DC Voltage Drop Calculator

Work out the voltage lost along a DC cable run: panel to charge controller, controller to battery, battery to inverter, or any 12V, 24V or 48V circuit. Low-voltage DC is where voltage drop bites hardest, because the same volt lost is a far bigger share of the supply. Single-phase and three-phase AC are supported too, for the AC side of a hybrid system.

Enter the one-way distance, not the total length of cable. The calculator doubles it for the return conductor.

How It Works

The formula

Voltage drop is conductor resistance multiplied by current and by the length of conductor the current actually travels through. On a DC circuit the current goes out along one conductor and back along the other, so the one-way run is counted twice:

  • DC: Vdrop = 2 × L × I × R
  • Single-phase AC: Vdrop = 2 × L × I × (R·cosφ + X·sinφ)
  • Three-phase AC: Vdrop = √3 × L × I × (R·cosφ + X·sinφ)

R and X are the conductor resistance and reactance per unit length, L is the one-way run length and I is the current. DC has no reactance term and no power factor, so only resistance matters.

Worked example

A 12 V camper runs 20 A from the battery to a distribution panel 20 ft away in 10 AWG copper (R about 3.9 ohm/km, so 0.00119 ohm/ft):

  • Vdrop = 2 × 20 ft × 20 A × 0.00119 = 0.95 V
  • Percentage = 0.95 ÷ 12 = 7.9%, well over the 3% target
  • The same run on a 24 V system would be 3.96%, and on 48 V it would be 1.98%

That last line is the whole argument for higher system voltages. Doubling the system voltage halves the current for the same power, which halves the volts dropped and halves the percentage again, so the percentage drop falls by four times. It is why 48 V has become the default for anything larger than a van.

What limit should I use?

A 3% drop is the usual target for each leg of a solar system, and 2% is the common design goal for the battery-to-inverter run because it carries the highest current in the system. On the panel side a 3% loss is money straight off the array. If your circuit is AC rather than DC, the guidance is the same 3% branch and 5% total convention used in building wiring, which the AC voltage drop calculator at Electrical Calculators is set up for.

Maximum DC Run Length at 3% Drop

The longest one-way run each copper cable can serve before voltage drop reaches 3%, at 12 V, 24 V and 48 V. Read across from your current, then down to the system voltage you are building.

12 V system

Maximum one-way run length in feet at 3 percent voltage drop on a 12 volt DC circuit, copper conductors
Copper size 10 A 20 A 30 A 40 A 60 A 80 A 100 A
10 AWG 15 ft 7 ft 5 ft — — — —
8 AWG 23 ft 11 ft 7 ft 5 ft — — —
6 AWG 36 ft 18 ft 12 ft 9 ft 6 ft — —
4 AWG 57 ft 28 ft 19 ft 14 ft 9 ft 7 ft —
2 AWG 95 ft 47 ft 31 ft 23 ft 15 ft 11 ft 9 ft
1/0 AWG 151 ft 75 ft 50 ft 37 ft 25 ft 18 ft 15 ft
2/0 AWG 178 ft 89 ft 59 ft 44 ft 29 ft 22 ft 17 ft
4/0 AWG 290 ft 145 ft 96 ft 72 ft 48 ft 36 ft 29 ft

24 V system

Maximum one-way run length in feet at 3 percent voltage drop on a 24 volt DC circuit, copper conductors
Copper size 10 A 20 A 30 A 40 A 60 A 80 A 100 A
10 AWG 30 ft 15 ft 10 ft — — — —
8 AWG 46 ft 23 ft 15 ft 11 ft — — —
6 AWG 73 ft 36 ft 24 ft 18 ft 12 ft — —
4 AWG 115 ft 57 ft 38 ft 28 ft 19 ft 14 ft —
2 AWG 190 ft 95 ft 63 ft 47 ft 31 ft 23 ft 19 ft
1/0 AWG 302 ft 151 ft 100 ft 75 ft 50 ft 37 ft 30 ft
2/0 AWG 357 ft 178 ft 119 ft 89 ft 59 ft 44 ft 35 ft
4/0 AWG 581 ft 290 ft 193 ft 145 ft 96 ft 72 ft 58 ft

48 V system

Maximum one-way run length in feet at 3 percent voltage drop on a 48 volt DC circuit, copper conductors
Copper size 10 A 20 A 30 A 40 A 60 A 80 A 100 A
10 AWG 60 ft 30 ft 20 ft — — — —
8 AWG 92 ft 46 ft 30 ft 23 ft — — —
6 AWG 146 ft 73 ft 48 ft 36 ft 24 ft — —
4 AWG 231 ft 115 ft 77 ft 57 ft 38 ft 28 ft —
2 AWG 381 ft 190 ft 127 ft 95 ft 63 ft 47 ft 38 ft
1/0 AWG 605 ft 302 ft 201 ft 151 ft 100 ft 75 ft 60 ft
2/0 AWG 715 ft 357 ft 238 ft 178 ft 119 ft 89 ft 71 ft
4/0 AWG 1,163 ft 581 ft 387 ft 290 ft 193 ft 145 ft 116 ft

Greyed cells are currents the cable cannot carry at a 75°C termination, so run length is moot. Resistance figures are NEC Chapter 9 Table 9 values; DC resistance is marginally lower, so these lengths are slightly conservative. Fine-stranded welding and battery cable of the same AWG behaves the same way electrically. Need the size rather than the length? Use the solar cable size calculator.

Frequently Asked Questions

What is an acceptable voltage drop on a solar system?

Aim for 3% or less on each leg: array to charge controller, controller to battery, and battery to inverter. Many installers design the battery-to-inverter run to 2% because it carries the highest current in the system and any drop there shows up as reduced inverter output under load. Local wiring rules take precedence over any rule of thumb.

Why does voltage drop matter so much on 12V systems?

Because the percentage is what counts, not the volts. Losing 0.5 V on a 12 V circuit is a 4.2% drop; the same 0.5 V on a 48 V circuit is 1%. Low-voltage systems also need four times the current for the same power, and drop rises with current, so the two effects compound. This is why van and boat wiring uses cable that looks absurdly oversized next to a house circuit.

Do I measure one-way or round-trip cable length?

Enter the one-way distance, the straight-line run from source to load. The calculator doubles it, because the current has to travel out on the positive conductor and back on the negative. Measure the actual routed path including any vertical rises, not the direct distance between the two ends.

Should I use 12V, 24V or 48V?

Use the lowest voltage your loads need and the highest your budget allows. As a rough guide: 12 V for systems under about 1.2 kW where 12 V appliances dominate (vans, small boats, small RVs), 24 V from 1 to 3 kW, and 48 V above that or wherever cable runs are long. Moving from 12 V to 48 V cuts the current to a quarter, which cuts the percentage drop by sixteen times for the same cable, and usually pays for itself in copper saved.

Copper or aluminium for DC runs?

Copper, in nearly every case. Aluminium has about 1.6 times the resistance of copper for the same size, so it needs to go up roughly two AWG sizes to match, and it is harder to terminate reliably at the small lug sizes DC systems use. Aluminium becomes worth considering only on long, large-gauge runs where the cost of copper dominates the build.

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