How many amp in 1 volt?

There is no fixed conversion — amps and volts are independent quantities. Volts measure electrical pressure (potential difference); amps measure current (rate of charge flow). The two are linked by Ohm's law (I = V / R) and by Joule's law (P = V × I). For a known resistance R: 1 V drives I = 1 / R amps. For a known load power P: a 1 V supply must deliver I = P / 1 = P amps. Asking "how many amps in a volt" is like asking "how many miles per hour in a kilometre" — the units measure different things.

How do you calculate amp hours?

Amp-hours (Ah) = current (A) × time (h). A battery rated 100 Ah can theoretically deliver 100 A for 1 hour, or 10 A for 10 hours, or 1 A for 100 hours. In practice, the actual runtime depends on the discharge rate (Peukert effect for lead-acid) and the cutoff voltage. Use the embedded battery calculator above for runtime and required Ah for any load.

How do I calculate the amp-hour rating I need?

Compute energy (Wh) = load (W) × hours, then Ah = Wh / battery voltage. A 200 W refrigerator running 24 h on a 12 V battery: 200 × 24 / 12 = 400 Ah. Apply depth-of-discharge: lead-acid usable = 50 % → 800 Ah nameplate; LFP = 90 % → 444 Ah. Apply Peukert: for lead-acid at C/24 discharge, derate another 10–15 % → 880–920 Ah lead-acid; LFP no derating.

How do I convert kW to amps?

Use I = P / (V × PF) for AC: a 5 kW load at 240 V single-phase, PF 0.95 → I = 5 000 / (240 × 0.95) = 21.9 A. For 3-phase: I = P / (√3 × V_LL × PF). A 50 kW motor at 480 V × PF 0.9: I = 50 000 / (1.732 × 480 × 0.9) = 66.8 A. kw to amp formula: I (A) = P (W) / (V × PF) for 1-φ; or P (W) / (√3 × V × PF) for 3-φ.

How do I size 20 amp wiring?

20 amp wiring: per NEC 310.16, a 20 A circuit needs minimum #12 AWG copper (25 A @ 75 °C) or #10 AWG aluminium. NEC 210.21 also requires that 20 A receptacles be installed only on 20 A circuits (not 15 A). Common application: kitchen countertop receptacles (NEC 210.52(B)) and bathroom receptacle circuits (NEC 210.11(C)(3)). Use a 20 A breaker and 12-2 or 12-3 NM-B cable for residential.

What ampacity does 2 AWG aluminium have?

2 aluminum ampacity: per NEC 310.16, 2 AWG aluminium / copper-clad aluminium = 75 A at 60 °C, 90 A at 75 °C, 100 A at 90 °C. Common use: 100 A residential service-entrance feeders per NEC 310.12 (which allows 2 AWG Al for 100 A dwelling services as a special case, or 4 AWG Al for the 100 A "120 % of service" calculation). For 100 A subpanel feeders use 2 AWG Al at the 75 °C column (90 A) — verify by 125 % continuous-load rule.

How do I calculate amps for a 3-phase load?

3 phase amperage formula: I = P / (√3 × V_LL × PF). For a 75 kW load at 480 V PF 0.92: I = 75 000 / (1.732 × 480 × 0.92) = 98 A. The 3 phase amperage calculator, amperage calculator 3 phase, and current calculator for 3 phase use cases all reduce to this single formula. For motor loads, NEC 430.250 gives standardised FLA — use the table value for branch sizing.

What is the amperage of a typical AC unit?

AC unit amperage depends on tonnage: 1.5 ton ~ 8 A @ 230 V, 2 ton ~ 12 A, 3 ton ~ 17 A, 4 ton ~ 22 A, 5 ton ~ 28 A FLA. For sizing the breaker, use the equipment nameplate MCA (Minimum Circuit Ampacity) and MOP (Maximum Overcurrent Protection). A 3-ton condenser typically: MCA = 18 A → use #12 AWG copper; MOP = 30 A → use a 30 A HACR breaker. See the MCA / MOP calculator.

How much amperage does a 5 amp 125 V fuse protect?

A 5 amp 125 volt fuse opens at any current above 5 A (typically within a few cycles for fast-blow types, or several seconds for time-delay types per UL 198 / IEC 60269). Common uses: appliance internal protection, electronic equipment, dental and medical equipment power supplies. The voltage rating (125 V) is the maximum circuit voltage the fuse can safely interrupt — never use a lower-voltage fuse on a higher-voltage circuit.

How do I convert HP to amps?

HP amp calculator: amps = (HP × 746) / (V × η × PF) where η is motor efficiency. For a 10 HP 230 V single-phase motor at η = 0.88, PF = 0.85: I = (10 × 746) / (230 × 0.88 × 0.85) = 43.5 A — close to the NEC 430.248 value of 50 A (NEC values are conservative). Use the hp amperage chart from NEC 430.248 (1-φ) and 430.250 (3-φ) for code-compliant sizing.

Reference · Electrical · Battery · NEC 310.16 · NEC 430

Amp — Hour Calculator, Ampacity & Amp-Volt Conversions

Working reference for the four meanings of "amp" you encounter on the job: the unit of current (the ampere itself), the amp hour calculator for battery capacity (Ah), the amp per hour calculator for runtime sizing, and the conductor ampacity from NEC 310.16. Includes hp amp calculator, kw to amp formula, 3 phase amperage calculator, 2 aluminum ampacity, 20 amp wiring, and the 5 amp 125 volt fuse rating. Reviewed by a licensed PE.

Amp hour calculator + power calculator

Use the amp hour calculator for battery sizing (load + runtime → required Ah, with depth-of-discharge and Peukert correction), and the power calculator below it for amps ↔ kW ↔ HP ↔ V conversions.

CALC.008 Battery · Runtime + Sizing + Energy · 6 chemistries · Peukert

Chemistry[Chem]

Battery voltage[V]

Depth of discharge[DoD]

Runtime

— h

—

Linear runtime (no Peukert): —
Peukert-corrected runtime: —
Usable energy at DoD: — Wh
Total stored energy: — Wh
C-rate (load / capacity): —
Estimated cycles to 80%: —
Estimated weight: — kg
Standard 100 Ah modules: —

FORMULA · t = (C × DoD) / I × Peukert SOURCE · IEEE 485 · IEC 60896 · PEUKERT 1897

Power / current calculator

CALC.007 Universal Power · V · I · P · R · 3 modes · 6 solve combos

I know:[Given]

Voltage V[V]

Current I[I]

Power P[P]

Resistance R[R]

Pure DC: P = V · I. Resistance shown is V/I (Ohm's law equivalent).

Voltage V

— V

Current I

— A

Power P

— W

Resistance R

— Ω

Apparent power S: — kVA
Reactive power Q: — kVAR
Power factor used: —
Mechanical equivalent: — HP
Heat output: — BTU/hr

Show your work

P = V · I = ...

FORMULA · P = V · I (DC) SOURCE · OHM 1827 · IEEE STD 100

Amp formulas — current, charge, ampacity

Eq. 01 — Amp from current definition SI

I = \frac{dQ}{dt} \quad (1 \, \text{A} = 1 \, \text{C/s})

·: I in amperes (A), Q in coulombs (C), t in seconds.
·: Defined since 2019 SI revision via the elementary charge: 1 A = 1/(1.602 × 10⁻¹⁹) e/s.
·: Foundation of all electrical measurement.

Eq. 02 — Amp-hour from load and runtime SI

Ah = I_{load} \cdot t_{(h)} = \frac{P_{load} \cdot t_{(h)}}{V_{battery}}

·: Used for battery capacity sizing.
·: For a 60 W load on 12 V running 5 h: Ah = 60 × 5 / 12 = 25 Ah.
·: Multiply by 1/DoD (e.g., 1/0.5 for lead-acid, 1/0.9 for LFP) to get nameplate Ah.

Eq. 03 — kW to amps (single-phase and three-phase) SI

I = \frac{P}{V \cdot \cos \varphi} \quad (1\text{-}\varphi); \quad I = \frac{P}{\sqrt{3} \cdot V_{LL} \cdot \cos \varphi} \quad (3\text{-}\varphi)

·: For 10 kW @ 240 V 1-φ PF 0.95: I = 10 000 / (240 × 0.95) = 43.9 A.
·: 10 amp to kw (at 240 V PF 0.95): P = 10 × 240 × 0.95 = 2.28 kW.
·: For motor loads, use NEC 430.248/250 standardised FLA — not the formula.

Eq. 04 — HP to amps SI

I = \frac{HP \cdot 746}{V \cdot \eta \cdot \cos \varphi}

·: η = motor efficiency (0.85–0.95 typical for premium induction motors).
·: For 5 HP × 230 V × η 0.88 × PF 0.85: I = 5 × 746 / (230 × 0.88 × 0.85) = 21.7 A.
·: NEC Table 430.248 lists 28 A for 5 HP 230 V 1-φ — use NEC value for sizing.

Standards governing amp ratings

Standard	Scope	Region
SI / BIPM	Definition of the ampere via elementary charge (2019)	Worldwide
NEC / NFPA 70 — Table 310.16	Conductor ampacity by AWG and insulation	USA
NEC Table 430.248 / 430.250	Motor full-load amperes (1-φ and 3-φ)	USA
NEC 240.6(A)	Standard fuse and breaker ratings	USA
UL 198 / IEC 60269	Low-voltage fuses (5 A 125 V class)	USA / Worldwide
IEC 60364	Low-voltage installation ampacity	Worldwide
BCI / IEC 60896	Battery ampere-hour rating standards	USA / Worldwide

HP amperage chart + kW amperage chart

The hp amperage chart for single-phase and three-phase induction motors per NEC 430.248 / 430.250 (use these values for branch-circuit sizing, not nameplate). The kw amperage chart follows the same data converted via 1 HP = 0.746 kW.

HP	kW	1-φ 240 V (A)	3-φ 230 V (A)	3-φ 460 V (A)
1	0.75	8	4.2	2.1
2	1.5	12	6.8	3.4
5	3.7	28	15.2	7.6
10	7.5	50	28	14
20	15	100	54	27
30	22	—	80	40
50	37	—	130	65
100	75	—	248	124
200	150	—	480	240

Aluminium ampacity reference

The 2 aluminum ampacity and 6 aluminum ampacity values per NEC 310.16. Aluminium runs ~ 22 % lower ampacity than copper at the same gauge — typical residential 200 A service uses 4/0 aluminium SE cable.

AWG Al	60 °C (A)	75 °C (A)	90 °C (A)
12 Al	15	20	25
10 Al	25	30	35
8 Al	35	40	45
6 Al	40	50	55
4 Al	55	65	75
2 Al	75	90	100
1/0 Al	100	120	135
2/0 Al	115	135	150
4/0 Al	150	180	205
250 kcmil Al	170	205	230

Decide whether you need amps (current), amp-hours (charge), or amp-rating (ampacity). Amps (A) = instantaneous current = charge per second. Amp-hours (Ah) = battery capacity = the steady current a battery can deliver for one hour (or 0.5 A for two hours, etc.). Ampacity = the maximum current a conductor can carry continuously without insulation damage (NEC 310.16). The same word "amp" gets used for all three.
For amps from a load: use the V-I-P-R formulas. DC: I = V / R or I = P / V. Single-phase AC: I = P / (V × PF). Three-phase AC: I = P / (√3 × V_LL × PF). For motor loads, NEC 430.250 gives the standardised FLA — use that, not nameplate.
For amp-hours from a battery + load: divide energy by voltage. Required Ah = (load in W × runtime in hours) / battery voltage. A 60 W LED running 5 h on a 12 V battery: Ah = 60 × 5 / 12 = 25 Ah. Apply DoD (depth-of-discharge): lead-acid 50 % usable → 50 Ah nameplate; LiFePO₄ 90 % → 28 Ah nameplate.
For ampacity from a wire size: use NEC 310.16. Look up the AWG row × insulation column. #12 Cu THWN-2 @ 75 °C = 25 A. #2 Cu @ 75 °C = 115 A. For aluminium: derate ~ 22 % (#2 Al = 90 A). For 90 °C insulation: use the 90 °C column only if both terminations are 90 °C-rated.
Match standard breaker / fuse to the calculated current. NEC 240.6: round up the calculated I (× 1.25 for continuous loads) to the next standard size — 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 A, etc. The breaker size cannot exceed the wire ampacity unless an exception applies.

Worked example — 30 A 240 V circuit + 100 Ah battery backup

Spec the branch wiring and battery backup for a 5 kW continuous load on a 240 V single-phase service:

Step 1 — load current: I = 5 000 / (240 × 0.95) = 21.9 A. Continuous-load multiplier: 21.9 × 1.25 = 27.4 A → use a 30 amp 240 volt breaker per NEC 240.6.

Step 2 — wire size: 27.4 A → #10 AWG copper THWN-2 (35 A @ 75 °C per NEC 310.16). Voltage drop on 50 ft round-trip: 21.9 × 2 × 50 × 1.24 / 1000 = 2.7 V (1.1 %). OK.

Step 3 — battery backup sizing: 8 hours of runtime backup. Energy = 5 kW × 8 h = 40 kWh. At 48 V battery system: required Ah = 40 000 / 48 = 833 Ah. Apply DoD: lead-acid 50 % → 1 666 Ah nameplate; LFP 90 % → 926 Ah nameplate.

Step 4 — battery configuration: 926 Ah at 48 V = ~ 6 × 200 Ah LFP modules in series-parallel (4 in series for 51.2 V × 4 banks paralleled = 800 Ah, slightly under-sized; or 4S × 5P = 1 000 Ah at 51.2 V, 5 % over-sized — pick 5P for margin).

Step 5 — fuse on battery side: short-circuit current of 200 Ah LFP = ~ 2 000–4 000 A. Pick a 200 A Class T fuse rated 80 V DC, 20 kA AIC.

Amps vs amp-hours vs ampacity

Quantity	Symbol / unit	What it measures	Used for
Current	I / A (ampere)	Instantaneous rate of charge flow	Sizing wires (must be ≤ ampacity), reading clamp meter
Charge / capacity	Q / Ah (ampere-hour)	Total charge transferable (current × time)	Battery rating, runtime calculation
Ampacity	I_amp / A	Maximum sustained current allowed in a conductor (NEC 310.16)	Picking AWG for a given load
HP / kW (related, not amps)	HP, kW	Power — rate of energy transfer	Motor sizing; converted to amps via I = P / (V × PF)

Variants and special amp cases

Amperage — calculations and meter readings

"Amperage" is the everyday English word for current (measured in amperes, A). The most common engineering lookups are: ampacity (NEC 310.16 conductor capacity — see /wire-size/), amperage drop on a long run (use the voltage-drop calculator at /voltage-drop/ and convert the result to percent current loss), and DC amperage which is the simple I = V / R or P / V with no power factor. Typical residential heat pump amperage runs 12 A at 240 V for a 2-ton unit and 32 A at 240 V for a 5-ton — size MCA / MOP per NEC 440. A "200 amp meter can" is the utility meter socket rated for 200 A continuous, common on residential service entrances.

Amp hours — battery capacity and runtime

Amp-hours (Ah) measure battery capacity. To size a battery: required Ah = (load W × runtime h) / battery V, then divide by usable depth-of-discharge (lead-acid 0.5, AGM 0.6, LFP 0.9). For a 100 W load at 12 V running 10 hours on lead-acid: nameplate Ah = (100 × 10 / 12) / 0.5 = 167 Ah. The reverse question — how many amps per hour does my system draw — gives the average current as load W divided by system V, so a 1 200 W load at 12 V averages 100 A.

Amp ↔ volt conversions

The query "how many amps in a volt" reflects a common confusion: amps and volts measure independent quantities and aren\'t interchangeable. They\'re linked by Ohm\'s law (I = V / R, requires resistance), Joule\'s law (I = P / V, requires power), and the AC power formula (P = V × I × PF). So "20 amp to volts" only makes sense given a load — a 20 A current on a 240 V circuit is 4 800 W; a 20 A current through a 6 Ω resistor produces a V = 20 × 6 = 120 V drop. Use the embedded calculator above for the exact conversion in any direction.

Frequently asked questions

How many amp in 1 volt?: There is no fixed conversion — amps and volts are independent quantities. Volts measure electrical pressure (potential difference); amps measure current (rate of charge flow). The two are linked by Ohm's law (I = V / R) and by Joule's law (P = V × I). For a known resistance R: 1 V drives I = 1 / R amps. For a known load power P: a 1 V supply must deliver I = P / 1 = P amps. Asking "how many amps in a volt" is like asking "how many miles per hour in a kilometre" — the units measure different things.
How do you calculate amp hours?: Amp-hours (Ah) = current (A) × time (h). A battery rated 100 Ah can theoretically deliver 100 A for 1 hour, or 10 A for 10 hours, or 1 A for 100 hours. In practice, the actual runtime depends on the discharge rate (Peukert effect for lead-acid) and the cutoff voltage. Use the embedded battery calculator above for runtime and required Ah for any load.
How do I calculate the amp-hour rating I need?: Compute energy (Wh) = load (W) × hours, then Ah = Wh / battery voltage. A 200 W refrigerator running 24 h on a 12 V battery: 200 × 24 / 12 = 400 Ah. Apply depth-of-discharge: lead-acid usable = 50 % → 800 Ah nameplate; LFP = 90 % → 444 Ah. Apply Peukert: for lead-acid at C/24 discharge, derate another 10–15 % → 880–920 Ah lead-acid; LFP no derating.
How do I convert kW to amps?: Use I = P / (V × PF) for AC: a 5 kW load at 240 V single-phase, PF 0.95 → I = 5 000 / (240 × 0.95) = 21.9 A. For 3-phase: I = P / (√3 × V_LL × PF). A 50 kW motor at 480 V × PF 0.9: I = 50 000 / (1.732 × 480 × 0.9) = 66.8 A. kw to amp formula: I (A) = P (W) / (V × PF) for 1-φ; or P (W) / (√3 × V × PF) for 3-φ.
How do I size 20 amp wiring?: 20 amp wiring: per NEC 310.16, a 20 A circuit needs minimum #12 AWG copper (25 A @ 75 °C) or #10 AWG aluminium. NEC 210.21 also requires that 20 A receptacles be installed only on 20 A circuits (not 15 A). Common application: kitchen countertop receptacles (NEC 210.52(B)) and bathroom receptacle circuits (NEC 210.11(C)(3)). Use a 20 A breaker and 12-2 or 12-3 NM-B cable for residential.
What ampacity does 2 AWG aluminium have?: 2 aluminum ampacity: per NEC 310.16, 2 AWG aluminium / copper-clad aluminium = 75 A at 60 °C, 90 A at 75 °C, 100 A at 90 °C. Common use: 100 A residential service-entrance feeders per NEC 310.12 (which allows 2 AWG Al for 100 A dwelling services as a special case, or 4 AWG Al for the 100 A "120 % of service" calculation). For 100 A subpanel feeders use 2 AWG Al at the 75 °C column (90 A) — verify by 125 % continuous-load rule.
How do I calculate amps for a 3-phase load?: 3 phase amperage formula: I = P / (√3 × V_LL × PF). For a 75 kW load at 480 V PF 0.92: I = 75 000 / (1.732 × 480 × 0.92) = 98 A. The 3 phase amperage calculator, amperage calculator 3 phase, and current calculator for 3 phase use cases all reduce to this single formula. For motor loads, NEC 430.250 gives standardised FLA — use the table value for branch sizing.
What is the amperage of a typical AC unit?: AC unit amperage depends on tonnage: 1.5 ton ~ 8 A @ 230 V, 2 ton ~ 12 A, 3 ton ~ 17 A, 4 ton ~ 22 A, 5 ton ~ 28 A FLA. For sizing the breaker, use the equipment nameplate MCA (Minimum Circuit Ampacity) and MOP (Maximum Overcurrent Protection). A 3-ton condenser typically: MCA = 18 A → use #12 AWG copper; MOP = 30 A → use a 30 A HACR breaker. See the MCA / MOP calculator.
How much amperage does a 5 amp 125 V fuse protect?: A 5 amp 125 volt fuse opens at any current above 5 A (typically within a few cycles for fast-blow types, or several seconds for time-delay types per UL 198 / IEC 60269). Common uses: appliance internal protection, electronic equipment, dental and medical equipment power supplies. The voltage rating (125 V) is the maximum circuit voltage the fuse can safely interrupt — never use a lower-voltage fuse on a higher-voltage circuit.
How do I convert HP to amps?: HP amp calculator: amps = (HP × 746) / (V × η × PF) where η is motor efficiency. For a 10 HP 230 V single-phase motor at η = 0.88, PF = 0.85: I = (10 × 746) / (230 × 0.88 × 0.85) = 43.5 A — close to the NEC 430.248 value of 50 A (NEC values are conservative). Use the hp amperage chart from NEC 430.248 (1-φ) and 430.250 (3-φ) for code-compliant sizing.

The ampere — re-defined by elementary charge (2019)

The ampere, symbol A, is the SI unit of electric current. It is defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 × 10⁻¹⁹ when expressed in the unit C, which is equal to A·s. This 2019 redefinition replaces the earlier (1948) definition based on the magnetic force between parallel wires, originally derived from André-Marie Ampère\'s 1820 quantification of the force between current-carrying conductors.

26th Conférence Générale des Poids et Mesures (CGPM) → Resolution 1 (2018), entered force 20 May 2019 — SI re-definition

Related calculators and references

Sources and further reading

BIPM. SI Brochure (9th edition, 2019). Redefinition of the ampere via elementary charge.
NFPA. NFPA 70 — National Electrical Code (2023), Articles 240, 310, 430, 440.
NEMA. NEMA MG 1-2021 — Motors and Generators, Tables 12-12 / 12-13 — full-load currents.
BCI. BCI Battery Council International — Group Size Specifications. Amp-hour and reserve-capacity definitions.
IEEE. IEEE Std 485-2020 — Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications.
UL. UL 198 — Low-Voltage Fuses; UL 489 — Molded-Case Circuit Breakers. Standard fuse / breaker rating tests.