Voltage Drop in 12V Systems

Why 12V systems are so voltage-drop-sensitive

Voltage drop scales with current and length, not with system voltage - but the percentage impact is inversely proportional to your baseline voltage. The same physical current and cable length that causes a barely-noticeable 1% drop on a 230V circuit can easily exceed 10% on a 12V circuit, enough to dim lights, cause water pumps to underperform, and confuse battery monitoring electronics.

How much drop is acceptable in a 12V system

A 3% drop target - the same recommendation used for AC branch circuits - translates to only 0.36V on a 12V system, an extremely tight budget for any meaningful cable run. Many mobile/RV electrical designers work to a slightly more permissive 5-10% target for non-critical lighting circuits while holding to 3% or better for sensitive electronics and charging circuits.

Why cable runs matter more than you'd expect

A 12V system with the battery bank at one end of a van and a load at the other end may have a combined (out-and-back) run of 10-15 meters even in a relatively small vehicle - use the Voltage Drop Calculator with your actual current draw and full round-trip length (not just the one-way distance to the load) to check real-world performance.

Sizing wire generously pays for itself

Because the acceptable voltage budget is so small in absolute terms, 12V systems often need conductors one or two gauge sizes larger than the same current would require on a 120V/230V circuit - the extra copper cost is minor compared to the performance and battery-monitoring accuracy problems caused by chronic under-sizing.

Frequently Asked Questions