Low voltage does not mean low hazard. The same power at a lower voltage produces higher current, so cable length, protection and connections become critical.
1. Convert the load into battery-side current
Begin with the loads that may run together and the inverter efficiency near that operating point. Divide output power by efficiency to estimate battery-side input power, then divide by the actual DC voltage.
Nominal voltage is not constant. Current rises when battery voltage falls for the same delivered power. Starting surges, charger operation and other DC loads need separate checks against the inverter, battery-management system and protection ratings.
2. Calculate the complete conductor loop
Current leaves the source and returns, so a simple two-conductor circuit contains both positive and negative cable lengths. The voltage-drop calculator asks for one-way length and doubles it internally.
The estimate uses copper resistivity at a reference temperature. Real resistance also comes from warmer conductors, terminals, fuses, switches, shunts and poor connections. Victron’s wiring guide recommends measuring voltage drop under high load after installation.
Victron Energy · Current, resistance and voltage drop ↗3. Select cable and protection as one design
Acceptable voltage drop does not prove acceptable ampacity. Conductor insulation, installation method, ambient temperature, bundling, termination ratings and local wiring rules all affect capacity.
Place correctly rated overcurrent protection and isolation where the applicable design requires them, and use components rated for the DC voltage and prospective fault current. Battery fault current can be severe. Final conductor and protective-device selection belongs to a qualified designer or installer.
4. Commission the system under representative load
Inspect polarity, routing, support, abrasion protection, clearances, terminal preparation and specified torque before energising. Record battery voltage and inverter-terminal voltage under a representative high load; their difference reveals total path drop.
Repeat checks after initial service and investigate heat, odour, discoloration, alarms or unexpected shutdowns immediately. Runtime should also be measured because temperature, battery age, load cycling and standby demand are not captured by the ideal calculation.
Keep these electrical checks separate
| Check | Calculator can estimate | Must still be verified |
|---|---|---|
| DC current | Load, voltage and efficiency relationship | Surge and low-voltage limits |
| Voltage drop | Copper conductor loss at steady current | Connections, temperature and installed measurement |
| Cable capacity | Not determined | Ampacity and installation rules |
| Protection | Not determined | Fuse/breaker rating and fault duty |
| Runtime | Ideal energy balance | Battery limits and real load profile |
Worked example
A 1.2 kW load on a 24 V system
Assume 90% inverter efficiency, 3 m one-way cable length and 16 mm² copper conductors.
DC input = 1,200 / 0.90 = 1,333 W Current = 1,333 / 24 = 55.6 A Cable drop = 2 × 3 × 55.6 × 0.0175 / 16 = 0.365 V (1.52%)
This screens voltage drop only. Check low-voltage current, surge, ampacity, protection, terminals and the manufacturer’s instructions before selecting the circuit.
What to have ready
- Concurrent and surge loads
- Minimum expected battery voltage
- Actual cable route and conductor material
- Applicable cable, protection and termination requirements
Common mistakes
- Sizing from nominal AC watts without inverter loss.
- Using one-way length without a return path.
- Treating voltage drop as proof of cable ampacity.
Reference check: 5 September 2026. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.