Harmonics

Harmonics are voltage or current components at integer multiples of the fundamental line frequency — on a 60 Hz North American system, the 3rd harmonic sits at 180 Hz, the 5th at 300 Hz, the 7th at 420 Hz, and so on. They appear whenever a nonlinear load draws current in pulses rather than in a smooth sine wave, and the switch-mode power supplies inside ASIC miners are textbook nonlinear loads: a rectifier front-end charges bulk capacitors in short gulps near each voltage peak. One miner's distortion is a rounding error; a room full of them can inject significant harmonic current onto shared wiring, and the effects show up as heat, nuisance trips, and unhappy transformers.

How harmonics are quantified

Total harmonic distortion (THD) is the standard metric: the RMS sum of all harmonic components expressed as a percentage of the fundamental, measured separately for voltage and for current. IEEE 519 is the reference standard, setting recommended distortion limits at the point of common coupling — the spot where a facility meets the utility. Voltage THD is mostly the utility's and transformer's problem; current THD is yours, because your loads create it. Keeping current distortion within the recommended envelope is part of being a good neighbor on a shared transformer, and utilities serving large mining loads increasingly check. Note that a basic clamp meter reads misleadingly here: only a true-RMS meter captures the real heating value of a distorted waveform, and a cheap averaging meter can under-read a harmonic-rich current substantially.

Why harmonics matter in a mining deployment

Harmonic currents cause extra heating everywhere they flow. Conductor losses rise because higher frequencies crowd current toward the conductor surface (skin effect). Transformers suffer added eddy-current losses and are commonly derated — or specified as K-rated — when feeding heavy nonlinear load. The neutral conductor deserves special fear: triplen harmonics (the 3rd, 9th, 15th…) do not cancel between phases the way balanced fundamental currents do; they add arithmetically in the neutral of a three-phase wye system, so a neutral sized on fundamental assumptions can run hotter than any phase conductor while a simple ammeter on the phases shows nothing wrong. Harmonic-rich current also has a high crest factor — tall, narrow peaks — which stresses components rated for sinusoidal duty and can trip peak-sensing protection below its nominal rating. A circuit breaker that trips mysteriously at apparent 70% load in a hot hashroom is often reacting to exactly this combination.

The PSU's role, and mitigation

Modern high-power miner supplies — the PSU class that includes Bitmain's APW12 — incorporate active power factor correction (PFC), a boost stage that forces input current to track the voltage waveform. Good PFC delivers near-unity power factor and low current THD, which is why a healthy modern fleet is far kinder to the wiring than the equal wattage of cheap uncorrected supplies would be. Mitigation beyond the PSU follows a standard ladder: oversize or double the neutral on shared circuits, specify K-rated or oversized transformers, spread nonlinear load across phases, and — at facility scale — deploy passive tuned filters or active harmonic filters that inject canceling current. For a home miner on a 240 V branch circuit, the practical checklist is short: run quality PFC-equipped supplies, use true-RMS measurement, and do not daisy-chain a wall of miners onto wiring that was sized for resistive loads. See power quality for the wider family of disturbances harmonics belong to.