Harness Voltage Drop Is a Geometry Problem

The number everybody computes

Voltage drop on a DC feed is not difficult. Resistance per unit length comes from the conductor gauge, you double the length because current has to return, and Ohm's law does the rest:

drop_V = current_A × resistance_per_m × length_m × 2

Every term on the right is easy to look up except one. Gauge is a design decision. Current is a requirement. Length is geometry, and geometry is the term that models routinely get wrong, not by a little, and always in the same direction.

Straight lines are not wire

If a model stores a connection as a pair of endpoints, the only length available is the straight-line distance between them. No harness has ever taken that path. Wire follows structure: around the inside of a shell, along a chassis rail, through a grommet, down the side that has the tie-down points rather than the side that is shorter.

The straight-line figure is therefore not an estimate of the run. It is a strict lower bound on it, and a bound that gets looser exactly when the routing gets constrained, which is to say, on the runs that matter.

The fix is unglamorous: store the route. A connection carries an ordered list of waypoints in world coordinates, and its length is the polyline through its endpoints and those waypoints, plus a stated slack for service loops and strain relief.

length = Σ |pᵢ₊₁ − pᵢ|  over the routed polyline
       + slack_m

The point is not that the polyline is sophisticated. It is that it is recorded, so the length is a measurement of a decision somebody made rather than a consequence of the model's storage format.

A worked case

A roof-mounted sensor pod. The compute module draws 22 A from a 12 V distribution unit, and the interface control document allows a 5 % drop.

The two parts are about 0.6 m apart inside the shell. Straight-line, on AWG 14, that is comfortably inside budget and nobody looks again.

The routed run is 3.29 m, because the feed goes the long way round the pod to stay clear of the lidar and pick up the existing tie-downs. At 22 A that is a 1.2 V drop, 9.99 % of nominal, against a 5 % limit. The requirement is violated by a factor of two, and the only thing that changed was the path.

Length used	Drop at 22 A, AWG 14	Against a 5 % limit
Straight line, ~0.6 m	~1.8 %	Passes comfortably
Routed polyline, 3.29 m	9.99 %	Fails by 2×

The part that surprises people

The drop is only half of what that routing decision costs. The same resistance that produces the voltage drop is also dissipating power, and it has to come from somewhere upstream:

loss_W = current_A² × resistance_per_m × length_m × 2

On this run that is 26.39 W of the pod's 26.77 W total harness loss, one feed accounting for essentially all of it, because loss goes as the square of current and this is the only run carrying tens of amps.

Now the power budget. The pod's components draw 316 W in autonomy mode against a 320 W allocation. That passes. Add the harness and the source has to supply 342.8 W, which does not.

A power budget that counts only loads and not conductors would have cleared this design. The two failures, the interface drop limit and the system power budget, have a single cause, and it is a routing choice that no electrical spreadsheet was tracking.

Why this belongs in the geometry model

The reason to compute drop where the parts live, rather than in a separate electrical tool, is that every input is already there and every output feeds something else that is already there.

• Route length falls out of the same coordinates that drive clearance.
• Copper mass falls out of gauge and length, and lands in the mass roll-up and the centre of gravity.
• Dissipated watts land in the power budget and in the thermal screen, as heat at a location.
• Moving a part re-routes its feeds, so the drop updates while you drag it.

Split those across tools and each one is individually correct while the system is wrong. Keep them in one model and moving the power distribution unit 200 mm visibly changes the drop, the loss, the budget and the CG at once, which is the actual trade being made.

What to take from it

If you record only the endpoints of a connection, you have not modelled the harness; you have modelled a wish about the harness. The gap between the two is where the drop lives.

And when a limit is exceeded, the finding is worth nothing unless it says by how much and over what. “Voltage drop exceeded” sends someone hunting. “Compute power feed drops 1.2 V of 12 V, 9.99 % against a 5 % limit, over 3.29 m of AWG 14” tells them the run, the path length, the gauge and the margin, which is enough to decide between a heavier conductor and a shorter route without opening anything else.