300 kcmil | 300000 | 0.60 | 0.25% | ✓ VD OK |
| 350 kcmil | 350000 | 0.51 | 0.21% | ✓ VD OK |
| 400 kcmil | 400000 | 0.45 | 0.19% | ✓ VD OK |
| 500 kcmil | 500000 | 0.36 | 0.15% | ✓ VD OK |
| 600 kcmil | 600000 | 0.30 | 0.12% | ✓ VD OK |
| 700 kcmil | 700000 | 0.26 | 0.11% | ✓ VD OK |
| 750 kcmil | 750000 | 0.24 | 0.10% | ✓ VD OK |
| 800 kcmil | 800000 | 0.22 | 0.09% | ✓ VD OK |
| 900 kcmil | 900000 | 0.20 | 0.08% | ✓ VD OK |
| 1000 kcmil | 1000000 | 0.18 | 0.07% | ✓ VD OK |
Circular-mil areas per NEC Chapter 9 Table 8. The recommended size is the smallest size meeting BOTH the VD target and the 75°C ampacity floor.
How this was calculated
- Select the voltage-drop multiplier: √3 (three-phase, line-to-line circuit).
- K = 12.9 (copper at 75°C, per Ugly's circular-mil-foot resistance: K = R × CM ÷ 1000).
- For each size from NEC Chapter 9 Table 8: VD = √3 × 12.9 × 40 × 200 ÷ CM, then VD% = VD ÷ 240 V.
- Find the smallest size with VD% ≤ 5% — 8 AWG.
- Find the 75°C ampacity floor from NEC Table 310.16 (copper column): 8 AWG (50 A). The recommendation is the larger of the two by circular mils.
- Recommended: 8 AWG with VD 10.83 V (4.51%) — governed by ampacity (the ampacity size is the larger one).
- NEC 210.19(A)(1) Informational Note 4: the 3% branch / 5% feeder-plus-branch guidance is a recommendation, not a requirement. The authority having jurisdiction may still require conformance.
Engineering disclaimer: Voltage-drop sizing is a design aid. K values assume 75°C conductor temperature; actual resistance varies with temperature, loading, and power factor. Verify final sizing with the authority having jurisdiction — the 3% / 5% figures in NEC 210.19(A)(1) IN 4 and 215.2(A)(1) are informational guidance, not mandatory limits.