"How is the ampere officially defined in the SI system?"

"Since 2019, the ampere is defined by fixing the elementary charge (e) at 1.602176634 × 10⁻¹⁹ coulombs, so 1 ampere is the current that transports 1 coulomb per second. This links the unit directly to a fundamental constant of nature."

"Why are amperes important in aircraft electrical design?"

"Amperes specify the maximum safe current for wires, circuit breakers, and loads. ICAO and FAA standards require all aircraft circuits to be rated and protected in amperes to prevent overheating, fire, and ensure safety-critical systems remain operational."

"How do I convert between amperes and other units?"

"1 ampere (A) = 1,000 milliamperes (mA). 1 coulomb (C) = 1 ampere × 1 second. 1 ampere-hour (Ah) = 3,600 coulombs. For AC systems, use RMS values for accurate current measurements."

"What is the difference between amperes (A) and ampere-hours (Ah)?"

"Amperes measure current (rate of charge flow), while ampere-hours measure total charge delivered over time (battery capacity). For example, a 10 Ah battery can supply 10 A for 1 hour, or 1 A for 10 hours."

"How is current measured in practice?"

"Current is measured using ammeters (inserted in series), clamp meters (detecting magnetic fields), or multimeters. In aviation, only calibrated, approved instruments may be used for maintenance and system checks."

Ampere (A)

The ampere (A) measures electric current—the amount of charge flowing per second. It is the SI base unit essential for electrical system design, safety, and compliance, especially in aviation and engineering.

Electricity SI units Aviation Current

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Ampere (A) – The SI Unit of Electric Current

Definition and Origin

The ampere (symbol: A) is the fundamental SI unit for measuring electric current. It quantifies the rate at which electric charge flows through a conductor:
1 ampere = 1 coulomb of charge passing a point per second.

The name honors André-Marie Ampère (1775–1836), a French physicist and mathematician who founded the science of classical electromagnetism (electrodynamics).

Modern SI Definition (2019–present)

Since 2019, the ampere is defined by fixing the value of the elementary charge (e) at exactly 1.602176634 × 10⁻¹⁹ coulombs.
Thus, 1 ampere is the electric current corresponding to the flow of 1/(1.602176634 × 10⁻¹⁹) elementary charges per second.

Source: BIPM SI base unit ampere

Key Point:

1 ampere = 1 coulomb/second
1 A = 1 C/s

Historical Definition

Before 2019, the ampere was defined by the force between two parallel conductors:

The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, 1 meter apart in vacuum, would produce a force equal to 2 × 10⁻⁷ newtons per meter of length.

This definition highlights the link between electricity and magnetism, as described by Ampère’s Law.

Where Are Amperes Used?

Electrical engineering: Circuit design, wiring, protection devices
Aviation: Aircraft electrical systems, load and wiring calculations (per ICAO and FAA standards)
Power systems: Household, industrial, and transportation
Science and industry: Physics, chemistry, electronics

In aviation, amperes are crucial for specifying battery capacity, alternator/generator output, circuit breaker settings, and wire sizing.

Core Formula

Current (I) = Charge (Q) / Time (t)

I (amperes, A)
Q (coulombs, C)
t (seconds, s)

Example: A current of 5 A means 5 coulombs of charge flow each second.

Quantity	Symbol	SI Unit	Formula / Example	What It Measures
Current	I	A	I = Q / t	Charge flow per second
Voltage	V	V	V = I × R	Electric potential difference
Resistance	R	Ω	R = V / I	Opposition to current
Power	P	W	P = V × I	Rate of energy transfer
Charge	Q	C	Q = I × t	Total charge transferred
Capacity	Ah	Ah	Ah = I × time (h)	Battery charge over time

Aviation & ICAO Context

Aircraft electrical systems are rated in amperes for:

Wiring: Wire size chosen for maximum expected current to avoid overheating
Protection: Circuit breakers/fuses marked in amperes (OCPDs)
Battery capacity: Ampere-hours (Ah) specify emergency power duration
Load analysis: Current draw of each system (e.g., lights, avionics, motors) calculated for safe operation

ICAO Annex 6, Part I:
Requires electrical load analysis in amperes, proper circuit protection, and minimum battery endurance in Ah (ICAO Annex 6 ).

Practical Examples in Aviation

Device/Scenario	Typical Current (A)	Aviation Relevance
Navigation light	1–3	Circuit/wire sizing, CB selection
Cockpit panel	0.5–2	Bus load calc, redundancy
Avionics system	5–20	Load shedding, backup, isolation
Starter motor	100–800	Surge rating, relay/cable selection
Cabin lighting	10–30	Emergency power, load estimation
Pitot heater	5–15	De-icing, monitoring
Ground power unit	up to 1,000	Supply limits, external power

All protection devices (fuses, breakers) are rated in amperes per ICAO/FAA.

Measuring Current

Ammeter

Measures current in amperes when inserted in series with circuit.

Clamp Meter

Detects magnetic field of current; no circuit disconnect needed—ideal for field and aircraft maintenance.

Multimeter

Measures current (A), voltage (V), resistance (Ω). Set to correct range and connect in series for current.

Best Practice:
Always use calibrated, approved instruments per aviation maintenance standards.

Sample Calculations

Example 1:
A 140 W landing light on a 28 V bus:
I = P / V = 140 / 28 = 5 A

Example 2:
Emergency bus must supply 20 A for 15 minutes:
t = 15 min = 900 s
Q = I × t = 20 × 900 = 18,000 C

Example 3:
Pitot heater, R = 7 Ω, V = 28 V:
I = V / R = 28 / 7 = 4 A

Aircraft Wire Ampacity Table (Sample, per FAA AC 43.13-1B)

Wire Gauge (AWG)	Max Current (A)	Typical Use
22	5	Avionics, signals
18	10	Small lights, switches
14	20	Cabin lighting, loads
10	30	Main bus, relays
6	55	Power, batteries

Always use official charts for your installation.

Water Pipe Analogy

Electrical Concept	Water Analogy
Voltage	Water pressure
Current (A)	Flow rate (L/sec)
Resistance (Ω)	Pipe restriction

Just as water flows faster with higher pressure or wider pipes, more electric current (amperes) flows with higher voltage or lower resistance.

Conversion Table

From	To	Multiply By
Amperes (A)	Milliamperes	1,000
Milliamperes	Amperes	0.001
Amperes (A)	Coulombs/sec	1
Amperes (A)	Electrons/sec	6.241×10¹⁸

Electrical Safety

0.05–0.1 ampere (50–100 mA) through the body can be fatal.
Aircraft wiring and protection must prevent overcurrent (fire/equipment risk).
All aviation maintenance follows strict current (A) limits and lockout/tagout procedures.

Summary

The ampere is the SI base unit for electric current—central to all electrical calculations, system design, and safety. In aviation, every wire, breaker, and battery is specified by amperes to ensure reliability and compliance. Understanding amperes is essential for everyone involved in electrical systems, especially in regulated fields like aviation.

Frequently Asked Questions

How is the ampere officially defined in the SI system?: Since 2019, the ampere is defined by fixing the elementary charge (e) at 1.602176634 × 10⁻¹⁹ coulombs, so 1 ampere is the current that transports 1 coulomb per second. This links the unit directly to a fundamental constant of nature.
Why are amperes important in aircraft electrical design?: Amperes specify the maximum safe current for wires, circuit breakers, and loads. ICAO and FAA standards require all aircraft circuits to be rated and protected in amperes to prevent overheating, fire, and ensure safety-critical systems remain operational.
How do I convert between amperes and other units?: 1 ampere (A) = 1,000 milliamperes (mA). 1 coulomb (C) = 1 ampere × 1 second. 1 ampere-hour (Ah) = 3,600 coulombs. For AC systems, use RMS values for accurate current measurements.
What is the difference between amperes (A) and ampere-hours (Ah)?: Amperes measure current (rate of charge flow), while ampere-hours measure total charge delivered over time (battery capacity). For example, a 10 Ah battery can supply 10 A for 1 hour, or 1 A for 10 hours.
How is current measured in practice?: Current is measured using ammeters (inserted in series), clamp meters (detecting magnetic fields), or multimeters. In aviation, only calibrated, approved instruments may be used for maintenance and system checks.

Master electrical systems with confidence

Understand ampere ratings to ensure safe, compliant, and efficient system design—essential for aviation, engineering, and electrical maintenance.

Learn More Explore Aviation Standards

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