DC wiring loss is one of the few system losses that is entirely decided before anyone turns the system on. Every foot of conductor between the modules and the inverter has resistance, and resistance turns part of your production into heat for as long as current flows. Unlike soiling or shading, it never improves with weather, never washes off, and cannot be fixed by monitoring. It is a design decision.
Why resistance costs more than it first appears
Power lost in a conductor rises with the square of the current. Double the current in the same wire and you lose four times as much. That single relationship explains most of the design choices in a photovoltaic array. It is why designers prefer longer series strings, which deliver the same power at higher voltage and lower current. It is why a fat conductor on a long run is cheaper over twenty years than a thin one. And it is why the losses on a small residential array with short runs are usually modest, while a ground mount two hundred feet from the house can bleed a meaningful fraction of its output into the earth as heat if the conductors were chosen casually. The loss is also not constant: it peaks at midday when current is highest, which is exactly when the energy is most valuable.
What actually sets the number
Four things: the current the circuit carries, the round-trip length of the conductor, the conductor’s cross-sectional area, and the conductor material. Copper has lower resistivity than aluminum for the same size, so an aluminum run needs to be larger to achieve the same drop, which is a normal and legitimate trade when the run is long and the cost difference is large. Temperature matters too, because conductor resistance rises as it heats, and conduit on a hot roof gets very hot indeed. A design calculation that ignores the operating temperature of a rooftop raceway will understate the loss. This is why designers use the correction factors in the code and the manufacturer’s tables rather than a simple resistance-per-foot figure from memory.
How designers keep the loss small
The first lever is voltage: within the limits set by cold-temperature open-circuit voltage and the inverter’s input window, longer strings mean lower current for the same power. The second is placement: put the inverter near the array rather than at the far end of the property, and accept a longer alternating-current run instead, because the alternating-current side usually operates at a higher voltage and lower current for the same power. The third is conductor size, which is chosen to satisfy both the safety requirements in the code and a separate voltage-drop target the designer sets for efficiency. Those are two different calculations with two different purposes, and the larger of the two results governs. A conductor can be perfectly safe and still be a poor economic choice.
Where the loss hides in practice
Not all resistance is in the wire itself. Every connector, terminal, splice and lug is a potential resistance, and a marginal crimp or a mismatched connector pair can develop far more drop than a long run of cable. Those points also heat, and heat accelerates the degradation that created the problem. Corroded connections in a rooftop combiner, a lug torqued by feel rather than by specification, or a mix of connector brands that mate mechanically but not reliably will all show up as lost production and, eventually, as a thermal event. This is a good reason for a thermal scan at commissioning and again a few years in: a hot spot at a termination is visible long before it becomes a failure.
Modeling it honestly before you sign
Production models include a wiring loss assumption, and installers sometimes leave it at a default that does not match the actual run. Ask what loss percentage the model used for direct-current wiring and how the conductor size was chosen. For a long ground-mount run in particular, ask for the voltage-drop calculation at maximum current and at the expected conductor operating temperature. Upsizing conductors is cheap while the trench is open and expensive afterward. If the array might be expanded later, sizing the conduit and conductors for the future system now is almost always the right call.
What to check before you act
- Ask what wiring loss percentage the production model assumed.
- Request the voltage-drop calculation for any run over a modest distance.
- Place the inverter near the array where the site allows it.
- Have terminations thermally scanned at commissioning and a few years later.
Common questions
Is a small voltage drop worth upsizing conductors for?
On short residential runs, usually not. On long ground-mount runs, the energy lost over decades commonly exceeds the one-time cost of the larger conductor, so run the numbers rather than guessing.
Can I measure my wiring loss myself?
Measuring drop means working on energized direct-current circuits, which do not self-extinguish an arc. Have a qualified installer take the measurement with proper test equipment.
The practical takeaway
Treat direct-current wiring loss as a permanent design choice, not a maintenance item. Ask for the voltage-drop calculation, keep the inverter close to the array, size conduit generously while the work is open, and get terminations scanned. All conductor work on a photovoltaic system belongs to a qualified installer.
References and further reading
- NREL PVWatts: production modeling including system loss assumptions
- NFPA: how the National Electrical Code addresses conductor sizing and protection
- NREL: photovoltaics research including balance-of-system performance
Numerical scenarios are illustrative unless identified otherwise. Follow the exact product instructions; component ratings and local installation requirements can differ.
