Definition
Voltage drop (Vd) is the loss of voltage along a conductor due to its resistance and the current flowing through it. NEC recommends ≤ 3% for branch circuits; solar industry targets ≤ 2% DC and ≤ 1% AC for efficiency and MPPT margin.
Quick Facts
| Field | Detail |
|---|---|
| Term | Voltage Drop |
| Category | Solar Engineering |
| Engineering Discipline | Electrical Design |
| NEC Reference | 215, Chapter 9 Table 8/9 |
| Target | ≤ 2% DC, ≤ 1% AC |
| Difficulty Level | Beginner to Intermediate |
Voltage Drop Formula
DC voltage drop
Vd = 2 × I × L × (R / 1000)
where:
- I = current (A)
- L = one-way length (ft)
- R = conductor resistance (Ω/1000 ft)
- factor 2 accounts for the round-trip (out and back)Voltage drop percentage
Vd_% = (Vd / V_string) × 100Worked example
String at Vmp = 600 V, Imp = 13.5 A. Run to inverter: 80 ft (one-way). 10 AWG copper, R = 1.0 Ω/kft.
Vd = 2 × 13.5 × 80 × 1.0 / 1000 = 2.16 V
Vd_% = 2.16 / 600 = 0.36% → ✓ excellentResistance Values (Copper, NEC Chapter 9 Table 8)
| AWG / kcmil | Ω/kft | Suitable for |
|---|---|---|
| 14 | 2.52 | Short residential |
| 12 | 1.59 | Residential DC |
| 10 | 1.0 | DC home runs ≤100 ft |
| 8 | 0.63 | DC home runs ≤200 ft |
| 6 | 0.40 | Combiner outputs |
| 4 | 0.25 | Sub-feeder |
| 2 | 0.16 | Sub-feeder |
| 1/0 | 0.099 | Feeder |
| 4/0 | 0.049 | Feeder |
| 250 kcmil | 0.0419 | Feeder |
| 500 kcmil | 0.0214 | Sub-station |
Aluminum is ~60% the conductivity of copper — adjust upsizing accordingly.
Design Targets
- DC home run from combiner to inverter: ≤ 2%.
- AC from inverter to main service: ≤ 1%.
- AC sub-feeder to commercial inverter pad: ≤ 0.5%.
How to Reduce Voltage Drop
- Shorter wire runs — locate combiner near inverter.
- Larger conductors — doubling size halves Vd.
- Higher string voltage — 1,500 V plants have 1/4 the current of 750 V for same power, dropping voltage drop by 75%.
- Aluminum upsizing — economical for large feeders, requires 1 AWG up for equivalent ampacity.
- Multiple parallel runs — splitting current among multiple conductors.
Common Mistakes
- Forgetting the factor of 2 (round-trip) in DC voltage drop calculation.
- Using one-way length only for AC three-phase circuits where neutral/return doesn’t apply.
- Ignoring voltage drop when checking MPPT minimum.
- Designing AC drop within limits but DC drop over limits.
- Using cold-temperature resistance when hot operating conditions yield higher R.
Best Practices
- Show voltage drop calculation on the SLD for DC and AC runs.
- Use high-temperature resistance values (75°C–90°C) for derated conditions.
- Cross-check string Vmp at site hot temperature plus expected Vd vs. inverter MPPT_min.
- Use voltage drop calculators (NREL Wire Sizer, Mike Holt, Bluebeam plugins).
Standards & Certifications
- NEC 215 Informational Note No. 2 — Voltage drop recommendation.
- NEC Chapter 9 Table 8 — Conductor resistance values.
- NEC Chapter 9 Table 9 — AC resistance and reactance (large feeders).
Key Takeaways
- Voltage drop = 2 × I × L × R / 1000 for DC round-trip.
- Solar industry targets: ≤ 2% DC, ≤ 1% AC; NEC recommendation is ≤ 3% branch.
- Include Vd in MPPT lower bound checking for hot-day Vmp.
- Reduce Vd by larger conductors, shorter runs, or higher string voltages.
- Always show the Vd calc on the SLD or in the conductor schedule.
Frequently Asked Questions
8 commonly searched questions about Voltage Drop.
What is voltage drop?
The reduction in voltage along a conductor due to its resistance and the current flowing through it. Calculated as Vd = I × R, where R = (2 × length × ohms/kft) / 1000 for DC round-trip.
What's the NEC voltage drop limit?
NEC 215 informational note recommends ≤ 3% for branch circuits and ≤ 5% combined (feeder + branch). Solar industry uses tighter targets: ≤ 2% DC, ≤ 1% AC for efficiency.
Why is solar voltage drop important?
Excess voltage drop reduces energy delivered to the inverter, can push DC voltage below MPPT minimum (especially hot afternoons), and wastes energy as heat in the wire.
How do I reduce voltage drop?
Use larger conductors, shorter runs, higher string voltages, or aluminum-to-copper conversion. Doubling conductor size halves resistance and voltage drop.
Is voltage drop different for AC and DC?
DC voltage drop is V = I × R (round-trip). AC voltage drop adds reactance: Vd = I × (R × cos θ + X × sin θ) where θ is power factor angle. For solar inverters near unity PF, AC ≈ DC voltage drop.
What's a typical PV wire resistance?
12 AWG: 1.59 ohm/kft. 10 AWG: 1.0 ohm/kft. 8 AWG: 0.63 ohm/kft. 6 AWG: 0.40 ohm/kft. 4 AWG: 0.25 ohm/kft. Higher temperature increases resistance slightly.
Does voltage drop affect MPPT?
Yes. If string Vmp at high temperature already approaches MPPT minimum, additional voltage drop on the home run can push the system out of tracking range. Include Vd when checking MPPT lower bound.
When does voltage drop matter most?
Long DC home runs from string combiner to inverter (>100 ft). Long AC runs from inverter to main service panel (>50 ft). Both can cost 1–3% energy if undersized.
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