Volts to Amps: How to Calculate Current from Voltage
Volts alone can't make amps — add watts or ohms and the calculator does the rest, with NEC breaker sizing built in.
"How many amps is 240 volts?" is the most-asked impossible question in the trade. Voltage is pressure, not flow — the same 240 V circuit carries 8 A for a water heater element and 40 A for a range. To get amps from volts you need one more quantity: the load's wattage (the common case) or its resistance (Ohm's-law case). The calculator below handles both paths instantly. This is the sibling of our amps-to-watts guide — same power law, run in the opposite direction.
⚡ Volts to Amps Calculator
Volts alone can't make amps — add either the wattage (most common) or the resistance, and get the current instantly with breaker sizing.
From watts: A = W ÷ V · From ohms: A = V ÷ Ω
Instant results as you type. Breaker suggestion = NEC 125% continuous rule, next standard rating per NEC 240.6. For three-phase AC, use our kVA-to-amps calculator.
From watts: A = W ÷ V · From ohms: A = V ÷ Ω
The watts path (power law). P = V × I, so I = P ÷ V. Every appliance nameplate gives you the inputs: 1,800 W ÷ 120 V = 15 A. This is the formula behind circuit sizing, generator loading, and inverter output checks. On DC and on resistive AC loads (heaters, incandescent, most electronics with PFC) it is exact.
The ohms path (Ohm's law). V = I × R, so I = V ÷ R. A 12 V battery across a 4 Ω heating element pushes 3 A. This path dominates DC work, element testing, and troubleshooting — measuring a heating element's resistance tells you its current draw before you ever energize it.
The AC caveat. With motors, compressors, and other reactive loads, some current circulates without doing work (power factor below 1.0). The complete single-phase form is I = P ÷ (V × PF), and three-phase adds the √3 factor: I = P ÷ (1.732 × V × PF). A 5 HP motor (≈3,730 W) at 240 V and 0.85 PF draws about 18.3 A — not the 15.5 A the naive formula returns. Nameplate FLA on the motor itself always wins over either formula; NEC 430.6 requires sizing from the tables, not the nameplate, for conductors — see our breaker sizing guide for that workflow.
| Watts | 12 V DC | 24 V DC | 48 V DC | 120 V AC | 240 V AC |
|---|---|---|---|---|---|
| 100 W | 8.3 A | 4.2 A | 2.1 A | 0.8 A | 0.4 A |
| 300 W | 25 A | 12.5 A | 6.3 A | 2.5 A | 1.3 A |
| 600 W | 50 A | 25 A | 12.5 A | 5 A | 2.5 A |
| 1,200 W | 100 A | 50 A | 25 A | 10 A | 5 A |
| 1,800 W | 150 A | 75 A | 37.5 A | 15 A | 7.5 A |
| 2,400 W | 200 A | 100 A | 50 A | 20 A | 10 A |
| 4,800 W | — | — | 100 A | 40 A | 20 A |
| 9,600 W | — | — | 200 A | 80 A | 40 A |
Table 1 — I = P ÷ V at common system voltages. The 12 V column shows why low-voltage DC systems need fat cable: modest watts, enormous amps.
| Resistance | 12 V | 24 V | 48 V | 120 V | 240 V |
|---|---|---|---|---|---|
| 1 Ω | 12 A | 24 A | 48 A | 120 A | 240 A |
| 2 Ω | 6 A | 12 A | 24 A | 60 A | 120 A |
| 4 Ω | 3 A | 6 A | 12 A | 30 A | 60 A |
| 8 Ω | 1.5 A | 3 A | 6 A | 15 A | 30 A |
| 12 Ω | 1 A | 2 A | 4 A | 10 A | 20 A |
| 24 Ω | 0.5 A | 1 A | 2 A | 5 A | 10 A |
| 48 Ω | 0.25 A | 0.5 A | 1 A | 2.5 A | 5 A |
Table 2 — I = V ÷ R. Note how dangerous low resistance is at line voltage: a 2 Ω fault at 240 V tries to draw 120 A — that is what breakers exist for.
Solar and battery DC sizing
DC-side solar work is a volts-to-amps drill. A 400 W panel operating at 31 V Vmp pushes 12.9 A; three in series still push 12.9 A at 93 V (series adds volts, not amps) while three in parallel push 38.7 A at 31 V. Battery banks follow the same logic: 5,000 W from a 48 V bank is 104 A — cable, fusing, and battery accessories all sized from that amp figure. Our panel wiring guide and 200 W / 12 V guide apply exactly this math.
Circuit and breaker sizing
Every residential circuit question resolves to volts-to-amps: "Can this 1,500 W heater share a 15 A, 120 V circuit?" 1,500 ÷ 120 = 12.5 A; at 125% continuous sizing that is 15.6 A — over the 15 A breaker, so the answer is no. EV charging is the same arithmetic at 240 V: a 7,680 W charger needs a 40 A circuit — see the EV charger circuit chart for the full table.
Troubleshooting with a multimeter
Techs reverse the flow constantly: measure resistance of a suspect element, compute expected current at system voltage, compare against measured current. A water heater element reading 14 Ω should draw 240 ÷ 14 ≈ 17 A; if it draws 9 A, the element is failing or the voltage is low.
Example 1 — Space heater on a 15 A circuit
- Load: 1,500 W at 120 V.
- Current: 1,500 ÷ 120 = 12.5 A.
- Continuous rule: 12.5 × 1.25 = 15.6 A → next standard size 20 A breaker and 12 AWG copper.
- Verdict: legal on a dedicated 15 A circuit only if non-continuous; as a continuous heater it belongs on 20 A.
Example 2 — Inverter draw from a battery bank
- Inverter output: 3,000 W; battery bank: 48 V DC.
- DC current (ignoring losses): 3,000 ÷ 48 = 62.5 A; at 90% inverter efficiency ≈ 69.4 A.
- Protection: 69.4 × 1.25 ≈ 87 A → 90 A fuse; cable from the ampacity chart (2 AWG copper class).
Example 3 — Heating element from resistance
- Measured element: 10 Ω on a 240 V circuit.
- Current: 240 ÷ 10 = 24 A; power check: 240 × 24 = 5,760 W.
- Circuit: 24 × 1.25 = 30 A → 30 A breaker, 10 AWG copper.
- NEC 210.20 / 215.3 (125% rule): continuous loads (3+ hours) get conductors and overcurrent devices at 125% of load current. Every "breaker hint" in the calculator applies this automatically.
- NEC 240.6 (Standard ratings): 15, 20, 25, 30, 35, 40, 45, 50 A and up. When your computed 125% value lands between sizes, round up.
- NEC 310.16 (Ampacity): the amp-to-wire bridge — 14 AWG/15 A, 12 AWG/20 A, 10 AWG/30 A copper at 60 °C terminations. Our ampacity chart reproduces the table with derating notes.
- NEC 210.23 (Permissible loads): on multi-outlet branch circuits, cord-and-plug equipment may not exceed 80% of circuit rating (12 A on a 15 A circuit, 16 A on 20 A) — the volts-to-amps math tells you whether your device fits.
| Computed amps | 125% design | Breaker (240.6) | Min. copper (310.16, 60 °C) | Typical use |
|---|---|---|---|---|
| 5 A | 6.25 A | 15 A | 14 AWG | Lighting, small loads |
| 12 A | 15 A | 15 A | 14 AWG | Space heater (continuous check!) |
| 16 A | 20 A | 20 A | 12 AWG | Microwave, small tools |
| 24 A | 30 A | 30 A | 10 AWG | Dryer, water heater |
| 32 A | 40 A | 40 A | 8 AWG | EV charger (32 A), range |
| 40 A | 50 A | 50 A | 6 AWG | Range, large EVSE |
Table 3 — From computed amps to a complete code-compliant circuit: breaker size and minimum copper conductor.
| Configuration (3 × 400 W panels, 31 Vmp / 12.9 A each) | Array volts | Array amps | Array watts |
|---|---|---|---|
| 3 in series | 93 V | 12.9 A | 1,200 W |
| 3 in parallel | 31 V | 38.7 A | 1,200 W |
| 2S (2 in series) | 62 V | 12.9 A | 800 W |
| 2S2P (4 panels) | 62 V | 25.8 A | 1,600 W |
| 3S2P (6 panels) | 93 V | 25.8 A | 2,400 W |
Table 4 — Series adds volts at constant amps; parallel adds amps at constant volts. Watts always multiply by panel count — V × A reconciles every row.
⚠ The five errors behind most blown calcs
1. Asking "how many amps is X volts" with no second number. Volts never convert to amps alone. Always grab the watts (nameplate) or ohms (meter) first.
2. Ignoring power factor on AC motor loads. I = P ÷ V is exact only at PF = 1.0. A 1,200 W compressor at 0.85 PF on 120 V draws 11.8 A — size from nameplate FLA, not the naive division.
3. Using peak watts instead of running watts. Motor starting surge (2–3×) is brief and handled by breaker trip curves; size the circuit from running watts at 125%, not from the surge.
4. Forgetting series/parallel rules. Series strings add volts (amps stay constant); parallel strings add amps (volts stay constant). Three 40 V / 10 A panels in series are 120 V at 10 A — not 30 A.
5. Sizing wire to the load instead of the breaker. The conductor must be protected by the overcurrent device: a 15 A load on a 20 A breaker still needs 12 AWG, not 14 AWG.
Can you convert volts to amps directly?
No — volts alone cannot be converted to amps. Current depends on either power or resistance: A = W ÷ V (watts mode) or A = V ÷ Ω (ohms mode). Voltage is electrical pressure; amperage is the resulting flow, and the flow is determined by what the pressure is pushing through. The calculator above handles both paths.
How many amps is 120 volts?
The question needs one more number. At 120 V, a 1,500 W space heater draws 12.5 A; a 60 W bulb draws 0.5 A; a 4 Ω load draws 30 A. Same voltage, three different currents — the wattage or resistance decides. This is why breaker sizing always starts from the load's watts or amps, never the voltage.
What is the formula for amps from volts and watts?
Amps = Watts ÷ Volts (I = P ÷ V), from the power law P = V × I. Example: 1,800 W on a 120 V circuit is 1,800 ÷ 120 = 15 A. For three-phase AC systems, the formula becomes I = P ÷ (1.732 × V × PF), which adds the √3 factor and power factor.
Does the formula change for DC and AC?
DC is always I = P ÷ V. Single-phase AC uses the same formula when power factor is 1.0 (resistive loads like heaters); motors, compressors, and electronics introduce power factor, so real current is I = P ÷ (V × PF) — a 1,200 W motor at 0.85 PF on 120 V draws 11.8 A, not 10 A.
How do I find amps from volts and ohms?
Use Ohm's law: I = V ÷ R. A 12 V battery across a 4 Ω load pushes 3 A. This mode is most useful for DC circuits, heating elements, and test loads. For AC, impedance (Z, including reactance) replaces resistance: I = V ÷ Z.
What wire and breaker do I need once I know the amps?
Multiply continuous loads by 1.25 (NEC 210.20), round up to the next standard breaker size in NEC 240.6 (15, 20, 30, 40, 50 A…), then pick conductor ampacity from NEC 310.16 — 14 AWG copper for 15 A, 12 AWG for 20 A, 10 AWG for 30 A. Our wire ampacity chart and breaker sizing guide walk both tables.
- Amps to Watts: The Reverse Calculation
- kWh to Amps Guide
- kVA to kW Conversion (Power Factor)
- Wire Ampacity Chart (NEC 310.16)
- NEC Wire Sizing Guide
- Breaker Sizing: NEC 125% Rule
- How to Wire a 240V Outlet
- How to Calculate Electrical Load (NEC)
- Can a 200W Panel Run a 12V System?
- EV Charger Circuit & Breaker Size Chart
- Shop Circuit Breakers
- Shop THHN Wire
- Shop PV Wire
- Shop EV Chargers
Have your amps? Get the gear: request a quote from Portlandia Electric Supply for breakers, wire, EV chargers, and complete solar kits.
