hectoamp

A hectoamp is a unit of electrical current equal to 100 amperes, derived from the SI prefix hecto (meaning 10²) applied to the ampere. The symbol is hA. While it sits correctly within the metric hierarchy alongside kiloamps and megaamps, the hectoamp occupies an awkward middle ground where it is almost never used in industrial measurement or specification.

The SI metric system includes ten standard prefixes for multiples: deca, hecto, kilo, mega, and so on. In electrical work, however, the jump from amperes to kiloamperes (1,000 A) covers most practical needs. A hectoamp falls between them: too small to be convenient for high-current applications like welding or power distribution, yet too large to subdivide common laboratory and control circuits. This awkwardness is reflected in real-world practice, where you will encounter amps, milliamps, and kiloamps regularly, but virtually never read a specification written in hectoamps.

Where it might appear: A hectoamp could theoretically describe a current in the range of 50 to 200 A, such as that drawn by large industrial motors or arc furnaces. A 150 kW three-phase motor running at 380 V might draw roughly 230 A, which could be written as 2.3 hA, but this is not standard practice. Equipment nameplates and technical drawings use amperes directly.

Metrologists and standards bodies maintain the hectoamp as a formally correct unit because it preserves the logical completeness of the SI system. However, the electrical trades have converged on a practical hierarchy: amperes for control circuits and measurement instruments, kiloamperes for power transmission and heavy equipment, and megaamperes for phenomena like lightning or pulsed power research. The hectoamp remains a ghost unit: technically valid but functionally unnecessary.