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Convert voltage to current (amps) using resistance or power. Calculate amperage using Ohm's Law or power formulas.
I(A) = V(V) ÷ R(Ω)
Current equals voltage divided by resistance
I(A) = P(W) ÷ V(V)
Current equals power divided by voltage
| Voltage | @ 1Ω | @ 5Ω | @ 10Ω | @ 100Ω |
|---|---|---|---|---|
| 12V | 12 A | 2.4 A | 1.2 A | 0.12 A |
| 24V | 24 A | 4.8 A | 2.4 A | 0.24 A |
| 120V | 120 A | 24 A | 12 A | 1.2 A |
| 240V | 240 A | 48 A | 24 A | 2.4 A |
Volts to amps conversion calculates electrical current (amperes) from voltage (volts) using either resistance or power as a second known variable. This calculation is based on Ohm's Law, one of the most fundamental relationships in electrical engineering, formulated by Georg Simon Ohm in 1827. The law states that current flowing through a conductor is directly proportional to the voltage across it and inversely proportional to the resistance: I = V ÷ R. Alternatively, if you know the power consumed, you can use the power formula: I = P ÷ V. These relationships are essential for sizing conductors per NEC Table 310.16, selecting circuit breakers, calculating voltage drop, and designing safe electrical circuits. In AC systems, the calculation must also account for power factor and whether the system is single-phase or three-phase, which affects the current by a factor of √3 for three-phase loads.
You need the voltage plus either the resistance (in ohms) or the power (in watts). Voltage alone cannot determine current — you always need a second electrical quantity.
If you know resistance: I = V ÷ R (Ohm's Law). If you know power: I = P ÷ V. For three-phase AC: I = P ÷ (V × √3 × PF). Select the formula that matches your known quantities.
Perform the division. For example, a 240V circuit with a 24Ω load draws 240 ÷ 24 = 10 amps. Or a 1800W appliance on 120V draws 1800 ÷ 120 = 15 amps.
Use the calculated amperage to select the correct wire gauge per NEC Table 310.16 and choose an appropriate circuit breaker. Per NEC 210.20, continuous loads should not exceed 80% of the overcurrent device rating.
Current (amps) is what creates heat in conductors and can cause electrical fires. Knowing the exact amperage ensures proper wire sizing per NEC and prevents dangerous overloading of circuits and connections.
Circuit breakers are rated in amps. Calculating the current draw of your loads ensures you select the right breaker size — large enough to avoid nuisance trips but small enough to protect the wiring.
Voltage drop in long cable runs depends on current (amps), wire gauge, and distance. Accurate amperage calculations prevent excessive voltage drop that can cause equipment malfunction, especially in 12V and 24V DC systems.
| Wire Gauge (AWG) | Copper @ 60°C | Copper @ 75°C | Typical Breaker Size | Common Use |
|---|---|---|---|---|
| 14 AWG | 15 A | 20 A | 15 A | General lighting circuits |
| 12 AWG | 20 A | 25 A | 20 A | Kitchen, bathroom outlets |
| 10 AWG | 30 A | 35 A | 30 A | Electric dryer, water heater |
| 8 AWG | 40 A | 50 A | 40 A | Electric range, EV charger |
| 6 AWG | 55 A | 65 A | 60 A | Subpanels, large appliances |
| 4 AWG | 70 A | 85 A | 70 A | Service entrance feeders |
| 2 AWG | 95 A | 115 A | 100 A | 100A service entrance |
* Values per NEC Table 310.16 for single conductors in raceway. Consult local codes for specific installations.
You need a second variable besides voltage. With resistance, use Ohm's Law: Amps = Volts ÷ Ohms. With power, use: Amps = Watts ÷ Volts. For example, a 120V circuit with a 12Ω load draws 10 amps, and a 1200W device on 120V also draws 10 amps. Voltage alone cannot be converted to amps without knowing resistance or power.
Standard US household circuits are either 15A or 20A at 120V. Per NEC Section 210.20, continuous loads should not exceed 80% of the circuit rating (12A for a 15A breaker, 16A for a 20A breaker). Kitchen countertop and bathroom receptacles require dedicated 20A circuits per NEC 210.11(C).
The amperage determines the required wire gauge per NEC Table 310.16. Higher current requires thicker wire to prevent overheating. For example, 15A requires 14 AWG minimum, 20A requires 12 AWG, and 30A requires 10 AWG (copper at 60°C). Using undersized wire is a fire hazard and NEC violation.
Power (watts) equals voltage times current. If an appliance uses 2400 watts, it draws 20 amps on a 120V circuit but only 10 amps on a 240V circuit (2400 ÷ 240 = 10). This is why large appliances like dryers and stoves use 240V — the lower current allows smaller, less expensive wiring.
For three-phase motors: Amps = Watts ÷ (Volts × √3 × Power Factor × Efficiency). For example, a 10 HP motor (7460W) at 480V, 0.85 PF, and 90% efficiency draws 7460 ÷ (480 × 1.732 × 0.85 × 0.90) = 11.7 amps per phase. Always use NEC Table 430.250 for motor full-load currents when sizing overcurrent protection.