If you are sizing wiring and fuses for a 600W DC DC converter power supply on a 12V nominal system, the direct answer is 55.0 Amps at worst-case low voltage (11.5V), or 52.6 Amps at a standard lead-acid nominal voltage of 12.0V (assuming 95% efficiency). For a 24V nominal system, those numbers halve to 27.5 Amps and 26.3 Amps, respectively. The formula used is I = P / (V × η). Substituting the worst-case 12V values: 600W / (11.5V × 0.95) = 55.0A. Always size your input wire and overcurrent protection for the lowest expected voltage, because as voltage drops, current must increase to deliver the same wattage.

The Core Formula and the Voltage Assumption

The conversion from Watts to Amps in a DC-DC converter is entirely fixed by two assumptions: input voltage and conversion efficiency (η). Unlike resistive loads where current remains static, a DC-DC converter is a constant-power load. If your battery bank sags under heavy load, the converter pulls more amps to maintain its 600W output.

⚠️ Wire Sizing Warning: Never use the 'nominal' voltage (12.8V for LiFePO4 or 12.0V for Lead-Acid) to size your input fuses. Use the inverter or converter's low-voltage cutoff. For most 12V units, this is 11.0V to 11.5V. At 11.0V and 95% efficiency, a 600W unit pulls 57.4 Amps. Your wire ampacity and fuse must exceed this worst-case scenario.

Efficiency is the second critical assumption. High-frequency switching converters from top-tier manufacturers like Texas Instruments or Victron easily hit 96-98% efficiency. However, cheaper unbranded buck/boost modules found on Amazon often operate at 85-90% efficiency, which drastically increases input current and heat generation.

Real-World DC-DC Converter Power Supply Specifications

Before finalizing your wire gauge, check the specific data sheet of your unit. Here is a spec-sheet-table of common DC-DC converters in the 400W–700W range, showing how real-world efficiency shifts the input current requirements.

Manufacturer & Model Max Output Power Peak Efficiency Input Voltage Range Max Input Current (Worst Case) Recommended Input Fuse
Victron Orion-Tr Smart 12/12-30 400W (30A out) 98% 8V - 17V 51.0A @ 8.0V 60A ANL
Renogy 60A DC-DC Charger 720W (60A out) 96% 9V - 16V 83.3A @ 9.0V 100A ANL
Samlex SDC-600-12 (Isolated) 600W 90% 10V - 16V 66.6A @ 10.0V 80A ANL
EPSOLAR 40A IP68 DC-DC 480W (40A out) 95% 9V - 17V 56.1A @ 9.0V 70A ANL

Note: Max input current is calculated at the absolute lowest specified input voltage to ensure safe wire and fuse sizing. Data sourced from manufacturer Victron Energy and Renogy technical manuals.

Sizing Chart: ±20% Power Range for 12V and 24V Systems

If you are scaling your system up or down, or if your 600W load is actually a 500W continuous draw with a 700W surge, use this neighboring values table. This assumes a standard 95% conversion efficiency.

Output Power (W) Input Amps @ 12.0V (Lead-Acid Nominal) Input Amps @ 11.5V (Low Cutoff) Input Amps @ 24.0V (24V Nominal) Input Amps @ 23.0V (24V Low Cutoff)
480W (-20%) 42.1 A 43.9 A 21.1 A 22.0 A
540W (-10%) 47.4 A 49.4 A 23.7 A 24.7 A
600W (Base) 52.6 A 55.0 A 26.3 A 27.5 A
660W (+10%) 57.9 A 60.5 A 28.9 A 30.2 A
720W (+20%) 63.2 A 66.0 A 31.6 A 33.0 A
💡 Wire Gauge Quick Pick: For the base 600W / 55A scenario on a 12V system, 6 AWG THHN in conduit is sufficient (rated 65A at 75°C). If you are using NM-B (Romex) inside a wall or enclosure, you must step up to 4 AWG, as NM-B is limited to the 60°C column (55A max for 6 AWG, which leaves no safety margin).

Crossing the AC Boundary: 120V, 230V, and 3-Phase Shifts

A pure DC-DC converter power supply does not interact with AC mains. However, if you are sizing an AC-DC power supply to feed your DC loads, or calculating the AC input draw of an inverter connected to your DC bus, the math shifts dramatically based on the AC grid standard.

When AC enters the picture, the assumption that fixes the answer changes from simple efficiency to Power Factor (PF) and Phase Count.

  • 120V Single-Phase (US/Canada): The formula becomes I = P / (V × PF × η). For a 600W load on 120V with a 0.90 PF and 95% efficiency, the AC draw is 600 / (120 × 0.90 × 0.95) = 5.8 Amps.
  • 230V Single-Phase (EU/UK/AU): Using the same 600W load, the current drops to 600 / (230 × 0.90 × 0.95) = 3.0 Amps. This is why 230V systems can use much thinner input wiring for the same power delivery.
  • 400V 3-Phase (Industrial): For 3-phase AC-DC rectifiers, the formula incorporates the square root of 3: I = P / (√3 × V_line × PF × η). A 6000W (6kW) 3-phase supply pulls only about 10.1 Amps per leg, vastly reducing conductor sizing compared to single-phase.

Frequently Asked Questions

When is the Watts-to-Amps conversion meaningless?
The conversion becomes useless if you do not know the Power Factor (PF) on the AC side of a power supply, or if the efficiency (η) of the DC-DC converter is undocumented. Cheap, unbranded DC-DC buck converters often lack published efficiency curves. If η is unknown, you must assume a worst-case 80% efficiency to prevent undersizing your wire, which would otherwise result in melted terminal lugs or voltage drop brownouts.

Does the output voltage of the DC-DC converter change the input amp calculation?
No. The input current is dictated strictly by the total output wattage and the input voltage. Whether your 600W DC-DC converter is stepping 12V up to 24V, or stepping 24V down to 12V, the input side only 'sees' the total power demanded plus the efficiency loss. A 600W output at 95% efficiency always requires 631W of input power, regardless of the output voltage target.