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Key takeaways
- Voltage drop is the voltage lost along a cable. It grows with cable length and with current, and shrinks as cable gets thicker.
- The run between charge controller and battery is where it often hurts, because that part of the circuit carries the most current at the lowest voltage.
- You can check it yourself by reading voltage at both ends of a cable while the circuit is under load and comparing the two readings.
- Use the cable size tables in your controller, inverter and battery manuals for your run length. Do not copy a size from a forum or a blog post, including this one.
- Poor lugs and loose connections add the same kind of loss as a long cable, so fix the terminations before you buy thicker wire.
What voltage drop actually is
Copper is a good conductor, not a perfect one. Push current through a cable and some voltage is used up along the way, which shows up as a little heat in the cable. The equipment at the far end then sees a lower voltage than the source supplies. The longer the cable, the thinner the cable and the more current it carries, the bigger that loss gets.
Victron's wiring guide, Wiring Unlimited, lays out the same relationships. Resistance rises with cable length, thinner cable has more resistance than thicker cable, and voltage drop rises with current, so it is worst at full charging current or maximum inverter load. The same guide notes that a higher system voltage carries the same load at a lower current, which lowers the drop.
Note that the circuit is a loop. Current travels out on the positive cable and back on the negative one, so both conductors count toward the loss, not just the one you can see.
Where long runs hurt in an RV solar system
An RV solar system has three stretches of cable, and each one behaves differently.
| Stretch | What it carries | Why length matters |
|---|---|---|
| Panels to controller | Panel-side voltage, which is higher when panels are wired in series | Higher voltage means lower current for the same power, so this stretch tolerates length better. Your controller manual sets the limits. |
| Controller to battery | Battery-side voltage at the controller's full charging current | Low voltage and high current make this stretch the most sensitive to length. A long run here can cost charging performance. |
| Battery to inverter | The inverter's full load, in bursts or for long stretches | Very high current for its voltage. Keep it as short and direct as the layout allows, and follow the inverter manual. |
Many installers place the charge controller close to the battery so the longer run falls on the panel side. Whether that suits your rig depends on your controller's documented input and output limits, so read the manual before you choose where it goes. Our series versus parallel wiring guide explains how panel wiring changes the voltage on the panel-side cable, and the 12V versus 24V comparison covers the system-voltage question.
Big coaches, fifth wheels and skoolies are the usual victims. A panel on the front of the roof and a battery bay in the rear basement can mean a route that is much longer than the straight line between them, once you add bends, roof entry and wall chases.
Signs your cable runs are costing you
Voltage drop rarely announces itself. Look for these hints and then measure.
- Charging seems weaker than the panels and controller should deliver in good sun, with no shade or dirt to explain it.
- The controller's display shows a different battery voltage than a meter does at the battery terminals while charging.
- The inverter throws a low-voltage warning when a big load starts, even though the battery reads healthy at rest.
- A cable, lug or fuse holder feels warm when the circuit is under load.
- A new bigger inverter or controller does not behave as the box promised.
A warm connection is a stop-and-investigate signal, not a curiosity. Heat at a lug usually means a poor connection, an undersized cable or both. Switch off the load and have a qualified RV technician or electrician inspect it.
How to check it with a meter
You do not need a calculator to see whether a run has a problem. You need a decent multimeter and a safe way to load the circuit.
- Read your equipment manuals first to see where their voltage and current limits sit, and where the maker says to measure.
- With the circuit running under a real load, such as the controller charging in good sun or a modest load on the inverter, read the voltage at the source end of the cable.
- Read the voltage at the far end of the same cable, at the same moment if you can, using the same meter.
- Subtract one reading from the other. That difference is the drop across that run.
- Compare it with the target in your manual. If the manual gives none, ask the equipment maker or a qualified technician what they accept.
Victron's guide advises keeping voltage drop to 2.5 percent or less. That is one maker's recommendation for its own wiring guidance, and other makers may set a different target. The point is to have a documented target for your equipment, then measure against it.
Take care when you measure. Panels produce voltage whenever light hits them. One panel maker's installation page says to keep your hands off the terminals while the panel is lit and to cover the panel face with an opaque material when you work on wiring. Meter leads on high-current terminals can short them if they slip, so if you are not comfortable, hire a professional.
Ways to cut voltage drop, and what each one costs
No single change is right for every rig. These are the levers, with their trade-offs.
| Lever | What it does | Trade-off |
|---|---|---|
| Shorten the route | Less cable means less resistance | May mean moving the controller or inverter, and the move has to suit the manual and the layout. |
| Use thicker cable | Lower resistance for the same length | Stiffer to route, larger lugs, a bigger hole in the roof entry gland, and cost. |
| Raise system voltage | Same power at lower current | Affects every component you own. A bigger decision than a cable swap. |
| Run two cables together | Adds cross-section when one thick cable is hard to get | Victron notes this works if the combined cross-section meets the recommendation. Check your maker's wording. |
| Fix the connections | Removes loss at lugs, busbars and fuse holders | Needs the right crimp tool and a careful hand. |
Choose cable size from the tables in your controller, inverter or battery manual, which are written for the equipment you own. Our RV solar wiring guide and the guide to solar extension cables help with the layout and the parts, but the numbers belong to your manuals and a qualified RV technician or electrician.
Do not forget the connections
Cable length gets the blame, but a bad termination can waste as much power as a long run. Victron's guide recommends a ratcheting crimp tool with the correct die, a gentle tug on every crimped joint and eye lugs that match the cable, and it advises against bullet and butt splice terminals because they often make poor contact.
Use that as a reason to inspect, not to improvise. Look for lugs that spin on the cable, discolored insulation, fuse holders that are loose and terminals that are not tight to the maker's instruction. Our guide to MC4 connectors and solar tools covers the panel-side connectors, and the multimeter and clamp meter guide helps pick a meter for the job.
Frequently asked questions
What is voltage drop in RV solar wiring?
It is the voltage lost along a cable because the cable has resistance. It grows with cable length and current, and it falls as cable gets thicker.
Which part of an RV solar system is most sensitive to long cable runs?
The stretch between the charge controller and the battery is usually sensitive, because it carries the highest current at the lowest voltage. The battery-to-inverter run is also demanding. Your equipment manuals give the limits.
How do I measure voltage drop?
Read the voltage at both ends of the same cable while the circuit is under a real load, then subtract. Compare the difference with the target in your equipment manual, and ask a qualified technician if you are unsure.
Does a higher system voltage reduce voltage drop?
Yes. For the same power, a higher voltage means lower current, which lowers the drop, according to Victron's wiring guide. Changing system voltage affects every component, so treat it as a design decision.
Should I just buy the thickest cable I can fit?
Not without checking. Thicker cable is harder to route and needs matching lugs and a suitable crimp tool. Follow the cable sizes in your controller, inverter and battery manuals and have a qualified technician review the run.
Sources and verification
- Victron Energy, Wiring Unlimited: resistance rises with cable length and falls with thicker cable, voltage drop rises with current, higher system voltage lowers current for the same load, a 2.5 percent drop target as that guide's recommendation, parallel cables when combined cross-section meets the recommendation, ratcheting crimp tool and tug test, eye lugs sized to the cable, caution about bullet and butt splice terminals.
- Go Power! support, cautions and safety for solar kits: panels generate DC electricity in light, do not touch terminals while lit, cover panel faces with an opaque material when working on wiring.
No cable gauge, fuse size, amperage or length figures are given beyond the quoted percentage. Your controller, inverter and battery manuals set cable sizes for your run, and local code and a qualified RV technician or electrician govern the installation. Remarks about warm connections and about where installers place controllers are general explanations, not sourced figures.