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Size low-voltage DC cable for 12 V, 24 V and 48 V systems from run length and amps — get the required mm², the matching AWG, voltage drop and a full size chart.
Inputs
1
System
2
Cable run
Distance from battery/source to load — the calculator doubles it for the return path.
Use 45–60 °C in engine bays, roof spaces and enclosed conduit.
How to use this calculator: DC Cable Size Calculator (12V / 24V / 48V)
Size low-voltage DC cable for 12 V, 24 V and 48 V systems from run length and amps — get the required mm², the matching AWG, voltage drop and a full size chart. The example below is calculated by this page's real engine from the displayed inputs.
1Confirm that the DC Cable Size Calculator (12V / 24V / 48V) matches the quantity or design check you need.
2Enter Load current, One-way run length, and Conductor temperature using the units printed beside each field.
3Select the applicable System voltage, Load entered as, and Allowable voltage drop options; these choices change the calculation method or factors.
4Calculate, then follow the substituted equations in the worked example and compare the result with any stated limit.
5Read the assumptions, warnings and cited references before using the result for a financial, medical or engineering decision.
Input guide and example values
Use values from the same measurement basis and time period. Conditional fields appear only when the related option is selected.
Input
Example value
Why it matters
System voltage
12 V — car, RV, small solar, battery bank
Select the option that matches the real installation or scenario.
Load entered as
Current (amps)
Select the option that matches the real installation or scenario.
Load current
20 A
Measured or known load current used by the calculation engine.
One-way run length
5 m
Distance from battery/source to load — the calculator doubles it for the return path.
Allowable voltage drop
3% — general branch circuits (typical)
Select the option that matches the real installation or scenario.
Conductor material
Copper
Select the option that matches the real installation or scenario.
Conductor temperature
45 °C
Use 45–60 °C in engine bays, roof spaces and enclosed conduit.
Formula, derivation and worked example
At 12, 24 or 48 volts there is very little voltage to spare, so DC cable size is decided by voltage drop long before it is decided by heating. A 20 A load that is happy on 2.5 mm² at 230 V AC needs 6–10 mm² at 12 V over the same distance, because the same 0.4 V lost is 0.17% of 230 V but 3.3% of 12 V. The calculator works out the current, adds the go-and-return path (DC always counts both conductors), corrects copper or aluminium resistivity for the working temperature, and then returns the smallest standard conductor that satisfies both the voltage-drop target and a safe current rating. Doubling the system voltage from 12 V to 24 V quarters the required copper for the same wattage, which is why larger off-grid systems move to 24 V and 48 V.
I = P / V (when power is entered)
ρ_T = ρ₂₀ × (1 + α (T − 20))
Required area A = 2 × ρ_T × L × I / (ΔV% × V / 100)
Actual drop ΔV = 2 × ρ_T × L × I / A; loss = 2 I² ρ_T L / A
Substitution steps
1. Current
given
= 20 A
2. Resistivity at temperature
ρ₂₀ (1 + α(T − 20))
= 0.01889 Ω·mm²/m
3. Allowable drop
3% × 12 V
= 0.36 V
4. Required area
2 ρ L I / ΔV
= 10.49 mm²
5. Standard size chosen
next size up
= 16 mm² (6 AWG)
6. Actual drop
2 ρ L I / A
= 0.236 V (1.97%)
Computed example results
Recommended cable
16 mm² (6 AWG)
Copper, round trip 10 m
Minimum required area
10.49 mm²
for 3% drop at 12 V
Actual voltage drop
0.236 V
1.97% of 12 V
Voltage at the load
11.76 V
Design current
20 A
240 W at 12 V
Power lost in the cable
4.7 W
1.97% of the load
Loop resistance
11.8 mΩ
Cable current rating
65 A
indicative, 30 °C — derate for heat and bunching
Suggested fuse / breaker
25 A
≥ 1.25 × load current and ≤ cable rating
Understanding the result
Read the main result together with supporting checks, assumptions, limits and intermediate values.
For a manual check, repeat the first equation, confirm the units and change one input at a time.
Common mistakes when using DC Cable Size Calculator (12V / 24V / 48V)
Do not mix units for Load current (A), One-way run length (m), Conductor temperature (°C). A unit mismatch changes the input magnitude even when the typed number looks reasonable.
Do not leave System voltage on the default choice unless that choice matches the real scenario; the selected option can change the calculation path or factor.
Do not replace the displayed I = P / V (when power is entered) relationship with a different convention without also changing the underlying assumptions; compare like-for-like methods when checking the result.
Do not treat Recommended cable = 16 mm² (6 AWG) from the worked example as a universal answer. It belongs to the displayed example inputs and must be recalculated for the actual case.
When the DC Cable Size Calculator (12V / 24V / 48V) is useful
DC Cable Size Calculator (12V / 24V / 48V) is designed for cases where System voltage, Load entered as, Load current, One-way run length are known and you need Recommended cable, Minimum required area, Actual voltage drop. The page keeps the live calculator, calculation method and worked example together so the result can be checked instead of treated as a black-box number.
Use the calculator for the scope described by its inputs and notes. The displayed method is I = P / V (when power is entered). If the real project or decision needs factors that are not represented here, treat the result as an estimate and add the missing checks separately.
System voltage and Load entered as: what changes the answer
The worked example uses System voltage = 12 V — car, RV, small solar, battery bank, Load entered as = Current (amps), Load current = 20 A, One-way run length = 5 m. With those values, Recommended cable is 16 mm² (6 AWG). Changing an input should be interpreted according to that field's unit, range, option and hint rather than by the number alone.
For this calculator, the main input roles are: System voltage: Select the option that matches the real installation or scenario. Available choices include 12 V — car, RV, small solar, battery bank, 24 V — larger solar, marine, telecom, 48 V — off-grid inverters, e-bikes, server DC, Other DC voltage. Load entered as: Select the option that matches the real installation or scenario. Available choices include Current (amps), Power (watts). Load current (A): Measured or known load current used by the calculation engine. One-way run length (m): Distance from battery/source to load — the calculator doubles it for the return path.
How to sanity-check a DC Cable Size Calculator (12V / 24V / 48V) result
Start by confirming the entered values and units, then compare the substituted working with the displayed formula or calculation steps. Pay particular attention to Recommended cable, because it is the first worked-example output shown by the live engine.
Finally, compare the result with the assumptions, warnings and related calculators on this page. A nearby calculator can be useful as a cross-check when it measures the same workflow from a different input or output direction.
Next logical calculator
Continue with Wire Size Calculator (AWG & mm²)
Useful next check because both tools use System voltage and Load entered as, while Wire Size Calculator (AWG & mm²) answers a different part of the same workflow.
Required area A = 2 × ρ_T × L × I / (ΔV% × V / 100)
Actual drop ΔV = 2 × ρ_T × L × I / A; loss = 2 I² ρ_T L / A
At 12, 24 or 48 volts there is very little voltage to spare, so DC cable size is decided by voltage drop long before it is decided by heating. A 20 A load that is happy on 2.5 mm² at 230 V AC needs 6–10 mm² at 12 V over the same distance, because the same 0.4 V lost is 0.17% of 230 V but 3.3% of 12 V. The calculator works out the current, adds the go-and-return path (DC always counts both conductors), corrects copper or aluminium resistivity for the working temperature, and then returns the smallest standard conductor that satisfies both the voltage-drop target and a safe current rating. Doubling the system voltage from 12 V to 24 V quarters the required copper for the same wattage, which is why larger off-grid systems move to 24 V and 48 V.
Engineering notes
Copper resistivity is 0.0172 Ω·mm²/m at 20 °C and rises about 0.39% per °C; a hot 70 °C cable has roughly 20% more resistance than a bench-temperature one.
12 V / 24 V / 48 V DC cable size chart (3% drop, copper)
Minimum copper conductor for a 3% voltage drop at the stated one-way run length and current. Round up if the cable runs hot or the load is continuous.
Current
System
3 m run
5 m run
10 m run
20 m run
10 A
12 V
1.5 mm² (16 AWG)
2.5 mm² (14 AWG)
4 mm² (12 AWG)
10 mm² (8 AWG)
20 A
12 V
4 mm² (12 AWG)
6 mm² (10 AWG)
10 mm² (8 AWG)
16 mm² (6 AWG)
30 A
12 V
6 mm² (10 AWG)
10 mm² (8 AWG)
16 mm² (6 AWG)
25 mm² (4 AWG)
50 A
12 V
10 mm² (8 AWG)
16 mm² (6 AWG)
25 mm² (4 AWG)
50 mm² (1/0 AWG)
100 A
12 V
25 mm² (4 AWG)
35 mm² (2 AWG)
70 mm² (2/0 AWG)
120 mm² (4/0 AWG)
10 A
24 V
1.0 mm² (17 AWG)
1.5 mm² (16 AWG)
2.5 mm² (14 AWG)
4 mm² (12 AWG)
20 A
24 V
2.5 mm² (14 AWG)
2.5 mm² (14 AWG)
4 mm² (12 AWG)
10 mm² (8 AWG)
30 A
24 V
2.5 mm² (14 AWG)
4 mm² (12 AWG)
6 mm² (10 AWG)
16 mm² (6 AWG)
50 A
24 V
4 mm² (12 AWG)
6 mm² (10 AWG)
16 mm² (6 AWG)
25 mm² (4 AWG)
100 A
24 V
10 mm² (8 AWG)
16 mm² (6 AWG)
35 mm² (2 AWG)
70 mm² (2/0 AWG)
20 A
48 V
1.5 mm² (16 AWG)
1.5 mm² (16 AWG)
2.5 mm² (14 AWG)
4 mm² (12 AWG)
50 A
48 V
2.5 mm² (14 AWG)
4 mm² (12 AWG)
6 mm² (10 AWG)
16 mm² (6 AWG)
100 A
48 V
6 mm² (10 AWG)
10 mm² (8 AWG)
16 mm² (6 AWG)
35 mm² (2 AWG)
Formulas explained
A = 2 ρ_T L I / ΔV
Both conductors of a DC circuit carry the full current, so the resistive path is twice the route length. Rearranging Ohm's law for the allowed drop gives the minimum cross-section directly.
A
conductor cross-sectional area, mm²
ρ_T
resistivity at working temperature, Ω·mm²/m
L
one-way run length, m
I
continuous DC current, A
ΔV
allowable drop in volts = % × system voltage
Round up to the next standard mm² / AWG size — never down.
3A = 2 × 0.01889 × 5 × 20 / 0.36 = 10.5 mm² → use 16 mm² (6 AWG) as the next standard size.
Assumptions
Continuous DC current with both conductors in the same run; ampacity figures are indicative 30 °C free-air / conduit values.
AWG equivalents are the nearest commercial size, not exact metric conversions.
Tips
If the required cable looks absurd, raise the system voltage: 24 V needs a quarter of the copper of 12 V for the same watts.
Measure the actual routed length including vertical drops, not the straight-line distance.
Solar array and charge-controller wiring is usually held to 1–2% so you do not throw away harvest.
Warnings
The fuse or breaker protects the cable, not the load — never fit protection larger than the chosen conductor's rating.
Battery-to-inverter cables can see several hundred amps; keep them short, correctly crimped and fused close to the battery.
Aluminium needs about one size larger than copper and proper anti-oxidant terminations.
Standards & references
IEC 60364-5-52
ABYC E-11 (marine DC)
NEC Article 690 (PV DC)
Frequently asked questions
What size cable do I need for a 12V 20A load 5 metres away?+
About 10.5 mm² is required for a 3% drop, so use 16 mm² (6 AWG) copper. At 24 V the same load needs only 4 mm² (12 AWG).
Why does 12V need such thick cable?+
Voltage drop is a percentage of the supply. Losing 0.4 V is 0.17% at 230 V but 3.3% at 12 V, so low-voltage circuits must keep resistance tiny — meaning thicker conductors.
Is DC cable sized on the one-way or the round-trip length?+
The round trip. Enter the one-way distance here and the calculator doubles it, because current flows out on the positive and back on the negative conductor.
How much voltage drop is acceptable in a 12V or 24V system?+
3% is the usual target for branch circuits, 2% for battery-to-inverter runs and critical loads, and 1% for solar array wiring where every lost watt is lost harvest. 5% is tolerable only for simple lighting.
What is 6 mm² in AWG?+
6 mm² is closest to 10 AWG. Other common pairs: 2.5 mm² ≈ 14 AWG, 4 mm² ≈ 12 AWG, 16 mm² ≈ 6 AWG, 25 mm² ≈ 4 AWG and 50 mm² ≈ 1/0 AWG.
Can I use the same cable for a 48V battery bank?+
Yes, but 48 V circuits carry a quarter of the current of a 12 V circuit of the same power, so the conductor is usually far smaller. Insulation must still be rated for the maximum system voltage.
What inputs does the DC Cable Size Calculator (12V / 24V / 48V) use?+
It uses System voltage, Load entered as, Load current, One-way run length, Allowable voltage drop, Conductor material, and Conductor temperature. Follow the unit printed for each field and choose any selectable option to match the real scenario.
What does the DC Cable Size Calculator (12V / 24V / 48V) calculate?+
It calculates Recommended cable, Minimum required area, Actual voltage drop, Voltage at the load, Design current, Power lost in the cable, Loop resistance, Cable current rating, and Suggested fuse / breaker. With the displayed default inputs, Recommended cable is 16 mm² (6 AWG).
