The voltage lost along a cable because of its resistance. Too much (over ~3% on the main runs) and gear under-performs or trips — which is why long runs need thicker wire.
Every cable loses a little voltage to its own resistance. Let it grow past about 3% on the main runs and gear under-performs, chargers misbehave, and you waste battery you paid for.
It scales with current and length and shrinks with thicker wire — which is exactly why long runs in a design call for a heavier gauge.
Design your van, boat, cabin or RV system in Wattonomy and it checks voltage drop on every run against a 3% budget and up-sizes the cable when a run is too long — from the appliances you actually run, sized to the recognized standard for your region. You see it on the wiring diagram, in the sized parts list, and in a plain-English build pack that explains the reasoning behind every choice. No account, no email — about a minute to a complete, validated design.
About 3% or less on the main and charging runs is the common target; critical circuits sometimes aim tighter. Wattonomy holds each run to budget.
Use thicker wire, shorten the run, or raise the system voltage. The tool up-sizes the gauge automatically when a run would exceed its budget.
It takes about a minute. No account, no email.