Is it okay to daisy chain a UPS?

Original Article ↗ Hacker News Discussion ↗

Daisy‑chaining uninterruptible power supplies (UPS units), surge protectors and power strips raises both safety and reliability concerns, especially when users exceed current ratings or violate electrical codes that require “relocatable power taps” to be plugged directly into permanent receptacles. Commenters note that chaining can confuse UPS electronics (particularly with modified sine wave outputs), create large inrush and recharge currents that trip upstream units, and increase fire risk at plug connections, even if surge components themselves are passive. Many argue it’s technically possible if you thoroughly understand load, wiring gauge and UPS design, but for most people the safer and more robust approach is a single appropriately sized UPS, possibly combined with redundant power feeds or dual‑PSU equipment instead of serial UPS chains.

Daisy‑chaining surge protectors / power strips / extension cords

  • Several comments say chaining surge protectors is electrically fine if:
    • Total current stays below the lowest‑rated device in the chain.
    • Wire gauge is adequate for length and load.
    • Devices are reputable and actually contain surge components.
  • Others stress safety and code:
    • NEC explicitly forbids daisy‑chaining extension cords and “relocatable power taps” (power strips/surge protectors); they must plug directly into a permanent receptacle.
    • Rationale: extra connections and splices increase chances of loose contacts, arcing, and fire; users often ignore amp ratings.
  • Voltage drop and heating:
    • Longer, thin, or coiled cords increase resistance, causing voltage drop and potentially higher current draw and overheating, especially for motors and heating tools.
    • Coiled “power wheels” can overheat because inner turns can’t dissipate heat.

Surge protector behavior and aging

  • Surge protectors typically use MOVs that shunt surges to ground and degrade over time.
  • Failure modes:
    • Trigger voltage drifts lower until MOV overheats and fails open, removing protection.
    • Better designs fuse against overheated MOVs and cut power; many have indicator lights showing protection status, but cheap units may not.
  • Used “power conditioners”/surge strips can be partially worn with no external sign; multiple large surges are not expected use.

Plugging surge protectors into a UPS

  • One view: safe in practice; surge components are passive and main concern is total load on the UPS.
  • Another view: some UPS vendors advise against it because surge circuitry can confuse the UPS’s sensing and may void warranty; recommendation is to put surge protection upstream of the UPS.

Daisy‑chaining UPS units

  • Strong consensus: generally a bad idea.
  • Technical issues cited:
    • Many consumer UPS outputs use modified or imperfect sine waves. Downstream UPS line‑monitoring often rejects this as “bad power” and stays on battery or shuts down.
    • When mains return, each UPS simultaneously powers its load and recharges its battery, creating a large transient draw that can overload upstream UPS or circuits.
    • Typical UPS input capacity is not sized to comfortably handle another UPS’s full output plus recharge current; two identically sized units in series are particularly problematic.
    • Offline/line‑interactive UPSs forward input directly and only switch to battery on out‑of‑tolerance conditions; their tolerances often don’t accept another UPS’s waveform.
  • Reliability/architecture concerns:
    • Chaining adds another single point of failure and extra conversion stages; better patterns mentioned:
      • Use one appropriately sized UPS instead of two in series.
      • Use equipment with dual PSUs fed by two independent UPS/circuits.
      • Use ATS/PDUs and parallel UPSs rather than serial chaining.
  • Some off‑grid users do run multiple UPSs, but keyed clarification: they don’t chain UPS AC outputs; instead, they treat large inverters/battery banks as the “upstream UPS” feeding ordinary UPSs normally.