DC Wire Size Calculator
Minimum conductor size for a two-wire DC circuit. Solar PV, battery banks, 12/24/48 V systems, telecom DC plant. Checks both voltage drop (NEC Chapter 9 Table 8 DC resistance, purely resistive) and derated ampacity (Table 310.16 with ambient and conductor-count adjustment), and reports which constraint governed.
How to use this calculator
- Enter the system voltage, load current, and one-way circuit distance. For PV source/output circuits, enter the NEC 690.8(A) current (125% of Isc) and check the continuous-load box.
- Pick the voltage-drop target. 3% matches the NEC recommendation; battery charging and critical electronics often warrant 1-2%.
- Set the conductor material, insulation and termination ratings, ambient temperature, and conductor count for derating.
- Read the minimum size and the governing constraint. At 12-48 V the answer is usually voltage-drop-governed; the notes explain what drove the result.
NEC reference
Voltage drop uses NEC 2020 Chapter 9, Table 8 DC resistance (75°C, uncoated stranded) with VD = 2 × R × I × L / 1000. The round-trip resistive drop, with no reactance or power factor because the circuit is DC. Ampacity uses Table 310.16 with the 310.15(B)(1) ambient correction, 310.15(C)(1) conductor-count adjustment, and the 110.14(C) termination cap, plus the 240.4(D) small-conductor limits. Continuous loads apply 125% per 210.19(A)(1). The recommended size is the smaller conductor that satisfies both constraints.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
Why voltage drop governs low-voltage DC
The same absolute volt drop costs ten times more headroom at 12 V than at 120 V. A drop of 0.5 V is 0.4% on a 120 V circuit and 4.2% on a 12 V circuit. Since drop scales with current and distance but the percentage scales inversely with system voltage, low-voltage DC circuits hit the 3% recommendation long before they exhaust conductor ampacity. The practical consequences: solar charge controllers mis-reading battery voltage, inverters tripping on low-voltage cutoff under load, and LED drivers flickering. All classic symptoms of an undersized low-voltage DC run.
Doubling the system voltage (12 V → 24 V) at the same power halves the current and doubles the allowable drop. A 4× reduction in required copper area. This is why moving battery banks from 12 V to 24 V or 48 V is usually cheaper than upsizing cable on long runs.
Quick reference. 12 V copper, 3% drop, 20 A
Minimum copper size by one-way distance for a 20 A load on a 12 V system at a 3% target, computed with the same Table 8 method as the calculator:
| One-way distance | Minimum copper size |
|---|---|
| 5 ft | 10 AWG |
| 10 ft | 8 AWG |
| 15 ft | 6 AWG |
| 25 ft | 4 AWG |
| 40 ft | 2 AWG |
| 60 ft | 1/0 AWG |
Derived from NEC Chapter 9 Table 8 stranded-copper DC resistance at 75°C; VD = 2·R·I·L/1000 against a 0.36 V budget. Use the calculator for your actual current and conditions.
Frequently asked questions
What size wire do I need for a 12V DC circuit?
Distance decides it. At 12 V the 3% budget is only 0.36 V, so a 20 A load needs 6 AWG copper at just 15 ft one-way. Even though 12 AWG carries 20 A comfortably by ampacity. Run the calculator with your actual distance; at low voltage, rules of thumb by amperage alone consistently undersize.
How do I size wire for a solar PV circuit?
NEC 690.8(A) sets the PV circuit current at 125% of Isc, and 690.8(B) treats it as continuous (another 125% for conductor sizing). Enter Isc × 1.25 as the load current, check the continuous box, and set your string's round-trip distance. Conditions matter on rooftops: PV conductors in sun-baked attics or on-roof raceways routinely see 45-60°C ambient. Enter the real ambient, not 86°F.
Why does the calculator use 75°C resistance?
Chapter 9 Table 8 publishes DC resistance at 75°C, and using it without downward temperature correction is the conservative choice. Correcting resistance down for a cooler conductor lowers the reported drop, which is the unsafe direction for a tool that cannot measure your operating temperature. This matches standard industry practice.
Can I use this for automotive or marine wiring?
The voltage-drop math is identical, and the 12V Voltage Drop Calculator is purpose-built for checking an existing automotive/RV run. Note that the ampacity side of this tool applies NEC Table 310.16, which governs premises wiring. Marine installations follow ABYC E-11 ampacity tables, and automotive practice uses SAE ratings. For those systems, treat the voltage-drop result as authoritative and verify ampacity against the applicable standard.
Related calculators
- DC Voltage Drop Calculator: Check the drop on a conductor size you already have.
- 12V Voltage Drop Calculator: Automotive, RV, and battery-bank runs.
- Wire Size Calculator: The AC sibling: ampacity + voltage drop with OCPD selection.
- Wire Ampacity Calculator: Full Table 310.16 derating detail for a specific conductor.
Studying for the licensing exam? Our own NEC Code Quickstart works all twelve exam calculations start to finish, each with the code reference. Also in paperback.
Running an electrical contracting business? Jobber handles scheduling, invoicing, and job management. Start a 14-day free trial.
For deeper NEC training on DC and solar wire sizing, Mike Holt's NEC courses are the industry standard.
Working from the printed code? NEC code books and tab sets are available from Mike Holt.
This page contains affiliate links. ElectricalToolbox may earn a commission when you sign up or buy through them, at no additional cost to you.