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SolarCalcWorks

Solar Wire Size Calculator

Find the smallest wire gauge that is safe and efficient for any DC circuit in a solar system. The calculator runs all three checks the job actually requires — NEC ampacity, overcurrent device coordination and voltage drop — and tells you which one decided your answer.

  • NEC 2023 tables
  • 12 V · 24 V · 48 V · custom
  • AWG and mm²
  • Full working shown

Solar Wire Size Calculator

2/0 AWG1.61% drop
Inputs2 % target
System voltage
Conductor material

2% of 12 V = 0.24 V of budget across the whole round trip.

Installation conditionsdefaults are code-safe

Already have wire? Check the drop on a size you own.

Conductor67.4 mm²

2/0AWG · Cu · 90 °C

Voltage drop sets this size

Ampacity alone would allow 1 AWG. Holding the drop at or below 2.0% is what forces 2/0 AWG. A shorter run or a higher system voltage makes this cheaper; more copper is the expensive fix.

Voltage drop
1.61%
0.19 V lost
At the load
11.81 V
from 12 V
Amps at terminals
175 A
needs 125 A
Heat in the wire
19 W
at 100 A
Calculation

1Maximum circuit current

100 A rated

No 690.8(A) multiplier applies outside PV source and output circuits.

2Test A — 125 % at the terminals

100 A × 1.25 = 125 A required
2/0 AWG @ 75 °C = 175 A ✓

Continuous loads need 125 % headroom, and NEC 110.14(C) caps the usable column at the lowest-rated termination. No derating applies to this test.

3Test B — derated ampacity

195 A @ 90 °C = 195 A
must be ≥ 100 A ✓

30 °C ambient falls in the 26–30 °C band of Table 310.15(B)(1).

4Test C — can a device protect it?

device needed = 125 A (NEC 240.6(A))
2/0 AWG may be protected at up to 175 A ✓

NEC 240.4(B) permits rounding up to the next standard rating; 240.4(D) then caps 14, 12 and 10 AWG regardless — except on PV circuit conductors, which 240.4(G) exempts. A conductor with adequate ampacity can still fail here.

5Voltage drop

2 × 10.0 ft × 100 A × 0.0967 Ω/kft ÷ 1000
= 0.19 V = 1.61% of 12 V

The run is doubled because current returns on the second conductor. Resistance from NEC Chapter 9, Table 8. Voltage drop is a design target, not a code requirement — NEC 210.19(A) Informational Note 4 recommends 3 % or less.

6The answer

Ampacity needs 1 AWG, protection coordination needs 1 AWG, and the 2.0% drop target needs 2/0 AWG. The conductor has to satisfy all three, so the answer is the largest: 2/0 AWG.

Reference 210.19(A)(1) · 310.15 · 706.30 · 110.14(C) · 240.4 · Ch.9 T.8 — NEC 2023. Sources and limitations.

Overcurrent protection125 A

Calculated minimum 125 A, rounded up to the 125 A standard rating in NEC 240.6(A).

NEC 210.20(A): the device rating must be at least 125 % of the continuous load. NEC 240.4(D) additionally caps the device on 14, 12 and 10 AWG conductors.

NextSize the fuse or breaker for this circuitA conductor is only protected once a device is sized to it.
Nearby sizes100 A · 10 ft
Conductor sizes compared by ampacity margin and voltage drop
SizeAmps spareDropVerdict
1 AWG52.6%Drop too high
1/0 AWG252.0%Drop too high
2/0 AWGpick501.6%Meets all
3/0 AWG751.3%Meets all
4/0 AWG1051.0%Meets all
250 kcmil1300.9%Meets all

Amps spare is the margin on the tighter of the two NEC ampacity tests.

How solar wire sizing actually works

A conductor has to pass three independent tests, and the wire you buy is whichever demands the most copper. Most calculators run one and ignore the others, which is why they disagree with each other.

Test 1 — ampacity: will it overheat?

Mandatory. The NEC publishes an allowable ampacity for every conductor size in Table 310.16 for conductors in a raceway or cable, and Table 310.17 for single conductors in free air. Both assume a 30 °C (86 °F) ambient, and both get modified by where you actually install the wire.

For a solar circuit the NEC states the requirement as two conditions that must both hold — 690.8(B) for PV circuits, 210.19(A)(1) for branch circuits, worded almost identically:

  1. Before derating: the table ampacity, read in the column matching the lowest-rated termination, must be at least 125 % of the maximum circuit current. NEC 110.14(C) is what caps you to that column — a 90 °C wire landing on a 75 °C lug is a 75 °C circuit at that point.
  2. After derating: the table ampacity, read in the column matching the conductor’s own insulation rating, then multiplied by the ambient correction factor from Table 310.15(B)(1) and the fill adjustment from Table 310.15(C)(1), must be at least the maximum circuit current.

Applying 125 % and the derates to the same number in one step — a common shortcut — overstates the requirement and sells people wire they do not need.

Test 2 — can a device actually protect it?

This is the check almost nobody runs, and it changes real answers. NEC 240.4 requires a conductor to be protected in accordance with its ampacity, and 240.4(D) caps the device on 14, 12 and 10 AWG regardless of what the ampacity table says.

A 20 A continuous load needs a 25 A device under 210.20(A). 12 AWG copper carries 25 A at 75 °C, so it passes both ampacity tests — but 240.4(D) caps its device at 20 A. The pairing is illegal, and a calculator that stops at ampacity will happily recommend it.

Test 3 — voltage drop: will the load still work?

Every conductor has resistance, so some voltage is lost as heat on the way to the load:

Vdrop = 2 × L × I × R ÷ 1000 — where L is the one-way length in feet, I the current in amps, and R the resistance in ohms per 1000 ft

The 2 is there because the current has to come back. R comes from NEC Chapter 9, Table 8, which publishes DC resistance directly — meaning DC voltage drop is exactly computable, with no power factor or reactance term to estimate.

Unlike ampacity, this is not a code requirement. It is a design target, and on low-voltage DC it is usually the target that decides your wire.

Why 12 V systems eat so much copper

A 3 % drop is 3 % of whatever your system voltage is. On a 48 V bank that is 1.44 V of budget. On 12 V it is 0.36 V — one quarter of the allowance.

It compounds, because moving the same power at a quarter of the voltage means four times the current, and drop is proportional to current. Four times the current across a quarter of the budget means roughly sixteen times the conductor area for the same watts over the same distance.

Common solar wire sizes

Generated by the same engine as the calculator above, so they cannot drift out of sync with it.

12 V DC circuits

Wire size for 12 V DC circuits by current and one-way run length.
12 V5 ft10 ft15 ft25 ft40 ft
5 A14 AWG14 AWG12 AWG10 AWG8 AWG
10 A14 AWG10 AWG8 AWG6 AWG4 AWG
20 A10 AWG8 AWG6 AWG4 AWG2 AWG
30 A8 AWG6 AWG4 AWG2 AWG1/0 AWG
50 A6 AWG4 AWG2 AWG1/0 AWG3/0 AWG
80 A3 AWG2 AWG1/0 AWG3/0 AWG250 kcmil
Wire size for 12 V DC circuits by current and one-way run length. Copper, 90 °C insulation on 75 °C terminals, in conduit at 30 °C, two current-carrying conductors, 3 % drop target.Amber means voltage drop rather than ampacity forced that size. Run lengths are one-way.

24 V DC circuits

Wire size for 24 V DC circuits by current and one-way run length.
24 V10 ft20 ft30 ft50 ft75 ft
5 A14 AWG14 AWG12 AWG10 AWG8 AWG
10 A14 AWG10 AWG8 AWG6 AWG4 AWG
20 A10 AWG8 AWG6 AWG4 AWG2 AWG
30 A8 AWG6 AWG4 AWG2 AWG1 AWG
50 A6 AWG4 AWG2 AWG1/0 AWG3/0 AWG
80 A3 AWG2 AWG1/0 AWG3/0 AWG250 kcmil
Wire size for 24 V DC circuits by current and one-way run length. Copper, 90 °C insulation on 75 °C terminals, in conduit at 30 °C, two current-carrying conductors, 3 % drop target.Amber means voltage drop rather than ampacity forced that size. Run lengths are one-way.

48 V DC circuits

Wire size for 48 V DC circuits by current and one-way run length.
48 V10 ft25 ft50 ft75 ft100 ft
10 A14 AWG12 AWG10 AWG8 AWG6 AWG
20 A10 AWG10 AWG6 AWG4 AWG4 AWG
30 A8 AWG8 AWG4 AWG4 AWG2 AWG
50 A6 AWG6 AWG3 AWG1 AWG1/0 AWG
80 A3 AWG3 AWG1 AWG2/0 AWG3/0 AWG
120 A1/0 AWG1/0 AWG2/0 AWG3/0 AWG250 kcmil
Wire size for 48 V DC circuits by current and one-way run length. Copper, 90 °C insulation on 75 °C terminals, in conduit at 30 °C, two current-carrying conductors, 3 % drop target.Amber means voltage drop rather than ampacity forced that size. Run lengths are one-way.

A worked example you can check by hand

A 2000 W 12 V inverter at 90 % efficiency draws 2000 ÷ 0.9 ÷ 12, about 185 A at full output. The battery sits 6 ft away. What cable does it need?

Ampacity. Inverter load is continuous, so 185 A × 1.25 = 231.25 A required at the 75 °C terminals. In Table 310.16, 4/0 AWG copper at 75 °C is 230 A — 1.25 A short. 250 kcmil at 255 A clears it.

Voltage drop. A 2 % target on 12 V is 0.24 V. For 4/0 AWG:

2 × 6 ft × 185 A × 0.0608 Ω/1000 ft = 0.135 V = 1.12 % — passes

4/0 passes voltage drop easily. Ampacity forces the size up to 250 kcmil, and no amount of shortening the run will change that — exactly the distinction the result panel calls out.

This is also why serious off-grid systems are built at 48 V. The same 2000 W at 48 V is 46 A, and 6 AWG handles it.

Which wire type to use where

Ampacity tables assume a conductor listed for the environment it is in. The insulation matters as much as the gauge.

Where Typical type Why
Between modules, exposed under the array PV Wire or USE-2 Sunlight resistant, 90 °C wet, required by NEC 690.31(C) for exposed single-conductor runs.
In conduit, outdoors or underground THWN-2 or XHHW-2 90 °C wet rating. Plain THHN is not rated for wet locations.
In conduit, indoors and dry THHN 90 °C dry. The usual choice inside a building.
Battery to inverter Listed battery cable, RHW-2 or THW-2 Fine stranding survives vibration and tight bends. Welding cable is common in RV work but is generally not listed as building wire.

Mistakes that cost people money

  • Entering round-trip length. The calculator doubles the one-way figure for you.
  • Sizing to the fuse instead of the load. The conductor is sized to the circuit current; the device is then sized to protect that conductor. Backwards leads to conductors no device can legally protect.
  • Ignoring the termination rating. A 90 °C conductor on 75 °C lugs is limited to the 75 °C column at those lugs. The 90 °C rating still earns its keep — it is the column used for derating.
  • Forgetting the rooftop adder. Conduit in direct sun within 3/4 in. of a roof takes a 33 °C adder under NEC 310.15(B)(2). On a 40 °C day that is 73 °C, where even 90 °C wire loses half its ampacity.
  • Counting grounding conductors in the fill. Equipment grounding conductors are not current-carrying and are excluded under 310.15(E).

What to calculate next

NextSize the fuse or breaker for this circuitA conductor is only protected once a device is sized to it.

Frequently asked questions

What size wire do I need for a 12V solar panel?

It depends far more on the run length than on the panel. A single 100 W panel at 12 V draws roughly 6 A, which 14 AWG carries comfortably. But at 20 ft one way, holding voltage drop to 3 % of 12 V — just 0.36 V — already needs 10 AWG. Enter your actual current and length above; on 12 V systems the distance is almost always what decides the answer.

Does the calculator use one-way length or total wire length?

Enter the one-way distance, measured from the source to the load along the route the cable actually takes. The calculator doubles it internally, because current flows out on one conductor and back on the other. Entering round-trip length is the single most common way people end up buying twice the copper they need.

Why does the calculator apply 125 % twice to a PV circuit?

They are two different rules solving two different problems. NEC 690.8(A)(1) multiplies module short-circuit current by 125 % because real irradiance can exceed the 1000 W/m² test condition — snow reflection and cloud-edge effects push panels above their rated Isc. NEC 690.8(B)(1) then applies a second 125 % because a PV circuit is a continuous load. The combined factor on Isc is 1.5625.

Is 3 % voltage drop a code requirement?

No. Voltage drop limits are not enforceable NEC requirements. NEC 210.19(A) Informational Note 4 and 215.2(A)(2) Informational Note 2 recommend a maximum of 3 % on a branch circuit or feeder and 5 % total, for reasonable efficiency of operation. Informational notes are advisory. Ampacity, by contrast, is mandatory — which is why this tool treats the two differently and tells you which one produced your answer.

Why did the calculator reject a wire that has enough ampacity?

Because a conductor also has to be protectable. NEC 240.4(D) caps the overcurrent device on 12 AWG copper at 20 A, so a 20 A continuous load — which needs a 25 A device under 210.20(A) — cannot legally use 12 AWG even though it carries 25 A at 75 °C. Sizing on ampacity alone produces that illegal pairing silently, so the calculator runs the coordination check too.

Should I use copper or aluminum for solar wiring?

Copper for anything under about 100 A, which covers nearly all DIY off-grid work. Aluminum needs roughly 1.6 times the cross-sectional area for the same ampacity, and it requires lugs and terminals listed for aluminum plus antioxidant compound per NEC 110.14. Most small charge controllers, inverters and busbars have copper-only terminals, so aluminum is simply not an option there. On long, large feeders it can cut cost meaningfully.

What happens if my wire is undersized?

Two separate failures. Undersized for ampacity, the conductor runs hot, degrades its insulation and eventually becomes a fire risk — this is what the NEC exists to prevent. Undersized for voltage drop, nothing burns, but the system quietly underperforms: a charge controller reads a low battery voltage and terminates absorption early, and an inverter hits low-voltage cutout well before the batteries are actually empty.

Does the calculator work for AC circuits?

No. It is built for DC and uses the DC resistance values in NEC Chapter 9, Table 8. AC voltage drop additionally involves conductor reactance and the load power factor, which this tool does not model. Use it for PV strings, charge controller runs, battery cables and DC loads — not for the AC output of an inverter.

Last reviewed 2026-08-19. Calculations reference NFPA 70 (NEC) 2023 where a code section applies. Sources, and what these tools deliberately do not model.