Solar design tools
Voltage drop calculator
Pick the equipment and drag it along the wall. The dashed line is the farthest you can mount it and stay inside your target — go past it and the run turns red, with the wire size that fixes it.
The circuit
- Farthest you can mount it
- Smallest wire for this run
- Battery placement
Show the math with your numbers
Checks voltage drop only. Size the conductor for ampacity first — inverter output and EV charging circuits are sized at 125% of continuous current — then check the drop. Battery placement is checked along the wall only. A design aid, not a sealed engineering document; confirm against the code edition your jurisdiction adopted.
How the calculator works
Voltage drop is the voltage lost in the wire between the panel and the equipment. For a single-phase or DC circuit it is twice the one-way length — the current flows out and back — times the conductor's resistance, times the current:
| Circuit | Voltage drop |
|---|---|
| Single-phase or DC | VD = 2 × L × R × I |
| Three-phase | VD = √3 × L × R × I |
L is the one-way length in feet, R the resistance in ohms per foot, and I the current in amps. Resistance comes from NEC Chapter 9, Table 8, for stranded uncoated copper or aluminum at 75 °C. The percentage is the drop divided by the system voltage.
That percentage is also the share of power lost: at full current, a 2% drop means about 2% of the power is turned into heat in the wire instead of reaching the panel.
How much voltage drop is acceptable?
For most circuits the NEC doesn't set a hard limit. Informational notes recommend about 3% on a branch circuit or feeder and 5% overall — recommendations, not requirements. Many solar designers hold the inverter output circuit to 2% or less, and some jurisdictions and utilities set their own limits, so check what yours expects.
Why it matters more for inverters
On a load, voltage drop means less voltage at the equipment. On an inverter it works the other way: to push power back to the grid, the inverter has to raise its own voltage above the panel's by the amount lost in the wire. If the utility voltage already runs high, that extra rise can push the inverter past its overvoltage limit, and it disconnects — usually on the sunniest afternoons, when production should be at its best. A home battery exporting through its own inverter behaves the same way.
Battery placement
Pick Battery and the drawing adds a door and a window with a 3-foot clearance around each. Under the 2021 International Residential Code, section R328, a battery mounted outdoors or on an exterior wall has to be at least 3 feet from doors and windows that open into the home. The calculator checks that distance along the wall; a battery well below a window may still comply, so confirm with your jurisdiction. See battery storage design for the location and capacity limits.