Wire Distance Calculator

None of those texts prints a figure for a feeder alone, a solar array run to a charge controller, a well pump or motor feeder, or an EV charger circuit: for those, enter the figure your adopted code or your equipment maker prints. The New York City code prints the combined 5% as "shall not exceed 5 percent", and in its 210.19(A) and 215.2(A)(1) it prints "Informational Note No. 3: DELETED." and "Informational Note No. 2: DELETED."; the informational note itself says only that conductors sized this way "provide reasonable efficiency of operation". A motor fed through a 5%+ drop circuit gets less voltage than it was designed for. It draws more current to compensate, overheats, and fails early. Lights dim noticeably. Electronics may malfunction or refuse to operate.

1. Identify the current (amps) and distance (feet). Use the actual expected load current, not the breaker rating. If the circuit will carry 20A, use 20A, not 30A just because you have a 30A breaker. For the distance, measure the one-way run from the source (panel, battery, charge controller) to the load. The calculator doubles it internally because current must flow out and back.
2. Pick your maximum drop percentage. The table above lists the figures that are printed: 3% at the farthest outlet of a branch circuit, 5% for feeders and branch circuits combined, and below 2.5% for the drop over both cables of a DC run, as the Victron toolkit app calculates it. Where your adopted code or your equipment maker prints a figure, use theirs.
3. Read the recommended gauge from the calculator. The result is the thicker of two minimums: the smallest gauge that keeps voltage drop under your target, and the smallest gauge whose ampacity carries the current. A smaller gauge number means thicker wire. A thinner wire than the result either exceeds your drop target or carries more current than its ampacity allows; the result line says which of the two sets the gauge. Using a thicker wire reduces the drop further.
4. Know which ampacity column applies. The calculator checks ampacity against NEC Table 310.16 copper. With the terminal select at No it reads the 60 °C column for 14 through 1 AWG and the 75 °C column from 1/0 up, the terminal rule printed by Schneider Electric's data bulletin 0110DB9901, Cerro Wire and the City of Long Beach plan-review checklist. This calculator reads the terminal rule by the size of the conductor (60 °C for 14 to 1 AWG, 75 °C from 1/0 AWG); the breaker size calculator reads the same rule by the rating of the circuit. Set it to Yes only when the equipment terminals are marked 75 °C; the 75 °C column is then read for every size. On either setting no load over 15A gets 14 AWG, over 20A 12 AWG, or over 30A 10 AWG: the small-conductor limits HELUKABEL's print of the table carries with both columns. For most residential and solar applications, voltage drop is the controlling factor on long runs, so the wire will be thicker than ampacity alone requires. On short runs at high current, ampacity is the controlling factor instead. Our breaker sizing tool starts from the load instead: it sizes the breaker to the load current, then the wire to the breaker.
5. Know the temperature basis. Copper resistance rises about 4% for every 10C of conductor temperature, and this calculator already uses the fully loaded 75C value from NEC Chapter 9 Table 8. Lightly loaded wire running cool drops less than the estimate.

A common off-grid scenario: four 420W solar panels on a ground mount, 100 feet from the battery shed where the charge controller lives. The array produces about 35A at 48V (panels wired in series for a 48V string).

At 48V, a 3% drop target allows only 1.44V of loss. With 35A over 100 feet (200 feet round trip), holding the drop inside that 1.44V budget takes 2 AWG copper wire, already a thick, costly conductor. Note this is set by voltage drop, not ampacity: 35A alone would need only 8 AWG, since 10 AWG stops at its 30A small-conductor limit.

Compare the same panels wired at 24V (two parallel strings of two series panels): now they produce 70A at 24V. The 3% drop limit is just 0.72V, and holding it would take a conductor beyond a single 4/0. The calculator caps out and flags the run as impractical without parallel conductors or a rethink.

This is why experienced solar installers wire panels in high-voltage strings (often 150-600V) and use MPPT charge controllers that step down the voltage near the battery. A 150V string carrying 11.2A (the same 1,680 W) needs only 12 AWG for the same 100-foot run at 3% drop. The MPPT controller handles the conversion to 48V battery charging voltage efficiently.

If you are designing the full solar and battery system, our solar panel and battery sizing calculator helps you figure out the right number of panels and batteries. Then come back here to size the wire runs.

Already know your panel output and want to convert watts to amps for the wire calculation? Our solar watts to amps calculator does that conversion at any voltage.