Is it okay to daisy chain a UPS?

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.