Voltage drop steals headroom at the load—especially on 12 V and long runs. This guide shows how to calculate wire gauge for DC voltage drop from current, cable length, and bus voltage so inverters and controllers see stable DC.
Benefits
- Targets ~3% drop as a common DC planning limit.
- Uses round-trip resistance for out-and-back conductors.
- Pairs ampacity check with drop on the recommended AWG.
How it works
- Enter DC load current and one-way cable distance.
- Add system voltage (12 V, 24 V, 48 V, etc.).
- Read AWG recommendation and estimated drop percent.
FAQ
How do I calculate wire gauge for DC voltage drop?
Drop % ≈ (2 × I × R × length) ÷ V × 100 for round-trip copper (I in amps, length one-way in feet, V system voltage). Pick the smallest AWG whose resistance keeps drop near 3%. The DC Cable Size tool applies ampacity tables first, then reports drop for the suggested gauge.
Why is 3% drop common for DC wiring?
On 12 V, 3% is ~0.36 V—meaningful for inverter low-voltage cutoff. On 48 V, 3% allows ~1.4 V loss with more headroom. Tighter drops (1–2%) help sensitive loads; longer runs or high current often need larger AWG than ampacity alone.
Does temperature affect gauge choice for drop?
Copper resistance rises when hot—field runs in engine bays or roof conduit see higher drop than cold calculations. If measured drop is borderline, step up one AWG or shorten the run. For temperature-adjusted resistance math, see Conductor Resistance at Temperature.
Technical specifications
- Drop % ∝ I × R_wire × length / V.
- Round-trip: double one-way length for + and −.
- Planning target: ~3% on many DC installs.
- Related: dc-cable-size, dc-cable-voltage-drop, conductor-resistance-temperature.
Drop is current times resistance over distance
To calculate wire gauge for DC voltage drop, model the loop: amps through copper resistance over round-trip length. Thin wire on a long 12 V feed can lose a volt before the inverter—enough to throttle output or trip BMS warnings. Higher system voltage tolerates the same absolute volt loss as a smaller percentage.
Gauge up until drop fits the budget
Start from load amps, then try standard AWG sizes until drop falls near your target—often 3% for DC planning. Ampacity still matters: a gauge that passes drop but not current is unsafe. The calculator balances both so the recommended AWG carries the amps and reports approximate drop for documentation.
Document drop on the as-built sketch
Note calculated drop % on one-line diagrams beside fuse ratings and cable lengths. If field measurements exceed estimates, check connections, crimps, and parallel path length. For array homeruns with MPPT input limits, cross-check with DC Cable Size & Voltage Drop when you need drop on a pre-selected spool gauge.