You cannot convert DC volts directly to amps without a third variable—either power (watts) or resistance (ohms). However, if you are sizing wire for a common 100-watt load on a 12V DC system, the direct answer is 8.33 amps (100W ÷ 12V). For a 24V system, that same 100W load drops to 4.16 amps. This guide gives you the exact formulas, a quick-reference table for voltage sag, and a decision tree to lock in your breaker and wire size without guessing.
The Core Formula (and When the Conversion is Meaningless)
Volts measure electrical pressure, while amps measure electrical flow. To bridge the two, you need to know either the work being done (watts) or the restriction in the path (ohms). The foundational equations from Ohm's Law and Watt's Law are:
- If you know Watts: Amps = Watts ÷ Volts (I = P / V)
- If you know Ohms: Amps = Volts ÷ Ohms (I = V / R)
If you only know the voltage (e.g., "I have a 12V battery") but do not know the wattage of the connected device or the resistance of the circuit, calculating amps is physically impossible. A 12V battery connected to a 5W LED draws 0.41A; that same battery connected to a 2000W inverter draws 166A. The voltage remains 12V in both scenarios. You must identify the load's power rating or measure the resistance with a multimeter before proceeding.
Worked Example: You are wiring a 120-watt DC water pump to a 12-volt nominal battery bank.
Calculation: 120W ÷ 12V = 10 amps.
This 10A figure is your baseline for selecting fuses and wire gauges.
DC vs. AC: Why 120V, 230V, and 3-Phase Don't Apply Here
A common point of confusion is asking how the amp calculation shifts when moving from 120V to 230V, or into 3-phase power. Those are strictly AC (Alternating Current) topologies. In DC (Direct Current) circuits, the math is entirely linear and immune to the penalties that complicate AC systems.
Here is how the assumptions differ between DC and AC environments:
- Power Factor (PF): In AC circuits, inductive loads (like motors) introduce a power factor (often 0.8 to 0.9), meaning you must divide by the PF to find true amps (I = P / (V × PF)). In DC, Power Factor is always exactly 1.0. There is no phase shift between voltage and current.
- 3-Phase Multipliers: AC 3-phase systems use the square root of 3 (≈1.732) in their denominator. DC systems are inherently single-phase (two wires: positive and negative).
- Voltage Levels: While AC steps up to 120V, 230V, or 480V to reduce current over long distances, DC systems in mobile, solar, and marine applications typically operate at 12V, 24V, or 48V. Because DC voltage is low, the amperage for a given wattage is drastically higher, demanding much thicker copper wire.
For context, a 2000W load at 12V DC pulls 166.6 amps. That exact same 2000W load on a 230V AC single-phase European circuit pulls only 8.69 amps (assuming a 1.0 PF). This is why DC wire sizing is so critical; the heat generated by high-amperage DC can melt undersized terminals rapidly.
Quick-Reference Table: The ±20% Voltage Sag Reality
The biggest mistake DIYers make is using the "nominal" battery voltage for their math. A 12V lead-acid or LiFePO4 battery rarely sits at exactly 12.0V. Under heavy load, voltage sags; during charging, it spikes. Because I = P / V, as voltage drops, amperage increases for constant-power loads like inverters.
Below is a spec-sheet-table showing the amp draw for common loads across a ±20% voltage range. Always size your wire for the highest amp value (which occurs at the lowest voltage).
| System State | Actual Voltage (12V Nominal) | Actual Voltage (24V Nominal) | Amps (100W Load) | Amps (500W Load) | Amps (1000W Load) |
|---|---|---|---|---|---|
| Heavy Sag (-20%) | 9.6V | 19.2V | 10.41A | 52.08A | 104.16A |
| Nominal (Baseline) | 12.0V | 24.0V | 8.33A | 41.66A | 83.33A |
| Charging (+20%) | 14.4V | 28.8V | 6.94A | 34.72A | 69.44A |
Note: If your load is purely resistive (like a DC heating element or basic LED strip without a driver), it acts as a constant resistance. In that specific edge case, current will actually drop as voltage sags, per Ohm's Law principles. However, for wire sizing, always assume constant power to maintain a safety margin.
Decision Tree: Sizing Your Wire and Breaker
Once you have calculated your baseline amps using the lowest expected voltage, you must apply the NEC continuous load rule. If the load will run for 3 hours or more, multiply your calculated amps by 1.25. Use this final number to pick your hardware from the decision matrix below.
| Calculated Max Amps | Continuous Multiplier (×1.25) | Target Breaker / Fuse Size | Minimum Copper Wire (AWG) |
|---|---|---|---|
| 1A to 8A | 1.25A to 10A | 10A or 15A | 14 AWG |
| 8.1A to 12A | 10.1A to 15A | 15A or 20A | 12 AWG |
| 12.1A to 16A | 15.1A to 20A | 20A | 12 AWG |
| 16.1A to 24A | 20.1A to 30A | 30A | 10 AWG |
| 24.1A to 32A | 30.1A to 40A | 40A | 8 AWG |
| 32.1A to 40A | 40.1A to 50A | 50A | 6 AWG |
| 40.1A to 80A | 50.1A to 100A | 100A | 2 AWG or 1/0 AWG |
Concrete Pick Example: You calculated 22 amps for a 12V fridge compressor. Because it cycles on and off, you treat it as a continuous load. 22A × 1.25 = 27.5A. Looking at the table, your target breaker is 30A, and your minimum wire size is 10 AWG THHN or marine-grade stranded copper.
Frequently Asked Questions
Can I use an AC breaker for a DC circuit?
No. AC breakers rely on the alternating current crossing zero volts 120 times a second (in a 60Hz system) to naturally extinguish the electrical arc when the contacts separate. DC current never crosses zero. If you trip a standard 120V AC breaker on a 24V DC battery bank, the arc can sustain itself, melt the breaker internals, and cause a fire. Always use DC-rated breakers (like the Blue Sea Systems MRBF or Eaton Bussmann series) or Class T fuses for DC systems.
Does temperature affect the DC amp calculation?
The calculation of how many amps the load draws does not change with ambient temperature. However, the wire's ability to carry those amps safely (ampacity) does. If your DC wiring is routed through an engine bay or a hot attic where ambient temperatures exceed 86°F (30°C), you must apply NEC Table 310.15(B)(1) temperature derating factors. For example, 10 AWG wire rated for 30A at 30°C drops to roughly 26A at 104°F (40°C). If your environment is hot, step up one wire gauge size.
How do I measure actual DC amps if my math seems wrong?
Do not rely solely on the manufacturer's sticker, which often lists "peak" or "ideal" wattage. To find the true amp draw, insert a digital multimeter in series with the circuit (for loads under 10A) or use a DC-compatible clamp meter around the positive conductor. Ensure the clamp meter is explicitly rated for DC current (using a Hall-effect sensor), as standard AC clamp meters use magnetic induction and will read 0.0A on a DC wire.
