What voltage drop actually is
Every conductor has resistance. Push current through it and some of your voltage is spent pushing rather than arriving — converted to heat along the way. What reaches the load is what is left.
Vdrop = 2 × L × I × R ÷ 1000 — L is the one-way length in feet, I the current in amps, R the resistance in ohms per 1000 ft
The 2 is there because the current has to come back. A 20 ft run is 40 ft of copper in the loop, and both conductors drop voltage. R comes from NEC Chapter 9, Table 8, which publishes DC resistance directly — so on a DC circuit this is exact, with no power factor or reactance term to estimate.
It is a design target, not a code requirement
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. Informational notes are advisory.
That matters practically. An inspector will not fail an installation for a 4 % drop, but your charge controller will still undercharge the bank. Voltage drop is your problem to manage, which is different from someone else’s rule to satisfy — and it is why this calculator separates the drop verdict from the ampacity verdict rather than blending them into one pass or fail.
The targets worth using
| Circuit | Target | Why |
|---|---|---|
| Charge controller to battery | 2 % or less | The controller senses battery voltage at its own terminals. Drop here makes it read high, end absorption early and chronically undercharge the bank. |
| Battery to inverter | 2 % or less | Highest current in the system. Drop shows up as premature low-voltage cutout and reduced surge capability. |
| Array to charge controller | 3 % | Usually the longest run. An MPPT controller recovers some loss by converting excess voltage, but it cannot recover power that became heat in the wire. |
| DC branch circuits | 3 % | Keeps loads inside their rated voltage window. Tighten to 2 % for LED lighting and sensitive electronics. |
Why this calculator also checks ampacity
Most voltage drop calculators answer one question and stop. That leaves a genuine gap, because voltage drop being acceptable tells you nothing about whether the conductor is safe.
Take 10 AWG copper carrying 60 A over a 3 ft run at 48 V. The voltage drop is about 0.15 %, which any calculator will report as excellent. That conductor is also rated for 40 A in the 90 °C column of NEC Table 310.16, and 35 A at the 75 °C terminals most equipment uses. It is 25 A beyond its limit. It will run hot, cook its own insulation, and eventually fail — and every voltage-drop-only tool would have told the person it was fine.
So this tool runs both checks on the size you enter: the drop, and the two mandatory NEC ampacity tests from 690.8(B) or 210.19(A)(1), with ambient correction and conduit fill applied. If the conductor fails ampacity, that warning appears above the voltage drop result, because it is the more important answer.
The three levers, and which one to pull
When the drop is too high there are exactly three things you can change, and the calculator computes all three exactly for your circuit rather than describing them in general terms.
- A bigger conductor. The obvious lever and usually the most expensive. Drop is inversely proportional to conductor area, so doubling the area halves the drop — roughly three AWG sizes up.
- A shorter run. Directly proportional, so halving the distance halves the drop. Often free, if the equipment can move. Shifting an inverter three feet closer to a bank beats buying a size up in 4/0 cable.
- A higher system voltage. The strongest lever by far, and the one people overlook. Doubling the system voltage halves the current and doubles the volts you can afford to lose, so the drop falls by a factor of four on the same wire.
Voltage drop by wire size
Generated by the same engine as the calculator above, for a 20 ft one-way run in copper.
| 12 V | 14 | 12 | 10 | 8 | 6 | 4 | 2 | 1/0 |
|---|---|---|---|---|---|---|---|---|
| 5 A | 5.2% | 3.3% | 2.1% | 1.3% | 0.8% | 0.5% | 0.3% | 0.2% |
| 10 A | 10.5% | 6.6% | 4.1% | 2.6% | 1.6% | 1.0% | 0.6% | 0.4% |
| 20 A | 20.9% | 13.2% | 8.3% | 5.2% | 3.3% | 2.1% | 1.3% | 0.8% |
| 30 A | 31.4% | 19.8% | 12.4% | 7.8% | 4.9% | 3.1% | 1.9% | 1.2% |
| 50 A | 52.3% | 33.0% | 20.7% | 13.0% | 8.2% | 5.1% | 3.2% | 2.0% |
| 24 V | 14 | 12 | 10 | 8 | 6 | 4 | 2 | 1/0 |
|---|---|---|---|---|---|---|---|---|
| 5 A | 2.6% | 1.7% | 1.0% | 0.6% | 0.4% | 0.3% | 0.2% | 0.1% |
| 10 A | 5.2% | 3.3% | 2.1% | 1.3% | 0.8% | 0.5% | 0.3% | 0.2% |
| 20 A | 10.5% | 6.6% | 4.1% | 2.6% | 1.6% | 1.0% | 0.6% | 0.4% |
| 30 A | 15.7% | 9.9% | 6.2% | 3.9% | 2.5% | 1.5% | 1.0% | 0.6% |
| 50 A | 26.2% | 16.5% | 10.3% | 6.5% | 4.1% | 2.6% | 1.6% | 1.0% |
| 48 V | 12 | 10 | 8 | 6 | 4 | 2 | 1/0 | 4/0 |
|---|---|---|---|---|---|---|---|---|
| 10 A | 1.7% | 1.0% | 0.6% | 0.4% | 0.3% | 0.2% | 0.1% | 0.1% |
| 20 A | 3.3% | 2.1% | 1.3% | 0.8% | 0.5% | 0.3% | 0.2% | 0.1% |
| 40 A | 6.6% | 4.1% | 2.6% | 1.6% | 1.0% | 0.6% | 0.4% | 0.2% |
| 60 A | 9.9% | 6.2% | 3.9% | 2.5% | 1.5% | 1.0% | 0.6% | 0.3% |
| 100 A | 16.5% | 10.3% | 6.5% | 4.1% | 2.6% | 1.6% | 1.0% | 0.5% |
What high voltage drop actually does
Nothing burns. That is what makes it insidious — an undersized-for-drop system works, just never quite right, and the symptoms rarely point at the wiring.
- Batteries that never reach full. Drop on the controller-to-battery run means the controller sees a higher voltage than the bank actually has, ends absorption early and leaves it chronically undercharged. On lead-acid this causes sulfation and permanent capacity loss.
- Inverters cutting out early. Under load, drop on the battery cable pulls the voltage the inverter sees below its cutoff while the bank still has usable capacity. It looks like a battery problem and is not.
- Weak motor starts. Inrush current is several times running current, so the bus sags exactly when a compressor or pump needs it most.
- Heat where you cannot see it. The lost power goes somewhere. On a high-current run that means a warm cable inside a conduit or a wall.