Voltage drop tables for solar circuits
Voltage drop wastes production and, on a long PV run, is the difference between a design that performs and one that quietly loses energy for 25 years. NEC 210.19 and 215.2 recommend (informational notes, not mandates) about 3% on a branch circuit and 5% total — most solar designers hold PV runs to 2%.
The formula
VD = (2 × I × L × R) ÷ 1000 for single-phase/DC (use √3 ≈ 1.732 instead of 2 for three-phase), where I is current in amps, L is the one-way run in feet, and R is ohms per 1,000 ft from NEC Chapter 9, Table 8. Percentage is VD ÷ nominal volts.
Voltage drop by conductor and run length — 20 A at 240 V, copper
| Conductor | 25 ft | 50 ft | 75 ft | 100 ft | 150 ft | 200 ft |
|---|---|---|---|---|---|---|
| 12 AWG | 0.83% | 1.65% | 2.48% | 3.3% | 4.95% | 6.6% |
| 10 AWG | 0.52% | 1.03% | 1.55% | 2.07% | 3.1% | 4.13% |
| 8 AWG | 0.32% | 0.65% | 0.97% | 1.3% | 1.95% | 2.59% |
| 6 AWG | 0.2% | 0.41% | 0.61% | 0.82% | 1.23% | 1.64% |
| 4 AWG | 0.13% | 0.26% | 0.39% | 0.51% | 0.77% | 1.03% |
| 2 AWG | 0.08% | 0.16% | 0.24% | 0.32% | 0.48% | 0.65% |
Amber > 2% (over the usual solar target) · red > 3% (over the NEC informational-note recommendation).
Voltage drop by current — 100 ft run at 240 V, copper
| Current | 12 AWG | 10 AWG | 8 AWG | 6 AWG | 4 AWG | 2 AWG |
|---|---|---|---|---|---|---|
| 10 A | 1.65% | 1.03% | 0.65% | 0.41% | 0.26% | 0.16% |
| 15 A | 2.48% | 1.55% | 0.97% | 0.61% | 0.39% | 0.24% |
| 20 A | 3.3% | 2.07% | 1.3% | 0.82% | 0.51% | 0.32% |
| 30 A | 4.95% | 3.1% | 1.95% | 1.23% | 0.77% | 0.48% |
| 40 A | 6.6% | 4.13% | 2.59% | 1.64% | 1.03% | 0.65% |
| 60 A | 9.9% | 6.2% | 3.89% | 2.46% | 1.54% | 0.97% |
Cutting voltage drop
- Upsize the conductor — the usual fix; each size up cuts resistance roughly a third.
- Shorten the run — relocate the inverter closer to the service.
- Raise the voltage — higher string voltage means lower current for the same power, and VD scales with current.
- Use copper over aluminum for the same size, or upsize the aluminum.
Note that voltage drop is a performance limit, not an ampacity limit — you still have to pass the 310.16 ampacity and 690.8 checks. On the DC side, size against Vmp (operating voltage), not Voc.
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