At the bench, you rarely need to derive formulas from scratch. You need to know exactly how much current a 12V LED strip will pull, what resistance is required to limit a 5V GPIO pin to 20mA, or how much heat a 48V e-bike motor controller will dump into a load resistor. While the fundamental equations are simple—V = I × R and P = V × I—translating them into real-world component selection requires a reliable lookup reference.
This ohms law chart provides exact calculated values for the most common DC system voltages used in automotive, solar, telecom, and embedded projects. Keep this page bookmarked to eliminate mental math errors during your next build.
The Practical Ohms Law Chart: DC System Lookup
How to read this table: The Nominal Voltage column is based on standard IEC 60038 system voltages, representing the expected operating baseline (not the absolute maximum charging voltage). The Required Resistance column gives the exact ohmic load needed to draw the target current. The Power Dissipation column shows the real power (in Watts) converted to heat or work. Assumption: This table assumes a purely resistive DC load at a 25°C ambient temperature. For wire sizing to carry these currents, you must separately consult NEC Table 310.16 ampacity charts; Ohm's law calculates the load, not the conductor capacity.
| Nominal Voltage (V) | Target Current (A) | Required Resistance (Ω) | Power Dissipation (W) | Common Application |
|---|---|---|---|---|
| 5V (USB) | 1.0 A | 5.0 Ω | 5.0 W | Raspberry Pi 4 under heavy load |
| 5V (USB) | 2.0 A | 2.5 Ω | 10.0 W | High-power USB-C PD hub |
| 12V (Auto/Solar) | 1.0 A | 12.0 Ω | 12.0 W | Small cooling fan / relay coil |
| 12V (Auto/Solar) | 5.0 A | 2.4 Ω | 60.0 W | 5-meter 5050 LED strip |
| 12V (Auto/Solar) | 10.0 A | 1.2 Ω | 120.0 W | Portable 12V compressor fridge |
| 12V (Auto/Solar) | 20.0 A | 0.6 Ω | 240.0 W | Automotive inverter (light load) |
| 24V (Industrial) | 5.0 A | 4.8 Ω | 120.0 W | 24V DC stepper motor driver |
| 24V (Industrial) | 10.0 A | 2.4 Ω | 240.0 W | Heavy-duty linear actuator |
| 24V (Industrial) | 20.0 A | 1.2 Ω | 480.0 W | 24V solar charge controller feed |
| 48V (Telecom/PoE) | 5.0 A | 9.6 Ω | 240.0 W | PoE++ high-power PTZ camera |
| 48V (Telecom/PoE) | 10.0 A | 4.8 Ω | 480.0 W | Light electric vehicle (scooter) |
| 48V (Telecom/PoE) | 20.0 A | 2.4 Ω | 960.0 W | Server rack battery backup (UPS) |
Applying the Chart: Derating, Columns, and Real-World Limits
A chart is only as good as the assumptions behind it. Before you order components or size fuses based on the numbers above, you must adjust for your specific physical environment and electrical topology.
Which Column Applies to Your Installation?
If you are wiring a DC battery bank, automotive accessory, or solar array, use the Nominal Voltage and Required Resistance columns exactly as written. However, if you are attempting to apply this chart to an AC mains circuit (like a 120V or 230V AC branch), stop. This table is strictly for DC. For AC circuits, you must use the AC RMS voltage and calculate Impedance (Z) rather than pure Resistance (R), factoring in the load's Power Factor (PF). A 120V AC motor drawing 5A does not present 24Ω of pure resistance; its impedance is 24Ω, but its true resistive heating will be lower due to inductive reactance.
How Derating Modifies the Base Power Value
The Power Dissipation (W) column assumes a standard 25°C to 70°C ambient environment. According to IEC 60115-1 standards for fixed resistors, a component's power rating must be linearly derated as ambient temperature rises above its rated threshold (usually 70°C), reaching 0W capacity at its maximum operating temperature (typically 155°C).
For example, if you are installing a 12V dummy load inside an enclosed automotive engine bay where ambient temperatures hit 105°C, a standard 60W rated resistor will safely dissipate only about 35W before thermal failure. Always multiply the table's base wattage by the manufacturer's derating curve for your specific enclosure temperature.
What the Table Cannot Tell You
This chart assumes a purely resistive, steady-state load. It cannot predict inrush current. When you first energize a cold incandescent bulb, a transformer, or a power supply with large input capacitors, the initial current spike can be 10 to 50 times higher than the steady-state value listed in the Target Current column. Furthermore, it does not account for inductive kickback. If your 12V 5A load is a relay coil or a DC motor, opening the circuit will generate a massive voltage spike that defies Ohm's law, requiring a flyback diode to protect your switching transistor.
Ohms Law Chart FAQ: Long-Tail Questions Answered
How do I use an ohms law chart for AC circuits instead of DC?
To adapt Ohm's law for AC, replace Resistance (R) with Impedance (Z), which combines resistance (R), inductive reactance (X_L), and capacitive reactance (X_C). The formula becomes V = I × Z. Furthermore, you must use the RMS (Root Mean Square) voltage, not the peak voltage. For standard US 120V AC mains, the peak voltage is actually ~170V (120 × √2). For a deeper breakdown of AC power calculations, refer to Georgia State University's HyperPhysics electric power module.
Why does my calculated wattage not match the breaker trip curve?
Ohm's law calculates the theoretical steady-state thermal output of a load, but a circuit breaker responds to thermal-magnetic trip curves, not instantaneous wattage. A 120V circuit drawing 15A calculates to 1800W. However, a standard 15A thermal breaker might hold 18A (2160W) for several minutes before the bimetallic strip bends enough to trip. Conversely, the magnetic trip mechanism will instantly open the circuit at 10x to 20x the rated current (150A-300A) during a dead short, regardless of the steady-state wattage calculation.
What is the difference between the PIE chart and the VIR chart?
Both are visual mnemonics for the same underlying physics, divided by what you are trying to solve. The VIR chart (Voltage, Current, Resistance) is used to find missing circuit parameters using V = I × R. The PIE chart (Power, Current, EMF/Voltage) is used when calculating energy consumption and heat using P = I × V. In practice, makers combine them into a single 12-spoke 'Ohm's Law Wheel' to solve for any of the four variables (V, I, R, P) when the other two are known.
